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Widespread Increase of Tree Mortality Rates in the Western United States
Abstract
Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.
... Across much of our planet, land use change and industrialization have resulted in dramatic declines in the area of old forests (mature and old growth, collectively) and degradation of desired old forest characteristics due to human use, invasive species, and long-term consequences of fire suppression (Williams, 2008;Davis et al., 2017;Hagmann et al., 2021). Rapidly changing climate threatens to further erode the integrity of old forests as trees that established decades or centuries ago become maladapted to the climate of current sites or are killed by uncharacteristic disturbance, drought, or pest and pathogen outbreaks (van Mantgem et al., 2009;Coop et al., 2020;Hartmann et al., 2022). ...
... Many of the studies used to characterize mature and old-growth forests and describe their species composition and function identified stands meeting criteria for age, dominant species composition, and lack of recent disturbance or human manipulation (e.g., Runkle, 1981;Ruggiero et al., 1991; 1997). Long-term plots have been useful for understanding tree demography and the spatial patterns of tree mortality and regeneration in these forests (e.g., Acker et al., 1998;van Mantgem et al., 2009;Sánchez Meador et al., 2010). Areas measured within selected stands (i. ...
... Contemporary data streams (e.g., annual inventories, remotely-sensed data) and refined modeling/inference approaches (e.g., Bayesian and machine learning) may afford novel and potentially more robust approaches to classifying mature and oldgrowth forests. First, rates of global change and associated effects on forest ecosystems (van Mantgem et al. 2009) may necessitate classification approaches that are equally dynamic in a manner similar to datadriven approaches to classifying forest types (Valle et al. 2014). The long-standing development patterns of site-specific forest types may rapidly change as species ranges shift resulting in novel systems. ...
Mature and old-growth forests are valued for biodiversity, carbon sequestration, habitat, hydrologic function, aesthetics, and spirituality, as well as Tribal and Indigenous histories, cultures, and practices. Over the last 500 years, land use change and industrialization have resulted in global declines in the area of older forests (however defined). The goal of this study was to identify concepts and indicators to define mature and old-growth forests across the vegetation types of the United States in order to quantify their abundance and distribution. Defining old growth has been described as a "wicked problem" that involves values, science, and management; requires multiple disciplines; and can be expressed from many contradictory approaches. The most common approach to defining mature and old-growth forests is to place them in a successional continuum of increases in tree size, biodiversity, habitat niches, and structural diversity with forest age. Time since severe disturbance, including human impact, is often a consideration, although humans have influenced the development of many forests for millennia. The successional framework is less useful in low-productivity or frequently-disturbed forests, or where current structural diversity under fire suppression may not reflect historic or desired future conditions. In order to classify forests into "old" and "not old", existing structure-based approaches apply minima of one or more structural or compositional criteria. Site productivity and/or plant association is an element of many definitions. Once defined, estimating the area of mature and old-growth forest presents challenges. The only comprehensive , consistent field data of US forests is the Forest Inventory and Analysis (FIA) network of >140,000 forested plots. While the 0.067 ha sample area of FIA plots limits the number of structural metrics that might be useful and the plot density cannot capture fine-scale spatial heterogeneity, measurements enable a granular application of multiple structural and compositional criteria by vegetation type at broad spatial extents, and the ability to track change consistently over time. Spatial models integrate field and remotely-sensed data to predict the distribution of structural classes at finer spatial grain, but with substantial error in high-resolution estimates. There does not seem to be a readily-available method to map mature and old-growth stands across a landscape with a high degree of accuracy. Identifying mature and old growth forests in a stand management context will likely require additional measurements, adjustments to criteria at local scales, and incorporation of social and traditional knowledge within a consistent definition framework.
... [15][16][17] Pine forests could be extremely affected by shifts in temperature, precipitation, and the concentrations of greenhouse gas. 18 Literatures 19,20 suggested that widespread forests mortality are caused by drought and hot weather conditions. It is also observed that extensive forests mortality is related to drought and global warming could exacerbate the vulnerability of vegetation. ...
... for ∈ [ − , ]. By the same logic, we have that (20) holds for all ∈ [ − 2 , − ]. Thus, by a finite number of iterations, we know that (20) holds for any ∈ [0, ]. ...
... By the same logic, we have that (20) holds for all ∈ [ − 2 , − ]. Thus, by a finite number of iterations, we know that (20) holds for any ∈ [0, ]. Furthermore, Equation (17) can be rewritten as ...
In this paper, considering the effect of environmental factors such as climate change, temperature and drought on the growth of forests and beetles as well as the pine forests death and economic loss caused by outbreaks of mountain pine beetles, we first develop a stochastic mountain pine beetle model with pesticide application, and then consider its near‐optimal control (NOC) problem. The problem NOC is formulated by decreasing the mountain pine beetles while keeping the cost of pesticide application to a minimum. By Pontryagin stochastic maximum principle, we establish some sufficient and necessary conditions of the near‐optimality. Some numerical simulations are carried out to illustrate the theoretical results, which indicate that the pesticide application could effectively control the number of beetles and relieve the pressure from the attacking of beetles on the pines. To some extent, this control strategy could protect the pine forests and reduce economic losses caused by the mountain pine beetles.
... Largescale forest die-off is of particular concern due to its dramatic disruption of forest function, with large consequences on biodiversity (Betts et al., 2017;Feng et al., 2021), ecosystem goods and services, and forests' prominent role in the global carbon cycle (Anderegg, Trugman, Badgley, Anderson, et al., 2020). The western US has experienced extensive drought-and insect-induced tree mortality over the past decades, associated with strong drying trends in the area (Van Mantgem et al., 2009;Williams et al., 2012;Zhang et al., 2021). ...
... Biotic agents, including diseases and insects can also drive largescale forest mortality, often in interaction with climate stress, which challenges disentangling their relative importance (Anderegg, Hicke, et al., 2015). Notably, western US forests have been particularly prone to extensive bark beetle outbreaks in the last decades, where drought was a key predisposing factor in many species (Chapman et al., 2012;Meddens et al., 2015;Raffa et al., 2008;Van Mantgem et al., 2009). Drought may facilitate biotic outbreaks by impairing tree defenses against attacks such as resin and defense chemical production, hence favoring infestations and eventually mortality (Gaylord et al., 2013;Kolb et al., 2019). ...
Climate change-triggered forest die-off is an increasing threat to global forests and carbon sequestration but remains extremely challenging to predict. Tree growth resilience metrics have been proposed as measurable proxies of tree susceptibility to mortality. However, it remains unclear whether tree growth resilience can improve predictions of stand-level mortality. Here, we use an extensive tree-ring dataset collected at ~3000 permanent forest inventory plots, spanning 13 dominant species across the US Mountain West, where forests have experienced strong drought and extensive die-off has been observed in the past two decades, to test the hypothesis that tree growth resilience to drought can explain and improve predictions of observed stand-level mortality. We found substantial increases in growth variability and temporal autocorrelation as well declining drought resistance and resilience for a number of species over the second half of the 20th century. Declining resilience and low tree growth were strongly associated with cross- and within-species patterns of mortality. Resilience metrics had similar explicative power compared to climate and stand structure, but the covariance structure among predictors implied that the effect of tree resilience on mortality could partially be explained by stand and climate variables. We conclude that tree growth resilience offers highly valuable insights on tree physiology by integrating the effect of stressors on forest mortality but may have only moderate potential to improve large-scale projections of forest die-off under climate change.
... Although overall tree density is up, the density of large trees has decreased over the last century, largely driven by past logging practices and climate-and stand density-driven water stress (Dolanc et al. 2014b, Easterday et al. 2018. Van Mantgem et al. (2009) documented widespread increases in tree mortality in old-growth forests across the west, including northern California, although their plots had not experienced increases in density or basal area during the 15-40-year period between first and last census. The highest mortality rates were documented in the Sierra Nevada, and in middle elevation forests (3300-6700 feet). ...
... Evidence suggests that old-growth forests can be susceptible to a wide range of stressors, including the disruption of historical disturbance regimes such as fire (Skinner et al. 2006), invasive species and pathogens (McDonald andHoff 2001, Rizzo andGarbelotto 2003), and increasing temperatures (van Mantgem et al. 2009, Allen et al. 2010, Peng et al. 2011, Williams et al. 2013. Work by van Mantgem and Sarr (2015) in diverse old-growth forests across a broad range of climates in the Klamath region further demonstrates the high correlation between forest structure and diversity with climate, as well as the complexity inherent in predicting future forest conditions in this region. ...
The climate of California is in a stage of rapid flux. This document highlights past, current, and projected climate change on the Six Rivers National Forest. It is divided into discussion of historical and current conditions and projected future trends by general resource area. This trend summary is produced by the US Forest Service Pacific Southwest Region (R5) Ecology Program to help forest managers plan for, and where possible, mitigate climate change-related ecosystem vulnerabilities. Climate change trend summaries are currently available for all the National Forests of California and are updated on approximately 5-year intervals as new climate science becomes available.
... Although overall tree density is up, the density of large trees has decreased over the last century, largely driven by past logging practices and climate-and stand density-driven water stress (Dolanc et al. 2014b, Easterday et al. 2018. Van Mantgem et al. (2009) documented widespread increases in tree mortality in old-growth forests across the west, including northern California, although their plots had not experienced increases in density or basal area during the 15-40-year period between first and last census. The highest mortality rates were documented in the Sierra Nevada, and in middle elevation forests (3300-6700 feet). ...
... Evidence suggests that old-growth forests can be susceptible to a wide range of stressors, including the disruption of historical disturbance regimes such as fire (Skinner et al. 2006), invasive species and pathogens (McDonald andHoff 2001, Rizzo andGarbelotto 2003), and increasing temperatures (van Mantgem et al. 2009, Allen et al. 2010, Peng et al. 2011, Williams et al. 2013. Work by van Mantgem and Sarr (2015) in diverse old-growth forests across a broad range of climates in the Klamath region further demonstrates the high correlation between forest structure and diversity with climate, as well as the complexity inherent in predicting future forest conditions in this region. ...
The climate of California is in a stage of rapid flux. This document highlights past, current, and projected climate change on the Mendocino National Forest. It is divided into discussion of historical and current conditions and projected future trends by general resource area. This trend summary is produced by the US Forest Service Pacific Southwest Region (R5) Ecology Program to help forest managers plan for, and where possible, mitigate climate change-related ecosystem vulnerabilities. Climate change trend summaries are currently available for all the National Forests of California and are updated on approximately 5-year intervals as new climate science becomes available.
