I am an Ecologist with the U.S. Geological Survey's Western Ecological Research Center, stationed at Sequoia and Kings Canyon National Parks. I study tree dynamics with a particular focus on tree mortality processes. My work is aimed at improving our understanding of western forests and increasing our capacity to assess how they will be affected by a changing climate.
RESEARCH INTERESTS
- Forest ecology
- Forest dynamics
- Tree mortality
- Tree growth
- Climatic change
Education and Certifications
PhD from University of California at Berkeley, Forest Ecology 2007
BS from University of Michigan, Chemistry and Cellular Molecular Biology, 1995
Science and Products
Filter Total Items: 44
Climate change risks to global forest health: Emergence of unexpected events of elevated tree mortality worldwide
Recent observations of elevated tree mortality following climate extremes, like heat and drought, raise concerns about climate change risks to global forest health. We currently lack both sufficient data and understanding to identify whether these observations represent a global trend towards increasing tree mortality. Here, we document events of sudden and unexpected elevated tree mortality follo
Empirically validated drought vulnerability mapping in the mixed conifer forests of the Sierra Nevada
Severe droughts are predicted to become more frequent in the future, and the consequences of such droughts on forests can be dramatic, resulting in massive tree mortality, rapid change in forest structure and composition, and substantially increased risk of catastrophic fire. Forest managers have tools at their disposal to try to mitigate these effects but are often faced with limited resources, f
Crowding, climate, and the case for social distancing among trees
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 for creating resilient forests often rely on retr
Forest resistance to extended drought enhanced by prescribed fire in low elevation forests of the Sierra Nevada
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. The current evidence for these effects is mixed an
Nonlinear shifts in infectious rust disease due to climate change
Range shifts of infectious plant disease are expected under climate change. As plant diseases move, emergent abiotic-biotic interactions are predicted to modify their distributions, leading to unexpected changes in disease risk. Evidence of these complex range shifts due to climate change, however, remains largely speculative. Here, we combine a long-term study of the infectious tree disease, whit
Mapping the vulnerability of giant sequoias after extreme drought in California using remote sensing
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 of the vulnerability of giant sequoia populations
Continent-wide tree fecundity driven by indirect climate effects
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 we find from a synthesis of tree species in North
Why is tree drought mortality so hard to predict?
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 at two levels stymie predictions of drought-driven
Seed production patterns of surviving Sierra Nevada conifers show minimal change following drought
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 conditions may be enhanced accordingly. We used a 20
Effects of postfire climate and seed availability on postfire conifer regeneration
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 increasingly hot and dry conditions. To better inform p
Compounding effects of white pine blister rust, mountain pine beetle, and fire threaten four white pine species
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 ponderosae; MPB) attacks and white pine blister rust, an infectious tree disease caused by the pathogen, Cronartium ribicola. We focused on four dominant white pine host species in Sequoia and Kings Canyon Nat
Mortality predispositions of conifers across western USA
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 (δ13C and δ18O, respectively) of dying and surviving conifers at eight old‐growth forest sites across a strong moisture gradient in the western USA to
Forest health and drought response
Forests provide society with economically important and often irreplaceable goods and services, such as wood products, carbon sequestration, clean water, biodiversity, and recreational opportunities. Yet hotter droughts (droughts in which unusually high temperatures exacerbate the effects of low precipitation) are projected to increase in frequency and intensity in coming decades, potentially...
Post-Fire Conifer Regeneration Under a Warming Climate: Will Severe Fire Be a Catalyst for Forest Loss?
The Southwest U.S. is experiencing hotter droughts, which are contributing to more frequent, severe wildfires. These droughts also stress vegetation, which can make it more difficult for forests to recover after fire. Forest regeneration in burned areas may be limited because seeds have to travel long distances to recolonize, and when they do arrive, conditions are often unfavorably hot and dry. C
Sequoia and Kings Canyon Field Station
The Sequoia and Kings Canyon Field Station is home to research programs that focus on wildfire patterns in Southern California, and the effects of drought on Sierra Nevada forests. Select the "Science" tab for a more comprehensive summary.
