Xanthe J. Walker's research while affiliated with Northern Arizona University and other places

Publications (21)

Preprint
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Fire is the dominant disturbance agent in Alaskan and Canadian boreal ecosystems and releases large amounts of carbon into the atmosphere. Burned area and carbon emissions have been increasing with climate change, which have the potential to alter the carbon balance and shift the region from a historic sink to a source. It is therefore critically i...
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In boreal North America, much of the landscape is covered by fire-adapted forests dominated by serotinous conifers. For these forests, reductions in fire return interval could limit reproductive success, owing to insufficient time for stands to reach reproductive maturity i.e., to initiate cone production. Improved understanding of the drivers of r...
Article
As climate warms, tree density at the taiga–tundra ecotone (TTE) is expected to increase, which may intensify competition for belowground resources in this nitrogen (N)‐limited environment. To determine the impacts of increased tree density on N cycling and productivity, we examined edaphic properties indicative of soil N availability along with ab...
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Resilience of plant communities to disturbance is supported by multiple mechanisms, including ecological legacies affecting propagule availability, species’ environmental tolerances, and biotic interactions. Understanding the relative importance of these mechanisms for plant community resilience supports predictions of where and how resilience will...
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Forest characteristics, structure, and dynamics within the North American boreal region are heavily influenced by wildfire intensity, severity, and frequency. Increasing temperatures are likely to result in drier conditions and longer fire seasons, potentially leading to more intense and frequent fires. However, an increase in deciduous forest cove...
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Thawing permafrost in northern latitudes has led to deepening active soil layers and fluctuating water tables. This could increase plant access to permafrost‐derived nitrogen (N), phosphorus (P), and other nutrients such as calcium (Ca) and magnesium (Mg), and subsequently increase plant productivity and ecosystem carbon storage and nutrient cyclin...
Preprint
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Deciduous tree cover is expected to increase in North American boreal forests with climate warming and wildfire occurrence. This shift in composition can generate biophysical cooling effects via increased land surface albedo. Here we use newly derived maps of continuous tree canopy and fractional deciduous cover to assess change over recent decades...
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Significance Black spruce is the dominant tree species in boreal North America and has shaped forest flammability, carbon storage, and other landscape processes over the last several thousand years. However, climate warming and increases in wildfire activity may be undermining its ability to maintain dominance, shifting forests toward alternative f...
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Climate change-driven droughts and insect outbreaks are becoming more frequent and widespread, increasing forest vulnerability to mortality. By addressing the impacts of climate and insects on tree growth preceding death, we can better understand tree mortality risk under a changing climate. Here, we used tree stature and interannual growth (basal...
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Data collected from research networks present opportunities to test theories and develop models about factors responsible for the long-term persistence and vulnerability of soil organic matter (SOM). Synthesizing datasets collected by different research networks presents opportunities to expand the ecological gradients and scientific breadth of inf...
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Increases in arctic-boreal fires can switch these biomes from a long-term carbon (C) sink to a source of atmospheric C through direct fire emissions and longer-term emissions from soil respiration. We here review advances made by the arctic-boreal fire science community over the last three years. Landscapes of intermediate drainage tend to experien...
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Carbon cycling after boreal forest fire Wildfire activity has been increasing in the boreal forests of the Northern Hemisphere, releasing carbon into the atmosphere from biomass and soil, with potential feedback to climate warming. In a long-term study, Mack et al. analyzed wildfire impacts on the carbon balance of boreal forest in Alaska, with par...
Article
The transition zone between the northern boreal forest and the arctic tundra, known as the tundra-taiga ecotone (TTE) has undergone rapid warming in recent decades. In response to this warming, tree density, growth, and stand productivity are expected to increase. Increases in tree density have the potential to negate the positive impacts of warmin...
Preprint
Full-text available
Data collected from research networks present opportunities to test theories and develop models about factors responsible for the long-term persistence and vulnerability of soil organic matter (SOM). Synthesizing datasets collected by different research networks presents opportunities to expand the ecological gradients and scientific breadth of inf...
Article
Full-text available
Increases in fire frequency, extent, and severity are expected to strongly impact the structure and function of boreal forest ecosystems. An important function of the boreal forest is its ability to sequester and store carbon (C). Increasing disturbance from wildfires, emitting large amounts of C to the atmosphere, may create a positive feedback to...
Article
Full-text available
Climate change is altering disturbance regimes outside historical norms, which can impact biodiversity by selecting for plants with particular traits. The relative impact of disturbance characteristics on plant traits and community structure may be mediated by environmental gradients. We aimed to understand how wildfire impacted understory plant co...
Article
Full-text available
Fungi play key roles in carbon (C) dynamics of ecosystems: saprotrophs decompose organic material and return C in the nutrient cycle, and mycorrhizal species support plants that accumulate C through photosynthesis. The identities and functions of extremophile fungi present after fire can influence C dynamics, particularly because plant-fungal relat...
Article
Fire is a primary disturbance in boreal forests and generates both positive and negative climate forcings. The influence of fire on surface albedo is a predominantly negative forcing in boreal forests, and one of the strongest overall, due to increased snow exposure in the winter and spring months. Albedo forcings are spatially and temporally heter...

