Climate Change and...
Effects of Climate Change
- Climate Variability
- Climate Models
Effects of Climate Change
Aber, J. D., Ollinger, S. V., Federer, C. A., Reich, P. B., Goulden, M. L., Kicklighter, D. W., Melillo, J. M., Lathrop, R. G., Jr. (1995). Predicting the effects of climate change on water yield and forest production in the northeastern United States. Climate Research 5 (3): 207-222
ABSTRACT: Rapid and simultaneous changes in temperature, precipitation and the atmospheric concentration of CO2 are predicted to occur over the next century. Simple, well-validated models of ecosystem function are required to predict the effects of these changes This paper describes an improved version of a forest carbon and water balance model (PnET-II) and the application of the model to predict stand- and regional-level effects of changes in temperature, precipitation and atmospheric CO2 (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements was parameterized and run for 4 forest/site combinations and validated against available data for water yield, gross and net carbon exchange and biomass production. The validation exercise suggests that the determination of actual water availability to stands and the occurrence or non-occurrence of soil-based water stress are critical to accurate modeling of forest net primary production (NPP) and net ecosystem production (NEP). The model was then run for the entire New England/New York (USA) region using a 1 km resolution geographic information system. Predicted long-term NEP ranged from -85 to +275 g C m-2 yr-1 for the 4 forest/site combinations, and from -150 to 350 g cm-2 yr-1 for the region, with a regional average of 76 g C m-2 yr-1 . A combination of increased temperature (+6 degree C), decreased precipitation (-15%) and increased Water use efficiency (2x, due to doubling of CO2 ) resulted generally in increases in NPP and decreases in water yield over the region
Bréda, N., Huc, R., Granier, A., Dreyer, E. (2006). Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences. Annals of Forest Science 63 (6): 625-644
ABSTRACT: The extreme drought event that occurred in Western Europe during 2003 highlighted the need to understand the key processes that may allow trees and stands to overcome such severe water shortages. We therefore reviewed the current knowledge available about such processes. First, impact of drought on exchanges at soil-root and canopy-atmosphere interfaces are presented and illustrated with examples from water and CO2 flux measurements. The decline in transpiration and water uptake and in net carbon assimilation due to stomatal closure has been quantified and modelled. The resulting models were used to compute water balance at stand level basing on the 2003 climate in nine European forest sites from the CARBOEUROPE network. Estimates of soil water deficit were produced and provided a quantitative index of soil water shortage and therefore of the intensity of drought stress experienced by trees during summer 2003. In a second section, we review the irreversible damage that could be imposed on water transfer within trees and particularly within xylem. A special attention was paid to the inter-specific variability of these properties among a wide range of tree species. The inter-specific diversity of hydraulic and stomatal responses to soil water deficit is also discussed as it might reflect a large diversity in traits potentially related to drought tolerance. Finally, tree decline and mortality due to recurrent or extreme drought events are discussed on the basis of a literature review and recent decline studies. The potential involvement of hydraulic dysfunctions or of deficits in carbon storage as causes for the observed long term (several years) decline of tree growth and development and for the onset of tree dieback is discussed. As an example, the starch content in stem tissues recorded at the end of the 2003's summer was used to predict crown conditions of oak trees during the following spring: low starch contents were correlated with large twig and branch decline in the crown of trees.
Calef, M. P., Mcguire, A. D., Epstein, H. E., Rupp, T. S., Shugart, H. H. (2005). Analysis of vegetation distribution in Interior Alaska and sensitivity to climate change using a logistic regression approach. Journal of Biogeography 32 (5): 863-878
ABSTRACT:To understand drivers of vegetation type distribution and sensitivity to climate change.Interior Alaska.A logistic regression model was developed that predicts the potential equilibrium distribution of four major vegetation types: tundra, deciduous forest, black spruce forest and white spruce forest based on elevation, aspect, slope, drainage type, fire interval, average growing season temperature and total growing season precipitation. The model was run in three consecutive steps. The hierarchical logistic regression model was used to evaluate how scenarios of changes in temperature, precipitation and fire interval may influence the distribution of the four major vegetation types found in this region.At the first step, tundra was distinguished from forest, which was mostly driven by elevation, precipitation and south to north aspect. At the second step, forest was separated into deciduous and spruce forest, a distinction that was primarily driven by fire interval and elevation. At the third step, the identification of black vs. white spruce was driven mainly by fire interval and elevation. The model was verified for Interior Alaska, the region used to develop the model, where it predicted vegetation distribution among the steps with an accuracy of 60–83%. When the model was independently validated for north-west Canada, it predicted vegetation distribution among the steps with an accuracy of 53–85%. Black spruce remains the dominant vegetation type under all scenarios, potentially expanding most under warming coupled with increasing fire interval. White spruce is clearly limited by moisture once average growing season temperatures exceeded a critical limit (+2 °C). Deciduous forests expand their range the most when any two of the following scenarios are combined: decreasing fire interval, warming and increasing precipitation. Tundra can be replaced by forest under warming but expands under precipitation increase.The model analyses agree with current knowledge of the responses of vegetation types to climate change and provide further insight into drivers of vegetation change.
Goldstein, A. H., Bauer, M. R., Panek, J. A., Xu, M., Qi, Y., Guenther, A. B., Baugh, W., Hultman, N. E., Fracheboud, J. M. (2000). Effects of climate variability on the carbon dioxide, water, and sensible heat fluxes above a ponderosa pine plantation in the Sierra Nevada (CA). Agricultural and Forest Meteorology 101 (2-3): 113-129
ABSTRACT: Fluxes of CO2 , water vapor, and sensible heat were measured by the eddy covariance method above a young ponderosa pine plantation in the Sierra Nevada Mountains (CA) over two growing seasons (1 June–10 September 1997 and 1 May–30 October 1998). The Mediterranean-type climate of California is characterized by a protracted summer drought, with precipitation occurring mainly from October through May. While drought stress increased continuously over both summer growing seasons, 1998 was wetter and cooler than average due to El Niño climate patterns and 1997 was hotter and drier than average. One extreme 3-day heat wave in 1997 (Days 218–221) caused a step change in the relationship between H2 O flux and vapor pressure deficit, resulting in a change in canopy conductance, possibly due to cavitation of the tree xylem. This step change was also correlated with decreased rates of C sequestration and evapotranspiration; we estimate that this extreme climatic event decreased gross ecosystem production (GEP) by roughly 20% (4μmol C m−2 s−1 ) for the rest of the growing season. In contrast, a cooler, wetter spring in 1998 delayed the onset of photosynthesis by about 3 weeks, resulting in roughly 20% lower GEP relative to the spring of 1997. We conclude that the net C balance of Mediterranean-climate pine ecosystems is sensitive to extreme events under low soil moisture conditions and could be altered by slight changes in the climate or hydrologic regime.
Grant, R.F., Black, T.A., Gaumont-Guay, D., Klujn, N., Barr, A.G., Morgenstern, K., Nesic, Z. (2006). Net ecosystem productivity of boreal aspen forests under drought and climate change: Mathematical modelling with Ecosys. Agricultural and Forest Meteorology 140 (1-4): 152-170
ABSTRACT: The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (Ea ) and precipitation (Pa ). Changes in Ea versus Pa during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of Ea versus Pa on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO2 uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO2 and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO2 effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO2 effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO2 exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in g C m−2 : 275 versus 367 ± 110 in 2001, 82 versus 144 ± 43 in 2002 and 23 versus 104 ± 31 in 2003). Lower modelled NEP was attributed to the larger modelled CO2 effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP = NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4 g C m−2 year−1 . NBP rose by 10, 108 and 126 g C m−2 year−1 during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP.
Krishnan, P., Black, T. A., Grant, N. J., Barr, A. G., Hogg, E. T. H., Jassal, R. S., Morgenstern, K. (2006). Impact of changing soil moisture distribution on net ecosystem productivity of a boreal aspen forest during and following drought. Agricultural and Forest Meteorology 139 (3-4): 208-223
ABSTRACT: The interannual and seasonal variability of gross ecosystem photosynthesis (P), ecosystem respiration (R) and evapotranspiration (E), and their relationships to environmental factors were used to explain changes in net ecosystem productivity (FNEP) at the onset of, during, and following a 3-year-long (2001–2003) drought in a mature boreal aspen stand in central Saskatchewan, Canada. The forest was a moderate carbon (C) sink over its entire 11-year data record (FNEP = 153 ± 99 g C m−2 year−1 ), including the peak drought years of 2002 and 2003. In 2001, the depletion of water near the soil surface likely reduced heterotrophic soil respiration while water remaining deep in the root zone maintained P above the pre-drought mean, resulting in above-average FNEP. In 2002 and 2003, the forest remained a C sink even though P was below average because R was also below average—a likely consequence of the influence of low soil water content in deep and shallow soil layers on both autotrophic and heterotrophic respiration. In 2004, the recharge of soil water in shallow soil layers allowed R to recover to its pre-drought values, whereas low spring temperatures, the slow recharge of soil water in deep soil layers in spring, late leaf emergence and diminished leaf area index combined to suppress P and produce the lowest annual FNEP of the 11-year record (4 g C m−2 year−1 ). The low FNEP and P were mirrored in the lowest stem growth and LAI values of the 11-year record. In 2005, a warm wet year, both the annual values and seasonal variations of FNEP, P and R returned to those of pre-drought years; the partial recovery of LAI to pre-drought values suggests that aspen P was able to adjust to this restriction on C assimilation. Growing season average dry surface conductance (gsv), the Priestley–Taylor coefficient (α) and light use efficiency (LUE) reached their lowest values in 2003 and became similar to pre-drought years in 2004–2005. Water use efficiency (WUE) was highest in 2003 and remained above average in 2004 and 2005. At the ecosystem scale, the above-average gains made in C sequestration in the first year of the drought were significantly offset by below-average stand FNEP in the final 2 years of the drought, and in the year following the drought.
Millard,Peter, Sommerkorn,Martin, Grelet,Gwen-Aelle (2007). Environmental change and carbon limitation in trees: a biochemical, ecophysiological and ecosystem appraisal. New Phytologist 175 (1): 11-28
ABSTRACT: As C3 photosynthesis is not yet CO2 -saturated, forests offer the possibility of enhanced growth and carbon (C) sequestration with rising atmospheric CO2 . However, at an ecosystem scale, increased photosynthetic rates are not always translated into faster tree growth, and in free air carbon enrichment (FACE) experiments with trees, the stimulation in above-ground growth often declines with time. So is tree growth C-limited? The evidence is reviewed here at three different scales. First, at the biochemical scale, the role of Rubisco is discussed by considering its evolution and role as a nitrogen (N) storage protein. Second, at the ecophysiological scale, C allocation to gain nutrients from the soil is considered and it is argued that any C limitation is only through a limitation to soil nutrient cycling. Finally, the response of forest ecosystems to rising atmospheric CO2 concentrations is considered and evidence from FACE experiments is discussed. From the three lines of evidence we conclude that the growth of trees is not C-limited, with the key to understanding future responses to climate change being turnover of soil organic matter and nutrient cycling.
G.M. Blate, L.A. Joyce, J. S. Littell, S.G. McNulty, C. I. Millar, S.C. Moser, R.P. Neilson, K. O’Halloran, D.L. Peterson (2009). Adapting to climate change in United States national forests. Unasylva 60 (231-232): 57-62
FIRST PARAGRAPH: Climate change is already affecting forests and other ecosystems, and additional, potentially more severe impacts are expected (IPCC, 2007; CCSP, 2008a, 2008b). As a result, forest managers are seeking practical guidance on how to adapt their current practices and, if necessary, their goals. Adaptations of forest ecosystems, which in this context refer to adjustments in management (as opposed to “natural” adaptation), ideally would reduce the negative impacts of climate change and help managers take advantage of any positive impacts.
This article summarizes key points from a review of climate change adaptation options for United States national forests (Joyce et al., 2008) produced under the auspices of the United States Climate Change Science Program (CCSP) (see Box). The study sought to provide practical information on potential adaptation options for resource managers by asking:
• How will climate change affect the ability of resource managers to achieve their management goals?
• What might a resource manager do to prepare the management system for climate change impacts while maintaining current goals (and constantly evaluating if these goals need to be modified or re-prioritized)?
