Nathan L. Stephenson

United States Geological Survey, Reston, Virginia, United States

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Publications (45)232.66 Total impact

  • Constance I Millar · Nathan L Stephenson
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    ABSTRACT: Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and uncharacteristically severe wildfire are resulting in forest mortality beyond the levels of 20th-century experience. Additional anthropogenic stressors, such as atmospheric pollution and invasive species, further weaken trees in some regions. Although continuing climate change will likely drive many areas of temperate forest toward large-scale transformations, management actions can help ease transitions and minimize losses of socially valued ecosystem services. Copyright © 2015, American Association for the Advancement of Science.
    No preview · Article · Aug 2015 · Science
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    Adrian J. Das · Nathan L. Stephenson
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    ABSTRACT: Tree growth rate is frequently used to estimate mortality probability. Yet, growth metrics can vary in form, and the justification for using one over another is rarely clear. We tested whether a growth index (GI) that scales the realized diameter growth rate against the potential diameter growth rate (PDGR) would give better estimates of mortality probability than other measures. We also tested whether PDGR, being a function of tree size, might better correlate with the baseline mortality probability than direct measurements of size such as diameter or basal area. Using a long-term dataset from the Sierra Nevada, California, U.S.A., as well as existing species-specific estimates of PDGR, we developed growth–mortality models for four common species. For three of the four species, models that included GI, PDGR, or a combination of GI and PDGR were substantially better than models without them. For the fourth species, the models including GI and PDGR performed roughly as well as a model that included only the diameter growth rate. Our results suggest that using PDGR can improve our ability to estimate tree survival probability. However, in the absence of PDGR estimates, the diameter growth rate was the best empirical predictor of mortality, in contrast to assumptions often made in the literature.
    Full-text · Article · Jul 2015 · Canadian Journal of Forest Research
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    ABSTRACT: Fire in high-elevation forest ecosystems can have severe impacts on forest structure, function and biodiversity. Using a 105-year data set, we found increasing elevation extent of fires in the Sierra Nevada, and pose five hypotheses to explain this pattern. Beyond the recognized pattern of increasing fire frequency in the Sierra Nevada since the late 20th century, we find that the upper elevation extent of those fires has also been increasing. Factors such as fire season climate and fuel build up are recognized potential drivers of changes in fire regimes. Patterns of warming climate and increasing stand density are consistent with both the direction and magnitude of increasing elevation of wildfire. Reduction in high elevation wildfire suppression and increasing ignition frequencies may also contribute to the observed pattern. Historical biases in fire reporting are recognized, but not likely to explain the observed patterns. The four plausible mechanistic hypotheses (changes in fire management, climate, fuels, ignitions) are not mutually exclusive, and likely have synergistic interactions that may explain the observed changes. Irrespective of mechanism, the observed pattern of increasing occurrence of fire in these subalpine forests may have significant impacts on their resilience to changing climatic conditions.
    Full-text · Article · Jul 2015 · Ecosphere
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    ABSTRACT: Climate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate-induced tree mortality and biotic disturbance agents are largely absent from process-based ecosystem models. Using data sets from the western USA and associated studies, we present a framework for determining the relative contribution of drought stress, insect attack, and their interactions, which is critical for modeling mortality in future climates. We outline a simple approach that identifies the mechanisms associated with two guilds of insects - bark beetles and defoliators - which are responsible for substantial tree mortality. We then discuss cross-biome patterns of insect-driven tree mortality and draw upon available evidence contrasting the prevalence of insect outbreaks in temperate and tropical regions. We conclude with an overview of tools and promising avenues to address major challenges. Ultimately, a multitrophic approach that captures tree physiology, insect populations, and tree-insect interactions will better inform projections of forest ecosystem responses to climate change. © 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.