Leaf to Landscape: Understanding and Mapping the Vulnerability of Forests to Hotter Droughts
Forests across the southwestern U.S. are crucial components of recreation and play an important role in state and local economies. Healthy forests also provide needed habitat for many wildlife species and contribute many other important services to our planet. “Hotter droughts” (otherwise normal droughts whose effects on ecosystems are exacerbated by higher temperatures) are an emerging climate ch...
Can Prescribed Fire Help Forests Survive Drought in the Sierra Nevada Mountains?
In 2017, California was experiencing its most severe drought in over a millennia. Low rainfall and record high temperatures resulted in increased tree mortality and complete forest diebacks across the West. Though land managers scrambled to respond, they lacked information needed to make informed decisions. Focusing on California’s central and southern Sierra Nevada Mountains, this project seeks t...
Fighting Drought with Fire: A Comparison of Burned and Unburned Forests in Drought-Impacted Areas of the Southwest
Drought is one of the biggest threats facing our forests today. In the western U.S., severe drought and rising temperatures have caused increased tree mortality and complete forest diebacks. Forests are changing rapidly, and while land managers are working to develop long-term climate change adaptation plans, they require tools that can enhance forest resistance to drought now. To address this imm
Characterizing a link in the terrestrial carbon cycle: a global overview of individual tree mass growth
Forests sequester the majority of the terrestrial biosphere’s carbon and are key components of the global carbon cycle, potentially contributing substantial feedbacks to ongoing climatic changes. It is therefore remarkable that no consensus yet exists about the fundamental nature of tree mass growth (and thus carbon sequestration rate). Specifically, does tree mass growth rate increase, decrease,
Calibration and Validation Data and Model Coefficients for Mixed Conifer Vulnerability Project from Sequoia and Kings Canyon National Park 2015 to 2019
These datasets contain the attributes of individual trees located in and around Sequoia and Kings Canyon national parks. Attributes include remote sensing indices, terrain characteristics, and-- for the calibration data-- tree size and growth rates. Calibration data (mixedconifervulnerability_calibrationdataset.csv) were collected in long term research plots where trees are visited annually to che
Sequoia and Yosemite National Parks Mortality and Fire Data (1990-2019) for Competition-Fire-Drought Interaction Analysis
This dataset records mortality-- including involvement of bark beetles-- and burn severity information for trees in long term forest dynamics plots in Sequoia National Park and Yosemite National Park that experienced fire.
These data support the following publication:
Furniss, T.J., Das, A.J., van Mantgem, P.J., Stephenson, N.L. and Lutz, J.A., 2021. Crowding, climate, and the case for soci
Data for Use in poscrptR Post-fire Conifer Regeneration Prediction Model
These data support poscrptR (Wright et al. 2021). poscrptR is a shiny app that predicts the probability of post-fire conifer regeneration for fire data supplied by the user. The predictive model was fit using presence/absence data collected in 4.4m radius plots (60 square meters). Please refer to Stewart et al. (2020) for more details concerning field data collection, the model fitting process, an
Forest Structure Data for Burned and Unburned Sites at Sequoia and Kings Canyon National Parks
These data represent tree diameter, species, mortality status, and plot attributes from 164 plots in Sequoia and Kings Canyon national parks, California. Plots were matched with local records of recent fire history to determine burn status and year burned if applicable.
These data support the following publication:
van Mantgem, P.J., Caprio, A.C., Stephenson, N.L. and Das, A.J., 2021. Forest Re
Post-fire conifer regeneration observations for National Forest land in California (2009 - 2017)
This data consists of presence/absence observations for post-fire conifer regeneration. The data also includes estimates of plot-level topography (slope, aspect), relativized differenced normalized burn ratio (RdNBR), post-fire climate, live basal area, and seed rain.
These data support the following publication:
Seed and Associated Tree Data from Long Term Research Plots in Sequoia and Yosemite national parks
This dataset was used as part of a continent-wide analysis of tree fecundity and its association with climate and tree size. This dataset consists of: plotinfo.csv, which contains basic attribute information for the field plots where the data were collected; seeddata.csv, which contains the data for seeds collected (how many, what type, etc.); trapxycoord.csv, which contains location and identific
Tree mortality in blue oak woodlands in Sequoia National Park during the extreme drought
These data were collected to quantify oak mortality during the 2014-2017 drought. 30 tenth hectare drought plots were established throughout the blue oak woodland of Sequoia National Park. Information on species, size, and mortality were collected for all standing trees in each plot. In addition, two long-term 2.25 ha plots were established, in which the same data were collected.
