Nathan L. Stephenson's research while affiliated with United States Geological Survey and other places
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Publications (91)
The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on...
In some areas burned by recent wildfires, most or all giant sequoias were killed. Sequoia managers wish to know whether post-fire seedling establishment in those areas has been adequate to regenerate the locally extirpated sequoias. To provide a yardstick for interpreting sequoia seedling densities measured after the recent severe wildfires, here w...
Fire is a critical driver of giant sequoia (Sequoiadendron giganteum [Lindl.] Buchholz) regeneration. However, fire suppression combined with the effects of increased temperature and severe drought have resulted in fires of an intensity and size outside of the historical norm. As a result, recent mega-fires have killed a significant portion of the...
The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundat...
Lack of tree fecundity data across climatic gradients precludes the analysis of how seed supply contributes to global variation in forest regeneration and biotic interactions responsible for biodiversity. A global synthesis of raw seedproduction data shows a 250‐fold increase in seed abundance from cold‐dry to warm‐wet climates, driven primarily by...
COVER PHOTO: Bark beetles, such as the Dendroctonus valens seen here, cause elevated tree mortality when forests are stressed by drought or fire. Furniss et al. (this issue; Article e2507; doi: 10.1002/eap.2507) investigated interactions between beetles, wildfire, and drought, demonstrating that tree crowding amplifies disturbance interactions and...
Significance
Suitable habitats for forest trees may be shifting fast with recent climate change. Studies tracking the shift in suitable habitat for forests have been inconclusive, in part because responses in tree fecundity and seedling establishment can diverge. Analysis of both components at a continental scale reveals a poleward migration of nor...
In an emerging era of megadisturbance, bolstering forest resilience to wildfire, insects, and drought has become a central objective in many western forests. Climate has received considerable attention as a driver of these disturbances, but few studies have examined the complexities of climate–vegetation–disturbance interactions. Current strategies...
Prescribed fire reduces fire hazards by removing dead and live fuels (small trees and shrubs). Reductions in forest density following prescribed fire treatments (often in concert with mechanical treatments) may also lessen competition so that residual trees might be more likely to survive when confronted with additional stressors, such as drought....
Between 2012 and 2016, California suffered one of the most severe droughts on record. During this period Sequoiadendron giganteum (Giant Sequoias) in the Sequoia and Kings Canyon National Parks (SEKI), California USA experienced canopy water content (CWC) loss, unprecedented foliage senescence, and, in a few cases, death. We present an assessment o...
A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-22025-2
Widespread tree mortality following droughts has emerged as an environmentally and economically devastating ‘ecological surprise’. It is well established that tree physiology is important in understanding drought-driven mortality; however, the accuracy of predictions based on physiology alone has been limited. We propose that complicating factors a...
Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here...
Reproduction is a key component of ecological resilience in forest ecosystems, so understanding how seed production is influenced by extreme drought is key to understanding forest recovery trajectories. If trees respond to mortality-inducing drought by preferentially allocating resources for reproduction, the recovery of the stand to pre-drought co...
Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by incre...
Invasive pathogens and bark beetles have caused precipitous declines of various tree species around the globe. Here, we characterized long-term patterns of mountain pine beetle (Dendroctonus pon-derosae; MPB) attacks and white pine blister rust, an infectious tree disease caused by the pathogen, Cronar-tium ribicola. We focused on four dominant whi...
Conifer mortality rates are increasing in western North America, but the physiological mechanisms underlying this trend are not well understood.
We examined tree‐ring‐based radial growth along with stable carbon (C) and oxygen (O) isotope composition ( δ ¹³ C and δ ¹⁸ O, respectively) of dying and surviving conifers at eight old‐growth forest sites...
Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire a...
Blue oak woodlands in California have been a focus of conservation concern for many years. Numerous studies have found that existing seedling and sapling numbers are inadequate to sustain current populations, and recent work has suggested that blue oak woodlands might be particularly vulnerable to a warming climate. California has recently experien...