... Although overall tree density is up, the density of large trees has decreased over the last century, largely driven by past logging practices and climate-and stand density-driven water stress (Dolanc et al. 2014b, Easterday et al. 2018. Van Mantgem et al. (2009) documented widespread increases in tree mortality in old-growth forests across the west, including northern California, although their plots had not experienced increases in density or basal area during the 15-40-year period between first and last census. The highest mortality rates were documented in the Sierra Nevada, and in middle elevation forests (3300-6700 feet). ...
... Evidence suggests that old-growth forests can be susceptible to a wide range of stressors, including the disruption of historical disturbance regimes such as fire (Skinner et al. 2006), invasive species and pathogens (McDonald andHoff 2001, Rizzo andGarbelotto 2003), and increasing temperatures (van Mantgem et al. 2009, Allen et al. 2010, Peng et al. 2011, Williams et al. 2013. Work by van Mantgem and Sarr (2015) in diverse old-growth forests across a broad range of climates in the Klamath region further demonstrates the high correlation between forest structure and diversity with climate, as well as the complexity inherent in predicting future forest conditions in this region. ...
The climate of California is in a stage of rapid flux. This document highlights past, current, and projected climate change on the Klamath National Forest. It is divided into discussion of historical and current conditions and projected future trends by general resource area. This trend summary is produced by the US Forest Service Pacific Southwest Region (R5) Ecology Program to help forest managers plan for, and where possible, mitigate climate change-related ecosystem vulnerabilities. Climate change trend summaries are currently available for all the National Forests of California and are updated on approximately 5-year intervals as new climate science becomes available.
... Although overall tree density is up, the density of large trees has decreased over the last century, largely driven by past logging practices and climate-and stand density-driven water stress (Dolanc et al. 2014b, Easterday et al. 2018. Van Mantgem et al. (2009) documented widespread increases in tree mortality in old-growth forests across the west, including northern California, although their plots had not experienced increases in density or basal area during the 15-40-year period between first and last census. The highest mortality rates were documented in the Sierra Nevada, and in middle elevation forests (3300-6700 feet). ...
... ), invasive species and pathogens(McDonald and Hoff 2001, Rizzo andGarbelotto 2003), and increasing temperatures(van Mantgem et al. 2009, Allen et al. 2010, Peng et al. 2011, Williams et al. 2013. Work by van Mantgem and Sarr ...
The climate of California is in a stage of rapid flux. This document highlights past, current, and projected climate change on the Shasta-Trinity National Forest. It is divided into discussion of historical and current conditions and projected future trends by general resource area. This trend summary is produced by the US Forest Service Pacific Southwest Region (R5) Ecology Program to help forest managers plan for, and where possible, mitigate climate change-related ecosystem vulnerabilities. Climate change trend summaries are currently available for all the National Forests of California and are updated on approximately 5-year intervals as new climate science becomes available.
... Natural and urban ecosystems are already impacted by climate change, resulting in suboptimal tree growth and increased mortality 9,10 . Climate change is increasing the frequency and severity of extreme events-such as heatwaves, fire and drought 8,11,12 -which contribute to extensive tree dieback and mortality globally 9,13 . ...
... Natural and urban ecosystems are already impacted by climate change, resulting in suboptimal tree growth and increased mortality 9,10 . Climate change is increasing the frequency and severity of extreme events-such as heatwaves, fire and drought 8,11,12 -which contribute to extensive tree dieback and mortality globally 9,13 . Additionally, features of urban environments, including impervious surfaces and the urban heat island (UHI) effect, can locally exacerbate climatic extremes 8 . ...
Climate change threatens the health and survival of urban trees and the various benefits they deliver to urban inhabitants. Here, we show that 56% and 65% of species in 164 cities across 78 countries are currently exceeding temperature and precipitation conditions experienced in their geographic range, respectively. We assessed 3,129 tree and shrub species, using three metrics related to climate vulnerability: exposure, safety margin and risk. By 2050 under Representative Concentration Pathway 6.0, 2,387 (76%) and 2,220 (70%) species will be at risk from projected changes in mean annual temperature and annual precipitation, respectively. Risk is predicted to be greatest in cities at low latitudes—such as New Delhi and Singapore—where all urban tree species are vulnerable to climate change. These findings aid the evaluation of the impacts of climate change to secure long-term benefits provided by urban forests
... Increased drought, caused by recent regional warming, is believed to be one of the underlying causes of tree mortality in forest ecosystems of western North America (van Mantgem et al. 2009) and worldwide (Allen et al. 2010;Choat et al. 2018). Kozlowski et al. (1991) define drought from a forest perspective as a period of below-average precipitation that reduces soil moisture and results in prolonged plant water stress and reduced growth. ...
... However, an increase in temperatures can also cause drought conditions by increasing evapotranspiration (ET) (Hember et al. 2017). Drought can therefore be caused by an increase in evaporative demand due to increases in temperature, decreases in water availability, or both (van Mantgem et al. 2009;Choat et al. 2018). The incidence and effects of drought vary with site characteristics such as soil depth, texture, exposure, and slope, as well as biological determinants such as forest cover and stand/ tree characteristics (Kozlowski et al. 1991;Klinka et al. 1999). ...
... Increased drought, caused by recent regional warming, is believed to be one of the underlying causes of tree mortality in forest ecosystems of western North America (van Mantgem et al. 2009) and worldwide (Allen et al. 2010;Choat et al. 2018). Kozlowski et al. (1991) define drought from a forest perspective as a period of below-average precipitation that reduces soil moisture and results in prolonged plant water stress and reduced growth. ...
... However, an increase in temperatures can also cause drought conditions by increasing evapotranspiration (ET) (Hember et al. 2017). Drought can therefore be caused by an increase in evaporative demand due to increases in temperature, decreases in water availability, or both (van Mantgem et al. 2009;Choat et al. 2018). The incidence and effects of drought vary with site characteristics such as soil depth, texture, exposure, and slope, as well as biological determinants such as forest cover and stand/ tree characteristics (Kozlowski et al. 1991;Klinka et al. 1999). ...
... °C till 2100 with an elevated CO2 concentration 790 ppm (µmol mol −1 )) [35,[37][38][39]. In addition to this, increased global average surface and atmospheric temperature, evaporation and plant transpiration will be affected [40,41]. This would inevitably lead to variations in humidity and rainfall [42]. ...
... This would inevitably lead to variations in humidity and rainfall [42]. In some areas, this would lead to extreme rainfall events [34], while in other areas seasonal droughts are likely to be more severe [40,43,44]. Hence, GCC is a serious risk as well as a threat to plant growth and development, because forest ecosystems are highly sensitive to changes in environmental characteristics, which directly affect tree mortality and health condition [36,45]. ...
Increased urbanization means human beings become the dominant species and reduction in canopy cover. Globally, urban trees grow under challenging and complex circumstances with urbanization trends of increasing anthropogenic carbon dioxide (CO2) emissions, high temperature and drought stress. This study aims to provide a better understanding of urban trees’ morpho-physio-biochemical attributes that can support sustainable urban greening programs and urban climate change mitigation policies. Globally, urban dwellers’ population is on the rise and spreading to suburban areas over time with an increase in domestic CO2 emissions. Uncertainty and less information on urban tree diversification and resistance to abiotic stress may create deterioration of ecosystem resilience over time. This review uses general parameters for urban tree physiology studies and employs three approaches for evaluating ecosystem resilience based on urban stress resistance in relation to trees’ morphological, physiological and biochemical attributes. Due to the lack of a research model of ecosystem resilience and urban stress resistance of trees, this review demonstrates that the model concept supports future urban tree physiology research needs. In particular, it is necessary to develop integral methodologies and an urban tree research concept to assess how main and combined effects of drought and/or climate changes affect indigenous and exotic trees that are commonly grown in cities.
... Moreover, whether or not vital rates will be maintained in the same way as the climate continues to warm has been called into question (Briscoe et al., 2019;Morin & Thuiller, 2009). In particular, climate (Davis et al., 2019;Shriver et al., 2021;Stanke et al., 2021;van Mantgem et al., 2009) and neighbourhood competition (Le Squin et al., 2021;Zhang et al., 2015) have increasingly impacted tree demographic rates in North American forests over the recent decades. In addition to the unknown role of demographic compensation in tree species range limit stabilisation, an unresolved topic in North American forests is how demographic compensation responds to changing climate and competition. ...
... Stand-level competition was calculated as the total basal area of living trees per plot. This is a proxy for the combined effect of inter-and intra-specific competition for light and other resources, which is considered as one of the best competition indices (van Mantgem et al., 2009;Zhang et al., 2015). ...
Demographic compensation—the opposing responses of vital rates along environmental gradients—potentially delays anticipated species’ range contraction under climate change, but no consensus exists on its actual contribution. We calculated population growth rate (λ) and demographic compensation across the distributional ranges of 81 North American tree species and examined their responses to simulated warming and tree competition. We found that 43% of species showed stable population size at both northern and southern edges. Demographic compensation was detected in 25 species, yet 15 of them still showed a potential retraction from southern edges, indicating that compensation alone cannot maintain range stability. Simulated climatic warming caused larger decreases in λ for most species and weakened the effectiveness of demographic compensation in stabilising ranges. These findings suggest that climate stress may surpass the limited capacity of demographic compensation and pose a threat to the viability of North American tree populations.
... Many of those FTGs have been subject to a series of interacting disturbances since at least the early 20th century, including fire suppression and mountain pine beetle, which have resulted in decreased extent of those forests (Stanke et al. 2021). In addition, increases in background mortality have been documented in many western tree species, with pines showing the greatest rates since the 1990s (Van Mantgem et al. 2009). The western white pine FTG lost the most area relative to its small 1977 range (78 percent loss, from 0.5 million acres in 1977 to 0.1 million acres in 2017), having faced threats from white pine blister rust in addition to area reductions due to fire and beetles (Dudney et al. , Schwandt et al. 2010). ...
... Both Lee (2018) and Jones et al. (2020) found that owls avoided post-fire landscapes where salvage logging occurred. If climate change continues to increase the frequency and magnitude of wildfires, their impact on spotted owl habitat may be exacerbated over time (Van Mantgem et al. 2009). ...
