Climate Change and...

Annotated Bibliography

Effects of Climate Change

Transpiration and Water Yield

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

Ferguson, P. R., N. Weinrauch, L. I. Wassenaar, B. Mayer, J. Veizer (2007). Isotope constraints on water, carbon, and heat fluxes from the northern Great Plains region of North America. Global Biogeochemical Cycles 21: GB2023

ABSTRACT: Water vapor flux facilitates the transfer of large amounts of mass and energy from terrestrial ecosystems to the atmosphere, yet the proportions of this flux ascribed to evaporation and plant transpiration are poorly constrained. Here we used a water-isotope mass balance approach to partition evaporation and transpiration water vapor fluxes in the northern Great Plains region of western Canada. Utilizing the proportion of gross watershed area that contributes to annual river flow, we estimated that of the mean annual precipitation (~490 mm), ~7% was transferred to the atmosphere via direct evaporation from water bodies and soils, whereas ∼50% was returned to the atmosphere by plant transpiration. Further, through the explicit coupling of transpiration and photosynthesis, we estimated that plant transpiration in the watersheds corresponded to an annual photosynthetic carbon flux of ~48.9 × 1012 g C or ~325 g C m−2 . Although uncertainty related to this estimate of photosynthetic carbon uptake was substantial owing to the paucity of regional estimates of plant water-use efficiency, it was similar to the flux of carbon released through soil respiration and other independent estimates of primary productivity. The water-isotope mass balance approach in partitioning evaporation and transpiration fluxes at a regional scale was promising, although results revealed that the flux estimates would be greatly improved by longer-term data on the isotope composition of river water, precipitation, and atmospheric moisture, as well as through detailed regional-scale measurements of plant water-use efficiency under various environmental and climatic conditions.

Fisher, R. A., Williams, M., Da Costa, A. L., Malhi, Y., Da Costa, R. F., Almeida, S., Meir, P. (2007). The response of an Eastern Amazonian rain forest to drought stress: results and modelling analyses from a throughfall exclusion experiment. Global Change Biology 13 (11): 2361-2378

ABSTRACT: Warmer and drier climates over Eastern Amazonia have been predicted as a component of climate change during the next 50-100 years. It remains unclear what effect such changes will have on forest-atmosphere exchange of carbon dioxide (CO2 ) and water, but the cumulative effect is anticipated to produce climatic feedback at both regional and global scales. To allow more detailed study of forest responses to soil drying, a simulated soil drought or 'throughfall exclusion' (TFE) experiment was established at a rain forest site in Eastern Amazonia, Brazil, for which time-series sap flow and soil moisture data were obtained. The experiment excluded 50% of the throughfall from the soil. Sap flow data from the forest plot experiencing normal rainfall showed no limitation of transpiration throughout the two monitored dry seasons. Conversely, data from the TFE showed large dry season declines in transpiration, with tree water use restricted to 20% of that in the control plot at the peak of both dry seasons. The results were examined to evaluate the paradigm that the restriction on transpiration in the dry season was caused by limitation of soil-to-root water transport, driven by low soil water potential and high soil-to-root hydraulic resistance. This paradigm, embedded in the soil-plant-atmosphere (SPA) model and driven using on-site measurements, provided a good explanation (R2 > 0.69) of the magnitude and timing of changes in sap flow and soil moisture. This model-data correspondence represents a substantial improvement compared with other ecosystem models of drought stress tested in Amazonia. Inclusion of deeper rooting should lead to lower sensitivity to drought than the majority of existing models. Modelled annual GPP declined by 13-14% in response to the treatment, compared with estimated declines in transpiration of 30-40%.

T. Grϋnwald, C. Bernhofer (2007). A decade of carbon, water and energy flux measurements of an old spruce forest at the Anchor Station Tharandt. Tellus B 59 (3): 387-396

ABSTRACT: At Tharandt/Germany eddy covariance (EC) measurements of carbon dioxide and heat fluxes are performed above an old spruce forest since 1996. The last ten years cover almost all meteorological extremes observed during the last 45 years: the coldest and warmest year with mean air temperature of 6.1°C (1996) and 9.6°C (2000) as well as the fourth wettest and the driest year with a precipitation of 1098 mm (2002) and 501 mm (2003), respectively. In general, the observed annual carbon net ecosystem exchange (NEE) indicates a high net sink from −395 g C m−2 a−1 (2003) to −698 g C m−2 a−1 (1999) with a coefficient of variation cv= 16.6% . The yearly evapotranspiration (ET) has a lower interannual variability (cv= 9.5%) between 389 mm (2003) and 537 mm (2000). The influence of flux correction and gap filling on the amount of annual NEE and ET is considerable. Using different methods of gap filling (non-linear regressions, mean diurnal courses) yields annual NEE totals that differ by up to 18%.