R.E.J. Boerner, J. Huang, S. C. Hart (2008). Fire, thinning, and the carbon economy: Effects of fire and fire surrogate treatments on estimated carbon storage and sequestration rate. Forest Ecology and Management 255 (8-9): 3081-3097
ABSTRACT: Changes in estimated standing stocks of carbon (C) in vegetation, forest floor, dead wood, and mineral soil for the fire and fire surrogate (FFS) network sites were evaluated in relation to the application of prescribed fire, mechanical treatments designed as surrogates for prescribed fire, and the combination of mechanical treatment and fire. Pre-treatment C stocks and changes in C stocks over two intervals (pre-treatment to first post-treatment year and first post-treatment to a 2nd, 3rd, or 4th post-treatment year, depending on site) were evaluated using meta-analytical methods. Total C storage across the network averaged 185 ± 8 (standard error) Mg C ha−1 , of which 45% was in vegetation, 38% in soil organic matter, 10% in the forest floor and 7% in dead wood. C stored in vegetation was not significantly affected by prescribed fire, but decreased ~30 Mg ha−1 as the result of mechanical or mechanical + fire treatment; in contrast, forest floor C storage was reduced by ~1–7 Mg ha−1 by fire or mechanical + fire treatment, but unaffected by mechanical treatment alone. Neither dead wood C nor soil organic C was significantly affected by the treatments. At the network scale, total ecosystem C was not significantly affected by fire, though four individual sites did exhibit significant C losses to fire. Mechanical treatment, with or without fire, produced significant reductions of 16–32 Mg ha−1 during the first post-treatment year, but this was partially balanced by enhanced net C uptake of ~12 Mg ha−1 during the subsequent 1–3 years. In terms of C storage and uptake, western coniferous forests responded differently to the FFS treatments than did eastern deciduous, coniferous, and mixed forests, suggesting that optimal management for fire, harvesting, and C sequestration may differ between regions.
ABSTRACT: Models that address the impacts of climate change on forests are reviewed at four levels of biological organization: global, regional or landscape, community, and tree. The models are compared for their ability to assess changes in fluxes of biogenic greenhouse gases, land use, patterns of forest type or species composition, forest resource productivity, forest health, biodiversity, and wildlife habitat. No one model can address all of these impacts, but landscape transition models and regional vegetation and land-use models have been used to consider more impacts than the other models. The development of landscape vegetation dynamics models of functional groups is suggested as a means to integrate the theory of both landscape ecology and individual tree responses to climate change. Risk assessment methodologies can be adapted to deal with the impacts of climate change at various spatial and temporal scales. Four areas of research needing additional effort are identified: (1) linking socioeconomic and ecologic models; (2) interfacing forest models at different scales; (3) obtaining data on susceptibility of trees and forest to changes in climate and disturbance regimes; and (4) relating information from different scales.
ABSTRACT: Characteristics of tree species may uniquely situate them to withstand environmental changes. Paleoecological evidence indicates that the geographic ranges of tree species have expanded and contracted several times since the last glacial epoch in response to directional environmental changes. For most tree species, these range fluctuations have been accomplished without any apparent loss of genetic diversity. A possible explanation that distinguishes most trees from many herbaceous plants is that much of the genetic variation within tree species is found within rather than among their populations. Thus, the extinction of a relatively large proportion of a tree species’ populations would result in relatively little overall loss of genetic diversity. Furthermore, phylogeographic studies indicate that for some tree species, habitat heterogeneity (elevation, slope aspect, moisture, etc.) in glacial refugia may have preserved adaptive genetic variation that, when recombined and exposed to selection in newly colonized habitats, gave rise to the local adaptation currently seen.
The maintenance of genetic diversity in the face of extensive habitat fragmentation is also a concern. Many forest trees, however, may be buffered from the adverse effects of habitat fragmentation. First, the longevity of individual trees may retard population extinction and allow individuals and populations to survive until habitat recovery occurs. Second, considerable evidence is available that both animal and wind-pollinated tree species in fragments experience levels of pollen flow that are sufficient to counteract the effects of genetic drift. The combination of individual longevity, high intra-population genetic diversity and the potential for high rates of pollen flow should make tree species especially resistant to extinction and the loss of genetic diversity during changing environmental conditions.
ABSTRACT: Our objective was to infer the climate drivers of regionally synchronous fire years in dry forests of the U.S. northern Rockies in Idaho and western Montana. During our analysis period (1650–1900), we reconstructed fires from 9245 fire scars on 576 trees (mostly ponderosa pine, Pinus ponderosa P. & C. Lawson) at 21 sites and compared them to existing tree-ring reconstructions of climate (temperature and the Palmer Drought Severity Index [PDSI]) and large-scale climate patterns that affect modern spring climate in this region (El Niño–Southern Oscillation [ENSO] and the Pacific Decadal Oscillation [PDO]). We identified 32 regional-fire years as those with five or more sites with fire. Fires were remarkably widespread during such years, including one year (1748) in which fires were recorded at 10 sites across what are today seven national forests plus one site on state land. During regional-fire years, spring–summers were significantly warm and summers were significantly warm-dry whereas the opposite conditions prevailed during the 99 years when no fires were recorded at any of our sites (no-fire years). Climate in prior years was not significantly associated with regional- or no-fire years. Years when fire was recorded at only a few of our sites occurred under a broad range of climate conditions, highlighting the fact that the regional climate drivers of fire are most evident when fires are synchronized across a large area. No-fire years tended to occur during La Niña years, which tend to have anomalously deep snowpacks in this region. However, ENSO was not a significant driver of regional-fire years, consistent with the greater influence of La Niña than El Niño conditions on the spring climate of this region. PDO was not a significant driver of past fire, despite being a strong driver of modern spring climate and modern regional-fire years in the northern Rockies.
T. Hudiburg, B. Law, D. P. Turner, J. Campbell, D. Donato, M. Duane (2009). Carbon dynamics of Oregon and Northern California forests and potential land-based carbon storage. Ecological Applications 19 (1): 163-180
ABSTRACT: Net uptake of carbon from the atmosphere (net ecosystem production, NEP) is dependent on climate, disturbance history, management practices, forest age, and forest type. To improve understanding of the influence of these factors on forest carbon stocks and flux in the western United States, federal inventory data and supplemental field measurements at additional plots were used to estimate several important components of the carbon balance in forests in Oregon and Northern California during the 1990s. Species- and ecoregion-specific allometric equations were used to estimate live and dead biomass stores, net primary productivity (NPP), and mortality. In the semiarid East Cascades and mesic Coast Range, mean total biomass was 8 and 24 kg C/m2 , and mean NPP was 0.30 and 0.78 kg C·m−2 ·yr−1 , respectively. Maximum NPP and dead biomass stores were most influenced by climate, whereas maximum live biomass stores and mortality were most influenced by forest type. Within ecoregions, mean live and dead biomass were usually higher on public lands, primarily because of the younger age class distribution on private lands. Decrease in NPP with age was not general across ecoregions, with no marked decline in old stands (>200 years old) in some ecoregions. In the absence of stand-replacing disturbance, total landscape carbon stocks could theoretically increase from 3.2 ± 0.34 Pg C to 5.9 ± 1.34 Pg C (a 46% increase) if forests were managed for maximum carbon storage. Although the theoretical limit is probably unattainable, given the timber-based economy and fire regimes in some ecoregions, there is still potential to significantly increase the land-based carbon storage by increasing rotation age and reducing harvest rates.
ABSTRACT: Management of forests for carbon uptake is an important tool in the effort to slow the increase in atmospheric CO2 and global warming. However, some current policies governing forest carbon credits actually promote avoidable CO2 release and punish actions that would increase long-term carbon storage. In fire-prone forests, management that reduces the risk of catastrophic carbon release resulting from stand-replacing wild-fire is considered to be a CO2 source, according to current accounting practices, even though such management may actually increase long-term carbon storage. Examining four of the largest wildfires in the US in 2002, we found that, for forest land that experienced catastrophic stand-replacing fire, prior thinning would have reduced CO2 release from live tree biomass by as much as 98%. Altering carbon accounting practices for forests that have historically experienced frequent, low-severity fire could provide an incentive for forest managers to reduce the risk of catastrophic fire and associated large carbon release events.
ABSTRACT: Projected climate warming will potentially have profound effects on the earth’s biota, including a large redistribution of tree species. We developed models to evaluate potential shifts for 80 individual tree species in the eastern United States. First, environmental factors associated with current ranges of tree species were assessed using geographic information systems (GIS) in conjunction with regression tree analysis (RTA). The method was then extended to better understand the potential of species to survive and/or migrate under a changed climate. We collected, summarized, and analyzed data for climate, soils, land use, elevation, and species assemblages for >2100 counties east of the 100th meridian. Forest Inventory Analysis (FIA) data for >100000 forested plots in the East provided the tree species range and abundance information for the trees. RTA was used to devise prediction rules from current species–environment relationships, which were then used to replicate the current distribution as well as predict the future potential distributions under two scenarios of climate change with twofold increases in the level of atmospheric CO2 . Validation measures prove the utility of the RTA modeling approach for mapping current tree importance values across large areas, leading to increased confidence in the predictions of potential future species distributions.
With our analysis of potential effects, we show that roughly 30 species could expand their range and/or weighted importance at least 10%, while an additional 30 species could decrease by at least 10%, following equilibrium after a changed climate. Depending on the global change scenario used, 4–9 species would potentially move out of the United States to the north. Nearly half of the species assessed (36 out of 80) showed the potential for the ecological optima to shift at least 100 km to the north, including seven that could move >250 km. Given these potential future distributions, actual species redistributions will be controlled by migration rates possible through fragmented landscapes.
Joyce, L.A., G.M. Blate, J. S. Littell, S.G. McNulty, C.I. Millar, S.C. Moser, R.P. Neilson, K. A. O’Halloran, D.L. Peterson, Julius, S.H., J.M. West (2008). National Forests. U.S. Environmental Protection Agency: 3-1 to 3-127
SUMMARY: The National Forest System (NFS) is composed of 155 national forests (NFs) and 20 national grasslands (NGs), which encompass a wide range of ecosystems, harbor much of the nation’s biodiversity, and provide myriad goods and services. The mission of the U.S. Forest Service (USFS), which manages the NFS, has broadened from water and timber to sustaining ecosystem health, diversity, and productivity to meet the needs of present and future generations. The evolution of this mission reflects changing societal values (e.g., increasing emphasis on recreation, aesthetics, and biodiversity conservation), a century of new laws, increasing involvement of the public and other agencies in NF management, and improved ecological understanding. Climate change will amplify the already difficult task of managing the NFS for multiple goals. This chapter offers potential adaptation approaches and management options that the USFS might adopt to help achieve its NF goals and objectives in the face of climate change.
R. E. Keane, L. M. Holsinger, R. A. Parsons, K. Gray (2008). Climate change effects on historical range and variability of two large landscapes in western Montana, USA. Forest Ecology and Management 254 (3): 375-389
ABSTRACT: Quantifying the historical range and variability of landscape composition and structure using simulation modeling is becoming an important means of assessing current landscape condition and prioritizing landscapes for ecosystem restoration. However, most simulated time series are generated using static climate conditions which fail to account for the predicted major changes in future climate. This paper presents a simulation study that generates reference landscape compositions for all combinations of three climate scenarios (warm-wet, hot-dry, and current) and three fire regime scenarios (half historical, historical, and double historical fire frequencies) to determine if future climate change has an effect on landscape dynamics. We applied the spatially explicit, state-and-transition, landscape fire succession model LANDSUM to two large landscapes in west-central Montana, USA. LANDSUM was parameterized and initialized using spatial data generated from the LANDFIRE prototype project. Biophysical settings, critical spatial inputs to LANDSUM, were empirically modeled across the landscape using environmental gradients created from historical and modeled future climate daily weather data summaries. Successional pathways and disturbance probabilities were assigned to these biophysical settings based on existing field data and extensive literature reviews. To assess the impact of changes in climate and fire regime, we compared simulated area burned and landscape composition over time among the different simulation scenario combinations using response variables of Sorenson's index (a global measure of similarity) and area occupied by the dominant vegetation class (simple indicator of change in landscape composition). Results show that simulated time series using future predicted climate scenarios are significantly different from the simulated historical time series and any changes in the fire regime tend to create more dissimilar and more variable simulated time series. Our study results indicate that historical time series should be used in conjunction with simulated future time series as references for managing landscapes.