Fire severity in forests is often defined in terms of post-fire tree mortality, yet the influences on tree mortality following fire are not fully understood. Pre-fire growth may serve as an index of vigour, indicating resource availability and the capacity to recover from injury and defend against pests. For trees that are not killed immediately by...
Understanding the response of forests to climate change is important for predicting changes in biodiversity and ecosystem services, including carbon storage. Seedlings represent a key demographic stage in these responses, because seedling establishment is necessary for population persistence and spread, and because the conditions allowing seedlings...
During drought, the tree subpopulations (such as size or vigour classes) that suffer disproportionate mortality can be conceptually arrayed along a continuum defined by the actions of biotic agents, particularly insects. At one extreme, stress dominates: insects are absent or simply kill the most physiologically stressed trees. At the opposite extr...
Tree mortality is an important outcome of many forest fires. Extensive tree injuries from fire may lead directly to mortality, but environmental and biological stressors may also contribute to tree death. However, there is little evidence showing how the combined effects of two common stressors, drought and competition, influence post-fire mortalit...
Hotter droughts are becoming more common as climate change progresses, and they may already have caused instances of forest dieback on all forested continents. Learning from hotter droughts, including where on the landscape forests are more or less vulnerable to these events, is critical to help resource managers proactively prepare for the future....
California experienced severe drought from 2012 to 2016, and there were visible changes in the forest canopy throughout the State. In 2014, unprecedented foliage dieback was recorded in giant sequoia (Sequoiadendron giganteum) trees in Sequoia National Park, in the southern California Sierra Nevada mountains. Although visible changes in sequoia can...
Ecologists who specialize in translational ecology (TE) seek to link ecological knowledge to decision making by integrating ecological science with the full complement of social dimensions that underlie today's complex environmental issues. TE is motivated by a search for outcomes that directly serve the needs of natural resource managers and decis...
Recent drought (2012-2016) caused unprecedented foliage dieback in giant sequoias (Sequoiadendron giganteum), a species endemic to the western slope of the southern Sierra Nevada in central California. As part of an effort to understand and map sequoia response to droughts, we studied the patterns of remotely sensed canopy water content (CWC), both...
Hotter droughts - droughts in which unusually high temperatures exacerbate the effects of low precipitation - are expected to increase in frequency and severity in coming decades, challenging scientists and managers to identify which parts of forested landscapes may be most vulnerable. In 2014, in the middle of California's historically unprecedent...
Severe drought has the potential to cause selective mortality within a forest, thereby inducing shifts in forest species composition. The southern Sierra Nevada foothills and mountains of California have experienced extensive forest dieback due to drought stress and insect outbreak. We used high-fidelity imaging spectroscopy (HiFIS) and light detec...
The drivers of background tree mortality rates – the typical low rates of tree mortality found in forests in the absence of acute stresses like drought – are central to our understanding of forest dynamics, the effects of ongoing environmental changes on forests, and the causes and consequences of geographical gradients in the nature and strength o...
Prescribed fire is a primary tool used to restore western forests following more than a century of fire exclusion, reducing fire hazard by removing dead and live fuels (small trees and shrubs). It is commonly assumed that the reduced forest density following prescribed fire also reduces competition for resources among the remaining trees, so that t...
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 unc...
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 la...
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...
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...
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,...
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 scale...
Projected Changes in Mortality Rate for predominantly Energy-Limited Sierra Nevada Conifer Forests (GFDL B1, Hypothetical). Mapped projections of average relative changes in mortality rate for the years 2090 to 2099 for energy-limited coniferous forests (≥2450 m) of California’s Sierra Nevada, using exponential models and the GFDL B1 emissions mode...