Consistency analysis regarding energy balance closure, comparisons with independent soil respiration and biomass increment measurements indicate reliability of the fluxes. The average gap of the energy balance is 15% of the available energy based on regression slope with an intercept of 3 to 16 W m−2 , but around zero for annual flux ratios. Between 47% and 63% of the net ecosystem productivity was fixed above ground according to up-scaled tree ring data and forest inventories, respectively. Chamber measurements of soil respiration yield up to 90% of nighttime EC based total ecosystem respiration. Thus, we conclude that the EC based flux represents an upper limit of the C sink at the site.

Ivans, S., Hipps, L., Leffler, A. J., Ivans, C. Y. (2006). Response of water vapor and CO2 fluxes in semiarid lands to seasonal and intermittent precipitation pulses. Journal of Hydrometeorology 7 (5): 995-1010

ABSTRACT: Precipitation pulses are important in controlling ecological processes in semiarid ecosystems. The effects of seasonal and intermittent precipitation events on net water vapor and CO2 fluxes were determined for crested wheatgrass (Agropyron desertorum ), juniper (Juniperus osteosperma ), and sagebrush (Artemisia tridentata ) ecosystems using eddy covariance measurements. The measurements were made at Rush Valley, Utah, in the northern Great Basin of the United States. Data were evaluated during the growing seasons of 2002 and 2003. Each of these communities responds to precipitation pulses in all seasons, but these responses vary among season and ecosystem, and differ for water vapor and CO2 . The degree and direction of response (i.e., net uptake or efflux) depended upon the timing and amount of precipitation. In early spring, both evapotranspiration (ET) and CO2 fluxes responded only slightly to precipitation pulses because soils were already moist from snowmelt and spring rains. As soils dried later in the spring, ET response to rainfall increased. The summer season was very warm and dry in both years, and both water and CO2 fluxes were generally reduced as compared to fluxes in the spring. Water vapor fluxes increased during and immediately after periodic summer rain events at all sites, especially at juniper, followed by the sagebrush and crested wheatgrass sites. Net CO2 exchange changed significantly at the juniper and sagebrush sites but changed very little at the crested wheatgrass site due to senescence of this grass. However, in the wetter summer of 2003, the grass species maintained physiological activity and responded to rain events. In the fall of both years, responses of ET and CO2 fluxes to precipitation were very similar for all three communities, with only small changes, presumably due to significantly lower temperatures in the fall. This research documents the importance of the temporal distribution of rainfall on patterns of ET and CO2 fluxes and suggests that soil moisture and stand-level leaf area index (LAI) are critical factors governing ET and CO2 responses to precipitation in these communities.

Leffler, A. J., Caldwell, M. M., Ryel, R. J., Hipps, L., Ivans, S. (2002). Carbon acquisition and water use in a northern Utah Juniperus osteosperma (Utah juniper ) population. Tree Physiology 22 (17): 1221-1230

ABSTRACT: Water use and carbon acquisition were examined in a northern Utah population ofJuniperus osteosperma (Torr.) Little. Leaf-level carbon assimilation, which was greatest in the spring and autumn, was limited by soil water availability. Gas exchange, plant water potential and tissue hydrogen stable isotopic ratio (D) data suggested that plants responded rapidly to summer rain events. Based on a leaf area index of 1.4, leaf-level water use and carbon acquisition scaled to canopy-level means of 0.59 mm day–1 and 0.13 mol m–2 ground surface day–1 , respectively. Patterns of soil water potential indicated thatJ. osteosperma dries the soil from the surface downward to a depth of about 1 m. Hydraulic redistribution is a significant process in soil water dynamics. Eddy covariance data indicated a mean evapotranspiration rate of 0.85 mm day–1 from March to October 2001, during which period the juniper population at the eddy flux site was a net source of CO2 (3.9 mol m–2 ground area). We discuss these results in relation to the rapid range expansion of juniper species during the past century.

