A5001464042- Climate change and permafrost
- Tree-ring climate responses
- Cryospheric studies and observations
- Geology and Paleoclimatology Research
- Plant Water Relations and Carbon Dynamics
- Fire effects on ecosystems
- Ecology and Vegetation Dynamics Studies
- Rangeland and Wildlife Management
- Hydrocarbon exploration and reservoir analysis
- Geological Studies and Exploration
- Peatlands and Wetlands Ecology
- Lichen and fungal ecology
- Forest Ecology and Biodiversity Studies
- Arctic and Antarctic ice dynamics
- Species Distribution and Climate Change
- Atmospheric and Environmental Gas Dynamics
- Plant responses to elevated CO2
- Astro and Planetary Science
Pacific Northwest Research Station
2017-2024
University of Alaska Anchorage
2016-2021
Pennsylvania State University
2011-2016
University of Maine
2016
University of Essex
2016
Loughborough University
2016
Boreal forests play critical roles in global carbon, water and energy cycles. Recent studies suggest drought is causing a decline boreal spruce growth, leading to predictions of widespread mortality shift dominant vegetation type interior Alaska. We took advantage large set tree cores collected from random locations across vast area Alaska examine long-term trends carbon isotope discrimination growth black white spruce. Our results confirm that both species sensitive moisture availability,...
Abstract Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts soil carbon (C). Evidence suggests that deciduous shrubs increasing in abundance, but implications for ecosystem C budgets remain uncertain. Using midsummer CO 2 flux data from 21 sites spanning 16° latitude biomes, we show air temperature explains c . one‐half variation respiration ( ER ) drives pattern net exchange across ecosystems. Woody were slightly...
The Kangerlussuaq area of southwest Greenland encompasses diverse ecological, geomorphic, and climate gradients that function over a range spatial temporal scales. Ecosystems from the microbial communities on ice sheet moisture-stressed terrestrial vegetation (and their associated herbivores) to freshwater oligosaline lakes. These ecosystems are linked by dynamic glacio-fluvial-aeolian geomorphic system transports water, geological material, organic carbon nutrients glacier surface adjacent...
Abstract Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with expansion of tall-statured shrubs trees in tundra. Changes vegetation structure function expected to alter soil thermal regimes, thereby modifying climate feedbacks related permafrost thaw carbon cycling. However, current understanding impacts on temperature is limited local or regional scales lacks generality necessary predict stability a pan-Arctic scale. Here we synthesize shallow...
Abstract Changes in the terrestrial carbon cycle may ameliorate or exacerbate future climatic warming. Research on this topic has focused almost exclusively abiotic drivers, whereas biotic factors, including trophic interactions, have received comparatively little attention. We quantified singular and interactive effects of herbivore exclusion simulated warming ecosystem CO 2 exchange over two consecutive growing seasons W est G reenland. Exclusion caribou muskoxen past 8 years led to...
Abstract Observational and experimental studies have generally shown that warming is associated with greater growth abundance of deciduous shrubs in arctic ecosystems. It uncertain, however, if this trend will persist the future. Our study examined responses to climate change over late 20th early 21st centuries near Kangerlussuaq western Greenland. We combined shrub dendrochronology, stable isotope analysis weekly measurements leaf gas exchange examine drivers secondary two widespread...
Boreal forests are critical sinks in the global carbon cycle. However, recent studies have revealed increasing frequency and extent of wildfires, decreasing landscape greenness, tree mortality declining growth black white spruce boreal North America. We measured ring widths from a large set increment cores collected across vast area interior Alaska examined implications data processing decisions for apparent trends growth. found that choice detrending method had important long-term strength...
Tree-ring time series provide long-term, annually resolved information on the growth of trees. When sampled in a systematic context, tree-ring data can be scaled to estimate forest carbon capture and storage landscapes, biomes, and-ultimately-the globe. A effort sample tree rings national inventories would yield unprecedented temporal spatial resolution dynamics help resolve key scientific uncertainties, which we highlight terms evidence for greening (enhanced growth) versus browning...
Biodiversity is declining globally in response to multiple human stressors, including climate forcing. Nonetheless, local diversity trends are inconsistent some taxa, obscuring contributions of processes global patterns. Arctic tundra diversity, plants, fungi, and lichens, declined during a 15-year experiment that combined warming with exclusion large herbivores known influence vegetation composition. Tundra regardless experimental treatment, as background growing season temperatures rose...
Abstract The boreal biome represents approximately one third of the world's forested area and plays an important role in global biogeochemical energy cycles. Numerous studies Alaska have concluded that growth black white spruce is declining as a result temperature‐induced drought stress. combined evidence changes fire regime favor establishment deciduous tree species has led some investigators to suggest region may be transitioning from dominance by forests and/or grasslands. Although trends...
Significance Whether climatic conditions or biotic interactions determine species abundances and distributions has been a persistent question in ecology. Furthermore, its answer long considered scale-dependent, with climate presumably constraining abundance at large scales determining them local scales. We conducted 15-y field experiment to test more recent theory that predicts abiotic can shape patterns of covariation both Our results affirm this prediction, offering insights will help...
Abstract Numerous recent studies have argued that moisture limitation is leading to growth declines and mortality of black spruce ( Picea mariana ) white glauca in the North American boreal forest. A parallel line research suggests increasingly common severe wildfires are altering successional pathways long‐term replacement forests with those dominated by paper birch Betula papyrifera , neoalaskana trembling aspen Populus tremuloides ). When both conifers deciduous trees establish after...
Abstract The recent expansion of deciduous shrubs is a common observation throughout the Arctic. However, we lack complete understanding how physiological differences between and coexisting species may confer competitive advantages to shrubs. We combined leaf gas exchange stable isotope analyses two important species, Betula nana Poa pratensis , elucidate processes governing seasonal carbon (C) gain in West Greenland. tested competing hypotheses. On one hand, anticipated cooler, drier soils...
Abstract Trees growing near the Arctic treeline have long been used to reconstruct past climates. However, recent studies shown deterioration of historically strong positive correlations between air temperature and tree growth (known as “divergence”). Divergence has important implications for confidence in paleoclimate reconstructions ecosystem–atmosphere carbon exchange. Studies Brooks Range northern Alaska showed that white spruce west increased response late 20th century warming, whereas...
Abstract Boreal forest soils contain large stocks of soil carbon (C) that may be sensitive to changes in climate and disturbance. Destablization boreal C through inputs, belowground pools and/or wildfire could feedback accelerate rising atmospheric CO 2 concentration. Additionally, increasing frequency severe fires changing the dominant types reshaping aboveground stocks. Although controls on ecosystem have received considerable attention, many studies been limited locations near road...
Alaska’s boreal forests are experiencing rapid changes in climate that may favor deciduous-dominated systems, with important implications for global biogeochemical and energy cycles. However, aspen ( Populus tremuloides Michx.) has experienced substantial defoliation from the leaf miner Phyllocnistis populiella Cham., hereafter ALM) Alaska, resulting significant growth reductions. We conducted a tree-ring Δ 13 C study to test hypothesis moisture limitation have predisposed damage. Contrary...