Differential declines in Alaskan boreal forest vitality related to climate and competition

Trugman, Anna T.; Medvigy, D.; Anderegg, William R. L.; Pacala, Stephen W.

doi:10.1111/gcb.13952

Cited by 41 publications

(37 citation statements)

References 58 publications

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“…Our results (Figure a) confirm the idea that continued warming will increase aboveground biomass throughout portions of Alaska (Ackerman, Griffin, Hobbie, & Finlay, ), which may partly help curb carbon emissions from thawing permafrost (Abbott et al., ). In contrast, our results (Figure c) also suggest that plant productivity may decline in warming regions due to drought stress which would predispose woody vegetation to the risk of disease, fire, and mortality (Rogers et al., ; Trugman, Medvigy, Anderegg, & Pacala, ). Further research is needed to better understand climates impact on short and long term shifts in vegetation phenology, as well as to identify and quantify higher order interactions among environmental factors (e.g., climate, soil moisture) that influence vegetation productivity at a multitude of scales.…”

Section: Discussionmentioning

confidence: 99%

Spatiotemporal remote sensing of ecosystem change and causation across Alaska

Pastick

Jorgenson²,

Goetz

et al. 2018

Global Change Biology

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Contemporary climate change in Alaska has resulted in amplified rates of press and pulse disturbances that drive ecosystem change with significant consequences for socio-environmental systems. Despite the vulnerability of Arctic and boreal landscapes to change, little has been done to characterize landscape change and associated drivers across northern high-latitude ecosystems. Here we characterize the historical sensitivity of Alaska's ecosystems to environmental change and anthropogenic disturbances using expert knowledge, remote sensing data, and spatiotemporal analyses and modeling. Time-series analysis of moderate-and high-resolution imagery was used to characterize land- and water-surface dynamics across Alaska. Some 430,000 interpretations of ecological and geomorphological change were made using historical air photos and satellite imagery, and corroborate land-surface greening, browning, and wetness/moisture trend parameters derived from peak-growing season Landsat imagery acquired from 1984 to 2015. The time series of change metrics, together with climatic data and maps of landscape characteristics, were incorporated into a modeling framework for mapping and understanding of drivers of change throughout Alaska. According to our analysis, approximately 13% (~174,000 ± 8700 km ) of Alaska has experienced directional change in the last 32 years (±95% confidence intervals). At the ecoregions level, substantial increases in remotely sensed vegetation productivity were most pronounced in western and northern foothills of Alaska, which is explained by vegetation growth associated with increasing air temperatures. Significant browning trends were largely the result of recent wildfires in interior Alaska, but browning trends are also driven by increases in evaporative demand and surface-water gains that have predominately occurred over warming permafrost landscapes. Increased rates of photosynthetic activity are associated with stabilization and recovery processes following wildfire, timber harvesting, insect damage, thermokarst, glacial retreat, and lake infilling and drainage events. Our results fill a critical gap in the understanding of historical and potential future trajectories of change in northern high-latitude regions.

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Section: Discussionmentioning

confidence: 99%

Spatiotemporal remote sensing of ecosystem change and causation across Alaska

Pastick

Jorgenson²,

Goetz

et al. 2018

Global Change Biology

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“…Conifer BA was our primary metric of competition. Although BA is a measure of stand productivity, it has also been successfully used as a measure of plot crowding as it integrates both stand density and tree size (Martin & Ek, ; Trugman et al, ). We focused on within conifer competition for two reasons.…”

Section: Methodsmentioning

confidence: 99%

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

Nicklen

Roland

Csank

et al. 2018

Global Change Biology

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The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against 13C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced 13C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.

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“…Plot-specific details can be found in Appendix S1. leading defoliator in interior Alaska compared with other pests (Dubois and Burr 2015), (2) has reduced aspen photosynthesis and growth in experimental studies (Wagner et al 2008) and (3) has recently been observed as a leading cause of mortality among inventory plots in the Tanana Valley (Trugman et al 2017). We examined the relationship between growth and ALM population (insects/m 2 of foliage) using ring width indices for cores collected from Bonanza Creek LTER in 2015 (n = 18).…”

Section: Tree Growth Responses To Biotic Agentsmentioning

confidence: 99%

“…Although climate appears to have more limited direct effects on aspen radial growth than the other species investigated, a warmer and drier climate in recent decades may have had a stronger indirect effect on aspen growth by increasing this species vulnerability to herbivory and by initiating or exacerbating the severity of the ALM outbreak. The interaction between defoliators and moisture limitation in aspen stands could have major implications for forest productivity (Hogg et al 2008), mortality (Hogg et al 2002, Trugman et al 2017 and feedbacks to climate (Michaelian et al 2011) in the form of altered biogeochemical cycles and surface energy budgets.…”

Section: Climatic and Biotic Drivers Of Aspen Growthmentioning

confidence: 99%

Contrasting drivers and trends of coniferous and deciduous tree growth in interior Alaska

Cahoon

Sullivan

Brownlee

et al. 2018

Ecology

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The boreal biome represents approximately one third of the world's forested area and plays an important role in global biogeochemical and energy cycles. Numerous studies in boreal Alaska have concluded that growth of black and white spruce is declining as a result of temperature-induced drought stress. The combined evidence of declining spruce growth and changes in the fire regime that favor establishment of deciduous tree species has led some investigators to suggest the region may be transitioning from dominance by spruce to dominance by deciduous forests and/or grasslands. Although spruce growth trends have been extensively investigated, few studies have evaluated long-term radial growth trends of the dominant deciduous species (Alaska paper birch and trembling aspen) and their sensitivity to moisture availability. We used a large and spatially extensive sample of tree cores from interior Alaska to compare long-term growth trends among contrasting tree species (white and black spruce vs. birch and aspen). All species showed a growth peak in the mid-1940s, although growth following the peak varied strongly across species. Following an initial decline from the peak, growth of white spruce showed little evidence of a trend, while black spruce and birch growth showed slight growth declines from ~1970 to present. Aspen growth was much more variable than the other species and showed a steep decline from ~1970 to present. Growth of birch, black and white spruce was sensitive to moisture availability throughout most of the tree-ring chronologies, as evidenced by negative correlations with air temperature and positive correlations with precipitation. However, a positive correlation between previous July precipitation and aspen growth disappeared in recent decades, corresponding with a rise in the population of the aspen leaf miner (Phyllocnistis populiella), an herbivorous moth, which may have driven growth to a level not seen since the early 20th century. Our results provide important historical context for recent growth and raise questions regarding competitive interactions among the dominant tree species and exchanges of carbon and energy in the warming climate of interior Alaska.

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Differential declines in Alaskan boreal forest vitality related to climate and competition

Cited by 41 publications

References 58 publications

Spatiotemporal remote sensing of ecosystem change and causation across Alaska

Spatiotemporal remote sensing of ecosystem change and causation across Alaska

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

Contrasting drivers and trends of coniferous and deciduous tree growth in interior Alaska

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