Ponton, S., L.B. Flanagan, K.P. Alstad, B.G. Johnson, K. Morgenstern, N. Kljun, T.A. Black, A.G. Barr (2006). Comparison of ecosystem water-use efficiency among Douglas-fir forest, aspen forest and grassland using eddy covariance and carbon isotope techniques. Global Change Biology 12 (2): 294-310

ABSTRACT: Comparisons were made among Douglas-fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C3) grassland for ecosystem-level water-use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO2 assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas-fir (located on Vancouver Island, BC), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (Dmax) during the mid-summer. The average (±SD) WUE at the grassland site was 2.6±0.7 mmol mol−1 , which was much lower than the average values observed for the two other sites (aspen: 5.4±2.3, Douglas-fir: 8.1±2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day−1 for the grassland, aspen and Douglas-fir sites, respectively. There was a strong negative correlation between WUE and Dmax for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO2 (dR ) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystem-integrated ratio of leaf intercellular to ambient CO2 concentration (ci /ca ). There was a consistent increase indR values in the grassland, aspen forest and Douglas-fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D anddR measurements. There was excellent agreement between WUE values calculated using the two techniques. OurdR measurements indicated that ci/ca values were quite similar among the Douglas-fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter-lived grass species had relatively high ci/ca values for the D of their habitat. By contrast, the longer-lived Douglas-fir trees were more conservative in water-use with lower ci /ca values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem-level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO2 is a useful ecosystem-scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.

K. V. R. Schäfer, R. Oren, C.-T. Lai, G. G. Katul (2002). Hydrologic balance in an intact temperate forest ecosystem under ambient and elevated atmospheric CO2 concentration.. Global Change Biology 8 (9): 895-911

ABSTRACT: Increasing atmospheric CO2 concentration decreases stomatal conductance in many species, but the savings of water from reduced transpiration may permit the forest to retain greater leaf area index (L). Therefore, the net effect on water use in forest ecosystems under a higher CO2 atmosphere is difficult to predict. The free air CO2 enrichment (FACE) facility (n = 3) in a 14-m tall (in 1996)Pinus taeda L. stand was designed to reduce uncertainties in predicting such responses. Continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient (CO2 a ) and elevated (CO2 e ) ambient + 200 µmol mol−1 ). Annual stand transpiration under ambient CO2 conditions accounted for 84–96% of latent heat flux measured with the eddy-covariance technique above the canopy. Under CO2 e ,P. taeda transpired less per unit of leaf area only when soil drought was severe.Liquidambar styraciflua , the other major species in the forest, used progressively less water, settling at 25% reduction in sap flux density after 3.5 years under CO2 e . BecauseP. taeda dominated the stand, and severe drought periods were of relatively short duration, the direct impact of CO2 e on water savings in the stand was undetectable. Moreover, the forest used progressively more water under CO2 e , probably because soil moisture availability progressively increased, probably owing to a reduction in soil evaporation caused by more litter buildup in the CO2 e plots. The results suggest that, in this forest, the effect of CO2 e on transpiration was greater indirectly through enhanced litter production than directly through reduced stomatal conductance. In forests composed of species more similar toL. styraciflua , water savings from stomatal closure may dominate the response to CO2 e .

Stoy, P. C., Katul, G. G., Siqueira, M. B. S., Juang, J. Y., Novick, K. A., Mccarthy, H. R., Oishi, A. C., Uebelherr, J. M., Kim, H. S., Oren, R. (2006). Separating the effects of climate and vegetation on evapotranspiration along a successional chronosequence in the southeastern U.S.. Global Change Biology 12 (11): 2115-2135

ABSTRACT: We combined Eddy-covariance measurements with a linear perturbation analysis to isolate the relative contribution of physical and biological drivers on evapotranspiration (ET) in three ecosystems representing two end-members and an intermediate stage of a successional gradient in the southeastern US (SE). The study ecosystems, an abandoned agricultural field [old field (OF)], an early successional planted pine forest (PP), and a late-successional hardwood forest (HW), exhibited differential sensitivity to the wide range of climatic and hydrologic conditions encountered over the 4-year measurement period, which included mild and severe droughts and an ice storm. ET and modeled transpiration differed by as much as 190 and 270 mm yr−1 , respectively, between years for a given ecosystem. Soil water supply, rather than atmospheric demand, was the principal external driver of interannual ET differences. ET at OF was sensitive to climatic variability, and results showed that decreased leaf area index (L) under mild and severe drought conditions reduced growing season (GS) ET (ETGS) by ca. 80 mm compared with a year with normal precipitation. Under wet conditions, higher intrinsic stomatal conductance (gs) increased ETGS by 50 mm. ET at PP was generally larger than the other ecosystems and was highly sensitive to climate; a 50 mm decrease in ETGS due to the loss of L from an ice storm equaled the increase in ET from high precipitation during a wet year. In contrast, ET at HW was relatively insensitive to climatic variability. Results suggest that recent management trends toward increasing the land-cover area of PP-type ecosystems in the SE may increase the sensitivity of ET to climatic variability.