    Full-text · Article · Jun 2015 · New Phytologist
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    ABSTRACT: Background/Question/Methods Giant sequoia is an iconic species that stores vast amounts of carbon and inspires awe and curiosity about the natural world. Much of its limited range is protected within federal, state, and tribal lands. Sequoia can live more than three millennia and have shown considerable resistance and resilience to drought, fire, insects, and disease. However, their limited distribution reflects narrow environmental tolerances, high moisture demand, and vulnerability to climate change. Indeed, giant sequoias were probably near extinction during a period of slightly warmer temperatures from about 10,000 to 4,000 years ago. Given a trajectory of rapid warming, the sustainability of this long-lived species is uncertain. Despite the high social value of this species, managers face critical problems that constrain our ability to manage giant sequoia forests sustainably in the face of increasing drought. What are these challenges and how can managers and scientists work together to overcome them? Results/Conclusions Fire management is one of the few tools we currently can use to reduce vulnerability of forests to climatic change. However, capacity to use fire is very limited and it is usually prioritized to protect the most socially valued sequoia groves. Managers might shift their focus to a longer term view of protecting likely climate refugia (i.e., low vulnerability areas) if we had reasonable confidence in how to identify them. Climate envelope models are one way to identify possible refugia, but they are not enough to convince managers to invest limited resources in these locations. Additional tools also are needed to identify areas that are not occupied by sequoia now, but might be suitable in the future. Currently, managers have very limited means to understand early warning signs of drought stress in giant sequoia. We don’t know what moisture thresholds will lead to mortality. It is critical that we develop efficient monitoring programs to detect subtle warning signs before loss is imminent. Proxies developed through remote sensing products could become a powerful tool to monitor moisture condition if we can validate and calibrate them with field data. We understand very little about genetic variability of giant sequoia and whether or not genotypes vary in their drought tolerance. This information is important to develop climate adapted seedbanks for future planting.
    No preview · Conference Paper · Aug 2014
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    ABSTRACT: Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest carbon cycle-particularly net primary productivity and carbon storage-increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.
    Full-text · Article · Jan 2014 · Nature
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    Tapash Das · Phillip J Van Mantgem · Nathan L Stephenson · Alan Flint · Adrian J Das

    Full-text · Dataset · Jul 2013
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    Adrian J Das · Nathan L Stephenson · Alan Flint · Tapash Das · Phillip J van Mantgem
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    ABSTRACT: Recent increases in tree mortality rates across the western USA are correlated with increasing temperatures, but mechanisms remain unresolved. Specifically, increasing mortality could predominantly be a consequence of temperature-induced increases in either (1) drought stress, or (2) the effectiveness of tree-killing insects and pathogens. Using long-term data from California's Sierra Nevada mountain range, we found that in water-limited (low-elevation) forests mortality was unambiguously best modeled by climatic water deficit, consistent with the first mechanism. In energy-limited (high-elevation) forests deficit models were only equivocally better than temperature models, suggesting that the second mechanism is increasingly important in these forests. We could not distinguish between models predicting mortality using absolute versus relative changes in water deficit, and these two model types led to different forecasts of mortality vulnerability under future climate scenarios. Our results provide evidence for differing climatic controls of tree mortality in water- and energy-limited forests, while highlighting the need for an improved understanding of tree mortality processes.
    Full-text · Article · Jul 2013 · PLoS ONE
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    ABSTRACT: Scope of analysis For natural resource managers in the southern Sierra Nevada, giant sequoia requires very little introduction. It receives great attention as an icon of western forests and as a common namesake with the areas where it occurs. While it is a single component of a very complex system, its attention in this assessment and in general is well deserved. Giant sequoia is one of the few ―destination species‖ that attracts a wide swath of the public by nature of it simply being present. It draws people, who otherwise may not travel, to a natural environment. The result is an expansion of the public’s sense of natural resource stewardship. Because park managers could not achieve their mission without public support, this fostering role of giant sequoia is critical for park natural resources and is important for natural resources in general. Despite its social relevance and physical size, we re-emphasize here that the giant sequoia resource is a relatively small component of the ecosystems of the southern Sierra Nevada. As is the case with all of the resources assessed in the NRCA, we focus on giant sequoia with the understanding that other resources will be considered simultaneously when evaluating management decisions that impact giant sequoia. While we attempt to explicitly address the interaction of giant sequoia with other resources and stressors, we also realize that ultimately managers will integrate much more information than is presented here when making decisions that influence giant sequoia. The autecology and management issues surrounding giant sequoia have been thoroughly reviewed elsewhere (Harvey et al. 1980, Aune 1994, Stephenson 1996). Stephenson (1996), in particular, should be reviewed when considering any management decisions that potentially impact giant sequoia. For those who may not be familiar with giant sequoia ecology, a summary of basic information is provided in a table below. In some parts of this assessment, we reproduce text from Stephenson’s review because it is still relatively current for addressing some of the stressors. Numerous recent studies reported since 1996 have confirmed and expanded the understanding of giant sequoia, especially in areas related to ecophysiology and the effectiveness of restoration treatments. These recent studies are integrated into this assessment. Additionally, much unpublished work has been done that is useful for establishing baselines and evaluating trends. This work is presented in detail in order to expand upon previous work and to inform the final assessments. Instead of providing an introductory description of giant sequoia distribution and the various landowners who manage groves, we refer readers to the more recent descriptions provided by Stephenson (1996) and Willard (2000). Some of the relevant points from these descriptions with respect to giant sequoia within SEKI and Giant Sequoia National Monument (GSNM) include: 3. Of the native giant sequoia grove area in SEKI and GSNM approximately 38% is within SEKI and 62% is within GSNM. 2. 35 of the groves that make up the entire population are all or partially managed by SEKI and 33 are managed by GSNM1. 3. As we have done above, reviewers addressing giant sequoia widely recognize its transcendence beyond an ecologically important species to one with considerable added cultural value.