T. G. F. Kittel, N. A. Rosenbloom, T. H. Painter, D. S. Schimel (1995). The VEMAP integrated database for modelling United States ecosystem/vegetation sensitivity to climate change. Journal of Biogeography 22 (4/5): 857-862
ABSTRACT: For the Vegetation/Ecosystem Modelling and Analysis Project (VEMAP), we developed a model database of climate, soils and vegetation that was compatible with the requirements of three ecosystem physiology models and three vegetation life-form distribution models. A key constraint was temporal, spatial and physical consistency among data layers to provide these daily or monthly time step models with suitable common inputs for the purpose of model inter-comparison. The database is on a 0.5° latitude/longitude grid for the conterminous United States. The set has both daily and monthly representations of the same long-term climate. Daily temperature and precipitation were stochastically simulated with WGEN and daily solar radiation and humidity empirically estimated with CLIMSIM. We used orographically adjusted precipitation, surface temperature and surface windspeed monthly means to maintain consistency among these fields and with vegetation distribution. Vegetation classes were based on physiognomic and physiological properties that influence biogeochemical dynamics. Soils data include characteristics of the 1-4 dominant soils per cell to account for subgrid variability.
W. A. Kurz, G. Stinson, G. J. Rampley, C. C. Dymond, E. T. Neilson (2008). Risk of natural disturbances makes future contribution of Canada's forests to the global carbon cycle highly uncertain. Proceedings of the National Academy of Sciences 105 (5): 1551-1555
ABSTRACT: A large carbon sink in northern land surfaces inferred from global carbon cycle inversion models led to concerns during Kyoto Protocol negotiations that countries might be able to avoid efforts to reduce fossil fuel emissions by claiming large sinks in their managed forests. The greenhouse gas balance of Canada's managed forest is strongly affected by naturally occurring fire with high interannual variability in the area burned and by cyclical insect outbreaks. Taking these stochastic future disturbances into account, we used the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to project that the managed forests of Canada could be a source of between 30 and 245 Mt CO2 e yr−1 during the first Kyoto Protocol commitment period (2008–2012). The recent transition from sink to source is the result of large insect outbreaks. The wide range in the predicted greenhouse gas balance (215 Mt CO2 e yr−1 ) is equivalent to nearly 30% of Canada's emissions in 2005. The increasing impact of natural disturbances, the two major insect outbreaks, and the Kyoto Protocol accounting rules all contributed to Canada's decision not to elect forest management. In Canada, future efforts to influence the carbon balance through forest management could be overwhelmed by natural disturbances. Similar circumstances may arise elsewhere if global change increases natural disturbance rates. Future climate mitigation agreements that do not account for and protect against the impacts of natural disturbances, for example, by accounting for forest management benefits relative to baselines, will fail to encourage changes in forest management aimed at mitigating climate change.
B. E. Law, D. Turner, J. Campbell, O. J. Sun, S. Van Tuyl, W. D. Ritts, W.B. Cohen (2004). Disturbance and climate effects on carbon stocks and fluxes across Western Oregon, USA. Global Change Biology 10 (9): 1429-1444
ABSTRACT: We used a spatially nested hierarchy of field and remote-sensing observations and a process model, Biome-BGC, to produce a carbon budget for the forested region of Oregon, and to determine the relative influence of differences in climate and disturbance among the ecoregions on carbon stocks and fluxes. The simulations suggest that annual net uptake (net ecosystem production (NEP)) for the whole forested region (8.2 million hectares) was 13.8 Tg C (168 g C m−2 yr−1 ), with the highest mean uptake in the Coast Range ecoregion (226 g C m−2 yr−1 ), and the lowest mean NEP in the East Cascades (EC) ecoregion (88 g C m−2 yr−1 ). Carbon stocks totaled 2765 Tg C (33 700 g C m−2 ), with wide variability among ecoregions in the mean stock and in the partitioning above- and belowground. The flux of carbon from the land to the atmosphere that is driven by wildfire was relatively low during the late 1990s (~0.1 Tg C yr−1 ), however, wildfires in 2002 generated a much larger C source (~4.1 Tg C). Annual harvest removals from the study area over the period 1995–2000 were ~5.5 Tg C yr−1 . The removals were disproportionately from the Coast Range, which is heavily managed for timber production (approximately 50% of all of Oregon's forest land has been managed for timber in the past 5 years). The estimate for the annual increase in C stored in long-lived forest products and land fills was 1.4 Tg C yr−1 . Net biome production (NBP) on the land, the net effect of NEP, harvest removals, and wildfire emissions indicates that the study area was a sink (8.2 Tg C yr−1 ). NBP of the study area, which is the more heavily forested half of the state, compensated for ~52% of Oregon's fossil carbon dioxide emissions of 15.6 Tg C yr−1 in 2000. The Biscuit Fire in 2002 reduced NBP dramatically, exacerbating net emissions that year. The regional total reflects the strong east–west gradient in potential productivity associated with the climatic gradient, and a disturbance regime that has been dominated in recent decades by commercial forestry.
ABSTRACT: The purpose of this work is to understand the nature of growth–climate relationships for Douglas-fir (Pseudotsuga menziesii ) across the climatic dimensions of its niche. We used a combination of biophysically informed sampling (to identify sample sites) and dendroclimatology (to identify growth–climate relationships) along a climate gradient in northwestern United States mountain ecosystems from the western Olympic Peninsula, Washington to the eastern Rocky Mountain Front, Montana. We used a multi-scale sampling strategy that accounted for continentality, physiography, and topography as non-climatic factors that could influence climate and alter tree growth. We developed a network of 124 Douglas-fir tree-ring chronologies and explored growth–climate correlations across the sampled gradients. We considered two different spatial scales of monthly and seasonal climate variables as potential controlling factors on tree growth. Annual radial growth in 60–65% of the plots across the region is significantly correlated with precipitation, drought, or water balance during the late summer prior to growth and the early summer the year of growth. In a few plots, growth is positively correlated with cool-season temperature or negatively correlated with snowpack. Water availability is therefore more commonly limiting to Douglas-fir growth than energy limitations on growing season length. The first principal component derived from the chronologies is significantly correlated with independent drought reconstructions. The sensitivity of Douglas-fir to summer water balance deficit (potential evapotranspiration minus actual evapotranspiration) indicates that increases in April to September temperature without increases in summer precipitation or soil moisture reserves are likely to cause decreases in growth over much of the sampled area, especially east of the Cascade crest. In contrast, Douglas-fir may exhibit growth increases at some higher elevation sites where seasonal photosynthesis is currently limited by growing-season length or low growing-season temperature. Life-history processes such as establishment, growth, and mortality are precursors to changes in biogeography, and measurements of climate effects on those processes can provide early indications of climate-change effects on ecosystems.
ABSTRACT: While current projections of future climate change associated with increases in atmospheric greenhouse gases have a high degree of uncertainty, the potential effects of climate change on forests are of increasing concern. A number of studies based on forest simulation models predict substantial alteration of forest composition, forest dieback, or even loss of forest cover in response to increased temperatures associated with increasing atmospheric carbon dioxide concentrations. However, the structure of these computer models may cause them to overemphasize the role of climate in controlling tree growth and mortality. Model functions that represent the influence of climate on tree growth are based on the geographic range limits of a species, predicting maximal growth in the center of the range and zero growth (100% mortality) at the range limits and beyond. This modeling approach ignores the fact that the geographic range of a species reflects the influence of both climate and other environmental factors, including competition with other tree species, soil characteristics, barriers to dispersal, and distributions of pests and pathogens. These climate-response functions in forest simulation models implicitly assume that tree species occur in all environments where it is possible for them to survive (their fundamental niche or potential habitat) and that these potential habitats are entirely defined by climate. Hence, any alteration of climate must result in a fairly rapid decline of species near their range limits and rapid alteration of forest composition and structure. The climate-response functions that lead to these unrealistic conclusions have no basis in plant physiology or actual measurements of tree responses to climate stressors. Rather, these functions were chosen as a necessary expedient for modeling the climatic responses of many tree species for which there were limited or no ecophysiological data. There is substantial evidence, however, that some tree species can survive, and even thrive, in climatic conditions outside their present range limits. This evidence suggests that nonclimatic factors exclude some species from natural forests beyond their present range limits and that climate may not be the only determinant of these limits. Hence, there is reason to suspect that published projections of forest responses to climate change based on forest simulation models may exaggerate the direct impact of climate on tree growth and mortality.
We propose that forest simulation models be reformulated with more realistic representations of growth responses to temperature, moisture, mortality, and dispersal. We believe that only when these models more accurately reflect the physiological bases of the responses of tree species to climate variables can they be used to simulate responses of forests to rapid changes in climate. We argue that direct forest responses to climate change projected by such a reformulated model may be less traumatic and more gradual than those projected by current models. However, the indirect effects of climate change on forests, mediated by alterations of disturbance regimes or the actions of pests and pathogens, may accelerate climate-induced change in forests, and they deserve further study and inclusion within forest simulation models.
Malmsheimer, R. W., Heffernan, P., Brink, S., Crandall, D., Deneke, F., Galik, C., Gee, E., Helms, J. A., McClure, N., Mortimer, M., Ruddell, S., Smith, M., Stewart, J. B. (2008). Forest management solutions for mitigating climate change in the United States. Journal of Forestry 106 (3): 115-173
Nabuurs, G.J., O. Masera, K. Andrasko, P. Benitez-Ponce, R. Boer, M. Dutschke, E. Elsiddig, J. Ford Robertson, P. Frumhoff, T. Karjalainen, O. Krankina, W.A. Kurz, M. Matsumoto, W. Oychantcabal, N.H. Ravindranath, M.J. Sanz Sanchez, X. Zhang, B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (2007). Forestry. Cambridge University Press: 541-584
FIRST PARAGRAPH: During the last decade of the 20th century, deforestation in the tropics and forest regrowth in the temperate zone and parts of the boreal zone remained the major factors responsible for emissions and removals, respectively. However, the extent to which the carbon loss due to tropical deforestation is offset by expanding forest areas and accumulating woody biomass in the boreal and temperate zones is an area of disagreement between land observations and estimates by top-down models. Emissions from deforestation in the 1990s are estimated at 5.8 GtCO2 /yr (medium agreement, medium evidence).
ABSTRACT: Recent concern over the ecological effects of future trace-gas-induced climate change has accelerated efforts to understand and quantify climate-induced vegetation change1–9 . Here we discuss new and published climate-model results indicating that global warming favours increased rates of forest disturbance, as a result of weather more likely to cause forest fires (drought, wind and natural ignition sources), convective wind storms, coastal flooding and hurricanes. New sensitivity tests carried out with a vegetation model indicate that climate-induced increases in disturbance could, in turn, significantly alter the total biomass and compositional response of forests to future warming. An increase in disturbance frequency is also likely to increase the rate at which natural vegetation responds to future climate change. Our results reinforce the hypothesis6 that forests could be significantly altered by the first part of the next century. Our modelling also confirms the potential utility of selected time series of fossil pollen data for investigating the poorly understood natural patterns of century-scale climate variability.
ABSTRACT: In the geologic past, natural climate changes have caused large-scale geographical shifts in species' ranges, changes in the species composition of biological communities, and species extinctions. If the widely predicted greenhouse effect occurs, natural ecosystems will respond in similar ways as in the past, but the effects will be more severe because of the extremely rapid rate of the projected change. Moreover, population reduction and habitat destruction due to human activities will prevent many species from colonizing new habitat when their old becomes unsuitable. The synergy between climate change and habitat destruction would threaten many more species than either factor alone.
These effects would be pronounced in temperate and arctic forests, where temperature increases are projected to be relatively large. Localized species might face extinction, while widespread forest trees are likely to survive in some parts of their range. New northward habitat will become suitable even as die-offs of tree species occur to the south. However, it may be difficult for many species to take advantage of this new habitat because dispersal rates for tree species are very slow relative to the rate of warming, and therefore ranges of even many widespread species are likely to show a net decrease during the next century. Range retractions will be proximally caused by temperature and precipitation changes, increases in fires, changes in the ranges and severity of pests and pathogens, changes in competitive interactions, and additional effects of non-climatic stress such as acid rain and low-level ozone. Changes in species composition will have large effects on local and regional economies and biological diversity.