The North Cascades Ecosystem is one of the largest and most intact wilderness areas in the contiguous United States. It spans 34,965 km2 across the U.S.-Canada border between central Washington State and southern British Columbia and is bisected north to south by the Cascade Mountain range. The North Cascades National Park Service Complex (hereafter, the Park) lies in the heart of the ecosystem and is comprised of North Cascades National Park (2,044 km2), Ross Lake National Recreation Area (NRA) (473 km2), and Lake Chelan NRA (251 km2). This report focuses on terrestrial wildlife species in the Park that are federally listed under the Endangered Species Act of 1973 (ESA), state-listed under Washington Administrative Code, and/or designated by NPS as Management Priority species. We provide a detailed synthesis of information around NPS records from 1995–2020 for nine bird and nine mammal species and one mammal Order (Chiroptera [bats]), as well as a suite of invertebrate pollinators in the Park where data are available. Information for each species includes life history information, occurrence in the Park, protective status, trends when known, a summary of known threats, and a summary of conservation and research needs. We also provide brief summaries for an additional five bird and two mammal species, and one taxa group (woodpeckers) where data are more limited.
... Aspen also has important aesthetic and cultural value, making issues of aspen forest health relevant to the general public and to local communities that benefit from aspen-driven tourism and recreation . Aspen mortality events across western North America have been thoroughly studied, revealing two pathways of stand-scale mortality: (1) long-term successional replacement of aspen by conifers in the absence of stand-replacing disturbance (Kay 1997) and (2) acute mortality events caused by interactions between predisposing, inciting, and contributing factors (Manion 1991;Worrall et al. 2013). Drought is the inciting factor driving acute aspen mortality, while specific site factors (e.g., aspen at low elevations and on south-facing aspects) predispose aspen to mortality and biotic agents (e.g., Cytospora canker [caused by Valsa sordida] and bark beetles) contribute to mortality Marchetti et al. 2011;Worrall et al. 2013;Singer et al. 2019). ...
Quaking aspen (Populus tremuloides) ecosystems are highly valued in the southwestern United States because of the ecological, economic, and aesthetic benefits they provide. Concerningly, aspen has experienced extensive mortality in recent decades, and there is evidence that many areas in Arizona, USA lack adequate recruitment to replace dying overstory trees. Maintaining sustainable levels of regeneration and recruitment is necessary for facilitating resilience to biotic and abiotic disturbance agents and for maximizing aspen’s ability to adapt in an increasingly uncertain future. However, questions remain about which factors currently limit aspen regeneration and recruitment in Arizona and which strategies are appropriate for promoting aspen sustainability. Moreover, recent outbreaks of an invasive insect, oystershell scale (Lepidosaphes ulmi; OSS) pose a new threat to aspen forest health. Because these are the first documented outbreaks of OSS in aspen ecosystems, there is an urgent need to survey the extent and impacts of these invasions and to better understand the insect’s biology and ecology on aspen in Arizona. To fill these knowledge gaps, we conducted a systematic literature review of aspen in the Southwest to understand how biotic and abiotic factors, including management, influence aspen forest dynamics (Chapter 1). We also sampled aspen populations across Arizona to quantify the sustainability and drivers of aspen regeneration and recruitment (Chapter 2) and impacts and drivers of OSS invasions (Chapter 3). Finally, we used repeated measurements of OSS-infested aspen stands to quantify short-term rates of OSS intensification on trees, OSS spread among trees, and aspen mortality, and we collected OSS from these stands to document the insect’s phenology in northern Arizona (Chapter 4).
We found that many aspen populations in Arizona lack sustainable regeneration and recruitment. The status of recruitment was especially dire, with 40% of study plots lacking a single recruiting stem. Aspen regeneration was less abundant on warmer, drier sites, highlighting the threat that a warming climate poses to aspen sustainability. Aspen recruitment was significantly more abundant in areas with recent fire and more severe fire. The most important factors limiting recruitment were OSS and browsing by ungulates, especially Rocky Mountain elk (Cervus canadensis). OSS was widespread in Arizona and was associated with increased aspen crown damage and mortality. Climate was the most important driver of OSS abundance, with warmer, drier conditions resulting in significantly more OSS. OSS was also associated with less recent fire, presence of ungulate management strategies such as fenced exclosures, and stands with a greater density of aspen saplings. We also found that immature OSS life stages persist throughout the year and that there are two waves of first-instar crawlers in northern Arizona, one throughout the summer and the second in mid-winter. The first wave seemed to be driven by warming temperatures, but the cause of the second wave is unknown and might represent the initiation of a second generation. We also found that OSS causes high levels of mortality and spreads rapidly within aspen stands. We conclude by discussing how our findings can inform contemporary management of aspen and OSS (Chapter 5).
... Climate changes influence forests directly. For example, drought and heat stress have been linked to increased tree mortality, shifts in species distributions, and decreased productivity (Allen et al. 2010; Van Mantgem et al. 2009;Williams et al. 2012). Climate changes also indirectly influence forests via wildfires, through changes in fire timing and seasonality, frequency, behavior, and spatial burn pattern ( fig. 1). ...
Decision makers need better methods for identifying critical ecosystem vulnerabilities to changing climate and fire regimes. Climate-wildfire-vegetation interactions are complex and hinder classification and projection necessary for development of management strategies. One such vulnerability assessment (VA) is FireCLIME VA, which allows users to compare management strategies under various climate scenarios and gauge the potential effectiveness of those strategies for reducing undesirable impacts of climate on wildfire regimes and resulting impacts of wildfire on natural ecosystems. Developed as part of the SW FireCLIME science-management partnership, FireCLIME is meant to be quick, flexible, and amendable to a range of data inputs (literature review, expert, and modeling or monitoring activities). These inputs allow users to easily compare various fire-climate outcomes for one or more ecosystems of interest. Users can use literature, hypothetical scenarios, or quantitative data to implement the FireCLIME VA tool. This tool, unlike other vulnerability assessment, is best used iteratively to explore a range of possible scenarios and management strategies.
... Browning drivers (i.e., drought, wildfire, and biotic agents) may intensify size-dependent tree browning trends [121], dramatically altering the ecosystem structure. Although both empirical [122] and theoretical [123] studies have shown a high risk for large-sized tree browning, other studies have also identified a vulnerability for small-sized tree browning [124]. Similarly, observations [125] have shown a lower browning risk for older trees and a higher browning risk for younger trees; however, a higher risk for old-growth forest browning has also been reported [126]. ...
As global climate conditions continue to change, disturbance regimes and environmental drivers will continue to shift, impacting global vegetation dynamics. Following a period of vegetation greening, there has been a progressive increase in remotely sensed vegetation browning globally. Given the many societal benefits that forests provide, it is critical that we understand vegetation dynamic alterations. Here, we review associative drivers, impacts, and feedbacks, revealing the complexity of browning. Concomitant increases in browning include the weakening of ecosystem services and functions and alterations to vegetation structure and species composition, as well as the development of potential positive climate change feedbacks. Also discussed are the current challenges in browning detection and understanding associated impacts and feedbacks. Finally, we outline recommended strategies.
... Analysis of the US Historical Climatology Network (USHCN) database between 1990 and 2020 for the Western North America (WNA) region reveals significant changes in temperature and precipitation [1]. The WNA has experienced severe droughts and gradual changes in hydrological regimes, leading to reduced population growth and increased mortality [2,3,4,5,6]. There is also evidence that climate change impacts not only the most vulnerable animal species in this region but also regionally adapted species with varying levels of physiological adaptability [7,8]. ...
Studies on the impacts of climate change rely on the global projections of the future climate made by General Circulation Models (GCMs). Despite the availability of a growing number of climate model outputs and the continual development of their process representations, considerable uncertainty cannot be eliminated from their future climate projections. This circumstance, combined with other variables, e.g., time and computing limitations, necessitates the selection of appropriate representative GCM-runs (RGCM-runs), reflecting the past and projected future climate, to assess the effects of climate change on different infrastructures. This study is undertaken to select RGCM-runs for Western North America (WNA) based on their ability to simulate past climates between 1981 and 2005 and climatic changes between two 30-year periods, 2071–2100 and 1981–2010, under two representative concentration pathways (RCPs) scenarios, RCP4.5 and RCP8.5. GCMs and their various runs are independently treated as standalone potentials in the selection process, and the initial pool (i.e., full-set) for RCP4.5 and RCP8.5 include 105 and 77 GCM-runs, respectively. We examine GCM-runs reduction by evaluating RGCM-runs performance on three criteria: (i) capturing changes in the climatology of monthly mean precipitation and air temperature, (ii) capturing changes in the monthly mean extreme indices, and (iii) matching the historical and reference datasets (i.e., history matching). For the first two criteria, we employ an envelope-based selection technique, and for the last criterion, gleaning the final RGCM-runs, we present a multi-objective distance-based approach comparing the GCM-runs to reference data sets (i.e., monthly average of temperature and precipitation). This framework selects four RGCM-runs for each RCP to represent the full-set and capture the full range of climatic conditions, including wet-warm, wet-cold, dry-warm, and dry-cold scenarios, which represent the extremes of the climatic spectrum. The results demonstrate that the RGCM-runs can simulate previous climatic conditions and projected changes in key climate variables in WNA, such as temperature and precipitation. This indicates that the selected RGCM-runs are suitable for conducting climate impact assessments and developing adaptation plans for the WNA region.
... Among these 41,518 selected plots, 33% (13,854 PSPs) were excluded due to the presence of severe natural (e.g., insects, disease, and fire) or anthropogenic (e.g., silviculture) disturbance reported during data collection. In the absence of disturbance records, we excluded another 18.2% (7,575 PSPs) with unusually high annual mortality rates above 6%⋅y −1 suggesting nonreported disturbance (SI Appendix, Table S3, Fig. S4, and Text S2) (95)(96)(97)(98). These selection criteria resulted in 20,089 repeatedly measured PSPs remaining for further analysis, including 611,554 individual trees. ...
Large projected increases in forest disturbance pose a major threat to future wood fiber supply and carbon sequestration in the cold-limited, Canadian boreal forest ecosystem. Given the large sensitivity of tree growth to temperature, warming-induced increases in forest productivity have the potential to reduce these threats, but research efforts to date have yielded contradictory results attributed to limited data availability, methodological biases, and regional variability in forest dynamics. Here, we apply a machine learning algorithm to an unprecedented network of over 1 million tree growth records (1958 to 2018) from 20,089 permanent sample plots distributed across both Canada and the United States, spanning a 16.5 °C climatic gradient. Fitted models were then used to project the near-term (2050 s time period) growth of the six most abundant tree species in the Canadian boreal forest. Our results reveal a large, positive effect of increasing thermal energy on tree growth for most of the target species, leading to 20.5 to 22.7% projected gains in growth with climate change under RCP 4.5 and 8.5. The magnitude of these gains, which peak in the colder and wetter regions of the boreal forest, suggests that warming-induced growth increases should no longer be considered marginal but may in fact significantly offset some of the negative impacts of projected increases in drought and wildfire on wood supply and carbon sequestration and have major implications on ecological forecasts and the global economy.