Projected Changes in Mortality Rate for Sierra Nevada Conifer Forests (PCM A2, Hypothetical). Mapped projections of average relative changes in mortality rate for the years 2090 to 2099 for coniferous forests of California’s Sierra Nevada, using exponential and the PCM A2 emissions model. Elevations generally increase from left to right. (A) Change...
Plot Details. Characteristics of the 21 forest plots used for model development.
(DOC)
Projected Changes in Mortality Rate for Sierra Nevada Conifer Forests (PCM B1, Hypothetical). Mapped projections of average relative changes in mortality rate for the years 2090 to 2099 for coniferous forests of California’s Sierra Nevada, using exponential and the PCM B1 emissions model. Elevations generally increase from left to right. (A) Change...
Projected Changes in Mortality Rate for Sierra Nevada Conifer Forests (GFDL B1, Hypothetical). Mapped projections of average relative changes in mortality rate for the years 2090 to 2099 for coniferous forests of California’s Sierra Nevada, using exponential and the GFDL B1 emissions model. Elevations generally increase from left to right. (A) Chan...
Plot Mortality Data. Counts of mortalities and live trees for each plot for each census year.
(DOC)
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 lo...
Forecasted change in Sierra Nevada climate through time from different model scenarios. Climate data is averaged by decade for all 33,594 gridpoints. Gap between standard error bars for each point was too small to distinguish so they have not been included. A) temperature forecasts; B) precipitation forecasts; C) climatic water deficit forecasts.
(...
Projected Changes in Mortality Rate for predominantly Energy-Limited Sierra Nevada Conifer Forests (PCM B1, Hypothetical). Mapped projections of average relative changes in mortality rate for the years 2090 to 2099 for energy-limited coniferous forests (≥2450 m) of California’s Sierra Nevada, using exponential models and the PCM B1 emissions model....
Tree Size Distribution for Each Plot. Shows the size distribution of trees in each of the A) water-limited and B) energy-limited plots. The y-axis is given in a log base 10 scale.
(TIF)
Projected Changes in Mortality Rate for predominantly Energy-Limited Sierra Nevada Conifer Forests (PCM A2, Hypothetical). Mapped projections of average relative changes in mortality rate for the years 2090 to 2099 for energy-limited coniferous forests (≥2450 m) of California’s Sierra Nevada, using exponential models and the PCM A2 emissions model....
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...
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 a...
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 envi...
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...
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...
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 immedia...
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 co...
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 deter...
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) c...
The major challenge to stewardship of protected areas is to decide where, when, and how to intervene in physical and biological processes, to conserve what we value in these places. To make such decisions, planners and managers must articulate more clearly the purposes of parks, what is valued, and what needs to be sustained. A key aim for conserva...
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...
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...
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 ra...
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 ra...
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...
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...
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...
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...
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 a...
Summary 1 We assess the use of simple, size-based matrix population models for projecting population trends for six coniferous tree species in the Sierra Nevada, California. We used demographic data from 16 673 trees in 15 permanent plots to create 17 separate time-invariant, density-independent population projection models, and determined differen...
Using a global database, we found that forest turnover rates (the average of tree mortality and recruitment rates) parallel broad-scale patterns of net primary productivity. First, forest turnover was higher in tropical than in temperate forests. Second, as recently demonstrated by others, Amazonian forest turnover was higher on fertile than infert...
An introduced pathogen, white pine blister rust (Cronartium ribicola), has caused declines in five-needled pines throughout North America. Simultaneously, fire ex- clusion has resulted in dense stands in many forest types, which may create additional stress for these generally shade-intolerant pines. Fire exclusion also allows fuels to accu- mulate...
THE GIANT SEQUOIA NATIONAL MONUMENT SCIENTIFIC ADVISORY BOARD
Final Report
In the Beginning
On April 15, 2000 the President ofthe United States issued a Proclamation in which he declared , "The rich and varied landscape ofthe Giant Sequoia National Monument holds a diverse array of scientific and historic resources. Magnificent groves oftowering gi...