N. Chattopadhyay, M. Hulme (1997). Evaporation and potential evapotranspiration in India under conditions of recent and future climate change. Agricultural and Forest Meteorology 87 (1): 55-73

ABSTRACT: Long-term changes in evaporation and potential evapotranspiration can have profound implications for hydrologic processes as well as for agricultural crop performance. This paper analyses evaporation time series data for different stations in India, and for the country as a whole, for different seasons on both a short-term (15 years) and long-term (32 years) basis for pan evaporation and on a short-term basis alone for potential evapotranspiration. The analysis shows that both pan evaporation and potential evapotranspiration have decreased during recent years in India. The likely causative meteorological parameters for such changes are identified. Future scenarios of potential evapotranspiration, and its component energy and aerodynamic terms, for India based on results from six global climate model climate change experiments are also calculated and intercompared. Future warming seems likely to lead in general to increased potential evapotranspiration over India, although this increase will be unequal between regions and seasons. Such changes could have marked implications for economic and environmental welfare in the country, especially if the increases in evaporation are not compensated by adequate increases in rainfall.

A. F. Hamlet, P. W. Mote, M. P. Clark, D. P. Lettenmaier (2007). Twentieth-century trends in runoff, evapotranspiration, and soil moisture in the western United States. Journal of Climate 20 (8): 1468-1486

ABSTRACT: A physically based hydrology model is used to produce time series for the period 1916–2003 of evapotranspiration (ET), runoff, and soil moisture (SM) over the western United States from which long-term trends are evaluated. The results show that trends in ET in spring and summer are determined primarily by trends in precipitation and snowmelt that determine water availability. From April to June, ET trends are mostly positive due primarily to earlier snowmelt and earlier emergence of snow-free ground, and secondarily to increasing trends in spring precipitation. From July to September trends in ET are more strongly influenced by precipitation trends, with the exception of areas (most notably California) that receive little summer precipitation and have experienced large changes in snowmelt timing. Trends in the seasonal timing of ET are modest, but during the period 1947–2003 when temperature trends are large, they reflect a shift of ET from midsummer to early summer and late spring. As in other studies, it is found that runoff is occurring earlier in spring, a trend that is related primarily to increasing temperature, and is most apparent during 1947–2003. Trends in the annual runoff ratio, a variable critical to western water management, are determined primarily by trends in cool season precipitation, rather than changes in the timing of runoff or ET. It was found that the signature of temperature-related trends in runoff and SM is strongly keyed to mean midwinter [December–February (DJF)] temperatures. Areas with warmer winter temperatures show increasing trends in the runoff fraction as early as February, and colder areas as late as June. Trends toward earlier spring SM recharge are apparent and increasing trends in SM on 1 April are evident over much of the region. The 1 July SM trends are less affected by snowmelt changes and are controlled more by precipitation trends.

E. S. Zavaleta, B. D. Thomas, N. R. Chiariello, G.P. Asner, M. R. Shaw, C.B. Field (2003). Plants reverse warming effect on ecosystem water balance. Proceedings of the National Academy of Sciences 100 (17): 9892-9893

ABSTRACT: Models predict that global warming may increase aridity in water-limited ecosystems by accelerating evapotranspiration. We show that interactions between warming and the dominant biota in a grassland ecosystem produced the reverse effect. In a 2-year field experiment, simulated warming increased spring soil moisture by 5–10% under both ambient and elevated CO2 . Warming also accelerated the decline of canopy greenness (normalized difference vegetation index) each spring by 11–17% by inducing earlier plant senescence. Lower transpirational water losses resulting from this earlier senescence provide a mechanism for the unexpected rise in soil moisture. Our findings illustrate the potential for organism–environment interactions to modify the direction as well as the magnitude of global change effects on ecosystem functioning.

Fay, P. A., Kelley, A. M., Proctor, A. C., Hui, D., Jin, V. L., Jackson, R. B., Johnson, H. B., Polley, H. W. (2009). Primary productivity and water balance of grassland vegetation on three soils in a continuous CO2 gradient: initial results from the lysimeter CO2 gradient experiment. Ecosystems 12 (5): 699-714