... Nevertheless, an increase across the West in the incidence of large, high-severity fires in most forest types suggests that climate change is also responsible (Westerling et al. 2006;Littell et al. 2009), and projections are for increases in fire severity and frequency in many western forests (Westerling et al. 2011). Climate change is also implicated in the global (Allen et al. 2010) and western U.S. (van Mantgem et al. 2009) decreases in tree survival. In part, this has manifested itself by increasing drought stress and making trees more vulnerable to native and nonnative insects and pathogens. ...
... However, as previously explained (see Methods), a HW occurred before the first drought and caused 50% mortality in white spruce seedlings; all white spruce seedlings were subsequently replaced. This extreme and unpredictable event probably reflects the future summer climate in North America, which is expected to be more and more prone to HW events (Auclair et al., 1997;van Mantgem et al., 2009). The high drought tolerance observed in this study for white spruce applies only to the replaced seedlings (i.e., those that had not been subjected to the HW). ...
An increase in the frequency and magnitude of drought events threatens the health of forests and the economic, ecological, and societal services they provide. It has been widely demonstrated that trees undergoing a succession of stresses may accumulate lesions that in turn lead to a decrease in their vigor and eventually to death. However, recent studies have shown that a nonlethal stress should also initiate a stress memory, which triggers a faster and stronger plant defensive response when a new stress occurs. Although this mechanism is well understood in many herbaceous plants, a better understanding in trees is needed. The aim of our study was to explore the capacity of two forest tree species to develop a stress memory. A greenhouse experiment was conducted to evaluate the tree seedlings' vigor after one or two consecutive droughts separate from a rehydration period during the same growing season. No stress memory pattern was observed for the two tree species as, on the contrary, we even observed a stress accumulation pattern in sugar maple. It remains possible that some individuals in our study developed stress memory, but that we were not able to detect it. The fine‐tuning of experimental parameters and the conducting of longitudinal studies would be helpful to detect individual capacity in stress memory activation.
... Increased tree mortality and changes in forest structure and composition have been observed in different biomes (Allen et al. 2015;Michel and Seidling 2018). In the western United States, van Mantgem et al. (2009) demonstrated a doubling of the rate of background tree mortality every 17 to 29 years, while Peng et al. (2011) observed a 4.9% increase in tree mortality per year from 1963 to 2008 in Canadian boreal forests. In Europe, remote sensing data have revealed a 2.4% increase of canopy mortality per year between 1984 and 2016 (Senf et al. 2018), and different hotspots have been identified in the north or the south of the continent (Neumann et al. 2017). ...
ContextA significant forest decline has been noticed these last years in Europe. Managers need tools to better anticipate these massive events.Objectives
We evaluated the efficiency of easily available data about environmental conditions and stand characteristics to determine different levels of vulnerability.Methods
We combined remote sensing images, photo-interpretation, and digital models describing environmental conditions within a modelling approach to achieve spatial vulnerability assessment of the stands. We focused on silver fir and Norway spruce stands in the Vosges mountains (8900 km2, northeastern France), where severe symptoms of decline are visible.ResultsSilver fir were predicted highly vulnerable on 7% of their area versus 33% for Norway spruce. Using an independent dataset, we observed ten-times (silver fir) and two-times (Norway spruce) higher mortality rates in the units with a high level of vulnerability than in the others. About half of the model deviance was directly or indirectly explained by variables related to water stress (soils displaying low water availability, having suffered severe drying events these last years). Furthermore, the stands acclimatised to drought conditions were more resilient. Stand characteristics also influenced dieback spread, suggesting that an evolution of silvicultural practices toward mixed stands with broadleaved species and uneven-aged trees can contribute to better adapt to future climate conditions.Conclusion
Vulnerability maps based on easily available geographic information describing climate, soil, and topography can efficiently discriminate canopy mortality patterns over broad areas, and can be useful tools for managers to mitigate the effects of climate change on forests.
... Widespread and large-scale tree mortality events have occurred across forested biomes over the past several decades (Breshears et al. 2005;Allen et al. 2010). In temperate forests of North America, these mortality events have often been associated with drought and resulted in rapid shifts in ecosystem structure and function (Shaw et al. 2005;Williams et al. 2010;van Mantgem et al. 2016). Drought poses a complex, multi-dimensional challenge for trees because it impacts the tightly interrelated systems of water and carbon (C). ...
Interactions between water and carbon dynamics underlie drought-related tree mortality. While whole-tree water relations have been shown to play a key role in the response to and recovery from drought, the role of nonstructural carbohydrates (NSC) and how their storage and allocation changes surrounding drought events deserves further attention and is critical for understanding tree survival. Here, we quantified in situ NSC responses of temperate forest trees to the 2016 drought in the northeastern United States. Sugar and starch concentrations were measured in the stemwood of five tree species from 2014 to 2019, which allowed us to monitor NSCs in relation to climatic conditions before, during, and after the natural drought. We found that immediately following the drought, measured stemwood NSC concentrations decreased. However, NSC concentrations rebounded quickly within three years. Notably, trees allocated proportionally more to starch than to sugars following the 2016 drought. In winter 2017, starch comprised 45% of total stemwood stores, whereas starch made up only 1–2% in other years. Further, we modeled and assessed the climatic drivers of total NSC concentrations in the stem. Variation in total NSC concentrations was significantly predicted by the previous year’s temperature, precipitation, and standardized precipitation-evapotranspiration index (SPEI), with stemwood concentrations decreasing following hotter, drier periods and increasing following cooler, wetter periods. Overall, our work provides insight into the climatic drivers of NSC storage and highlights the important role that a tree’s carbon economy may play in its response and recovery to environmental stress.
... This conclusion, however, provides little context of the relative landscape importance fires play in driving large changes in stream conditions. We found trees proximate to the stream (King et al. 2013) subject to mortality or other small-scale disturbances (Van Mantgem et al. 2009) that resulted in their falling into streams brought about more changes in stream reach conditions than fire. This is because burned forested riparian zones represent only a small portion of the streams with forested buffers. ...
Hydrologic, terrestrial and biologic disturbances are factors influencing stream channel conditions important to the persistence of aquatic biota. Past studies of how disturbance events alter streams have focused on fires, floods, and debris torrents as the magnitude of these events make them easy to detect. This approach has led to a bias in understanding which disturbances are likely to affect stream conditions. To address this concern, we used stream habitat data to identify where a substantial change in stream conditions had occurred and then used photographs of the evaluated reach to determine the disturbance that potentially caused those changes. We evaluated conditions in over 2000 stream reaches and found nearly a quarter of them had seen a substantial change in at least one of the five stream channel characteristics ‐ bankfull width, wood frequency, median particle size, pool depth, and bank stability – in the two decades these streams were monitored. Although many stream reaches were affected by charismatic disturbance events such as fires, floods, and mass wasting, the majority of the substantial changes we observed in channel conditions were related to small scale disturbances. Mechanisms such as beavers, tree fall, vegetative growth, grazing, and active restoration all played an important role in bringing about large changes in stream channel conditions. While the majority of the disturbances were natural, some channel changes remain tied to anthropogenic activities. Our results suggest no single sampling approach can be used to evaluate how stream conditions respond to all disturbances as they vary in intensity, over space and time, and which stream attribute is measured. By better understanding the full range of possible disturbances, managers should be better able to use such events to improve outcomes for streams and aquatic biota.
... Snow cover plays a critical role in the Earth's hydrological processes and its impact on the broader global climate is of great interest (Barnett et al. 2005;Karl et al. 2009;Goudie 2018;Van Mantgem et al. 2009). Snow greatly influences the global energy balance due to its high albedo and insulating characteristics and is therefore a prominent indicator of climate change (Liston and Hiemstra 2011;Mote 2003;Lawrence and Slater 2010;Callaghan et al. 2011). ...
This paper develops a mathematical model and statistical methods to quantify trends in presence/absence observations of snow cover (not depths) and applies these in an analysis of Northern Hemispheric observations extracted from satellite flyovers during 1967-2021. A two-state Markov chain model with periodic dynamics is introduced to analyze changes in the data in a grid by grid fashion. Trends, converted to the number of weeks of snow cover lost/gained per century, are estimated for each study grid. Uncertainty margins for these trends are developed from the model and used to assess the significance of the trend estimates. Grids with questionable data quality are identified. Among trustworthy grids, snow presence is seen to be declining in almost twice as many grids as it is advancing. While Arctic and southern latitude snow presence is found to be rapidly receding, other locations, such as Eastern Canada, are experiencing advancing snow cover.
... Note that in this effort we define the 1 % reduction level as the CL. Relatively small demographic rate changes can alter forest structure and function due to compounding effects over time (Kobe, 1996) We selected the threshold used here as impactful to forest management objectives due to a significant loss of function, even at a relatively small absolute change (van Mantgem et al., 2009). This is similar in spirit to the lichen CLs where a 20 % change in the community is defined as the threshold and is different from Horn et al. . ...
Critical loads (CLs) of atmospheric deposition for nitrogen (N) and sulfur (S) are used to support decision making related to air regulation and land management. Frequently, CLs are calculated using empirical methods, and the certainty of the results depends on accurate representation of underlying ecological processes. Machine learning (ML) models perform well in empirical modeling of processes with non-linear characteristics and significant variable interactions. We used bootstrap ensemble ML methods to develop CL estimates and assess uncertainties of CLs for the growth and survival of 108 tree species in the conterminous United States. We trained ML models to predict tree growth and survival and characterize the relationship between deposition and tree species response. Using four statistical methods, we quantified the uncertainty of CLs in 95 % confidence intervals (CI). At the lower bound of the CL uncertainty estimate, 80 % or more of tree species have been impacted by nitrogen deposition exceeding a CL for tree survival over >50 % of the species range, while at the upper bound the percentage is much lower (<20 % of tree species impacted across >60 % of the species range). Our analysis shows that bootstrap ensemble ML can be effectively used to quantify critical loads and their uncertainties. The range of the uncertainty we calculated is sufficiently large to warrant consideration in management and regulatory decision making with respect to atmospheric deposition.