ABSTRACT: The occurrence and abundance of conifers along climate gradients in the Inland Northwest (USA) was assessed using data from 5082 field plots, 81% of which were forested. Analyses using the Random Forests classification tree revealed that the sequential distribution of species along an altitudinal gradient could be predicted with reasonable accuracy from a single climate variable, a growing-season dryness index, calculated from the ratio of degree-days >5°C that accumulate in the frost-free season to the summer precipitation. While the appearance and departure of species in an ascending altitudinal sequence were closely related to the dryness index, the departure was most easily visualized in relation to negative degree-days (degree-days <0°C). The results were in close agreement with the works of descriptive ecologists. A Weibull response function was used to predict from climate variables the abundance and occurrence probabilities of each species, using binned data. The fit of the models was excellent, generally accounting for >90% of the variance among 100 classes.
ABSTRACT: The earth's climate may currently be undergoing a warming in response to the well documented accumulation of CO2 and other greenhouse gases. Changes in forestland areas and biomass are playing a role in the accumulation. This paper reviews and offers some observations on estimates of the role of forests in the carbon cycle. The temperate forests are roughly in carbon balance, with biomass growth equaling or exceeding losses. The tropical forests, by contrast, are a carbon source with forest area declining due primarily to land-use changes. A number of carbon-sequestering sources, such as wood construction and landfills, may be sequestering more carbon than is commonly assumed. Climate change can also affect forests. A number of mechanisms that influence forest growth and composition are discussed. In the absence of increased precipitation and/or a CO2 “fertilization” effect, warming is likely to diminish forest area and biomass. Forest burning is part of the natural cycle. During a burn, carbon is released through the post-burning period and typically involves carbon sequestering as the result of regeneration and vigorous growth. In an undisturbed natural system, a steady-state level of global forest biomass would be reached. Anthropogenic factors can upset the natural steady state. In a period of rapid climate transition, such as might accompany a global warming, forests are likely to lose vigor and thus be particularly susceptible to wildfire.
Thornton, P. E., B. E. Law, H. L. Gholz, K.L. Clark, E. Falge, D.S. Ellsworth, A.H. Goldstein, R.K. Monson, D. Hollinger, M. Falk, J. Chen, J. P. Sparks (2002). Modeling and measuring the effects of disturbance history and climate on carbon and water budgets in evergreen needleleaf forests. Agricultural and Forest Meteorology 113 (1-4): 185-222
ABSTRACT: The effects of disturbance history, climate, and changes in atmospheric carbon dioxide (CO2 ) concentration and nitrogen deposition (Ndep ) on carbon and water fluxes in seven North American evergreen forests are assessed using a coupled water–carbon–nitrogen model, canopy-scale flux observations, and descriptions of the vegetation type, management practices, and disturbance histories at each site. The effects of interannual climate variability, disturbance history, and vegetation ecophysiology on carbon and water fluxes and storage are integrated by the ecosystem process model Biome-BGC, with results compared to site biometric analyses and eddy covariance observations aggregated by month and year. Model results suggest that variation between sites in net ecosystem carbon exchange (NEE) is largely a function of disturbance history, with important secondary effects from site climate, vegetation ecophysiology, and changing atmospheric CO2 and Ndep . The timing and magnitude of fluxes following disturbance depend on disturbance type and intensity, and on post-harvest management treatments such as burning, fertilization and replanting. The modeled effects of increasing atmospheric CO2 on NEE are generally limited by N availability, but are greatly increased following disturbance due to increased N mineralization and reduced plant N demand. Modeled rates of carbon sequestration over the past 200 years are driven by the rate of change in CO2 concentration for old sites experiencing low rates of Ndep . The model produced good estimates of between-site variation in leaf area index, with mixed performance for between- and within-site variation in evapotranspiration. There is a model bias toward smaller annual carbon sinks at five sites, with a seasonal model bias toward smaller warm-season sink strength at all sites. Various lines of reasoning are explored to help to explain these differences.
R. D. Westfall, C. I. Millar (2004). Genetic consequences of forest population dynamics influenced by historic climatic variability in the western USA. Forest Ecology and Management 197 (1-3): 159-170
ABSTRACT: We review recent advances in climate science that show cyclic climatic variation over multiple time scales and give examples of the impacts of this variation on plant populations in the western USA. The paleohistorical reconstructions we review and others indicate that plant species track these cycles in individualistically complex ways. These dynamic histories suggest that genetic structures are in a non-equilibrium state, with populations constantly lagging their environmental optima. Such dynamism may serve to maintain genetic variation in populations, which may be a hedge against rapid environmental change. We also discuss how population history affects the way we analyze and interpret genetic data and, conversely, the way genetic theory affects historical reconstructions.
C.W. Woodall, C.M. Oswalt, J.A. Westfall, C.H. Perry, M.D. Nelson, A.O. Finley (2009). An indicator of tree migration in forests of the eastern United States. Forest Ecology and Management 257 (5): 1434-1444
ABSTRACT: Changes in tree species distributions are a potential impact of climate change on forest ecosystems. The examination of tree species shifts in forests of the eastern United States largely has been limited to simulation activities due to a lack of consistent, long-term forest inventory datasets. The goal of this study was to compare current geographic distributions of tree seedlings (trees with a diameter at breast height ≤2.5 cm) with biomass (trees with a diameter at breast height > 2.5 cm) for sets of northern, southern, and general tree species in the eastern United States using a spatially balanced, region-wide forest inventory. Compared to mean latitude of tree biomass, mean latitude of seedlings was significantly farther north (>20 km) for the northern study species, while southern species had no shift, and general species demonstrated southern expansion. Density of seedlings relative to tree biomass of northern tree species was nearly 10 times higher in northern latitudes compared to southern latitudes. For forest inventory plots between 44° and 47° north latitude where southern tree species were identified, their biomass averaged 0.46 tonnes/ha while their seedling counts averaged 2600 ha−1 . It is hypothesized that as northern and southern tree species together move northward due to greater regeneration success at higher latitudes, general species may fill their vacated niches in southern locations. The results of this study suggest that the process of northward tree migration in the eastern United States is currently underway with rates approaching 100 km/century for many species.
ABSTRACT: The foliar compliment of evergreen conifer trees is dependent on the crown ratio and foliar longevity. Variation in foliage longevity is one of the important traits that often favours the genusPinus over its competitors. Healthy lodgepole pine trees throughout British Columbia, Canada typically retain needles for 4 to 5 years. Foliar diseases have a profound influence on foliar longevity and crown ratio, and lodgepole pine is susceptible to a large suite of foliar pathogens. We assessed the foliar longevity, live crown percent and mortality of lodgepole pine trees at 25 lodgepole pine provenance test sites in central BC, and correlated these values with changes in climate at each site between the decade of the 1920s and the 1990s. We found strong relationships between increases in August minimum temperatures and live crown percent (R = -0.75) and mortality (R = 0.75). Sites in Region 7 of the trial (Robson Valley) consistently have the least foliage and have consistently experienced the greatest increases in August minimum temperature and July precipitation, and the greatest decreases in May maximum temperature. Region 9 (Nechako Plateau) sites consistently have the most foliage and have consistently experienced the least change in August minimum temperature, July precipitation, and greatest increases in May maximum temperature. Future changes in climate in conjunction with foliar pathogens could have profound effects on the health of lodgepole pine in BC.
A. R. Keyser, J. S. Kimball, R. R. Nemani, S. W. Running (2000). Simulating the effects of climate change on the carbon balance of North American high-latitude forests. Global Change Biology 6 (S1): 185-195
ABSTRACT: The large magnitude of predicted warming at high latitudes and the potential feedback of ecosystems to atmospheric CO2 concentrations make it important to quantify both warming and its effects on high-latitude carbon balance. We analysed long-term, daily surface meteorological records for 13 sites in Alaska and north-western Canada and an 82-y record of river ice breakup date for the Tanana River in interior Alaska. We found increases in winter and spring temperature extrema for all sites, with the greatest increases in spring minimum temperature, average 0.47 °C per 10 y, and a 0.7-day per 10 y advance in ice breakup on the Tanana River. We used the climate records to drive an ecosystem process model, BIOME_BGC, to simulate the effects of climate change on the carbon and water balances of boreal forest ecosystems. The growing season has lengthened by an average of 2.6 days per 10 y with an advance in average leaf onset date of 1.10 days per 10 y. This advance in the start of the active growing season correlates positively with progressively earlier ice breakup on the Tanana River in interior Alaska. The advance in the start of the growing season resulted in a 20% increase in net primary production for both aspen (Populus tremuloides ) and white spruce (Picea glauca ) stands. Aspen had a greater mean increase in maintenance respiration than spruce, whereas spruce had a greater mean increase in evapotranspiration. Average decomposition rates also increased for both species. Both net primary production and decomposition are enhanced in our simulations, suggesting that productive forest types may not experience a significant shift in net carbon flux as a result of climate warming.
L. A. Joyce, G. M. Blate, S. G. McNulty, C. I. Millar, S. Moser, R. P. Neilson, D. L. Peterson (2009). Managing for multiple resources under climate change: National Forests. Environmental Management Online First
ABSTRACT: This study explores potential adaptation approaches in planning and management that the United States Forest Service might adopt to help achieve its goals and objectives in the face of climate change. Availability of information, vulnerability of ecological and socio-economic systems, and uncertainties associated with climate change, as well as the interacting non-climatic changes, influence selection of the adaptation approach. Resource assessments are opportunities to develop strategic information that could be used to identify and link adaptation strategies across planning levels. Within a National Forest, planning must incorporate the opportunity to identify vulnerabilities to climate change as well as incorporate approaches that allow management adjustments as the effects of climate change become apparent. The nature of environmental variability, the inevitability of novelty and surprise, and the range of management objectives and situations across the National Forest System implies that no single approach will fit all situations. A toolbox of management options would include practices focused on forestalling climate change effects by building resistance and resilience into current ecosystems, and on managing for change by enabling plants, animals, and ecosystems to adapt to climate change. Better and more widespread implementation of already known practices that reduce the impact of existing stressors represents an important “no regrets” strategy. These management opportunities will require agency consideration of its adaptive capacity, and ways to overcome potential barriers to these adaptation options.
ABSTRACT: Forests currently absorb billions of tons of CO2 globally every year, an economic subsidy worth hundreds of billions of dollars if an equivalent sink had to be created in other ways. Concerns about the permanency of forest carbon stocks, difficulties in quantifying stock changes, and the threat of environmental and socioeconomic impacts of large-scale reforestation programs have limited the uptake of forestry activities in climate policies. With political will and the involvement of tropical regions, forests can contribute to climate change protection through carbon sequestration as well as offering economic, environmental, and sociocultural benefits. A key opportunity in tropical regions is the reduction of carbon emissions from deforestation and degradation.
U.S. Global Change Research Group, (2000). Climate change impacts on the United States: The potential consequences of climate variability and change. National Assessment Synthesis Team, U.S. Global Change Research Program
ABOUT THIS DOCUMENT: What is this Assessment? The National Assessment of the Potential Consequences of Climate Variability and Change is a landmark in the major ongoing effort to understand what climate change means for the US. Climate science is developing rapidly and scientists are increasingly able to project some changes at the regional scale, identifying regional vulnerabilities, and assessing potential regional impacts. Science increasingly indicates that the Earth's climate has changed in the past and continues to change, and that even greater climate change is very likely in the 21st century. This Assessment has begun a national process of research, analysis, and dialogue about the coming changes in climate, their impacts, and what Americans can do to adapt to an uncertain and continuously changing climate. This Assessment is built on a solid foundation of science conducted as part of the United States Global Change Research Program (USGCRP).
What is this document and who is the NAST? This document is the Assessment Overview, written by the National Assessment Synthesis Team (NAST). The NAST is a committee of experts drawn from governments, universities, industry, and non-governmental organizations. It has been responsible for broad over-sight of the Assessment, with the Federal agencies of the USGCRP. This Overview is based on a longer, referenced "Foundation" report, written by the NAST in cooperation with independent regional and sector assessment teams. These two national-level, peer-reviewed documents synthesize results from studies conducted by regional and sector teams, and from the broader scientific literature.