Seedling and tree data from Sequoia National Park and Yosemite National Park
These tables include data from 25 long-term forest plots located in either Sequoia or Yosemite national park. Trees in these plots (established between 1982 and 2001) are censused annually for mortality and measured for diameter every 4 to 6 years. Plots were mostly 1 hectare (ha) in size (range 0.9 - 2.5 ha) and contained at least two 25 by 25 meter seedling sub-plots to monitor natural seedling
Tree mortality in Sequoia National Park from 2004 to 2007 and during severe drought in 2014 to 2017
These data describe tree mortality and the factors associated with tree mortality for a variety of plots in Sequoia National Park. Most of the data were collected between 2014 and 2017 (during an extremely severe drought), along with some comparison data from 2004 to 2007.
These data support the following publication:
Stephenson, N.L., Das, A.J., Ampersee, N.J., Bulaon, B.M., and Yee, J.L.,
Sequoia foliage dieback data from Sequoia National Park
These datasets provide spatially-explicit estimates of the magnitude of giant sequoia foliage dieback along selected trail corridors in Sequoia and Kings Canyon national parks, California, from 2014 through 2017. They additionally provide giant sequoia tree-ring measurements, through the year 1989, for two locations in the Giant Forest grove, Sequoia National Park, California.
Mortality factors for dead trees from a subset of plots from the Sierra Nevada Forest Dynamics Plot Network from 1998 to 2010
This dataset was used to summarize and analyze the mortality factors recorded on dead trees in the Sierra Nevada Forest Dynamics Plot Network, which is managed by the Sequoia and Kings Canyon Field station of the U.S. Geological Survey's Western Ecological Research Center. Each row of the dataset represents an individual dead tree. These are dead trees that were recorded in the network from 1998 t
poscrptR
poscrptR is a simple R package with the sole purpose of distributing a shiny app
for predicting post-fire conifer regeneration. Learn more about shiny apps here.
Seed source, not drought, determines patterns of seed production in Sierra Nevada conifers
This release consists of data collected from 26 plots in two national parks over a 19-year period. The data consists of plot-level seed counts for three genera, number of seed traps, live tree basal area, plot area, and climate metrics from the gridmet gridded data set, the daymet gridded data set, the PRISM gridded data set, and two nearby COOP stations.
Science and Products
- Publications
Filter Total Items: 44
Climate change risks to global forest health: Emergence of unexpected events of elevated tree mortality worldwide
Recent observations of elevated tree mortality following climate extremes, like heat and drought, raise concerns about climate change risks to global forest health. We currently lack both sufficient data and understanding to identify whether these observations represent a global trend towards increasing tree mortality. Here, we document events of sudden and unexpected elevated tree mortality folloEmpirically validated drought vulnerability mapping in the mixed conifer forests of the Sierra Nevada
Severe droughts are predicted to become more frequent in the future, and the consequences of such droughts on forests can be dramatic, resulting in massive tree mortality, rapid change in forest structure and composition, and substantially increased risk of catastrophic fire. Forest managers have tools at their disposal to try to mitigate these effects but are often faced with limited resources, fCrowding, climate, and the case for social distancing among trees
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 for creating resilient forests often rely on retrForest resistance to extended drought enhanced by prescribed fire in low elevation forests of the Sierra Nevada
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. The current evidence for these effects is mixed anNonlinear shifts in infectious rust disease due to climate change
Range shifts of infectious plant disease are expected under climate change. As plant diseases move, emergent abiotic-biotic interactions are predicted to modify their distributions, leading to unexpected changes in disease risk. Evidence of these complex range shifts due to climate change, however, remains largely speculative. Here, we combine a long-term study of the infectious tree disease, whitMapping the vulnerability of giant sequoias after extreme drought in California using remote sensing
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 of the vulnerability of giant sequoia populationsContinent-wide tree fecundity driven by indirect climate effects
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 we find from a synthesis of tree species in NorthWhy is tree drought mortality so hard to predict?