... However, in 2010, western Russia suffered from an even warmer summer, covering a record-breaking area of ~2 million km 2 , causing approximately 55,000 heat-related deaths and a 30 % loss of crop yield (Barriopedro et al., 2011;McMichael and Lindgren, 2011;Russo et al., 2014;Shaposhnikov et al., 2014). Water stress caused by regional drought leads to increased mortality in high-latitude boreal forests (Peng et al., 2011) and decreased water use efficiency in Asian drylands (Lu et al., 2019;Tian et al., 2011), resulting in dramatic carbon emission, and similar findings also have been reported in low-and midlatitude forests (Mantgem et al., 2009;Phillips et al., 2009). It is not only "dry" events but also "wet" events that have caused serious impacts. ...
Eurasian drylands are the regions that are most vulnerable to climate change. Climate extremes have caused enormous or even devastating impacts on ecosystems and the social economy in this region, and the compound climate extremes (com_CEs, two or more extreme events occurring simultaneously) and cascading climate extremes (cas_CEs, two or more extreme events occurring successively) have exacerbated these problems. However, little is known about the occurrence patterns of com_CEs and cas_CEs in the Eurasian drylands. Based on the ERA5 reanalysis data range from 1979 to 2020, we improved the methodology for the extraction of co-occurrence events and identified high-frequency types, their hotspots, and occurrence rhythms (seasonally and annually) in Eurasia drylands. Our results showed that com_CEs and cas_CEs have high similarities in the types and spatial hotspots of extreme events; however, the former has a wider geographical and spatial distribution, and the latter has a longer duration. Specifically, co-occurring drought and heatwave events (DH) frequently appear in South Asia and western mid-latitude regions during summer, while in the winter, high latitude regions should be alert to the co-occurrence of drought and low-temperature events (DT). Central Asia and the Mongolian Plateau regions are prone to frequent drought and wind events (DW), and wind and high precipitation events (WP) in the spring and autumn. We have noticed that mid-latitude may suffer from extreme events that have never occurred before, such as com_DH being scattered sporadically in the first two decades and suddenly surging in West Asia and East Asia after the year 2000, and com_DT migrating from high-latitude areas such as the Arctic Ocean coast to mid-latitudes. Our results contribute to understanding hotspots of co-occurring CEs in Eurasian drylands, where more efforts will be needed in the future, especially in mid-latitudes which may suffer extreme climate events that have never occurred before.
... Numerous studies have documented that browsing can cause reduced tree recruitment (Bradshaw & Waller, 2016;Faison et al., 2016), large-scale changes in forest demographics (Davis et al., 2011), and shifts in the structure and composition of understory plant communities (Martin et al., 2011;Tanentzap et al., 2011). In an era of unprecedented global change, ungulate browsing may have especially severe consequences for forests, as many tree species are already suffering declines due to a variety of anthropogenic stressors, including drought and high temperatures (Allen et al., 2010;Anderegg et al., 2015;van Mantgem et al., 2009), altered fire regimes (Cohen et al., 2016), and novel pests and pathogens (Kautz et al., 2017;Sturrock et al., 2011;Weed et al., 2013). The successful management of forests experiencing novel climatic and disturbance regimes will require a broader understanding of how the intensity and effects of ungulate herbivory vary across vast, heterogeneous landscapes and the extent to which actions to mitigate ungulate herbivory effectively promote the health and resilience of forests. ...
Herbivory by wild and domestic ungulates can influence tree recruitment and understory forest communities throughout the world. Herbivore‐driven declines in tree recruitment have been observed for quaking aspen (Populus tremuloides), a foundation species whose health and management is recognized as a critical priority throughout much of its range. Livestock fencing is commonly used to promote aspen regeneration, but its effectiveness is rarely assessed, especially across large spatial scales. Using a livestock‐reduction experiment, we evaluated the effects of ungulate herbivory on aspen in the Great Basin and southern Cascades, an expansive and environmentally heterogeneous region where aspen faces the interacting threats of climate change, conifer encroachment, and herbivory. We found that livestock fencing only reduced the intensity of herbivore browsing on aspen when wild ungulate abundance was low and did not increase stem densities of aspen recruits. Contrary to expectations, wild ungulate abundance was a strong driver of browsing intensity on juvenile aspen within fenced, but not unfenced, aspen stands, and when the abundance of these herbivores was high, browsing intensity in fenced stands exceeded that in unfenced stands. The density of aspen recruits decreased with browsing intensity on juvenile aspen and with the density of both adult aspen and conifers, suggesting that both herbivory and intra‐ and interspecific competition are important drivers of recruitment. Fire history was also an important driver of recruitment, with stands that burned 10–20 years ago having the greatest density of aspen recruits. Finally, in the stand understory, we found that livestock fencing decreased forb cover, increased shrub species richness, and increased the cover of exotic annual grasses, a group dominated by Bromus tectorum. This latter finding suggests that livestock fencing may not be appropriate in areas where controlling the spread of this invader is a priority. In sum, our findings indicate that aspen recruitment is limited by browsing by both wild and domestic ungulates, is mediated by competition with neighboring trees and fire history, and will require management actions beyond livestock fencing, as this approach does not control browsing by wild ungulates.
... On the other hand, considering that pathogens have more ability to survive, reproduce and migrate in changing climatic conditions compared to their hosts (Sturrock et al., 2011), it is estimated that the negative effects on forests may increase in this process. Thus, climate change associated with increasing forest damage has been shown as the source of recent tree mortality in forests in the west of the United States (van Mantgem et al., 2009). Therefore, when a general evaluation is made, it is possible to say that disease damages by various insect, pathogen, etc. in forests can be triggered by climate change in many regions. ...
... Repeat sampling of "legacy plots" -plots established by earlier ecologists decades ago that are spatially referenced and include documented sampling methods (Vellend et al. 2013, Stuble et al. 2021) -provide important observations of long-term changes in tree distributions (van Mantgem et al. 2009, McIntyre et al. 2015 and community composition and structure (Rogers et al. 2008, Dolanc et al. 2014b, Savage and Vellend 2015. A unique feature of studies that resample legacy plots is their emphasis on natural history, yielding detailed, fine-scale community composition data on a decadal time scale (Vellend et al. 2013); these data are not available with other common methods of assessing long-term vegetation change such as paleoecology and remote-sensing. ...
Climate warming and altered disturbance regimes are changing forest composition and structure worldwide. Given that species often exhibit individualistic responses to change, making predictions about the cumulative effects of multiple stressors across environmental gradients is challenging, especially in diverse communities. For example, warming temperatures are predicted to drive species upslope, while fire exclusion promotes expansion of species at lower elevations where fire was historically frequent. We resampled 148 vegetation plots to assess 46-years (1969 to 2015) of species and community-level response to warming and fire exclusion in a topographically complex landscape in the Klamath Mountains, California (USA), a diverse region that served as a climate refugia throughout the Holocene. We compared cover and assessed change in the elevational distributions of 12 conifer species at different life stages (i.e., seedlings, saplings, canopy). We observed consistent but non-significant shifts upward in elevation for eight species, and a significant shift upward for one species, all of which were far less than expectations based on recent warming. Six species declined in total cover and another five declined in at least one life stage, while the drought-and fire-intolerant Abies concolor increased by 30.7%. The largest declines were at lower elevations in drought-tolerant, early seral species (Pinus lambertiana and Pinus ponderosa) and at higher elevations for the shade-tolerant Abies magnifica var. shastensis and the regionally rare Abies lasiocarpa. Regionally rare (Picea engelmannii) and endemic (Picea breweriana) species had reductions in early life stages, portending future declines. Multivariate analyses revealed a high degree of inertia with a minor but significant shift in composition and a slight decrease in species turnover along the elevation gradient driven by expansion of A. concolor. Our results indicate that most species are declining, especially at lower-and mid-elevations where fire exclusion increased cover of shade-tolerant species and reduced recruitment for fire-adapted species. Collectively, declines in most species, insufficient upward movement to track warming, reductions in drought-and fire-tolerant early seral species, and an increase in a single, shade-tolerant species will leave these communities maladapted to projected climate scenarios and questions the potential for future climate refugia in this region. Word count: 348
... Growing evidence shows that forests in many regions are increasingly susceptible to intensified climate extremes, and their complex interactions with disturbances such as insect outbreaks and fires (Allen et al. 2015;Anderegg et al. 2015a;Frank et al. 2015;Anderegg et al. 2020b;Forzieri et al. 2021). This is exemplified by recent widespread forest dieback episodes, growth decline and tree mortality triggered primarily by droughts accompanied with increasing temperature (van Mantgem et al. 2009;Allen et al. 2010;Liu et al. 2013;Allen et al. 2015;Zhang et al. 2020). ...
Plain Language Summary
The occurrence patterns of seasonal extreme drought and wetness events are dramatically shifting with climate warming. However, how will different seasonal extreme climate regimes affect the bioclimatic sensitivity of tree growth remains poorly understood. In this study, we investigated the sensitivity of tree growth to different seasonal climate factors and preceding tree growth conditions during 1951–2013 under different seasonal extreme drought/wetness regimes, using 1,032 tree ring chronologies covering 121 gymnosperm and angiosperm species. We found the magnitude in tree growth reduction caused by seasonal extreme drought events is much larger than that in tree growth stimulation by seasonal extreme wetness events in arid and temperate regions. Tree growth in arid and temperate dry regions is more negatively impacted by extreme drought events in pre‐growing‐seasons (PGSs) than in growing‐seasons. We further found that angiosperms are more sensitive to PGS water availability, while gymnosperms are more sensitive to legacy effects of the preceding tree growth conditions in temperate dry and humid regions. These findings highlight divergent bioclimatic legacy effects on tree growth under different seasonal extreme climate regimes, and provide valuable insights into the future trajectories of forest growth across diverse ecoregions and functional groups in a more extreme climate.
... Repeat sampling of "legacy plots"-plots established by earlier ecologists decades ago that are spatially referenced and include documented sampling methods (Stuble et al., 2021;Vellend et al., 2013)-provide important observations of long-term changes in tree distributions (McIntyre et al., 2015;van Mantgem et al., 2009) and community composition and structure Rogers et al., 2008;Savage & Vellend, 2015). A unique feature of studies that resample legacy plots is their emphasis on natural history, yielding detailed, fine-scale community composition data on a decadal time scale (Vellend et al., 2013); these data are not available from other common methods of assessing long-term vegetation change such as paleoecology and remote sensing. ...