Why was this Assessment undertaken? The Assessment was called for by a 1990 law, and has been con-ducted under the USGCRP in response to a request from the President's Science Advisor. The NAST developed the Assessment's plan, which was then approved by the National Science and Technology Council, the cabinet-level body of agencies responsible for scientific research, including global change research, in the US government.
ABSTRACT: General warming in the Northern Hemisphere has been recorded since the end of the 1800s following the Little Ice Age. Records of glacier retreat during the last 100 years over the entire globe independently confirmed the recorded trend in global temperature rise. Several studies have illustrated various responses to this climate forcing, i.e., the recorded changes in temperature and precipitation concurrent with the increase in atmospheric CO2 concentration, increases in density of tree populations, declines in tree populations, treeline displacement or lack thereof, lengthening of the growing season, and enhanced tree growth. It is critical that we identify the tools needed to estimate potential consequences of climate change on forest ecosystems and develop management practices and policies adapted to projected drifts in the geographic distribution of ecosystems.
FIRST PARAGRAPH: When and if the United States decides on mandatory policies to address global climate change, it will be necessary to decide whether carbon sequestration should be part of the domestic portfolio of compliance activities. The potential costs of carbon sequestration policies will presumably be a major criterion, so it is important to assess the cost of supplying forest-based carbon sequestration in the United States. In this report we survey major studies, examine the factors that have affected their carbon sequestration cost estimates, and synthesize the results.
INTRODUCTION: Global change involves the simultaneous and rapid alteration of several key environmental parameters that control the dynamics of forests. We cannot predict with certainty, through direct experimentation, what the responses of forests to global change will be, because we cannot carry out the multisite, multifactorial experiments required for doing so. The physical extent, complexity, and expense of even single-factor experiments at the scale of the whole ecosystem challenge our abilities, although several such experiments have been successfully undertaken (e.g., DeLucia et al. 1999, Wright and Rasmussen 1998). To inform policy decisions, however, the scientific community can offer an interdisciplinary synthesis of existing information. When this synthesis takes the form of a computer model, quantitative predictions can be made that integrate what has been learned from single-factor experiments. The success of such an approach depends on the quality and completeness of the information base and on the rigor of the modeling effort.
The direct and secondary physiological effects of changes in the physical and chemical climate on plants and soils are relatively well known. We also know which primary environmental drivers—precipitation, temperature, and atmospheric concentrations of carbon dioxide (CO2 ), ozone (O3 ), and nitrogen (N), for example—are being altered by human activities, and we can directly measure temporal change in these parameters. Despite this relatively rich information base, predictions of future responses of forests to environmental change show significant variation. This is due in part to differences between the models of ecosystem function derived from the existing database and in part to differences in climate scenarios generated by the general circulation models (GCMs) used to predict future climates. Understanding both the trend in predicted futures and the uncertainties surrounding those trends is critical to policy formation. At this time, the major mechanism for determining the degree of uncertainty in predictions is through comparison of results from runs of different models using identical input parameters.
The purpose of this article is to review the state of prediction of forest ecosystem response to envisioned changes in the physical and chemical climate. These results are offered as one part of the forest sector analysis of the National Assessment of the Potential Consequences of Climate Variability and Change; other contributions to this assessment appear in this edition of BioScience. This article has three sections. The first offers a very brief review of the literature on the effects of environmental factors on forest ecosystem function (some references are also made to changes in species composition, but Hansen et al.  provide a more complete discussion). The second and largest part of the article is a summary of results from the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP), an integrated effort to predict ecosystem response to climate change. The third is a brief review of other regional modeling efforts that have addressed climate change or have looked at the possible effects of other components of global change, using tropospheric ozone and nitrogen deposition as examples, or both.
INTRODUCTION: Atmospheric concentrations of carbon dioxide (CO2 ) and other greenhouse gases have been increasing since the beginning of the industrial revolution in 1850. Over the next century, increasing gas concentrations could cause the temperature on the surface of the Earth to rise as much as 2–3°C over historic mean annual levels. Variation in annual climate could also increase.
The United States experienced one indication of climate change in 1988: The summer of that year was one of the hottest, driest ever recorded across the nation. Barges were stranded on the Mississippi River, and forest fires burned millions of acres in the western United States. In the eastern United States, temperatures were so high that many factory assembly lines had to be shut down. The former Soviet Union states and China also experienced severe drought, while Africa, India, and Bangladesh witnessed torrential rains and flooding.
These events triggered televised congressional debates, which concluded that atmospheric greenhouse gas inputs would very likely increase the intensity and severity of weather patterns during the next 100 years. The potential negative effects of global warming—melting of polar ice caps, a rise in the sea level, reduced agricultural and forest productivity, water shortages, and extinction of sensitive species—were also discussed.
These findings prompted the passage of the 1990 Global Change Research Act (GCRA) and the establishment of the US Global Change Research Program (USGCRP). The program sponsors ongoing research (over $1.6 billion in 2000) at several federal agencies, including the National Aeronautics and Space Administration, Department of Energy, US Department of Agriculture, Environmental Protection Agency, National Institutes of Health, Department of Commerce, and National Science Foundation, among others (USGCRP 1999). In addition to providing a mechanism for funding research on global change, the GCRA mandates that an assessment be conducted periodically to summarize research findings. Begun in 1997, the first National Assessment of the Potential Consequences of Climate Variability and Change was published in 2001 (USGCRP 2001). The assessment was a collaboration between federal and nonfederal researchers, resource managers, and users. The assessment is divided into five sectors: (1) water resources and availability, (2) agriculture and food production, (3) human health, (4) coastal areas, and (5) forests. These sectors represent important or potentially sensitive US resources that could be adversely affected by climate change. The assessment also includes over 20 regional studies, which examine the impacts of climate change for specific geographical areas of the United States. This special section of BioScience focuses on a summary of research findings from the forest sector and regional findings of the 2001 national assessment (USGCRP 2001).
The impacts of climate change on the forest sector are divided into four categories: (1) forest processes, (2) biodiversity change, (3) disturbance interactions, and (4) socioeconomic change. These categories represent key interactions between a changing climate, forest structure or function, and human interactions with forests.
C. I. Millar, R. D. Westfall, D. L. Delany, J. C. King, L.J. Graumlich (2004). Response of subalpine conifers in the Sierra Nevada, California, U.S.A., to 20th-century warming and decadal climate variability. Arctic, Antarctic, and Alpine Research 36 (2): 181-200
ABSTRACT: Four independent studies of conifer growth between 1880 and 2002 in upper elevation forests of the central Sierra Nevada, California, U.S.A., showed correlated multidecadal and century-long responses associated with climate. Using tree-ring and ecological plot analysis, we studied annual branch growth of krummholzPinus albicaulis ; invasion byP. albicaulis andPinus monticola into formerly persistent snowfields; dates of vertical branch emergence in krummholzP. albicaulis ; and invasion byPinus contorta into subalpine meadows. Mean annual branch growth at six treeline sites increased significantly over the 20th century (range 130–400%), with significant accelerations in rate from 1920 to 1945 and after 1980. Growth stabilized from 1945 to 1980. Similarly, invasion of six snowfield slopes began in the early 1900s and continued into snowfield centers throughout the 20th century, with significantly accelerated mean invasion from 1925 to 1940 and after 1980. Rate of snowfield invasion decreased between 1950 and 1975. Meadow invasion and vertical leader emergence showed synchronous, episodic responses.Pinus contorta invaded each of ten subalpine meadows in a distinct multidecadal pulse between 1945 and 1976 (87% of all trees) and vertical release in five krummholzP. albicaulis sites also occurred in one pulse between 1945 and 1976 (86% of all branches). These synchronies and lack of effect of local environments implicate regional climate control. Composite weather records indicated significant century-long increases in minimum monthly temperature and multidecadal variability in minimum temperature and precipitation. All ecological responses were significantly correlated with minimum temperature. Significant interactions among temperature, precipitation, Pacific Decadal Oscillation (PDO) indices, and multiyear variability in moisture availability further explained episodic ecological responses. Four multidecadal periods of the 20th century that are defined by ecological response (<1925; 1925–1944; 1945–1976; >1976) correlate with positive and negative PDO phases, as well as with steps in the rate of temperature increase. These diverse factors in spatially distributed upper-montane and treeline ecosystems respond directionally to century-long climate trends, and also exhibit abrupt and reversible effects as a consequence of interdecadal climate variability and complex interactions of temperature and moisture.
ABSTRACT: We reconstructed a 3500-yr history of fluctuations in treeline elevation and tree abundance in the southern Sierra Nevada. Treeline elevation was higher than at present throughout most of the last 3500 yr. Declines in the abundance of live trees and treeline elevation occurred twice during the last 1000 yr: from 950 to 550 yr BP and from 450 to 50 yr BP. The earlier decline coincided with a period of warm temperatures (relative to present) in which at least two severe, multidecadal droughts occurred. This decline was apparently triggered by an increase in the rate of adult mortality in treeline forests. The more recent decline occurred during a period of low temperatures lasting for up to 400 yr and was apparently caused by a sustained failure of regeneration in combination with an increased rate of adult mortality. The apparent past importance of precipitation in controlling the position and structure of the treeline ecotone suggests that climatic controls over treeline may be more complex than previously thought. In the Sierra Nevada, responses of high-elevation forests to future warming may depend strongly on water supply.
ABSTRACT: Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene. By tracking climatically sensitive forest boundaries, the moisture-controlled lower timberline and the temperature-controlled upper timberline, paleoclimatic estimates can be derived from modern temperature and precipitation lapse rates. Pollen data suggest that prior to 11 000 yr B.P., a subalpine forest dominated byPicea (spruce) andPinus (pine) grew 300–700 m below its modern limit. The inferred climate was 2–5 °C cooler and had 7–16 cm greater precipitation than today.Abies (fir) increased in abundance in the subalpine forest around 11 000 yr B.P., probably in response to cooler conditions with increased winter snow. Pollen and plant macrofossil data demonstrate that from 9000 to 4000 yr B.P. the subalpine forest occupied a greater elevational range than it does today. Upper timberline was 270 m above its modern limit, suggesting that mean annual and mean July temperatures were 1–2 °C warmer than today. Intensification of the summer monsoon, coupled with increased summer radiation between 9000 and 6000 yr B.P., raised mean annual precipitation by 8–11 cm and allowed the lower limit of the subalpine and montane forests to descend to lower elevations. The lower forest border began to retreat upslope between 6000 and 4000 yr B.P. in response to drier conditions, and the upper timberline descended after 4000 yr B.P., when temperatures cooled to about 1 °C warmer than today. The modern climatic regime was established about 2000 yr B.P., when the summer precipitation maxima of the early and middle Holocene were balanced by increased winter precipitation.
ABSTRACT: Ring width and density measurements from the same trees can produce distinctly different climatic information. Ring width variations and recorded data in central and northern Alaska indicate annual temperatures increased over the past century, peaked in the 1940s, and are still near the highest level for the past 3 centuries. Density variations indicate summer temperatures are now warm but not above some previous levels occurring prior to this century. The early cooler period, corresponding to the Little Ice Age, was interrupted by brief warm intervals. The recent increase in temperatures combined with drier years may be changing the tree response to climate and raising the potential for some forest changes in Alaskan and other boreal forests.
ABSTRACT: The projected response of coniferous forests to a climatic change scenario of doubled atmospheric CO2 , air temperature of +4 °C, and +10% precipitation was studied using a computer simulation model of forest ecosystem processes. A topographically complex forested region of Montana was simulated to study regional climate change induced forest responses. In general, increases of 10–20% in LAI, and 20–30% in evapotranspiration (ET) and photosynthesis (PSN) were projected. Snowpack duration decreased by 19–69 days depending on location, and growing season length increased proportionally. However, hydrologic outflow, primarily fed by snowmelt in this region, was projected to decrease by as much as 30%, which could virtually dry up rivers and irrigation water in the future. To understand the simulated forest responses, and explore the extent to which these results might apply continentally, seasonal hydrologic partitioning between outflow and ET, PSN, respiration, and net primary production (NPP) were simulated for two contrasting climates of Jacksonville, Florida (hot, wet) and Missoula, Montana (cold, dry). Three forest responses were studied sequentially from; climate change alone, addition of CO2 induced tree physiological responses of -30% stomatal conductance and +30% photosynthetic rates, and finally with a reequilibration of forest leaf area index (LAI), derived by a hydrologic equilibrium theory. NPP was projected to increase 88%, and ET 10%, in Missoula, MT, yet decrease 5% and 16% respectively for Jacksonville, FL, emphasizing the contrasting forest responses possible with future climatic change.