ABSTRACT: Field studies of atmospheric CO2 effects on ecosystems usually include few levels of CO2 and a single soil type, making it difficult to ascertain the shape of responses to increasing CO2 or to generalize across soil types. The Lysimeter CO2 Gradient (LYCOG) chambers were constructed to maintain a linear gradient of atmospheric CO2 (~250 to 500μl l−1 ) on grassland vegetation established on intact soil monoliths from three soil series. The chambers maintained a linear daytime CO2 gradient from 263μl l−1 at the subambient end of the gradient to 502μl l−1 at the superambient end, as well as a linear nighttime CO2 gradient. Temperature variation within the chambers affected aboveground biomass and evapotranspiration, but the effects of temperature were small compared to the expected effects of CO2 . Aboveground biomass on Austin soils was 40% less than on Bastrop and Houston soils. Biomass differences between soils resulted from variation in biomass ofSorghastrum nutans ,Bouteloua curtipendula ,Schizachyrium scoparium (C4 grasses), andSolidago canadensis (C3 forb), suggesting the CO2 sensitivity of these species may differ among soils. Evapotranspiration did not differ among the soils, but the CO2 sensitivity of leaf-level photosynthesis and water use efficiency in S. canadensis was greater on Houston and Bastrop than on Austin soils, whereas the CO2 sensitivity of soil CO2 efflux was greater on Bastrop soils than on Austin or Houston soils. The effects of soil type on CO2 sensitivity may be smaller for some processes that are tightly coupled to microclimate. LYCOG is useful for discerning the effects

L. R. Gardner (2009). Assessing the effect of climate change on mean annual runoff. Journal of Hydrology In Press, Corrected Proof

ABSTRACT: From published runoff measurements in catchments with a wide range of climatic conditions it is found that long-term mean annual runoff (R) can be closely fitted (r2 = 0.94) to measured climatic data by R = P*exp(-PET/P), where P is the mean annual precipitation and PET is the mean annual potential evapotranspiration (in mm) calculated via the Holland equation, PET = 1.2 * 1010 * exp(-4620/Tk), which is solely a function of the mean annual temperature in Kelvin, Tk. Application of the chain rule for partial differentiation to the combined equations gives the following equation for estimating the change in runoff due to changes in P and Tk:

dR = exp(-PET/P) * [1 + PET/P] * dP – [5544 * 1010 * exp(-PET/P) * exp(-4620/Tk) * Tk2 ] * dTk

By setting dR equal to zero, this equation can be used to estimate the increase in P required to maintain constant runoff for a small increase in T. It can also be used to estimate the decrease in runoff in a scenario with constant precipitation and increased temperature. It is shown herein that predictions of annual runoff changes for various climate change scenarios based on this simple model compare favorably with those based on more complex, calibrated hydrological models, as well as with those based on long-term historical observations of runoff and climate change. Application of the equation above also indicates that the IPCC projections for climate change under the A1B emissions scenario may underestimate the area of North America that is likely to suffer decreases in runoff.

K. A. Farley, E. G. Jobbágy, R. B. Jackson (2005). Effects of afforestation on water yield: a global synthesis with implications for policy. Global Change Biology 11 (10): 1565-1576

ABSTRACT: Carbon sequestration programs, including afforestation and reforestation, are gaining attention globally and will alter many ecosystem processes, including water yield. Some previous analyses have addressed deforestation and water yield, while the effects of afforestation on water yield have been considered for some regions. However, to our knowledge no systematic global analysis of the effects of afforestation on water yield has been undertaken. To assess and predict these effects globally, we analyzed 26 catchment data sets with 504 observations, including annual runoff and low flow. We examined changes in the context of several variables, including original vegetation type, plantation species, plantation age, and mean annual precipitation (MAP). All of these variables should be useful for understanding and modeling the effects of afforestation on water yield. We found that annual runoff was reduced on average by 44% (±3%) and 31% (±2%) when grasslands and shrublands were afforested, respectively. Eucalypts had a larger impact than other tree species in afforested grasslands (P=0.002), reducing runoff (90) by 75% (±10%), compared with a 40% (±3%) average decrease with pines. Runoff losses increased significantly with plantation age for at least 20 years after planting, whether expressed as absolute changes (mm) or as a proportion of predicted runoff (%) (P<0.001). For grasslands, absolute reductions in annual runoff were greatest at wetter sites, but proportional reductions were significantly larger in drier sites (P<0.01 and P<0.001, respectively). Afforestation effects on low flow were similar to those on total annual flow, but proportional reductions were even larger for low flow (P<0.001). These results clearly demonstrate that reductions in runoff can be expected following afforestation of grasslands and shrublands and may be most severe in drier regions. Our results suggest that, in a region where natural runoff is less than 10% of MAP, afforestation should result in a complete loss of runoff; where natural runoff is 30% of precipitation, it will likely be cut by half or more when trees are planted. The possibility that afforestation could cause or intensify water shortages in many locations is a tradeoff that should be explicitly addressed in carbon sequestration programs.

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