USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Tree Mortality


Aligning smoke management with ecological and public health goals (2017)

Past and current forest management affects wildland fire smoke impacts on downwind human populations. However, mismatches between the scale of benefits and risks make it difficult to proactively manage wildland fires to promote both ecological and public health. Building on recent literature and advances in modeling smoke and health effects, we outline a framework to more directly quantify and compare smoke impacts based on emissions, dispersion, and the size and vulnerability of downwind populations across time and space. We apply the framework in a case study to demonstrate how different kinds of fires in California's Central Sierra Nevada have resulted in very different smoke impacts.

Air quality (2014)

The major pollutants harming the Sierra Nevada are ozone, which can be toxic to plants, and nitrogen deposition, which can induce undesirable effects on terrestrial and aquatic ecosystems. Other airborne pollutants of concern include black carbon, particulate matter (PM), pesticides and heavy metals, including mercury. The highest potential for ozone to injure plants occurs on western, low-elevation slopes that have elevated daytime levels that coincide with the highest physiological activity of plants. Ozone and nitrogen deposition interact with other environmental stressors, especially drought and climate change, to predispose forests to impacts of pests and diseases.

Interactive effects of air pollution and climate change on forest ecosystems in the United States: current understanding and future scenarios (2013)

Simulations of past, present and future hydrologic, nutrient, and growth changes caused by interactive effects of air pollution and climate change are described for two U.S. forest ecosystems. The extent of these effects will vary depending on the future intensity and extent of climate change, air pollutant emission levels, the distribution of air pollution, and other factors such as drought, pest outbreaks, fire, etc.

Managing air pollution impacted forests of California (2009)

High ambient ozone concentration and elevated nitrogen deposition weakens and predisposes trees to bark beetle attacks, increases foliar senescence and fuel build-up, and increases water stress during drought periods. Climate variability is expected to increase beyond historic ranges of variation, resulting in more severe droughts. Combinations of future climate variability and air pollution are likely to increase risk of episodic tree mortality, long-term ecosystem changes, and frequency and severity of wildland fires. Air pollution and climate effects should be incorporated into forest mensuration models to predict stand development.

  • Arbaugh, Michael J.; Proctor, Trent; Esperanza, Annie. 2009. Managing air pollution impacted forests of California. In: Bytnerowicz, Andrzej; Arbaugh, Michael; Andersen, Christian; Riebau, Allen 2009. Wildland Fires and Air Pollution. Developments in Environmental Science 8. Amsterdam: Elsevier: 567-582. Chapter 25.
Within-Year Prediction of Tree Mortality from Bark Beetles in California (2018)

U.S. Forest Service researchers have developed a model and map of within-year tree mortality attributable to bark beetle and wood borer insects in California. The statistical model is based on bark beetle- and wood borer-related tree mortality as captured by aerial detection surveys, insect activity history and weather history. This is a first attempt at a statistical tool to aid forest managers in within-year prioritization of management activities.

  • Haiganoush Preisler, Pacific Southwest Research Station, Albany, California; Nancy Grulke, Western Wildland Threat Assessment Center, Prineville, Oregon; and Zachary Heath, Region 6 Forest Health Protection, Sandy, Oregon
Analysis and out-year forecast of beetle, borer, and drought-induced tree mortality in California (2017)

The level of tree mortality and drought observed over the past decade in North America has been described as 'unparalleled' in our modern history, in particular in the Sierra Nevada, California. Forest managers could use early warning of where and how much tree mortality to expect in the very near future to plan and prioritize hazard tree removal, pest suppression activities, infer location of funding needs and fuels reduction treatments as well as access for firefighting. To answer these needs, we developed an empirically-based forecast model for expected tree mortality for an upcoming year.