ABSTRACT: Predicted changes in climate have raised concerns about potential impacts on terrestrial forest ecosystem productivity, biogeochemical cycling, and the availability of water resources. This review summarizes characteristics of drought typical to the major forest regions of the United States, future drought projections, and important features of plant and forest community response to drought. Research needs and strategies for coping with future drought are also discussed. Notwithstanding uncertainties surrounding the magnitude and direction of future climate change, and the net impact on soil water availability to forests, a number of conclusions can be made regarding the sensitivity of forests to future drought. The primary response will be a reduction in net primary production and stand water use, which are driven by reductions in stomatal conductance. Mortality of small stature plants (i.e. seedlings and saplings) is a likely consequence of severe drought. In comparison, deep rooting and substantial reserves of carbohydrates and nutrients make mature trees less susceptible to water limitations caused by severe or prolonged drought. However, severe or prolonged drought may render even mature trees more susceptible to insects or disease. Drought-induced reductions in decomposition rates may cause a buildup of organic material on the forest floor, with ramifications for fire regimes and nutrient cycling. Although early model predictions of climate change impacts suggested extensive forest dieback and species migration, more recent analyses suggest that catastrophic dieback will be a local phenomenon, and changes in forest composition will be a relatively gradual process. Better climate predictions at regional scales, with a higher temporal resolution (months to days), coupled with carefully designed, field-based experiments that incorporate multiple driving variables (e.g. temperature and CO2 ), will advance our ability to predict the response of different forest regions to climate change.
DESCRIPTION: This report documents trends and impacts of climate change on America's forests as required by the Renewable Resources Planning Act of 1974. Recent research on the impact of climate and elevated atmospheric carbon dioxide on plant productivity is synthesized. Modeling analyses explore the potential impact of climate changes on forests, wood products, and carbon in the United States.
McKenzie, D., D. W. Peterson, D. L. Peterson, P. E. Thornton (2003). Climatic and biophysical controls on conifer species distributions in mountain forests of Washington state, USA. Journal of Biogeography 30 (7): 1093-1108
ABSTRACT:The purpose of this study was to quantify relationships between conifer species distributions and climatic and biophysical variables, in order to provide better insight into the potential for redistribution of species on the landscape in response to climatic change.Data are from 10,653 georeferenced sites in Washington State, USA, along a longitudinal gradient from west of the crest of the Cascade Range to the beginnings of the western slope of the Rocky Mountains, and across two physiographic provinces, the Northern Cascades, characterized by steep, rugged topography, and the Okanogan Highlands, presenting moderate slopes and broad rounded summits.Tree data were drawn from the USDA Forest Service Area Ecology Program database, collected in mature, undisturbed stands. We compared simple climatic variables (annual temperature, growing-degree days, annual and seasonal precipitation) to biophysical variables (soil, hydrologic, and solar radiation) derived from climatic variables. Climatic and biophysical variables were taken from the output of climatological and hydrological simulation models (DAYMET and VIC) and estimated for each plot in the tree database. Generalized linear models were used, for each of 14 tree species, at multiple spatial extents, to estimate the probability of occurrence of that species as a function of climatic and biophysical predictors. Models were validated by a combination of bootstrapping and estimating receiver operating characteristic (ROC) curves.For the majority of species, we were able to fit variables representing both moisture and temperature gradients, and in all but a few cases these models identified a unimodal response of species occurrence to these gradients. In some cases the ecological/environmental niche of a species had been clearly captured by the model, whereas in others a longer gradient in the predictor variable(s) would be needed. Responses of most species were consistent across three spatial scales.By identifying the ecological niches of multiple species, we can forecast their redistribution on the landscape in response to climatic change, evaluate the predictions of simulation models, and alert managers to particularly sensitive or vulnerable ecosystems and landscapes.
ABSTRACT: The vegetation of the montane and subalpine zones of the Rocky Mountains is a mosaic of conifer forests and large (1 ha to several square kilometers) treeless “parks” dominated by sagebrush (Artemisia spp.), grasses, and forbs. Three hypotheses for the origin of parks are proposed. The “permanent site hypothesis” states that the park–forest vegetation mosaic is a result of differences in physical characteristics of sites. In the “remnant hypothesis” parks are thought to be remnants of vegetation that was widespread under previous climate conditions. The “replacement hypothesis” states that parks replace forest vegetation in response to disturbance, climate change, or a combination of these two factors. Patterns in the past distribution of park and forest vegetation in the vicinity of Fish Creek Park (elevation 2750 m) were used to test these hypotheses.
Fossil pollen extracted from the sediments of five small ponds in and around Fish Creek Park was used to reconstruct Holocene vegetation changes. Changes in vegetation were reconstructed through the use of multivariate analyses and pollen ratios derived from modern surface samples and by comparison with pollen data from other studies. The pollen record indicates that shortly after deglaciation (11000 yr BP) the area supported alpine tundra, followed by whitebark pine–spruce–fir parkland at 9500 yr BP. From 8500 to 6000 yr BP, a pine parkland occupied the area, perhaps in response to climate conditions warmer than today. By 5000 yr BP a mixed pine–spruce–fir forest resembling the modern subalpine forest near Fish Creek Park probably replaced the pine parkland at all five sites. The modern park vegetation originated only within the last 2500 yr.
The conversion to park vegetation may not have been synchronous at all three sites, and the replacement of forest by park did not always result in a long-term conversion to park vegetation. The timing and pattern of changes in the vegetation mosaic eliminate the permanent site and remnant hypotheses and suggest instead that climatic cooling over the last several thousand years, possibly combined with removal of forest cover by fire or some other disturbance, could explain the origin of Fish Creek Park.
C.D. Allen, D. D. Breshears (1998). Drought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences 95 (25): 14839-14842
ABSTRACT: In coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects, particularly those caused by mortality largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and piñon-juniper woodland shifted extensively (2 km or more) and rapidly (<5 years) through mortality of ponderosa pines in response to a severe drought. This shift has persisted for 40 years. Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change.
ABSTRACT: Current models of climate change predict a reduction of area covered by northern coniferous forests and tundra, and an increase in grasslands. These scenarios also indicate a northerly shift in agricultural regions, bringing virgin soils under cultivation. The direct effects of man on tundra, boreal forest, and temperate grassland ecosystems are likely to result in less carbon mobilization from soils and vegetation than from tropical forests. However, as a consequence of climate change, carbon mineralization rates from arctic and sub-arctic soils could be very rapid under warmer and drier conditions because of low stabilization of soil organic matter (SOM) and enhanced microbial responses to small changes in soil moisture and temperature. Predicting the response of these systems to climate change is complicated where the edaphic environment regulating SOM dynamics is not a direct function of macroclimatic conditions. Grasslands contain a greater proportion of highly stabilized SOM than coniferous forests, distributed over greater depth in the soil profile, which is less susceptible to changes in mineralization rates. It is concluded that short-term responses of soil processes to climate change are more predictable in well-drained grassland and forest soils than in waterlogged soils of the tundra and boreal region. Over longer periods of time, however, plant species and soil types will alter in response to new temperature and moisture regimes above- and belowground interacting with the effects of carbon enrichment and changes in nutrient availability. The dynamics of these plant-soil interactions and the future status of soils in different life zones as sources or sinks of carbon is poorly understood. More data are also needed on the distribution of waterlogged forest soils in the boreal zone and responses to warming, which include the production of methane as well as CO2 . The primary recommendation for future research is for integrated studies on plant and soil processes.
Dale, V. H., Joyce, L.A., McNulty, S., Neilson, R. P., Ayres, M. P., Flannigan, M. D., Hanson, P. J., Irland, L.C., Lugo, A.E., Peterson, C. J., Simberloff, D., Swanson, Frederick J., Stocks, B. J., Wotton, M. (2001). Climate change and forest disturbances. BioScience 51 (9): 723-734
INTRODUCTION: Studies of the effects of climate change on forests have focused on the ability of species to tolerate temperature and moisture changes and to disperse, but they have ignored the effects of disturbances caused by climate change (e.g., Ojima et al. 1991). Yet modeling studies indicate the importance of climate effects on disturbance regimes (He et al. 1999). Local, regional, and global changes in temperature and precipitation can influence the occurrence, timing, frequency, duration, extent, and intensity of disturbances (Baker 1995, Turner et al. 1998). Because trees can survive from decades to centuries and take years to become established, climate-change impacts are expressed in forests, in part, through alterations in disturbance regimes (Franklin et al. 1992, Dale et al. 2000).
Disturbances, both human-induced and natural, shape forest systems by influencing their composition, structure, and functional processes. Indeed, the forests of the United States are molded by their land-use and disturbance history. Within the United States, natural disturbances having the greatest effects on forests include fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, windstorms, ice storms, and landslides (Figure 1). Each disturbance affects forests differently. Some cause large-scale tree mortality, whereas others affect community structure and organization without causing massive mortality (e.g., ground fires). Forest disturbances influence how much carbon is stored in trees or dead wood. All these natural disturbances interact with human-induced effects on the environment, such as air pollution and land-use change resulting from resource extraction, agriculture, urban and suburban expansion, and recreation. Some disturbances can be functions of both natural and human conditions (e.g., forest fire ignition and spread) (Figure 2).
Each disturbance has both social and economic effects (Table 1). Estimating the costs of each of these disturbances is very difficult; these estimates for the United States are illustrative only. Of the eight forest disturbances considered, ice storms are the least costly, averaging about $10 million and more than 180,000 ha annually (Michaels and Cherpack 1998). Insects and pathogens are the most expensive, with costs exceeding $2 billion and 20.4 million ha per year (USDA 1997). The socioeconomic aspects of these damages are only part of the cost. Costs of impacts to ecological services (e.g., water purification) can be large and long term.
This article examines how eight disturbances influence forest structure, composition, and function and how climate change may influence the severity, frequency, and magnitude of disturbances to forests. We focus on examples from the United States, although these influences occur worldwide. We also consider options for coping with disturbance under changing climate. This analysis points to specific research needs that should improve the understanding of how climate change affects forest disturbances.
This paper is one in a series developed by the forest sector of the US National Assessment of the Potential Consequences of Climate Variability and Change. In examining how forests may be affected by climate change, the Forest Sector Committee divided the topic into four areas (processes, diversity, disturbances, and socioeconomics), each of which is the focus of an article in this issue of BioScience. Impacts of climate changes on aquatic disturbances are critical, but this paper focuses on direct terrestrial impacts. The effects of a rise in sea level, coastal processes, and salinity on terrestrial systems are examined in the coastal sector of the national assessment (NAST 2000).
INTRODUCTION: Global change is often perceived as human-induced modifications in climate. Indeed, human activities have undeniably altered the atmosphere, and probably the climate as well (Watson et al. 1998). At the same time, most of the world's forests have also been extensively modified by human use of the land (Houghton 1994). Thus, climate and land use are two prongs of human-induced global change. The effect of these forces on forests is mediated by the organisms within forests. Consideration of climate, land use, and biological diversity is key to understanding forest response to global change.
Biological diversity refers to the variety of life at organizational levels from genotypes through biomes (Franklin 1993). The responses of ecological systems to global change reflect the organisms that are within them. While ecologists have sometimes not seen the forest for the trees, so to speak, it is also true that forests cannot be understood without knowledge of the trees and other component species. It is the responses of individual organisms that begin the cascade of ecological processes that are manifest as changes in system properties, some of which feed back to influence climate and land use (Figure 1). Beyond its role in ecosystems, biodiversity is invaluable to humans for foods, medicines, genetic information, recreation, and spiritual renewal (Pimentel et al. 1997). Thus, global changes that affect the distribution and abundance of organisms will affect future human well-being and land use, as well as, possibly, the climate.