Climate warming and altered disturbance regimes are changing forest composition and structure worldwide. Given that species often exhibit individualistic responses to change, making predictions about the cumulative effects of multiple stressors across environmental gradients is challenging, especially in diverse communities. For example, warming temperatures are predicted to drive species upslope, while fire exclusion promotes expansion of species at lower elevations where fire was historically frequent. We resampled 148 vegetation plots to assess 46‐years (1969 to 2015) of species and community‐level response to warming and fire exclusion in a topographically complex landscape in the Klamath Mountains, California (USA), a diverse region that served as a climate refugia throughout the Holocene. We compared cover and assessed change in the elevational distributions of 12 conifer species at different life stages (i.e., seedlings, saplings, canopy). We observed consistent but non‐significant shifts upward in elevation for eight species, and a significant shift upward for one species, all of which were far less than expectations based on recent warming. Six species declined in total cover and another five declined in at least one life stage, while the drought‐ and fire‐intolerant Abies concolor increased by 30.7%. The largest declines were at lower elevations in drought‐tolerant, early seral species (Pinus lambertiana and Pinus ponderosa) and at higher elevations for the shade‐tolerant Abies magnifica var. shastensis and the regionally rare Abies lasiocarpa. Regionally rare (Picea engelmannii) and endemic (Picea breweriana) species had reductions in early life stages, portending future declines. Multivariate analyses revealed a high degree of inertia with a minor but significant shift in composition and a slight decrease in species turnover along the elevation gradient driven by expansion of A. concolor. Our results indicate that most species are declining, especially at lower‐ and mid‐elevations where fire exclusion increased cover of shade‐tolerant species and reduced recruitment for fire‐adapted species. Collectively, declines in most species, insufficient upward movement to track warming, reductions in drought‐ and fire‐tolerant early seral species, and an increase in a single, shade‐tolerant species will leave these communities maladapted to projected climate scenarios and questions the potential for future climate refugia in this region.
... Because populations are genetically adapted to their local historical climates, the health and productivity of naturally regenerated or planted ecosystems using native local seed sources will likely decline. Recent observations of increased mortality for many forest tree species in North America can likely be attributed, at least in part, to changes in climate (Allen et al., 2010(Allen et al., , 2015S aenz-Romero et al., 2020;van Mantgem et al., 2009). Impacts due to climate change will likely only get worse-the amount of climate change to date is small compared to what is projected for the rest of the century (McDowell et al., 2016(McDowell et al., , 2020NOAA, 2021). ...
The Seedlot Selection Tool and Climate‐Smart Restoration Tool are web‐based tools designed to match seedlots with planting sites assuming that seedlots are adapted to the past climates in which they evolved, primarily with respect to temperature and aridity. The tools map the climatic match of seedlots with the past or projected climates of planting sites. The challenge is that future climates are a moving target, which means that seedlots must be adapted to the near‐term climates as well as the climates of the mid‐ to late‐21st century. Because climate projections are uncertain, the prudent approach is to aim for the warmest climate that may be expected while ensuring that seedlots moved from warmer to colder locales are not moved so far that they risk cold damage. Uncertainty in climate projections may be mitigated by ensuring genetic diversity through mixing seed sources and having collections from many parents per seed source. Three examples illustrate how to effectively use the web tools: (1) choosing seedlots targeting different future climates for a mid‐elevation Douglas‐fir site in the Washington Cascades, (2) finding current and future seed sources for restoration of big sagebrush after fires in the Great Basin and Snake River Plain, and (3) planning to ensure that a Douglas‐fir seed inventory includes seedlots suitable for future climates in western Oregon and Washington.
... Ideally, forest carbon reservoirs may be affected by major changes in mortality and growth rate associated with climate and forest structure and their interactions. (Van Mantgem et al., 2009;Dietze & Moorcroft, 2011;Ruiz-Benito et al., 2012). ...
The Mediterranean Basin covers more than 2 million square kilometres and is surrounded by three continents: Africa, Asia and Europe. The Basin that is rich in biodiversity has tilted towards warmer and drier conditions over the last decades. The emerging climatic conditions particularly the increase in the number of climate extremes are bringing new threats and risks that will exacerbate existing pressures. The present study thoroughly reviewed the recent scientific literature and synthesized existing body of knowledge on the impacts (direct and indirect) of climate change on forest ecosystem services in the Mediterranean Basin. Despite many uncertainties about climate change in the Basin, there appears to be a consensus among a number of studies that climate change is having and will continue to have mostly negative impacts on the Mediterranean forest ecosystem services (wood and non-wood forest products, water resources, carbon storage and recreation and tourism) with possible substantial impacts in the future. Further, evidence is mounting that climate-induced natural disturbances (fires, insect pests, and pathogenic diseases) are becoming frequent and severe. The Mediterranean plants are known for their resilience to natural disturbances. However, the novel climatic conditions may exceed their resilience and alter the ecosystem services. Therefore, there is the need to mitigate the challenges posed by climate change and adapt forest management practices to impending changes to sustain the forest ecosystem services.
... Moreover, climatic stressors might cause defoliation and tree mortality leading to declining forest productivity over large spatial scales (Brienen et al., 2015). Severe droughts have caused widespread forest degradation in Amazonia (Xu et al., 2011) and the Congo Basin (Zhou et al., 2014), and drought-induced large-scale tree mortality in the western United States (van Mantgem et al., 2009). Forest fires are also expected to increase with subsequent consequences for forest-dependent livelihoods and human health (Barbero et al., 2015;Alencar et al. 2015;Tan-Soo and Pattanayak, 2019). ...
Addressing poverty is an urgent global priority. Many of the world's poor and vulnerable people live in or near forests and rely on trees and other natural resources to support their livelihoods. Effectively tackling poverty and making progress toward the first of the United Nations 2030 Sustainable Development Goals to “end poverty in all its forms everywhere” must therefore consider forests and trees. But what do we know about the potential for forests and tree-based systems to contribute to poverty alleviation? This Special Issue responds to this question. It synthesises and presents available scientific evidence on the role of forests and tree-based systems in alleviating and, ultimately, eradicating poverty. The articles compiled here also develop new conceptual frameworks, identify research frontiers, and draw out specific recommendations for policy. The scope is global, although emphasis is placed on low- and middle-income countries where the majority of the world's poorest people live. This introductory article stakes out the conceptual, empirical and policy terrain relating to forests, trees and poverty and provides an overview of the contribution of the other seven articles in this collection. This Special Issue has direct implications for researchers, policymakers and other decision-makers related to the role of forests and tree-based systems in poverty alleviation. The included articles frame the relationships between forests, trees and poverty, identify research gaps and synthesize evidence to inform policy.
... It is a normal process within forest stands and is generally assumed to be in balance with growth to sustain ecosystem processes. Tree mortality affects stand structure, community composition, and stand development processes, and has been recently hypothesised to be increasing globally due to a range of anthropogenic factors and drought (van Mantgem et al. 2009). ...
... It is a normal process within forest stands and is generally assumed to be in balance with growth to sustain ecosystem processes. Tree mortality affects stand structure, community composition, and stand development processes, and has been recently hypothesised to be increasing globally due to a range of anthropogenic factors and drought (van Mantgem et al. 2009). ...
... In the temperate forest biome, increasing temperatures and rising evaporative demand are seen as the main drivers of widespread vitality loss and increasing mortality of drought-sensitive tree species, especially after extreme hot droughts (van Mantgem et al. 2009;Allen et al. 2010;Anderegg et al. 2013;Leuschner 2020;Schuldt et al. 2020). Elevated tree mortality rates have been observed in various forest types on the southern and continental edges of the distribution ranges (e.g., in European beech, Penuelas et al. 2007;Lakatos and Molnár 2009), but recently after the extreme 2018/19 drought also in Central Europe (Schuldt et al. 2020;Braun et al. 2021). ...
Human activities, especially forest conversion and degradation, are causing global declines in forest biodiversity. This review quantifies the current extent of the major forest biomes on earth and their area losses in historical and recent time. The importance of global forests for the earth’s terrestrial biodiversity is explored and the role of forest degradation, fragmentation, defaunation, and forest fires for forest biodiversity analyzed based on the comparison of managed and unmanaged forests and reported forest biodiversity trends. The outstanding role of the remaining primary forests for global forest biodiversity is highlighted, the imprint of millenia of forest use on forest biodiversity explored using Germany’s forests as an example, and a brief assessment of the impact of climate change on forest biodiversity given. We conclude that conserving the last remaining primary forests is of paramount importance for the future of biodiversity on earth. When a substantial part of the earth’s forest-related biodiversity is to be inherited to future generations, a global effort to establish at least ten effectively managed forest mega-reserves in the tropical and boreal forest biomes is urgently needed.KeywordsBiodiversity trendsBoreal forestDefaunationForest degradationForest fragmentationForest lossForest managementSpecies numbersTemperate forestTropical forest
Increasing temperatures and extreme heat episodes have become more common with climate change. While forests are known to be buffered from increasing temperatures compared to non-forested areas, whether this buffering is maintained under extreme temperature events, how such events influence forests, and how forest organisms respond to extreme heat is relatively unknown. Here we assess the effects of an extreme heat event (the Pacific Northwest (PNW) heatdome in June 2021) on forest microclimates, forests, and the organisms living within them. We first asked how the PNW heatdome affected microclimates in forests with differing canopy cover (including non-forests) and found that the buffering capacity of forests is greater under denser canopies, even under extreme heat events. We then combined this information with organismal temperature tolerance curves for 12 relevant species and found that canopy buffering can minimize the negative impacts of even extreme heat events on understory organisms, with greater canopy density providing greater microclimate moderation. Finally, we analyzed seasonal NDVI trends in recent years, and found signs of canopy stress following the extreme 2021 heat event. In all, this suggests that although forest canopies may buffer the negative effects of extreme heat events on understory organisms, a greater frequency of extreme heat events may threaten this capacity by damaging forest canopies.