Native bark beetles and wood borers in Mediterranean forests of California (2016)

While some 200 species are native to California, only a handful is capable of causing tree mortality in Mediterranean forests. Trees of all species, ages and size classes may be colonized and killed, but each bark beetle species exhibits unique host preferences, life history traits, and impacts. In most cases, the resultant tree mortality goes unnoticed until a large infestation or outbreak occurs, which generally requires several years of favorable weather conducive to beetle survival and population growth, and an abundance of susceptible host trees.

Forest health and bark beetles (2012)

In recent years, bark beetles have caused significant tree mortality in the Sierra Nevada, rivaling mortality caused by wildfire in some locations. This chapter addresses two important questions: How can managers prepare for and influence levels of bark beetle-caused tree mortality given current forest conditions and future climate uncertainties? And how would the variable forest conditions suggested by U.S. Forest Service General Technical Report PSW GTR- 220, "An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests," influence these dynamics?

Climate change and bark beetles of the western United States and Canada: Direct and indirect effects (2010)

Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships.

The effectiveness of vegetation management practices for prevention and control of bark beetle infestations in coniferous forests of the western and southern United States (2007)

Insects are major components of forest ecosystems, representing most of the biological diversity and affecting virtually all processes and uses. In the USA, bark beetles (Coleoptera: Curculionidae, Scolytinae) heavily influence the structure and function of these ecosystems by regulating certain aspects of primary production, nutrient cycling, ecological succession and the size, distribution and abundance of forest trees. The purpose of this report is to review tree and stand factors associated with bark beetle infestations and analyze the effectiveness of vegetation management practices for mitigating the negative impacts of bark beetles on forest ecosystems.We describe the current state of our knowledge and identify gaps for making informed decisions on proposed silvicultural treatments. This review draws from examination of 498 scientific publications (many of which are cited herein) on this and related topics.

  • Fettig, Christopher J.; Klepzig, Kier D.; Billings, Ronald f.; Munson, A. Steven; Nebeker, T. Evan; Negron, Jose F.; Nowak, John T. 2007. The effectiveness of vegetation management practices for prevention and control of bark beetle infestations in coniferous forests of the western and southern United States. Forest ecology and management. 238(1-3): 24-53
Effects of drought on forests and rangelands in the United States: a comprehensive science synthesis (2016)

Drought can be a severe natural disaster with substantial social and economic consequences. Large, stand-level impacts of drought are already underway in the West, but all U.S. forests are vulnerable to drought. Drought-associated forest disturbances are expected to increase with climatic change. Options to mitigate drought include altering structural or functional components of vegetation, minimizing drought-mediated disturbance such as wildfire or insect outbreaks, and managing for reliable flow of water.

Forest mortality in high-elevation whitebark pine (Pinus albicaulis) forests of eastern California, USA; influence of environmental context, bark beetles, climatic water deficit, and warming (2012)

Whitebark pine (Pinus albicaulis Engelm.) in subalpine zones of eastern California experienced significant mortality from 2007 to 2010. Although trees that survived had greater growth during the 20th century than trees that died, the 19th century trees that eventually died grew better than trees that survived, suggesting selection for genetic adaptation to current climates as a result of differential tree mortality. Scenarios for the future of whitebark pine in California are discussed.

Global warming and stress complexes in forests of western North America (2009)

A warmer climate in western North America will likely affect forests directly through soil moisture stress and indirectly through increased extent and severity of disturbances. Across western North America, increased water deficit will accelerate the normal stress complex experienced in forests, which typically involves some combination of multi-year drought, insects and fire. Air pollution and high stand densities from fire exclusion have compromised mixed-conifer forests of the Sierra Nevada. The effects of stress complexes will be magnified given a warming climate.

Response of high-elevation limber pine (Pinus flexilis) to multiyear droughts and 20th-century warming, Sierra Nevada, California, USA (2007)

Limber pine (Pinus flexilis James) stands along the eastern escarpment of the Sierra Nevada, California, experienced significant mortality from 1985 to 1995 during a period of sustained low precipitation and high temperature. The stands differ from old-growth limber pine forests in being dense, young, more even-aged, and located in warmer, drier microclimates. Significant correlations and interactions of growth and mortality dates with temperature and precipitation indicate that conditions of warmth plus sustained drought increased the likelihood of mortality in the 1985-1995 interval. However, the thinning effect of the drought-related mortality appears to have promoted resilience and improved near-term health of these stands, which suffered no additional mortality in the subsequent 1999-2004 drought.