This article serves as a primer on forest biodiversity as a key component of global change. We first synthesize current knowledge of interactions among climate, land use, and biodiversity. We then summarize the results of new analyses on the potential effects of human-induced climate change on forest biodiversity. Our models project how possible future climates may modify the distributions of environments required by various species, communities, and biomes. Current knowledge, models, and funding did not allow these analyses to examine the population processes (e.g., dispersal, regeneration) that would mediate the responses of organisms to environmental change. It was also not possible to model the important effects of land use, natural disturbance, and other factors on the response of biodiversity to climate change. Despite these limitations, the analyses discussed herein are among the most comprehensive projections of climate change effects on forest biodiversity yet conducted. We conclude with discussions of limitations, research needs, and strategies for coping with potential future global change.
Irland, L.C., D. Adams, R. Alig, C.J. Betz, C. Chen, M. Hutchins, B.A. McCarl, K. Skog, B.L. Sohngen (2001). Assessing socioeconomic impacts of climate change on US forests, wood-product markets, and forest recreation. BioScience 51 (9): 753-764
INTRODUCTION: Scientists have suggested that future climate change will significantly affect the distribution, condition, species composition, and productivity of forests (Aber et al. 2001, Dale et al. 2001, Hansen et al. 2001, McNulty and Aber 2001). These biological changes will set in motion complex regional changes in supplies of wood to sawmills and paper mills, producing effects on market prices. In turn, landowners and consumers will adapt in ways that cause further feedback effects on forests. For some time, social scientists have been assessing the manifold implications for social and economic welfare. In particular, they have been examining ways in which price responses to changing supplies cause timber growers, sawmills and pulpmills, producers, and consumers to adapt. This paper reviews this research, focusing on the forest benefits of timber production and outdoor recreation. Analyzing these sectors involves quite different methods and issues because wood products are primarily producer goods that reach consumers through a complex marketing chain, whereas forest-recreation experiences are directly consumed by visitors. As part of the national assessment of climate change, a socioeconomic team (the authors of this article) assembled existing data and conducted limited new analyses. In this short summary, many important topics must be left aside.
In this paper we discuss the problems of projecting social and economic changes affecting forests and review recent efforts to assess the wood-market impacts of possible climate changes. To illustrate the range of conditions encountered in projecting socioeconomic change linked to forests, we consider two markedly different uses: forest products markets and forest recreation. In the case of forest products, we use an existing forest-sector model to arrive at new simulation results concerning the impacts of climate change. The impact of climate change on recreation has received less attention; here we consider a case study of downhill skiing. Other important forest values were not treated explicitly in this research. Our primary emphasis is on methods and issues in the socioeconomic assessment process. Our efforts may be viewed as an exercise in human ecology, studying complex interactions between human societies and their forests. We close with suggestions for future research.
Mote, P. W., D. J. Canning, D. L. Fluharty, R.C. Francis, J. F. Franklin, A. F. Hamlet, M. Hershman, M. Holmberg, K. N. Ideker, W. S. Keeton, D. P. Lettenmaier, L. R. Leung, N. J. Mantua, E. L. Miles, B. Noble, H. Parandvash, D. W. Peterson, A. K. Snover, S. R. Willard (1999). Impacts of climate variability and change, Pacific Northwest. National Atmospheric and Oceanic Administration, Office of Global Programs, and JISAO/SMA Climate Impacts Group: 110 pp.
OVERVIEW: Experience of the recent past illustrates the impacts that the climate variations have on the Pacific Northwest, and illustrates that there are both winners and loser when the climate is different from the “average.” The mild winter and spring of 1997—98 saw an early snow melt, which strained regional water supplies during the summer and fall months. An especially warm and dry summer, coupled with the early melt, led to exceptionally low flows and high temperatures in many Northwest streams. These conditions in turn caused severe difficulties for salmon. However, 1997—98 also had benefits for the region, which avoided the damage and disruption caused by heavy snow fall and winter flooding during the previous two winters.
Climate is not a constant, and yet many aspects of human infrastructure and activities are planned with the assumption that it is constant. But what happens when climate produces a surprise? What if, furthermore, there are long-term changes in climate? Humans have altered the composition of Earth’s atmosphere to such an extent that climate itself appears to be changing. The consequences of a changing climate may be beneficial for some places and activities, and detrimental for others.
This report describes the possible impacts of human-induced climate change and of natural climate variability like El Niño, focusing on the water resources, salmon, forests, and coasts of the Pacific Northwest (PNW). It has been prepared largely by the Climate Impacts Group (CIG) at the University of Washington. The CIG, under the direction of Professor Edward L. Miles, is an interdisciplinary group of researchers from the physical, biological, and social sciences working together to understand the impacts of climate variability and change on the Northwest.
Looking at the recent past, much of the climate history of the PNW can be described by a few recurring patterns. The strongest pattern highlights the tendency for winter climate to be either relatively cool and wet or relatively warm and dry. Cool-wet winters are generally associated with increased risks of flooding and landslides, abundant summer water supply, more abundant salmon, reduced risk of forest fires, and improved tree growth (except at high elevation). Warm-dry winters are often followed by summer water shortages, less abundant salmon, and increased risk of forest fires. The occurrence of the cool-wet or warm-dry winter pattern is influenced by two main climate variations in the Pacific Basin: ENSO (El Niño-Southern Oscillation) primarily on year-to-year timescales and PDO (the Pacific Decadal oscillation) primarily on decade-to-decade timescales. ENSO and PDO cause variation sin snowpack and streamflow, and hence the ability to meet water resource objectives; with respect tot he region’s water resources, ENSO and PDO can reinforce or cancel each other. In contrast, the response of forests and salmon is correlated more strongly with the PDO than with ENSO. The magnitude of seasonal anomalies of temperature and precipitation leading to the above effects is strikingly small, but these past anomalies enable us to calibrate the possible responses to long-term climate change.
Looking to the future, computer models of climate generally agree that the PNW will become, over the next half century, gradually warmer and wetter, with most of the precipitation increase in winter. These trends mostly agree with observed changes over the past century. Wetter winters would likely mean more flooding of certain rivers, and landslides on steep coastal bluffs. The region’s warm, dry summers may see slight increases in rainfall, according to the models, but the gains in rainfall will be more than offset by losses due to increases in evaporation. Loss of moderate-elevation snowpack in response to warmer winter temperatures would have enormous and mostly negative impacts on the region’s water resources, forests, and salmon. Among these impacts are a diminished ability to store water in reservoirs for summer use, more drought-stressed tress leading to reductions in forested area, and spawning and rearing difficulties for salmon.
Knowing what changes might occur is only part of the challenge, however. This knowledge must make its way from the realm of research to the realm of decisions, and be used in decisions. Large practical and, in some cases, legal constraints prevent climate information from being fully utilized. Meeting the challenges posed by climate variations and climate change will require considerable revision of the policies and practices concerning how the region’s natural resources are managed. An indication of the scope of such revisions comes from considering how government agencies have handled climate-related stresses in the past, like droughts and coastal erosion. In many cases, agencies cannot even make use of a good seasonal forecast in making short-term planning decision: the operating assumption is often that climate is constant and extremes do not occur. There are wide variations among the four sectors considered here in how management presently makes use of climate information.
ABSTRACT: Because of the characteristically low temperatures and ambient CO2 concentrations associated with greater altitudes, mountain forests may be particularly sensitive to global warming and increased atmospheric CO2 . Moreover, the upper treeline is probably the most stressful location within these forests, possibly providing an early bellwether of forest response. Most treeline studies of the past century, as well as recently, have correlated temperatures with the altitudinal limits observed for treelines. In contrast, investigations on pre-establishment seedlings, the most vulnerable life stage of most tree species, are rare. There appears to be specific microclimatic factors dictated by wind and sky exposure that limit seedling survival, and also generate the distorted tree forms commonly observed at treeline. Seedling survival appears critical for creating the biological facilitation of microclimate at the community level which is necessary for the growth of seedlings to normal tree stature, forming new subalpine forest at a higher altitude.
ABSTRACT: Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 x 1015 to 1.6 x 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.
Dietze, M. C.., Hersh, M. H., Moran, E. V., Clark, J. S., Goldman, R. L. (2009). A predictive framework to understand forest responses to global change. Annals of the New York Academy of Sciences 1162 (The Year in Ecology and Conservation Biology 2009): 221-236
ABSTRACT: Forests are one of Earth's critical biomes. They have been shown to respond strongly to many of the drivers that are predicted to change natural systems over this century, including climate, introduced species, and other anthropogenic influences. Predicting how different tree species might respond to this complex of forces remains a daunting challenge for forest ecologists. Yet shifts in species composition and abundance can radically influence hydrological and atmospheric systems, plant and animal ranges, and human populations, making this challenge an important one to address. Forest ecologists have gathered a great deal of data over the past decades and are now using novel quantitative and computational tools to translate those data into predictions about the fate of forests. Here, after a brief review of the threats to forests over the next century, one of the more promising approaches to making ecological predictions is described: using hierarchical Bayesian methods to model forest demography and simulating future forests from those models. This approach captures complex processes, such as seed dispersal and mortality, and incorporates uncertainty due to unknown mechanisms, data problems, and parameter uncertainty. After describing the approach, an example by simulating drought for a southeastern forest is offered. Finally, there is a discussion of how this approach and others need to be cast within a framework of prediction that strives to answer the important questions posed to environmental scientists, but does so with a respect for the challenges inherent in predicting the future of a complex biological system.
ABSTRACT: As changes in climate become more apparent, ecologists face the challenge of predicting species responses to the new conditions. Most forecasts are based on climate envelopes (CE), correlative approaches that project future distributions on the basis of the current climate often assuming some dispersal lag. One major caveat with this approach is that it ignores the complexity of factors other than climate that contribute to a species' distributional range. To overcome this limitation and to complement predictions based on CE modeling we carried out a transplant experiment of resident and potential-migrant species. Tree seedlings of 18 species were planted side by side from 2001 to 2004 at several locations in the Southern Appalachians and in the North Carolina Piedmont (USA). Growing seedlings under a large array of environmental conditions, including those forecasted for the next decades, allowed us to model seedling survival as a function of variables characteristic of each site, and from here we were able to make predictions on future seedling recruitment. In general, almost all species showed decreased survival in plots and years with lower soil moisture, including both residents and potential migrants, and in both locations, the Southern Appalachians and the Piedmont. The detrimental effects that anticipated arid conditions could have on seedling recruitment contradict some of the projections made by CE modeling, where many of the species tested are expected to increase in abundance or to expand their ranges. These results point out the importance of evaluating the potential sources of migrant species when modeling vegetation response to climate change, and considering that species adapted to the new climate and the local conditions may not be available in the surrounding regions.
Ollinger, S. V., Goodale, C. L., Hayhoe, K., Jenkins, J. P. (2008). Potential effects of climate change and rising CO2 on ecosystem processes in northeastern U.S. forests. Mitigation and Adaptation Strategies for Global Change 13 (5-6): 467-485
ABSTRACT: Forest ecosystems represent the dominant form of land cover in the northeastern United States and are heavily relied upon by the region’s residents as a source of fuel, fiber, structural materials, clean water, economic vitality, and recreational opportunities. Although predicted changes in climate have important implications for a number of ecosystem processes, our present understanding of their long-term effects is poor. In this study, we used the PnET-CN model of forest carbon (C), nitrogen (N) and water cycling to evaluate the effects of predicted changes in climate and atmospheric carbon dioxide (CO2 ) on forest growth, C exchange, water runoff, and nitrate () leaching at five forest research sites across the northeastern U.S. We used four sets of statistically downscaled climate predictions from two general circulation models (the Hadley Centre Coupled Model, version 3 and the Parallel Climate Model) and two scenarios of future CO2 concentrations. A series of model experiments was conducted to examine the effects of future temperature, precipitation, CO2 , and various assumptions regarding the physiological response of forests to these changes. Results indicate a wide range of predicted future growth rates. Increased growth was predicted across deciduous sites under most future conditions, while growth declines were predicted for spruce forests under the warmest scenarios and in some deciduous forests when CO2 fertilization effects were absent. Both climate and rising CO2 contributed to predicted changes, but their relative importance shifted from CO2 -dominated to climate-dominated from the first to second half of the twenty-first century. Predicted runoff ranged from no change to a slight decrease, depending on future precipitation and assumptions about stomatal response to CO2 . Nitrate leaching exhibited variable responses, but was highest under conditions that imposed plant stress with no physiological effects of CO2 . Although there are considerable uncertainties surrounding predicted responses to climate change, these results provide a range of possible outcomes and highlight interactions among processes that are likely to be important. Such information can be useful to scientists and land managers as they plan on means of examining and responding to the effects of climate change.