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 at two levels stymie predictions of drought-drivenSeed production patterns of surviving Sierra Nevada conifers show minimal change following drought
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 conditions may be enhanced accordingly. We used a 20Effects of postfire climate and seed availability on postfire conifer regeneration
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 increasingly hot and dry conditions. To better inform pCompounding effects of white pine blister rust, mountain pine beetle, and fire threaten four white pine species
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 ponderosae; MPB) attacks and white pine blister rust, an infectious tree disease caused by the pathogen, Cronartium ribicola. We focused on four dominant white pine host species in Sequoia and Kings Canyon NatMortality predispositions of conifers across western USA
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 (δ13C and δ18O, respectively) of dying and surviving conifers at eight old‐growth forest sites across a strong moisture gradient in the western USA to - Science
Forest health and drought response
Forests provide society with economically important and often irreplaceable goods and services, such as wood products, carbon sequestration, clean water, biodiversity, and recreational opportunities. Yet hotter droughts (droughts in which unusually high temperatures exacerbate the effects of low precipitation) are projected to increase in frequency and intensity in coming decades, potentially...Post-Fire Conifer Regeneration Under a Warming Climate: Will Severe Fire Be a Catalyst for Forest Loss?
The Southwest U.S. is experiencing hotter droughts, which are contributing to more frequent, severe wildfires. These droughts also stress vegetation, which can make it more difficult for forests to recover after fire. Forest regeneration in burned areas may be limited because seeds have to travel long distances to recolonize, and when they do arrive, conditions are often unfavorably hot and dry. CSequoia and Kings Canyon Field Station
The Sequoia and Kings Canyon Field Station is home to research programs that focus on wildfire patterns in Southern California, and the effects of drought on Sierra Nevada forests. Select the "Science" tab for a more comprehensive summary.Leaf to Landscape: Understanding and Mapping the Vulnerability of Forests to Hotter Droughts
Forests across the southwestern U.S. are crucial components of recreation and play an important role in state and local economies. Healthy forests also provide needed habitat for many wildlife species and contribute many other important services to our planet. “Hotter droughts” (otherwise normal droughts whose effects on ecosystems are exacerbated by higher temperatures) are an emerging climate ch...Can Prescribed Fire Help Forests Survive Drought in the Sierra Nevada Mountains?
In 2017, California was experiencing its most severe drought in over a millennia. Low rainfall and record high temperatures resulted in increased tree mortality and complete forest diebacks across the West. Though land managers scrambled to respond, they lacked information needed to make informed decisions. Focusing on California’s central and southern Sierra Nevada Mountains, this project seeks t...Fighting Drought with Fire: A Comparison of Burned and Unburned Forests in Drought-Impacted Areas of the Southwest
Drought is one of the biggest threats facing our forests today. In the western U.S., severe drought and rising temperatures have caused increased tree mortality and complete forest diebacks. Forests are changing rapidly, and while land managers are working to develop long-term climate change adaptation plans, they require tools that can enhance forest resistance to drought now. To address this immCharacterizing a link in the terrestrial carbon cycle: a global overview of individual tree mass growth
Forests sequester the majority of the terrestrial biosphere’s carbon and are key components of the global carbon cycle, potentially contributing substantial feedbacks to ongoing climatic changes. It is therefore remarkable that no consensus yet exists about the fundamental nature of tree mass growth (and thus carbon sequestration rate). Specifically, does tree mass growth rate increase, decrease, - Data
Calibration and Validation Data and Model Coefficients for Mixed Conifer Vulnerability Project from Sequoia and Kings Canyon National Park 2015 to 2019
These datasets contain the attributes of individual trees located in and around Sequoia and Kings Canyon national parks. Attributes include remote sensing indices, terrain characteristics, and-- for the calibration data-- tree size and growth rates. Calibration data (mixedconifervulnerability_calibrationdataset.csv) were collected in long term research plots where trees are visited annually to cheSequoia and Yosemite National Parks Mortality and Fire Data (1990-2019) for Competition-Fire-Drought Interaction Analysis