Efficacy of variable density thinning and prescribed fire for restoring forest heterogeneity to mixed-conifer forest in the central Sierra Nevada, CA (2017)

Frequent-fire forests were historically characterized by lower tree density, a higher proportion of pine species, and greater within-stand spatial variability, compared to many contemporary forests where fire has been excluded. As a result, such forests are now increasingly unstable, prone to uncharacteristically severe wildfire or high levels of tree mortality in times of drought stress. While reducing tree density might help to restore resilience, thinning treatments are frequently seen as conflicting with management for other resources such as wildlife habitat, in part because standard thinning prescriptions don’t typically produce the degree of within-stand heterogeneity found in historical forests. In this study, we compare stand structures and heterogeneity produced by two different mechanical thinning treatments and in an unthinned control, all with or without prescribed fire as a follow-up treatment.

Forest thinning and subsequent bark beetle-caused mortality in Northeastern California (2010)

The Warner Mountains of northeastern California on the Modoc National Forest experienced a high incidence of tree mortality (2001-2007) that was associated with drought and bark beetle (Coleoptera: Curculionidae, Scolytinae) attack. Various silvicultural thinning treatments were implemented prior to this period of tree mortality to reduce stand density and increase residual tree growth and vigor. Fir engraver beetle-caused mortality occurred in direct proportion to the density of available white fir. In plantations, density of mountain pine beetle-caused mortality was associated with treatment and tree density of all species. In mixed conifer areas, density of fir engraver beetle-caused mortality had a positive association with white fir density and a curvilinear association with elevation.

Drought induced tree mortality and ensuing bark beetle outbreaks in southwestern pinyon-juniper woodlands (2008)

The current drought and ensuing bark beetle outbreaks during 2002 to 2004 in the Southwest have greatly increased tree mortality in pinyon-juniper woodlands. Stand densities did not impact mortality levels for either tree species, which does not support tree thinning to reduce susceptibility to drought-bark beetle outbreaks. Impacts of drought-induced tree mortality may not appear for years or decades after a major mortality event.

Patterns of mortality in an old-growth mixed-conifer forest of the Southern Sierra Nevada, California (2005)

Mortality patterns in an old-growth, mixed-conifer forest, in the absence of wildfire, were investigated at the Teakettle Experimental Forest from 2000 to 2002. Mortality for all conifers was higher than expected in areas of high stand density and lower in areas of low stand density. Mortality of small-diameter trees was clustered and particularly high in areas of high stand density. Our data suggest pathogen- and insect-associated mortality is significantly greater in areas of high stand density, but it is not higher for shade-tolerant species. Furthermore, mortality is higher than expected for large-diameter trees, suggesting an acceleration of old-tree mortality under current fire suppression conditions.

Drought, tree mortality, and wildfire in forests adapted to frequent fire (2018)

Massive tree mortality has occurred rapidly in frequent-fire-adapted forests of the Sierra Nevada, California. This mortality is a product of acute drought compounded by the long-established removal of a key ecosystem process: frequent, low- to moderate-intensity fire. The recent tree mortality has many implications for the future of these forests and the ecological goods and services they provide to society.

Observed and anticipated impacts of drought on forest insects and diseases in the United States (2016)

Future anthropogenic-induced changes to the earth’s climate will likely include increases in temperature and changes in precipitation that will increase the frequency and severity of droughts. Insects and fungal diseases are important disturbances in forests, yet understanding of the role of drought in outbreaks of these agents is limited. Current knowledge concerning the effects of drought on herbivorous insect and pathogen outbreaks in U.S. forests is reviewed, and compared between the relatively mesic and structurally diverse forests of the eastern U.S. and the more xeric forests of the western U.S.