ABSTRACT: The composition and successional status of a forest affect carbon storage and net ecosystem productivity, yet it remains unclear whether elevated atmospheric carbon dioxide (CO2 ) will impact rates and trajectories of forest succession. We examined how CO2 enrichment (+200μL CO2 /L air differential) affects forest succession through growth and survivorship of tree seedlings, as part of the Duke Forest free-air CO2 enrichment (FACE) experiment in North Carolina, USA. We planted 2352 seedlings of 14 species in the low light forest understory and determined effects of elevated CO2 on individual plant growth, survival, and total sample biomass accumulation, an integrator of plant growth and survivorship over time, for six years. We used a hierarchical Bayes framework to accommodate the uncertainty associated with the availability of light and the variability in growth among individual plants.We found that most species did not exhibit strong responses to CO2 .Ulmus alata (+21%),Quercus alba (+9.5%), and nitrogen-fixingRobinia pseudoacacia (+230%) exhibited greater mean annual relative growth rates under elevated CO2 than under ambient conditions. The effects of CO2 were small relative to variability within populations; however, some species grew better under low light conditions when exposed to elevated CO2 than they did under ambient conditions. These species include shade-intolerantLiriodendron tulipifera andLiquidambar styraciflua , intermediate-tolerantQuercus velutina , and shade-tolerantAcer barbatum ,A. rubrum ,Prunus serotina ,Ulmus alata , andCercis canadensis . Contrary to our expectation, shade-intolerant trees did not survive better with CO2 enrichment, and population-scale responses to CO2 were influenced by survival probabilities in low light. CO2 enrichment did not increase rates of sample biomass accumulation for most species, but it did stimulate biomass growth of shade-tolerant taxa, particularlyAcer barbatum andUlmus alata . Our data suggest a small CO2 fertilization effect on tree productivity, and the possibility of reduced carbon accumulation rates relative to today's forests due to changes in species composition.
ABSTRACT: The world's forests influence climate through physical, chemical, and biological processes that affect planetary energetics, the hydrologic cycle, and atmospheric composition. These complex and nonlinear forest-atmosphere interactions can dampen or amplify anthropogenic climate change. Tropical, temperate, and boreal reforestation and afforestation attenuate global warming through carbon sequestration. Biogeophysical feedbacks can enhance or diminish this negative climate forcing. Tropical forests mitigate warming through evaporative cooling, but the low albedo of boreal forests is a positive climate forcing. The evaporative effect of temperate forests is unclear. The net climate forcing from these and other processes is not known. Forests are under tremendous pressure from global change. Interdisciplinary science that integrates knowledge of the many interacting climate services of forests with the impacts of global change is necessary to identify and understand as yet unexplored feedbacks in the Earth system and the potential of forests to mitigate climate change.
L. A. Joyce, J. R. Mills, L. S. Heath, A. D. McGuire, R. W. Haynes, R. A. Birdsey (1995). Forest sector impacts from changes in forest productivity under climate change. Journal of Biogeography 22 (4/5): 703-713
ABSTRACT: The consequences of elevated carbon dioxide and climate change on forest systems and the role that economics could play in timber harvest and vegetation change have not been addressed together. A framework was developed to link climate change scenarios, an ecosystem model, a forest sector model and a carbon accounting model. Four climate scenarios were used to estimate net primary productivity (NPP) for forests in the United States. Changes in NPP were estimated using TEM, the Terrestrial Ecosystem Model which uses spatially referenced information on climate, soils and vegetation to estimate important carbon and nitrogen fluxes and pool sizes within ecosystems at the continental scale. Changes in NPP under climate change were used to modify timber growth within the Aggregate Timberland Assessment Model (ATLAS), which is a part of the forest sector model (TAMM-ATLAS) used by the Forest Service to examine timber policy questions. The changes in timber were the translated into changes in the amount of carbon stored on private timberlands using a national carbon model (FORCARB). Regional changes in productivity filter through the forest sector and result in changes in land use and timber consumption. Long-term changes in carbon storage indicate that these private timberlands will be a source of carbon dioxide for all but the most optimistic climate change scenario.
ABSTRACT: The carbon sequestered by restoring forests is greater than the emissions avoided by the use of the liquid biofuels.
ABSTRACT: A new ecosystem-based climate envelope modeling approach was applied to assess potential climate change impacts on forest communities and tree species. Four orthogonal canonical discriminant functions were used to describe the realized climate space for British Columbia's ecosystems and to model portions of the realized niche space for tree species under current and predicted future climates. This conceptually simple model is capable of predicting species ranges at high spatial resolutions far beyond the study area, including outlying populations and southern range limits for many species. We analyzed how the realized climate space of current ecosystems changes in extent, elevation, and spatial distribution under climate change scenarios and evaluated the implications for potential tree species habitat. Tree species with their northern range limit in British Columbia gain potential habitat at a pace of at least 100 km per decade, common hardwoods appear to be generally unaffected by climate change, and some of the most important conifer species in British Columbia are expected to lose a large portion of their suitable habitat. The extent of spatial redistribution of realized climate space for ecosystems is considerable, with currently important sub-boreal and montane climate regions rapidly disappearing. Local predictions of changes to tree species frequencies were generated as a basis for systematic surveys of biological response to climate change.
Crookston, N. L., Rehfeldt, G. E., Ferguson, D. E., Warwell, M. V. (2008). FVS and global warming: a prospectus for future development. U.S. Dept. of Agriculture, Forest Service, Rocky Mountain Research Station: 7-16
ABSTRACT: Climate change-global warming and changes in precipitation-will cause changes in tree growth rates, mortality rates, the distribution of tree species, competition, and species interactions. An implicit assumption in FVS is that site quality will remain the same as it was during the time period observations used to calibrate the component models were made and that the site quality will not be affected by climate change. This paper presents evidence of the impacts of climate change on forests and argues that FVS needs to be revised to account for these changes. The changes include modification of the growth, mortality, and regeneration establishment models, all of which need to account for changes in site quality and genetic adaptation. Criteria for modifying the model recognize that the models applications and uses will not diminish and need to be supported. The new process, climate change, needs to be recognized by the model because it influences all of the processes FVS currently represents. Plans are being made to address this major task.
Guardiola-Claramonte, M., Adams, H. D., Barron-Gafford, G. A., Villegas, J. C., Breshears, D. D., Zou, C. B., Troch, P.A., Huxman, T. E. (2009). Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought. Proceedings of the National Academy of Sciences 106 (17): 7063-7066
ABSTRACT: Large-scale biogeographical shifts in vegetation are predicted in response to the altered precipitation and temperature regimes associated with global climate change. Vegetation shifts have profound ecological impacts and are an important climate-ecosystem feedback through their alteration of carbon, water, and energy exchanges of the land surface. Of particular concern is the potential for warmer temperatures to compound the effects of increasingly severe droughts by triggering widespread vegetation shifts via woody plant mortality. The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates—temperature-sensitive carbon starvation in response to a period of protracted water stress or temperature-insensitive sudden hydraulic failure under extreme water stress (cavitation). Here we show that experimentally induced warmer temperatures (≈4 °C) shortened the time to drought-induced mortality inPinus edulis (piñon shortened pine) trees by nearly a third, with temperature-dependent differences in cumulative respiration costs implicating carbon starvation as the primary mechanism of mortality. Extrapolating this temperature effect to the historic frequency of water deficit in the southwestern United States predicts a 5-fold increase in the frequency of regional-scale tree die-off events for this species due to temperature alone. Projected increases in drought frequency due to changes in precipitation and increases in stress from biotic agents (e.g., bark beetles) would further exacerbate mortality. Our results demonstrate the mechanism by which warmer temperatures have exacerbated recent regional die-off events and background mortality rates. Because of pervasive projected increases in temperature, our results portend widespread increases in the extent and frequency of vegetation die-off.
Rhemtullaa, J. M., Mladenoff, D. J., Clayton, M. K. (2009). Historical forest baselines reveal potential for continued carbon sequestration. Proceedings of the National Academy of Sciences 106 (15): 6082-6087
ABSTRACT: One-third of net CO2 emissions to the atmosphere since 1850 are the result of land-use change, primarily from the clearing of forests for timber and agriculture, but quantifying these changes is complicated by the lack of historical data on both former ecosystem conditions and the extent and spatial configuration of subsequent land use. Using fine-resolution historical survey records, we reconstruct pre-EuroAmerican settlement (1850s) forest carbon in the state of Wisconsin, examine changes in carbon after logging and agricultural conversion, and assess the potential for future sequestration through forest recovery. Results suggest that total above-ground live forest carbon (AGC) fell from 434 TgC before settlement to 120 TgC at the peak of agricultural clearing in the 1930s and has since recovered to approximately 276 TgC. The spatial distribution of AGC, however, has shifted significantly. Former savanna ecosystems in the south now store more AGC because of fire suppression and forest ingrowth, despite the fact that most of the region remains in agriculture, whereas northern forests still store much less carbon than before settlement. Across the state, continued sequestration in existing forests has the potential to contribute an additional 69 TgC. Reforestation of agricultural lands, in particular, the formerly high C-density forests in the north-central region that are now agricultural lands less optimal than those in the south, could contribute 150 TgC. Restoring historical carbon stocks across the landscape will therefore require reassessing overall land-use choices, but a range of options can be ranked and considered under changing needs for ecosystem services.
Beckage, B., Osborne, B., Gavin, D. G., Pucko, C., Siccama, T. G., Perkins, T. (2008). A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont. Proceedings of the National Academy of Sciences 105 (11): 4197-4202
ABSTRACT: Detecting latitudinal range shifts of forest trees in response to recent climate change is difficult because of slow demographic rates and limited dispersal but may be facilitated by spatially compressed climatic zones along elevation gradients in montane environments. We resurveyed forest plots established in 1964 along elevation transects in the Green Mountains (Vermont) to examine whether a shift had occurred in the location of the northern hardwood–boreal forest ecotone (NBE) from 1964 to 2004. We found a 19% increase in dominance of northern hardwoods from 70% in 1964 to 89% in 2004 in the lower half of the NBE. This shift was driven by a decrease (up to 76%) in boreal and increase (up to 16%) in northern hardwood basal area within the lower portions of the ecotone. We used aerial photographs and satellite imagery to estimate a 91- to 119-m upslope shift in the upper limits of the NBE from 1962 to 2005. The upward shift is consistent with regional climatic change during the same period; interpolating climate data to the NBE showed a 1.1°C increase in annual temperature, which would predict a 208-m upslope movement of the ecotone, along with a 34% increase in precipitation. The rapid upward movement of the NBE indicates little inertia to climatically induced range shifts in montane forests; the upslope shift may have been accelerated by high turnover in canopy trees that provided opportunities for ingrowth of lower elevation species. Our results indicate that high-elevation forests may be jeopardized by climate change sooner than anticipated.
ABSTRACT: Climate change is already affecting America's forests. The fires of 2000 shocked the Nation, the fires of 2006 burned an area greater than in any year since 1954, and the 2007 fires in southern California forced the evacuation of more than a million residents. Some of the largest individual fires ever recorded in the Western United States and Alaska occurred in the first 5 years of the 21st century. Scientists have linked growing fire season severity with warming temperatures and earlier snowmelt (Westerling et al. 2006). Higher temperatures and drought also are blamed for unprecedented bark beetle outbreaks and tree mortality across the West (Breshears et al. 2005, Logan and Powell 2005). However, forest productivity is increasing in some temperate areas because of warmer temperatures, a longer growing season, and the "fertilizer effect" of increasing atmospheric carbon dioxide (Nemani et al. 2003).