This dataset records mortality-- including involvement of bark beetles-- and burn severity information for trees in long term forest dynamics plots in Sequoia National Park and Yosemite National Park that experienced fire. These data support the following publication: Furniss, T.J., Das, A.J., van Mantgem, P.J., Stephenson, N.L. and Lutz, J.A., 2021. Crowding, climate, and the case for sociData for Use in poscrptR Post-fire Conifer Regeneration Prediction Model
These data support poscrptR (Wright et al. 2021). poscrptR is a shiny app that predicts the probability of post-fire conifer regeneration for fire data supplied by the user. The predictive model was fit using presence/absence data collected in 4.4m radius plots (60 square meters). Please refer to Stewart et al. (2020) for more details concerning field data collection, the model fitting process, anForest Structure Data for Burned and Unburned Sites at Sequoia and Kings Canyon National Parks
These data represent tree diameter, species, mortality status, and plot attributes from 164 plots in Sequoia and Kings Canyon national parks, California. Plots were matched with local records of recent fire history to determine burn status and year burned if applicable. These data support the following publication: van Mantgem, P.J., Caprio, A.C., Stephenson, N.L. and Das, A.J., 2021. Forest RePost-fire conifer regeneration observations for National Forest land in California (2009 - 2017)
This data consists of presence/absence observations for post-fire conifer regeneration. The data also includes estimates of plot-level topography (slope, aspect), relativized differenced normalized burn ratio (RdNBR), post-fire climate, live basal area, and seed rain. These data support the following publication:Seed and Associated Tree Data from Long Term Research Plots in Sequoia and Yosemite national parks
This dataset was used as part of a continent-wide analysis of tree fecundity and its association with climate and tree size. This dataset consists of: plotinfo.csv, which contains basic attribute information for the field plots where the data were collected; seeddata.csv, which contains the data for seeds collected (how many, what type, etc.); trapxycoord.csv, which contains location and identificTree mortality in blue oak woodlands in Sequoia National Park during the extreme drought
These data were collected to quantify oak mortality during the 2014-2017 drought. 30 tenth hectare drought plots were established throughout the blue oak woodland of Sequoia National Park. Information on species, size, and mortality were collected for all standing trees in each plot. In addition, two long-term 2.25 ha plots were established, in which the same data were collected.Seedling and tree data from Sequoia National Park and Yosemite National Park
These tables include data from 25 long-term forest plots located in either Sequoia or Yosemite national park. Trees in these plots (established between 1982 and 2001) are censused annually for mortality and measured for diameter every 4 to 6 years. Plots were mostly 1 hectare (ha) in size (range 0.9 - 2.5 ha) and contained at least two 25 by 25 meter seedling sub-plots to monitor natural seedlingTree mortality in Sequoia National Park from 2004 to 2007 and during severe drought in 2014 to 2017
These data describe tree mortality and the factors associated with tree mortality for a variety of plots in Sequoia National Park. Most of the data were collected between 2014 and 2017 (during an extremely severe drought), along with some comparison data from 2004 to 2007. These data support the following publication: Stephenson, N.L., Das, A.J., Ampersee, N.J., Bulaon, B.M., and Yee, J.L.,Sequoia foliage dieback data from Sequoia National Park
These datasets provide spatially-explicit estimates of the magnitude of giant sequoia foliage dieback along selected trail corridors in Sequoia and Kings Canyon national parks, California, from 2014 through 2017. They additionally provide giant sequoia tree-ring measurements, through the year 1989, for two locations in the Giant Forest grove, Sequoia National Park, California.Mortality factors for dead trees from a subset of plots from the Sierra Nevada Forest Dynamics Plot Network from 1998 to 2010
This dataset was used to summarize and analyze the mortality factors recorded on dead trees in the Sierra Nevada Forest Dynamics Plot Network, which is managed by the Sequoia and Kings Canyon Field station of the U.S. Geological Survey's Western Ecological Research Center. Each row of the dataset represents an individual dead tree. These are dead trees that were recorded in the network from 1998 t - Software
poscrptR
poscrptR is a simple R package with the sole purpose of distributing a shiny app for predicting post-fire conifer regeneration. Learn more about shiny apps here.Seed source, not drought, determines patterns of seed production in Sierra Nevada conifers
This release consists of data collected from 26 plots in two national parks over a 19-year period. The data consists of plot-level seed counts for three genera, number of seed traps, live tree basal area, plot area, and climate metrics from the gridmet gridded data set, the daymet gridded data set, the PRISM gridded data set, and two nearby COOP stations. - News