    Full-text · Technical Report · Jun 2013
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    Nathan L. Stephenson
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    ABSTRACT: My colleagues and I have analyzed the age structure of four populations of giant sequoia (Sequoiadendron giganteum (Lindl.) Buchholz). We have found the following: (1) The amount of successful reproduction in a grove cannot be judged by the sizes of its trees. (2) Sequoia populations almost certainly were near equilibrium or increasing before the arrival of European settlers. (3) In this century, there has been a massive failure of sequoia reproduction in groves protected from fire but otherwise meant to be maintained in a natural state. (4) Before the arrival of European settlers, successful recru itment of mature sequoias d ep ended on fires intense enough to kill the forest canopy in small areas. Thus, sequoia is a pioneer species, and this conclusion has specific management implications.
    Preview · Article · May 2012
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    ABSTRACT: At global and regional scales, tree mortality rates are positively correlated with forest net primary productivity (NPP). Yet causes of the correlation are unknown, in spite of potentially profound implications for our understanding of environmental controls of forest structure and dynamics and, more generally, our understanding of broad-scale environmental controls of population dynamics and ecosystem processes. Here we seek to shed light on the causes of geographic patterns in tree mortality rates, and we consider some implications of the positive correlation between mortality rates and NPP. To reach these ends, we present seven hypotheses potentially explaining the correlation, develop an approach to help distinguish among the hypotheses, and apply the approach in a case study comparing a tropical and temperate forest. Based on our case study and literature synthesis, we conclude that no single mechanism controls geographic patterns of tree mortality rates. At least four different mechanisms may be at ...
    Full-text · Article · Nov 2011 · Ecological Monographs
  • Nathan L Stephenson · Adrian J Das
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    ABSTRACT: Crimmins et al. (Reports, 21 January 2011, p. 324) attributed an apparent downward elevational shift of California plant species to a precipitation-induced decline in climatic water deficit. We show that the authors miscalculated deficit, that the apparent decline in species' elevations is likely a consequence of geographic biases, and that unlike temperature changes, precipitation changes should not be expected to cause coordinated directional shifts in species' elevations.
    No preview · Article · Oct 2011 · Science
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    Adrian Das · John Battles · Nathan L. Stephenson · Phillip J. van Mantgem
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    ABSTRACT: a b s t r a c t Competition is a well-documented contributor to tree mortality in temperate forests, with numerous studies documenting a relationship between tree death and the competitive environment. Models fre-quently rely on competition as the only non-random mechanism affecting tree mortality. However, for mature forests, competition may cease to be the primary driver of mortality. We use a large, long-term dataset to study the importance of competition in determining tree mortality in old-growth forests on the western slope of the Sierra Nevada of California, U.S.A. We make use of the comparative spatial configuration of dead and live trees, changes in tree spatial pattern through time, and field assessments of contributors to an individual tree's death to quantify competitive effects. Competition was apparently a significant contributor to tree mortality in these forests. Trees that died tended to be in more competitive environments than trees that survived, and suppression frequently appeared as a factor contributing to mortality. On the other hand, based on spatial pattern analyses, only three of 14 plots demonstrated compelling evidence that competition was dominating mortality. Most of the rest of the plots fell within the expectation for random mortality, and three fit neither the random nor the competition model. These results suggest that while competition is often playing a significant role in tree mortality processes in these forests it only infrequently governs those processes. In addition, the field assessments indicated a substantial presence of biotic mortality agents in trees that died. While competition is almost certainly important, demographics in these forests cannot accurately be characterized without a better grasp of other mortality processes. In particular, we likely need a better understanding of biotic agents and their interactions with one another and with competition. Published by Elsevier B.V.
