Nathan L. Stephenson

United States Geological Survey, Reston, Virginia, United States

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Publications (41)190.21 Total impact

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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.
    Nature 01/2014; · 42.35 Impact Factor
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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.
    PLoS ONE 07/2013; 8(7):e69917. · 3.53 Impact Factor
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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 ...
    Ecological Monographs 11/2011; 81(4):527-555. · 7.11 Impact Factor
  • 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.
    Science 10/2011; 334(6053):177; author reply 177. · 31.48 Impact Factor
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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.
    Forest Ecology and Management 04/2011; 261:1203-1213. · 2.67 Impact Factor
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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.
    Fuel and Energy Abstracts 03/2011; 261(6):989-994.
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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 con-tribute 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 differ-ent environments may prove to be more sensitive to coring. Résumé : Les auteurs ont évalué la possibilité que la méthode de sondage qui consiste à prélever une carotte de bois dans le tronc, une méthode courante pour mesurer l'âge et la croissance des arbres, contribue à la mortalité. Ils ont uti-lisé jusqu'à 21 années de relevés annuels provenant de deux places-échantillons permanentes où les arbres avaient été sondés et deux autres où les arbres n'avaient pas été sondés dans la Sierra Nevada en Californie. L'objectif consistait à détecter les variations dans le taux de mortalité 12 ans après le sondage de tiges de plus de 5 cm au DHP appartenant à deux espèces de conifères : Abies concolor (Gordon & Glend.) Lindl. (sapin concolore) et Abies magnifica A. Murr. (sapin rouge). À l'aide d'un dispositif aléatoire de contrôle d'impact avant-après (BACI), nous n'avons observé aucune différence dans le taux de mortalité de 825 tiges sondées et 525 tiges non sondées d'A. concolor, et de 104 tiges son-dées et 66 tiges non sondées d'A. magnifica. Ces résultats supportent le point de vue que le prélèvement de carottes chez les arbres peut être considéré comme un échantillonnage non destructif. Cependant, nous insistons sur le fait que nos relevés, qui vont jusqu'à 12 ans après qu'il y ait eu des sondage, couvrent une courte période comparativement à la durée de vie maximum de ces arbres et que d'autres espèces dans différentes conditions environnementales pour-raient s'avérer plus sensibles au prélèvement de carottes.
    Canadian Journal of Forest Research 02/2011; 34(11). · 1.66 Impact Factor
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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.
    Canadian Journal of Forest Research 02/2011; 33(6):1029-1038. · 1.66 Impact Factor
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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.
    AGU Fall Meeting Abstracts. 12/2010;
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    Frontiers in Ecology and the Environment 11/2010; 8(9). · 8.41 Impact Factor
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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.,
    Eos Transactions American Geophysical Union 04/2010; 91:153-154.
  • 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.
    94th ESA Annual Convention 2009; 08/2009
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    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.
    Science 02/2009; 323(5913):521-4. · 31.48 Impact Factor
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    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.
    Science. 01/2009; 323:521-524.
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    ABSTRACT: For many species of long-lived organisms, such as trees, survival appears to be the most critical vital rate affecting population persistence. However, methods commonly used to quantify tree death, such as relating tree mortality risk solely to diameter growth, almost certainly do not account for important spatial processes. Our goal in this study was to detect and, if present, to quantify the relevance of such processes. For this purpose, we examined purely spatial aspects of mortality for four species, Abies concolor, Abies magnifica, Calocedrus decurrens, and Pinus lambertiana, in an old-growth conifer forest in the Sierra Nevada of California, USA. The analysis was performed using data from nine fully mapped long-term monitoring plots. In three cases, the results unequivocally supported the inclusion of spatial information in models used to predict mortality. For Abies concolor, our results suggested that growth rate may not always adequately capture increased mortality risk due to competition. We also found evidence of a facilitative effect for this species, with mortality risk decreasing with proximity to conspecific neighbors. For Pinus lambertiana, mortality risk increased with density of conspecific neighbors, in keeping with a mechanism of increased pathogen or insect pressure (i.e., a Janzen-Connell type effect). Finally, we found that models estimating risk of being crushed were strongly improved by the inclusion of a simple index of spatial proximity. Not only did spatial indices improve models, those improvements were relevant for mortality prediction. For P. lambertiana, spatial factors were important for estimation of mortality risk regardless of growth rate. For A. concolor, although most of the population fell within spatial conditions in which mortality risk was well described by growth, trees that died occurred outside those conditions in a disproportionate fashion. Furthermore, as stands of A. concolor become increasingly dense, such spatial factors are likely to become increasingly important. In general, models that fail to account for spatial pattern are at risk of failure as conditions change.