Citations

... To account for possible nonlinearities in the total and viable seed rain response, we compared alternative full-saturated models including linear or spline adjustments (with up to two degrees of freedom) for each continuous predictor. We selected the adjustment with the lowest Akaike Information Criterion (AIC) score as being the best supported model corrected for parsimony (Crawley 2013). ...
... Since the beginning of the monitoring (in 1990), the Minimal thaw area showed less ground subsidence and little-disturbed moist acidic tundra and is dominated by the sedge E. vaginatum and Sphagnum spp. mosses, coexisting with evergreen and deciduous shrubs (Jasinski et al., 2022;Schuur et al., 2007). The Moderate thaw area displays isolated areas of ground subsidence (Schuur et al., 2007) and remains dominated by the sedge E. vaginatum, with a lower moss cover than the Minimal thaw area (Jasinski et al., 2018). ...
... Wildfire and climate are drivers of change in boreal ecosystems (Foster et al., 2022). Understanding the tipping point of drought conditions at which the landscape becomes connected, and peatlands are susceptible to wildfire with deeper burning of the organic soil layers is important for understanding the potential future effects of climate change and projected increases in wildfire on peatlands. ...
... In northwestern North America, there has been particular interest in detecting changes to tree and tall shrub extent near forest-tundra boundaries along gradients of climate, latitude, and elevation (Brodie et al., 2019;Danby & Hik, 2007;Dial et al., 2007;Roland et al., 2016;Terskaia et al., 2020). Boreal forest wildfire and successional processes have also been the subject of numerous investigations in the subarctic (Baltzer et al., 2021;Kasischke et al., 2010;Mack et al., 2021;Walker et al., 2021). However, comparatively little is known about vegetation dynamics in subarctic riparian zones, despite their disproportionately high importance as "hotspots" of hydrological processes (Ploum et al., 2021), biogeochemical cycling (Blackburn et al., 2017), species diversity (Andersson et al., 2000;Johansson et al., 1996;Johnson & Almlöf, 2016;Nilsson & Svedmark, 2002), wildlife habitat use (Cooke & Tauzer, 2020), and ecological disturbance (Scrimgeour et al., 1994). ...
... Taken together, these results suggest that black spruce recruitment after fire may decline under projected increases in fire activity in this region (Wotton et al. 2017), most notably fire frequency and to a lesser extent fire size. Indeed, there is already evidence of changing regeneration patterns in black spruce across much of boreal North America (Baltzer et al. 2021). The non-linear nature of several of the modelled relationships highlights the importance of capturing broad environmental, stand structural, and climatic gradients to better reflect complex ecological relationships. ...
... Although there are several definitions for data reuse in the literature (Pasquetto et al., 2017), we are following the guidance of van (Carpenter et al., 2009) addressing complex questions at broad scales (Wieder et al., 2021) with long-term observations proving critical to the understanding of drivers of environmental change and its implications (Patel et al., 2021). Synthesis involves meta-analyses, reviews, new combinations of existing data, and advances in statistical methods (Collins, 2020). ...
... We developed time series of annual maximum vegetation greenness from 1985 to 2019 for 10 5 random sample sites using 30-m resolution measurements of land surface reflectance from Landsat 5, 7, and 8. We focused on annual maximum vegetation greenness because it correlates with vegetation productivity and mortality Boyd et al., 2021;Erasmi et al., 2021) and can be reliably estimated with Landsat data , whereas metrics like annual integrated growing season vegetation greenness are not readily derived using these satellite data given their low temporal resolution. Landsat surface reflectance data were produced by the United States Geological Survey (USGS) as part of the Landsat Collection 2 (Tier 1 and Tier 2) dataset that included corrections for atmospheric and terrain effects based on the Landsat Ecosystem Disturbance Adaptive Processing System (Masek et al., 2006) and ...
... (Whitman et al., 2019;Zhao et al., 2021). In Siberia, NECB source occurred primarily across the tundra, driven by R eco outpacing GPP, and within the southern boreal zone which has been impacted by drought and fire (Sun et al., 2021;Veraverbeke et al., 2021). ...
... In northwestern North America, there has been particular interest in detecting changes to tree and tall shrub extent near forest-tundra boundaries along gradients of climate, latitude, and elevation (Brodie et al., 2019;Danby & Hik, 2007;Dial et al., 2007;Roland et al., 2016;Terskaia et al., 2020). Boreal forest wildfire and successional processes have also been the subject of numerous investigations in the subarctic (Baltzer et al., 2021;Kasischke et al., 2010;Mack et al., 2021;Walker et al., 2021). However, comparatively little is known about vegetation dynamics in subarctic riparian zones, despite their disproportionately high importance as "hotspots" of hydrological processes (Ploum et al., 2021), biogeochemical cycling (Blackburn et al., 2017), species diversity (Andersson et al., 2000;Johansson et al., 1996;Johnson & Almlöf, 2016;Nilsson & Svedmark, 2002), wildlife habitat use (Cooke & Tauzer, 2020), and ecological disturbance (Scrimgeour et al., 1994). ...
... In contrary, in a closed stand nearby, an adequate growth response was almost lacking. This suggests that at treeline, due to competition for light, soil water availability and nutrients [39][40][41][42], the positive effect of climate warming on individual tree growth is counteracted by dense canopies. ...