Xanthe WalkerResearch Associate Senior

November 19, 2015

Portrait of Xanthe Walker, post-doctoral scientist, in rocky landscape.

19Nov

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11 - 2015

Job Focus: Wildfire disturbance Email: xanthe.walker@nau.edu

Research Interests

Walker’s research focuses on the impacts of climate change and disturbance from wildfires on tundra and boreal forest ecosystems. She has experience in fire ecology and using dendrochronology techniques to assess climate change impacts on ecosystem resilience and post-fire succession. Dr. Walker is currently working on the NASA Arctic Boreal Research project to study severe fires and the loss of old carbon from boreal forest and arctic tundra soils.

Full Curriculum Vitae

Selected Publications

Walker XJ, Baltzer JL, Cumming SG, Day NJ, Ebert C, Goetz SJ, Johnstone JF, Potter S, Rogers BM, Schuur EAG, Turetsky MR, Mack MC. 2018. Increasing wildfires threaten historic carbon sink of boreal forest soils. Nature. In press.

Walker XJ, Rogers BM, Baltzer JL, Cumming SG, Day NJ, Goetz SJ, Johnstone JF, Schuur EAG, Turetsky MR, Mack MC. 2018. Cross-scale controls on carbon emissions from boreal forest megafires. Global Change Biology, 24(9): 4251-4265. doi:0.1111/gcb.12893

Walker XJ, Baltzer JL, Cumming SG, Day NJ, Johnstone JF, Rogers BM, Solvik K, Turetsky MR, Mack MC. 2018. Soil organic layer combustion in boreal black spruce and jack pine stands of the Northwest Territories, Canada. Int J Wildland Fire 27:125–34.

Plants are thought to be limited by phosphorus (P) especially in tropical regions. Here, Hou et al. report a meta-analysis of P fertilization experiments to show widespread P limitation on plant growth across terrestrial ecosystems modulated by climate, ecosystem properties, and fertilization regimes

Pinyon-juniper (PJ) plant communities cover a large area across North America and provide critical habitat for wildlife, biodiversity and ecosystem functions, and rich cultural resources. These communities occur across a variety of environmental gradients, disturbance regimes, structural conditions and species compositions, including three species of juniper and two species of pinyon. PJ communities have experienced substantial changes in recent decades and identifying appropriate management strategies for these diverse communities is a growing challenge. Here, we surveyed the literature and compiled 441 studies to characterize patterns in research on PJ communities through time, across geographic space and climatic conditions, and among focal species. We evaluate the state of knowledge for three focal topics: 1) historical stand dynamics and responses to disturbance, 2) land management actions and their effects, and 3) potential future responses to changing climate. We identified large and potentially important gaps in our understanding of pinyon-juniper communities both geographically and topically. The effect of drought on Pinus edulis, the pinyon pine species in eastern PJ communities was frequently addressed, while few studies focused on drought effects on Pinus monophylla, which occurs in western PJ communities. The largest proportion of studies that examined land management actions only measured their effects for one year. Grazing was a common land-use across the geographic range of PJ communities yet was rarely studied. We found only 39 studies that had information on the impacts of anthropogenic climate change and most were concentrated on Pinus edulis. These results provide a synthetic perspective on PJ communities that can help natural resource managers identify relevant knowledge needed for decision-making and researchers design new studies to fill important knowledge gaps.

Plant species are characterized along a spectrum of isohydry to anisohydry depending on their regulation of water potential (?), but the plasticity of hydraulic strategies is largely unknown. The role of environmental drivers was evaluated in the hydraulic behavior of Larrea tridentata, a drought-tolerant desert shrub that withstands a wide range of environmental conditions. With a 1.5 yr time-series of 2324 in situ measurements of daily predawn and midday ?, the temporal variability of hydraulic behavior was explored in relation to soil water supply, atmospheric demand and temperature. Hydraulic behavior in Larrea was highly dynamic, ranging from partial isohydry to extreme anisohydry. Larrea exhibited extreme anisohydry under wet soil conditions corresponding to periods of high productivity, whereas partial isohydry was exhibited after prolonged dry or cold conditions, when productivity was low. Environmental conditions can strongly influence plant hydraulic behavior at relatively fast timescales, which enhances our understanding of plant drought responses. Although species may exhibit a dominant hydraulic behavior, variable environmental conditions can prompt plasticity in ? regulation, particularly for species in seasonally dry climates.