Relationship between precipitation and tree mortality levels in coastal California forests infested with sudden oak death (2013)

Phytophthora ramorum has caused extensive oak (Quercus) and tanoak (Notholithocarpus densiflorus mortality in portions of the central and north coasts of California. Aerial surveys conducted by the U.S. Department of Agriculture Forest Service, Forest Health Protection, have consistently documented the extent and intensity of hardwood mortality across forested areas affected by sudden oak death since 2005. The main objective is to determine whether oak and tanoak mortality levels are related to precipitation.

Tree mortality in drought-stressed mixed-conifer and ponderosa pine forests, Arizona, USA (2011)

We monitored tree mortality in northern Arizona mixed-conifer and ponderosa pine (Pinus ponderosa Dougl. ex Laws) forests from 1997 to 2007, a period of severe drought in this area. Mortality was pervasive, occurring on 100 and 98% of 53 mixed-conifer and 60 ponderosa pine plots (1-ha each), respectively. Most mortality was attributable to a suite of forest insects, mediated by drought stress. Mortality rates generally were not strongly related to either elevation or stand density. Mortality was nonrandom with respect to tree size classes and species.

Forest responses to increasing aridity and warmth in the southwestern United States (2010)

In recent decades, intense droughts, insect outbreaks and wildfires have led to decreasing tree growth and increasing mortality in many temperate forests. Forests within the southwestern United States appear particularly sensitive to drought and warmth. If temperature and aridity rise as they are projected to, southwestern trees will experience substantially reduced growth during this century. As tree growth declines, mortality rates may increase at many sites. Increases in wildfires and bark beetle outbreaks in the most recent decade are likely related to extreme drought and high temperatures during this period. Expected climatic changes will alter future forest productivity, disturbance regimes and species ranges throughout the Southwest.

California's forest resources: Forest Inventory and Analysis, 2001–2010 (2016)

This report highlights key findings from the most recent (2001–2010) data collected by the Forest Inventory and Analysis program across all forest land in California, updating previously published findings from data collected from 2001 through 2005. We summarize and interpret basic resource information such as forest area, ownership, volume….tree growth, mortality and removals for timber products. Consistent sampling across different ownerships and forest management regimes now allow comparisons.

Changing climates, changing forests: A western North American perspective (2013)

The Earth’s mean surface air temperature has warmed by ~1C within the last 100 years and is projected to increase at a faster rate in the future, accompanied by changes in precipitation patterns and increases in the occurrence of extreme weather events. Individual trees or populations exposed to climate conditions outside their climatic niches will be maladapted, resulting in compromised productivity and increased vulnerability to disturbance, specifically insects and pathogens. Droughts associated with higher temperatures may accelerate levels of tree mortality as elevated temperatures increase metabolic rates without increasing photosynthesis rates, thus compromising a tree’s ability to create defenses against insects and pathogens. As a result, distributions of the climatic niches of some tree species in western North America are predicted to change by up to 200% during this century based on bioclimate envelope modeling.

Species-specific response to climate reconstruction in upper-elevation mixed-conifer forests of the western Sierra Nevada, California (2007)

We sampled 579 trees representing five upper montane mixed-conifer species at the Teakettle Experimental Forest in California’s southern Sierra Nevada to determine species-specific responses to annual climatic fluctuations. Under current high-density conditions, shade-tolerant white fir (Abies concolor (Gord. & Glend.) provided the best model for climate reconstruction. Shade-intolerant Jeffrey pine (Pinus jeffreyi Grev. & Balf.) had a lagged response to annual climatic fluctuations, possibly because its roots may tap water reserves in granitic bedrock fissures. Open-grown trees provided more accurate records of climate. Changes in forest density in this forest may have resulted in changes in species-specific response to annual climatic fluctuations.