Wildfires and land use play a central role in the long‐term carbon (C) dynamics of forested ecosystems of the United States. Understanding their linkages with changes in biomass, resource use and consumption in the context of climate change mitigation is crucial. We reconstruct a long‐term C balance of forests in the contiguous U.S. using historical reports, satellite data and other sources at multiple scales (national scale 1926‐2017, regional level 1941‐2017) to disentangle the drivers of biomass C stock change. The balance includes removals of forest biomass by fire, by extraction of woody biomass, by forest grazing, and by biomass stock change, their sum representing the net ecosystem productivity (NEP). Nationally, the total forest NEP increased for most of the 20 th century while fire, harvest and grazing reduced total forest stocks on average by 14%, 51% and 6%, respectively, resulting in a net increase in C stock density of nearly 40%. Recovery from past land‐use, plus reductions in wildfires and forest grazing coincide with consistent forest regrowth in the eastern U.S. but associated C stock increases were offset by increased wood harvest. C stock changes across the Western U.S. fluctuated, with fire, harvest, and other disturbances (e.g., insects, droughts) reducing stocks on average by 14%, 81%, and 7%, respectively, resulting in a net growth in C stock density of 14%. Although wildfire activities increased in recent decades, harvest was the key driver in the forest C balance in all regions for most of the observed timeframe.
The ongoing rapid changes in climate pose significant pressures on forests’ health and growth with modalities that are still poorly understood 1–3 . Trees’ structural properties like tree height, leaf and root biomass, result from the interplay between plant physiology and key environmental factors like temperature and water. On the other hand, the combination of these structural properties and site characteristics controls the functioning of trees and their sensitivity to environmental stressors. The unbalance between tree architecture and environmental conditions driven by climate change may therefore affect plant growth, vulnerability and mortality, ultimately affecting the stability of the terrestrial carbon cycle and the mitigation potential of land-based climate solutions ⁴ . Here we use satellite data and machine learning to assess the response of European forests to changing climate over 2001-2020. We show that tree architecture, topography and soil characteristics largely control the recent trend in forest greenness and dryness. Specifically, our results show that tree height and the vertical distribution of leaves have a strong effect on the canopy decline observed in large areas of Europe, with toller trees having leaves close to the top that show browning and drying signals. In addition, we demonstrate that the negative impact of recent climate trends is exacerbated on south-facing slopes and in nutrient-rich soil with high bulk density. These architectural and environmental controls of the trees’ response to climate are currently not accounted for in vegetation models, which therefore poorly represent the recent pattern of forest dieback 5,6 , and may therefore underestimate the risks of natural disturbances and tree mortality. These findings unveil the drivers of the worrying trends in EU forest conditions. In parallel, they offer valuable insights for the development of climate-effective practices for the management of EU forests under the pressure of climate change.
This paper develops a mathematical model and statistical methods to quantify trends in presence/absence observations of snow cover (not depths) and applies these in an analysis of Northern Hemispheric observations extracted from satellite flyovers during 1967–2021. A two-state Markov chain model with periodic dynamics is introduced to analyze changes in the data in a cell by cell fashion. Trends, converted to the number of weeks of snow cover lost/gained per century, are estimated for each study cell. Uncertainty margins for these trends are developed from the model and used to assess the significance of the trend estimates. Cells with questionable data quality are explicitly identified. Among trustworthy cells, snow presence is seen to be declining in almost twice as many cells as it is advancing. While Arctic and southern latitude snow presence is found to be rapidly receding, other locations, such as eastern Canada, are experiencing advancing snow cover.
Significance Statement
This project quantifies how the Northern Hemisphere’s snow cover has recently changed. Snow cover plays a critical role in the global energy balance due to its high albedo and insulating characteristics and is therefore a prominent indicator of climate change. On a regional scale, the spatial consistency of snow cover influences surface temperatures via variations in absorbed solar radiation, while continental-scale snow cover acts to maintain thermal stability in the Arctic and subarctic regions, leading to spatial and temporal impacts on global circulation patterns. Changing snow presence in Arctic regions could influence large-scale releases of carbon and methane gas. Given the importance of snow cover, understanding its trends enhances our understanding of climate change.
The frequency and intensity of forest disturbances such as drought and fire are increasing globally, with an increased likelihood of multiple disturbance events occurring in short succession. Disturbances layered over one another may influence the likelihood or intensity of subsequent events (a linked disturbance) or impact response and recovery trajectories (a compound disturbance), with substantial implications for ecological spatiotemporal vulnerability. This study evaluates evidence for disturbance interactions of drought followed by wildfire in a resprouting eucalypt-dominated forest (the Northern Jarrah Forest) in southwestern Australia. Sites were stratified by drought (high, low) from previous modelling and ground validation, and fire severity (high, moderate, unburnt) via remote sensing using the relative difference normalised burn ratio (RdNBR). Evidence of a linked disturbance was assessed via fine fuel consumption and fire severity. Compound disturbance effects were quantified at stand scale (canopy height, quadratic mean diameter, stem density) and at the stem scale (mortality). There was no evidence of prior drought influencing fine fuel consumption or fire severity, hence no evidence of a linked disturbance. However, compound disturbance effects were evident; stands previously affected by drought experienced smaller shifts in canopy height, quadratic mean diameter, and stem density than stands without prior drought impact. At the stem scale, size and fire severity were the strongest determinants of stem survival. Proportional resprouting height was higher in high drought sites than in low drought sites (p<0.01), thus, structurally, the low drought stands decreased in height more than the high drought stands. Thus, a legacy of the drought was evident after the wildfire. While these resprouting eucalypt forests have been regarded as particularly resilient, this study illustrates how multiple disturbances can overwhelm the larger tree component and promote an abundance of smaller stems. We suggest that this is early evidence of a structural destabilisation of these forests under a more fire prone, hotter and drier future climate.
Aim
Species distribution models typically project climatically suitable habitat for trees in eastern North America to shift hundreds of kilometres this century. We simulated potential migration, accounting for various traits that affect species' ability to track climatically suitable habitat.
Location
Eastern Canada, covering ~3.7 million km².
Methods
We simulated migration‐constrained range shifts through 2100 using a hybrid approach combining projections of climatically suitable habitat based on two Representative Concentration Pathways (RCP4.5, RCP8.5) for three time periods and two species distribution modelling approaches with process‐based models parameterized using data related to dispersal ability and generation time. We developed a unique “migration kernel” that uses seed dispersal traits and observed migration velocities to obtain kernel shape and dispersal probabilities for each tree species. We then calculated lags between the migration‐constrained range limits obtained through simulations and limits of climatically suitable habitat.
Results
All species demonstrated northward range shifts at the leading edge of their simulated distribution through 2100, but the magnitude and rate of that shift varied by species and time period. Climatically suitable habitat limits were found to be north of simulated distribution limits across both RCPs, with lags increasing through time. On average, simulated distribution that remained within climatically suitable habitat declined more under RCP8.5 than RCP4.5, with large areas of the rear edge of the simulated distribution becoming partially or completely climatically unsuitable for many species.
Main conclusions
Climatically suitable habitat limits projected for 2100 far exceeded migration‐constrained range limits for all 10 tree species, particularly for temperate species. This study underlines the limited extent to which tree species will track climate change via natural migration. Integrating observed migration velocities, seed dispersal and generation time with SDM outputs allows for more realistic evaluations of tree migration ability under climate change and may help orient forest conservation and restoration efforts.
Climate change is altering the distribution of woody plants by influencing demographic processes and modifying disturbance regimes. Trailing-edge forests may be particularly vulnerable to these effects because they exist at warm, dry margins of tree distributions. To better understand recent climate-driven changes in trailing-edge forests, we used Landsat time series and 1558 field reference plots to develop annual land cover maps from 1985 to 2020 in two large, biodiverse landscapes in central Arizona, USA. We then combined annual land cover maps with tree ring records and spatial data describing interannual climate, terrain, bark beetle (Curculionidae: Scolytinae) activity, wildfire, and harvest to quantify drivers of forest change. Throughout the two landscapes, forest extent declined by 0.3 % and 0.8 % from 1985 to 2020. However, considerable variation occurred within the study period, with abrupt (ca. 1–2 years) declines in forest extent followed by gradual (ca. 10 years) recovery on each landscape. Pinyon-juniper (Pinus edulis, Pinus monophylla, and/or Juniperus spp.) cover increased from 1985 to ca. 2000 but declined after 2000, a period of extreme drought and regional tree die-off. In contrast, pine-oak (Pinus ponderosa and Quercus spp.) cover increased from 2000 to 2020, primarily due to declines in ponderosa pine and mixed conifer cover over the same period. Wildfire was a key driver of transitions from forest to non-forest cover in our study area, with the occurrence of multiple compounded drought years playing an important role in unburned areas. By driving transitions to alternative forest types or non-forest cover, disturbance and drought will increasingly shape forest dynamics and ecosystem transformations throughout the southwestern US.
Considerable uncertainty and debate exist in projecting the future capacity of forests to sequester atmospheric CO2. Here we estimate spatially explicit patterns of biomass loss by tree mortality (LOSS) from largely unmanaged forest plots to constrain projected (2015–2099) net primary productivity (NPP), heterotrophic respiration (HR) and net carbon sink in six dynamic global vegetation models (DGVMs) across continents. This approach relies on a strong relationship among LOSS, NPP, and HR at continental or biome scales. The DGVMs overestimated historical LOSS, particularly in tropical regions and eastern North America by as much as 5 Mg ha−1 y−1. The modeled spread of DGVM-projected NPP and HR uncertainties was substantially reduced in tropical regions after incorporating the field-based mortality constraint. The observation-constrained models show a decrease in the tropical forest carbon sink by the end of the century, particularly across South America (from 2 to 1.4 PgC y−1), and an increase in the sink in North America (from 0.8 to 1.1 PgC y−1). These results highlight the feasibility of using forest demographic data to empirically constrain forest carbon sink projections and the potential overestimation of projected tropical forest carbon sinks. Here the authors use broad-scale tree mortality data to estimate biomass loss, constraining uncertainty of projected forest net primary productivity in 6 models, finding weaker tropical forest carbon sinks with climate change.
The natural forest ecosystem has been affected by wind storms for years, which have caused several down wood (DW) and dramatically modified the fabric and size. Therefore, it is very important to explain the forest system by quantifying the spatial relationship between DW and environmental parameters. However, the spatial non-stationary characteristics caused by the terrain and stand environmental changes with distinct gradients may lead to an incomplete description of DW, the local neural-network-weighted models of geographically neural-network-weighted (GNNWR) models are introduced here. To verify the validity of models, our DW and environmental factors were applied to investigate of occurrence of DW and number of DW to establish the generalized linear (logistic and Poisson) models, geographically weighted regression (GWLR and GWPR) models and GNNWR (GNNWLR and GNNWPR) models. The results show that the GNNWR models show great advantages in the model-fitting performance, prediction performance, and the spatial Moran’s I of model residuals. In addition, GNNWR models can combine the geographic information system technology for accurately expressing the spatial distribution of DW relevant information to provide the key technology that can be used as the basis for human decision-making and management planning.