    Full-text · Article · Apr 2011 · Forest Ecology and Management
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    Phillip J. van Mantgem · Nathan L. Stephenson · Eric Knapp · John Battles · Jon E. Keeley
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    ABSTRACT: The capacity of prescribed fire to restore forest conditions is often judged by changes in forest structure within a few years following burning. However, prescribed fire might have longer-term effects on forest structure, potentially changing treatment assessments. We examined annual changes in forest structure in five 1ha old-growth plots immediately before prescribed fire and up to eight years after fire at Sequoia National Park, California. Fire-induced declines in stem density (67% average decrease at eight years post-fire) were nonlinear, taking up to eight years to reach a presumed asymptote. Declines in live stem biomass were also nonlinear, but smaller in magnitude (32% average decrease at eight years post-fire) as most large trees survived the fires. The preferential survival of large trees following fire resulted in significant shifts in stem diameter distributions. Mortality rates remained significantly above background rates up to six years after the fires. Prescribed fire did not have a large influence on the representation of dominant species. Fire-caused mortality appeared to be spatially random, and therefore did not generally alter heterogeneous tree spatial patterns. Our results suggest that prescribed fire can bring about substantial changes to forest structure in old-growth mixed conifer forests in the Sierra Nevada, but that long-term observations are needed to fully describe some measures of fire effects.
    Full-text · Article · Mar 2011 · Fuel and Energy Abstracts
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    ABSTRACT: Tree mortality is often the result of both long-term and short-term stress. Growth rate, an indicator of long-term stress, is often used to estimate probability of death in unburned stands. In contrast, probability of death in burned stands is modeled as a function of short-term disturbance severity. We sought to narrow this conceptual gap by determining (i) whether growth rate, in addition to crown scorch, is a predictor of mortality in burned stands and (ii) whether a single, simple model could predict tree death in both burned and unburned stands. Observations of 2622 unburned and 688 burned Abies concolor (Gord. & Glend.) Lindl. (white fir) in the Sierra Nevada of California, U.S.A., indicated that growth rate was a significant predictor of mortality in the unburned stands, while both crown scorch and radial growth were significant predictors of mortality in the burned stands. Applying the burned stand model to unburned stands resulted in an overestimation of the unburned stand mortality rate. While failing to create a general model of tree death for A. concolor, our findings underscore the idea that similar processes may affect mortality in disturbed and undisturbed stands.
    Full-text · Article · Feb 2011 · Canadian Journal of Forest Research
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    Phillip J. van Mantgem · Nathan L Stephenson
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    ABSTRACT: We assess the potential of increment coring, a common method for measuring tree ages and growth, to contribute to mortality. We used up to 21 years of annual censuses from two cored and two uncored permanent plots in the Sierra Nevada of California, to detect changes in mortality rates 12 years following coring for individuals >5 cm DBH from two coniferous species, Abies concolor (Gordon & Glend.) Lindl. (white fir) and Abies magnifica A. Murr. (red fir). Using a randomized before-after control impact (BACI) design, we found no differences in mortality rates following coring for 825 cored and 525 uncored A. concolor and 104 cored and 66 uncored A. magnifica. These results support the view that collecting tree cores can be considered nondestructive sampling, but we emphasize that our 12-year postcoring records are short compared with the maximum life-span of these trees and that other species in different environments may prove to be more sensitive to coring.
    Full-text · Article · Feb 2011 · Canadian Journal of Forest Research
  • N. B. English · N. McDowell · C. D. Allen · A. J. Das · C. I. Mora · N. L. Stephenson
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    ABSTRACT: Increasing rates of tree mortality in the western United States have potentially profound effects on native biodiversity and ecosystem structure and function. Regional drought and insect or pathogen outbreaks have been implicated; however, the underlying cause(s) of recent tree mortality remain unclear. Hypotheses include 1) hydraulic failure; 2) carbon starvation; or 3) increased susceptibility to pests due to (1) and/or (2). We examined evidence for and against these hypotheses by measuring patterns in the delta13C and delta18O of tree-rings from the last 50 years in eight species of isohydric and anisohydric trees, both living and dead, from across the western United States. We compared isotopic compositions of delta13C and delta18O between living and dead trees with models of expected isotope responses to hydraulic failure, carbon starvation and pest infestation to eliminate or support specific mechanisms of tree mortality. Our data thus far show that Pinaceae (including Abies, Picea, Pinus, and Psuedotsuga) respond isohydrically to drought, while Cupressaceae (including Calocedrus, Juniperus, and Sequoiadendron) respond anisohydrically to drought. We expect that in water-limited forests, Pinaceae will exhibit more rapid and pronounced increases in delta13C and delta18O, attributable to water stress, than in Cupressaceae. However, we observe variable results within Pinaceae and between live and dead trees within a given species. For example, in Piñon from various sites in New Mexico, dead trees had either higher or lower delta13C in the years preceding their death than living trees at the same site. The isotopic patterns associated with mortality varied between energy-limited (Oregon, Washington, high-elevation Colorado and California) and water-limited (low-elevation Colorado and New Mexico) forests in a predictable manner. We expect dying trees in energy-limited systems to show changes in delta13C but not in delta18O. Our preliminary conclusions are that either: 1) multiple mechanisms of mortality occur across the western United States; or 2) similar mechanisms may result in differing isotopic patterns between sites and species.