    Ecology 07/2008; 89(6):1744-56. · 5.00 Impact Factor
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    Constance I Millar, Nathan L Stephenson, Scott L Stephens
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    ABSTRACT: We offer a conceptual framework for managing forested ecosystems under an assumption that future environments will be different from present but that we cannot be certain about the specifics of change. We encourage flexible approaches that promote reversible and incremental steps, and that favor ongoing learning and capacity to modify direction as situations change. We suggest that no single solution fits all future challenges, especially in the context of changing climates, and that the best strategy is to mix different approaches for different situations. Resources managers will be challenged to integrate adaptation strategies (actions that help ecosystems accommodate changes adaptively) and mitigation strategies (actions that enable ecosystems to reduce anthropogenic influences on global climate) into overall plans. Adaptive strategies include resistance options (forestall impacts and protect highly valued resources), resilience options (improve the capacity of ecosystems to return to desired conditions after disturbance), and response options (facilitate transition of ecosystems from current to new conditions). Mitigation strategies include options to sequester carbon and reduce overall greenhouse gas emissions. Priority-setting approaches (e.g., triage), appropriate for rapidly changing conditions and for situations where needs are greater than available capacity to respond, will become increasingly important in the future.
    Ecological Applications 01/2008; 17(8):2145-51. · 4.13 Impact Factor
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    Phillip J van Mantgem, Nathan L Stephenson
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    ABSTRACT: 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.
    Ecology Letters 11/2007; 10(10):909-16. · 13.04 Impact Factor
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    ABSTRACT: We examined mortality of Abies concolor (Gord. & Glend.) Lindl. (white fir) and Pinus lambertiana Dougl. (sugar pine) by developing logistic models using three growth indices obtained from tree rings: average growth, growth trend, and count of abrupt growth declines. For P. lambertiana, models with average growth, growth trend, and count of abrupt declines improved overall prediction (78.6% dead trees correctly classified, 83.7% live trees correctly classified) compared with a model with average recent growth alone (69.6% dead trees correctly classified, 67.3% live trees correctly classified). For A. concolor, counts of abrupt declines and longer time intervals improved overall classification (trees with DBH ≥20 cm: 78.9% dead trees correctly classified and 76.7% live trees correctly classified vs. 64.9% dead trees correctly classified and 77.9% live trees correctly classified; trees with DBH <20 cm: 71.6% dead trees correctly classified and 71.0% live trees correctly classified vs. 67.2% dead trees correctly classified and 66.7% live trees correctly classified). In general, count of abrupt declines improved live-tree classification. External validation of A. concolor models showed that they functioned well at stands not used in model development, and the development of size-specific models demonstrated important differences in mortality risk between understory and canopy trees. Population-level mortality-risk models were developed for A. concolor and generated realistic mortality rates at two sites. Our results support the contention that a more comprehensive use of the growth record yields a more robust assessment of mortality risk.
    Canadian Journal of Forest Research 05/2007; 37(3):580-597. · 1.66 Impact Factor
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    Phillip J. van Mantgem, Nathan L. Stephenson, Jon E. Keeley
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    ABSTRACT: To elucidate broad-scale environmental controls of coniferous forest reproduction in the Sierra Nevada, California, we monitored reproduction for 5 years in 47 plots arrayed across a steep elevational (climatic) gradient. We found that both absolute seedling densities (stems < 1.37 m) and seedling densities relative to overstory parent tree basal area declined sharply with elevation. Rates of seedling turnover (the average of birth and death rates) also declined with elevation. In contrast, seed production was not predicted by elevation and was highly variable from year to year. During a mast year of seed production, the intensity of masting was uneven among plots. Seedling densities were elevated only during the single year immediately following the mast year, suggesting reproduction in our forests may be primarily limited by abiotic factors such as the availability of suitable sites and weather. Disturbance also clearly affected reproduction; plots that had recently burned had significantly higher seedling to parent tree ratios for Abies species, suggesting that even though established Abies concolor may be relatively susceptible to fire, the species can recover rapidly through prolific reproduction. Since reproductive failures may be our earliest signal of changing forest conditions, seedling dynamics could provide a sensitive, if variable, indicator of environmental changes.
    Forest Ecology and Management 04/2006; · 2.67 Impact Factor

Publication Stats

1k Citations
190.21 Total Impact Points

Institutions

  • 2007–2013
    • United States Geological Survey
      • Sequoia and Kings Canyon Field Station
      Reston, Virginia, United States
  • 2008
    • University of California, Berkeley
      • Department of Environmental Science, Policy, and Management
      Berkeley, MO, United States
  • 2005
    • USGS National Wetlands Research Center
      Lafayette, Louisiana, United States
  • 2000
    • Duke University
      • Nicholas School of the Environment
      Durham, North Carolina, United States
  • 1987
    • Cornell University
      Ithaca, New York, United States
  • 1979
    • University of California, Irvine
      • Department of Ecology and Evolutionary Biology
      Irvine, CA, United States