Context: A significant forest decline has been noticed these last years in Europe. Managers need tools to better anticipate these massive events.
Objectives: We evaluated the efficiency of easily available data about environmental conditions and stand characteristics to determine stand vulnerability and map different levels of risk.
Methods: We combined remote sensing images, photo-interpretation, and digital models describing environmental conditions within a modelling approach to achieve spatial risk assessment of stand vulnerability. We focused on silver fir and Norway spruce stands in the Vosges mountains (8.800 km², north-east of France), where severe symptoms of decline are visible.
Results: Easily available factors describing environmental conditions were used to design relevant vulnerability maps: using an independent dataset, we predicted two-times (Norway spruce) and ten-times (silver fir) higher mortality rates in the units with a high level of risk compared to the others. Norway spruce were predicted highly vulnerable on 33% of their area versus 7% for silver fir. Mortality was higher where the soils displayed low water availability and had suffered severe drying events these last years; it was lower when the species belonged to uneven-aged stands, in mixture, and in the absence of forest edges. Furthermore, the stands acclimatised to drought conditions were more resilient.
Conclusions: Vulnerability maps based on easily available geographic information describing climate, soil, and topography can efficiently discriminate canopy mortality patterns over broad areas, and can be useful tools for managers to mitigate the effects of climate change on forests.
There is broad consensus that active management through thinning and fire is urgently needed in many forests of the western United States. This consensus stems from physically based models of fire behavior and substantial empirical evidence. But the types of thinning and fire and where they are applied are the subjects of much debate. We propose that low thinning is the most appropriate type of thinning practice. Treating surface fuels, reducing ladder fuels, and opening overstory canopies generally produce fire-safe forest conditions, but large, fire-resistant trees are also important components of fire-safe forests. The context of place is critical in assigning priority for the limited resources that will be available for restoration treatments. Historical low-severity fire regimes, because of their current high hazards and dominance by fire-resistant species, are the highest priority for treatment. Mixed-severity fire regimes are of intermediate priority, and high-severity fire regimes are of lowest priority. Classification systems based on potential vegetation will help identify these fire regimes at a local scale.
Nerve terminals release neurotransmitters from vesicles into the synaptic cleft upon transient increases in intracellular
Ca2+. This exocytotic process requires the formation of trans SNARE complexes and is regulated by accessory proteins including
the complexins. Here we report the crystal structure of a squid core complexin-SNARE complex at 2.95-Å resolution. A helical
segment of complexin binds in anti-parallel fashion to the four-helix bundle of the core SNARE complex and interacts at its
C terminus with syntaxin and synaptobrevin around the ionic zero layer of the SNARE complex. We propose that this structure
is part of a multiprotein fusion machinery that regulates vesicle fusion at a late pre-fusion stage. Accordingly, Ca2+ may initiate membrane fusion by acting directly or indirectly on complexin, thus allowing the conformational transitions
of the trans SNARE complex that are thought to drive membrane fusion.
During neurotransmitter release at the synapse, influx of calcium ions stimulates the release of neurotransmitter. However,
the mechanism by which synaptic vesicle fusion is coupled to calcium has been unclear, despite the identification of both
the core fusion machinery [soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE)] and the principal calcium sensor (synaptotagmin). Here,
we describe what may represent a basic principle of the coupling mechanism: a reversible clamping protein (complexin) that
can freeze the SNAREpin, an assembled fusion-competent intermediate en route to fusion. When calcium binds to the calcium
sensor synaptotagmin, the clamp would then be released. SNARE proteins, and key regulators like synaptotagmin and complexin,
can be ectopically expressed on the cell surface. Cells expressing such “flipped” synaptic SNAREs fuse constitutively, but
when we coexpressed complexin, fusion was blocked. Adding back calcium triggered fusion from this intermediate in the presence
of synaptotagmin.
Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.
Neuronal signaling occurs through both action potential-triggered synaptic vesicle fusion and spontaneous release, although the fusion clamp machinery that prevents premature exocytosis of synaptic vesicles in the absence of calcium is unknown. Here we demonstrate that spontaneous release at synapses is regulated by complexin, a SNARE complex-binding protein. Analysis of Drosophila melanogaster complexin null mutants showed a marked increase in spontaneous fusion and a profound overgrowth of synapses, suggesting that complexin functions as the fusion clamp in vivo and may modulate structural remodeling of neuronal connections by controlling the rate of spontaneous release.
In regulated exocytosis, the core membrane fusion machinery proteins, the SNARE proteins, are assisted by a group of regulatory
factors in order to couple membrane fusion to an increase of intracellular calcium ion (Ca2+) concentration. Complexin-I and synaptotagmin-I have been shown to be key elements for this tightly regulated process. Many
studies suggest that complexin-I can arrest the fusion reaction and that synaptotagmin-I can release the complexin-I blockage
in a calcium-dependent manner. Although the actual molecular mechanism by which they exert their function is still unknown,
recent in vivo experiments postulate that domains of complexin-I produce different effects on neurotransmitter release. Herein, by using
an in vitro flipped SNARE cell fusion assay, we have identified and characterized the minimal functional domains of complexin-I necessary
to couple calcium and synaptotagmin-I to membrane fusion. Moreover, we provide evidence that other isoforms of complexin,
complexin-II, -III, and -IV, can also be functionally coupled to synaptotagmin-I and calcium. These correspond closely to
results from in vivo experiments, providing further validation of the physiological relevance of the flipped SNARE system.
Tree mortality is a critical but understudied process in coniferous forest development. Current successional models assume that mortality during early forest development is dominated by density-dependent processes, but few long-term studies exist to test this assumption. We examined changes in forest structure and patterns of tree mortality 14-38 years (1979-2001) after clear-cut logging of two experimental watersheds in the western Cascade Range of Oregon, USA. We sampled 193 permanent plots (250 m2) six times generating 75 126 data records and 7146 incidents of mortality. Mean density peaked at .3000 stems/ha (� 1.4 m tall) after 22-25 years; bole biomass increased continuously to .100 Mg/ha. At final sampling, stem density varied by two orders of magnitude and biomass by a factor of 10 among sample plots. Suppression mortality occurred in .80% of plots and was .2.5 times as frequent as mechanical damage (uprooting, stem snap, and crushing). However, biomass lost to mortality via mechanical damage was nearly four times that lost to suppression, a result of episodic storms that created windthrow patches, with some plots losing 30-50% of biomass. Total annual mortality increased from 1.0% to 5.3% of stems over the study period and was highly variable among species. Although mortality rates were highest for sprouting hardwoods (reaching 9.7% in Cornus nuttallii), biomass of most hardwood species increased through canopy closure as dominant stems achieved large sizes. Shade-tolerant conifers (Tsuga heterophylla and Thuja plicata), typically assumed to be absent or to play a minor role in early forest development, accounted for 26% of stems after 38 years. In regression tree models, environmental attributes of plots had limited ability to predict mortality. Instead, stem density prior to canopy closure was the strongest predictor of cumulative mortality (either suppression or mechanical damage). Our long-term studies suggest that current models of early forest development are overly simplistic, particularly in their treatment of mortality. Although suppression was the dominant demographic process, mechanical damage yielded greater loss of biomass and greater structural heterogeneity through creation of windthrow gaps. Thus, gap-forming processes that operate late in succession and contribute to structural complexity in old-growth forests can also occur early in stand development.
Mice carrying a mutation in the synaptotagmin I gene were generated by homologous recombination. Mutant mice are phenotypically normal as heterozygotes, but die within 48 hr after birth as homozygotes. Studies of hippocampal neurons cultured from homozygous mutant mice reveal that synaptic transmission is severely impaired. The synchronous, fast component of Ca(2+)-dependent neurotransmitter release is decreased, whereas asynchronous release processes, including spontaneous synaptic activity (miniature excitatory postsynaptic current frequency) and release triggered by hypertonic solution or alpha-latrotoxin, are unaffected. Our findings demonstrate that synaptotagmin I function is required for Ca2+ triggering of synchronous neurotransmitter release, but is not essential for asynchronous or Ca(2+)-independent release. We propose that synaptotagmin I is the major low affinity Ca2+ sensor mediating Ca2+ regulation of synchronous neurotransmitter release in hippocampal neurons.
Synaptic vesicle fusion at synapses is triggered by increases in cytosolic Ca2+ levels. However, the identity of the Ca2+ sensor and the transduction mechanism of the Ca2+ trigger are unknown. We show that Complexins, stoichiometric components of the exocytotic core complex, are important regulators of transmitter release at a step immediately preceding vesicle fusion. Neurons lacking Complexins show a dramatically reduced transmitter release efficiency due to decreased Ca2+ sensitivity of the synaptic secretion process. Analyses of mutant neurons demonstrate that Complexins are acting at or following the Ca2+-triggering step of fast synchronous transmitter release by regulating the exocytotic Ca2+ sensor, its interaction with the core complex fusion machinery, or the efficiency of the fusion apparatus itself.
According to a popular theory, the three presynaptic SNARE proteins (syntaxin 1, synaptobrevin 2 and SNAP-25) drive neuroexocytosis by forming a complex that forces vesicle and plasma membranes together. However, individual reactions in this process have been difficult to resolve. Now, two groups have succeeded in studying a very early and a very late step in the fusion process.
Since the discovery of SNARE proteins in the late 1980s, SNAREs have been recognized as key components of protein complexes that drive membrane fusion. Despite considerable sequence divergence among SNARE proteins, their mechanism seems to be conserved and is adaptable for fusion reactions as diverse as those involved in cell growth, membrane repair, cytokinesis and synaptic transmission. A fascinating picture of these robust nanomachines is emerging.
We provide a first detailed analysis of long-term, annual-resolution demographic trends in a temperate forest. After tracking the fates of 21,338 trees in a network of old-growth forest plots in the Sierra Nevada of California, we found that mortality rate, but not the recruitment rate, increased significantly over the 22 years of measurement (1983-2004). Mortality rates increased in both of two dominant taxonomic groups (Abies and Pinus) and in different forest types (different elevational zones). The increase in overall mortality rate resulted from an increase in tree deaths attributed to stress and biotic causes, and coincided with a temperature-driven increase in an index of drought. Our findings suggest that these forests (and by implication, other water-limited forests) may be sensitive to temperature-driven drought stress, and may be poised for die-back if future climates continue to feature rising temperatures without compensating increases in precipitation.
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