    No preview · Article · Dec 2010
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    Full-text · Article · Nov 2010 · Frontiers in Ecology and the Environment
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    ABSTRACT: One of the greatest uncertainties in global environmental change is predicting changes in feedbacks between the biosphere and the Earth system. Terrestrial ecosystems and, in particular, forests exert strong controls on the global carbon cycle and influence regional hydrology and climatology directly through water and surface energy budgets [Bonan, 2008; Chapin etal., 2008]. According to new research, tree mortality associated with elevated temperatures and drought has the potential to rapidly alter for-est ecosystems, potentially affecting feed-backs to the Earth system [Allen etal., 2010]. Several lines of recent research demonstrate how tree mortality rates in forests may be sen-sitive to climate change— particularly warm-ing and drying. This emerging consequence of global change has important effects on Earth system processes (Figure1). Observations and Patterns of Tree Mortality Reports of tree mortality associated with heat and drought from around the world have increased in the past decade, and although each cannot be conclusively linked to climate change, they collectively illustrate the vulnerability of many forested ecosys-tems to rapid increases in tree mortality due to warmer temperatures and more severe drought [Allen etal., 2010]. Recent examples include extensive "die-offs" in which high proportions of trees die at regional scales [Breshears etal., 2005]. In the southwestern United States, wide-spread drought and insect-driven mortality of piñon pine in the early 2000s affected more than 12,000 square kilometers in less than 3years, killing 40–97% of those trees at some sites [Breshears etal., 2005; McDowell etal.,
    Full-text · Article · Apr 2010 · Eos Transactions American Geophysical Union
  • Nathan L. Stephenson
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    ABSTRACT: Background/Question/Methods Management policies for protected areas (parks and wildernesses) in the United States usually direct natural resource managers to restore and maintain naturally-functioning ecosystems. When this is not possible, managers are directed to maintain the closest approximation of the natural condition. However, in the face of rapid global changes these management directions pose significant challenges. What is “natural?” What role might resilience play in maintaining naturalness? How does one maintain naturalness when future environmental conditions are expected to have no analog in the past? Has the concept of naturalness lost its value? Supported by examples from literature, I examine whether current management policies for protected areas are sensible (or even possible), and offer ideas for moving forward. Results/Conclusions Naturalness in the sense of historical fidelity (conditions as they were before the advent of modern technological society) will almost certainly become impossible to maintain. In fact, efforts to maintain a semblance of historical fidelity might result in ecosystems that are inherently unstable to novel environmental conditions. On the other hand, naturalness in the sense of freedom from intentional human intervention will remain possible, but may be undesirable if accompanied by the threat of sudden, catastrophic loss of native biodiversity or key ecosystem functions. Thus, impetus for management intervention may be high. However, precise characterization of appropriate management interventions and desired future conditions will be confounded by (1) the unprecedented nature of environmental changes (meaning we will have no precise reference analogs in the past), and (2) the unpredictability of future environmental and biotic changes at spatial and temporal scales useful for management. Managing for ecosystem resilience may be useful less as a desired end in its own right, and more as a means of buying time while a range of adaptation options are explored and implemented.
    No preview · Conference Paper · Aug 2009

Publication Stats

3k Citations
232.66 Total Impact Points

Institutions

  • 2008-2015
    • United States Geological Survey
      • • Sequoia and Kings Canyon Field Station
      • • Western Ecological Research Center
      Reston, Virginia, United States
  • 2005
    • USGS National Wetlands Research Center
      Lafayette, Louisiana, United States
  • 1987
    • Cornell University
      Ithaca, New York, United States