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

Abstract

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.

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

“…We show that trees in high BA sites are more sensitive to climate than those growing in more open stands. The greater climate sensitivity of trees in mature, high competition sites may increase vulnerability to disease and insect induced mortality (Anderegg et al, 2015;Cahoon et al, 2018;Csank et al, 2016;Mcdowell et al, 2008) and reduce resilience to disturbance (Johnstone, McIntire, Pedersen, King, & Pisaric, 2010) relative to trees in younger, less drought-stressed stands.…”

Regional variation in interior Alaskan boreal forests is driven by fire disturbance, topography, and climate

“…, Cahoon et al. ). The ultimate direction of these changes, and thus whether they represent a “state change” or a cyclical successional change is unknown (Pastick et al.…”

“…), particularly in interaction with intensified future warming (Wagner and Doak , Cahoon et al. ). Furthermore, interactive effects among multiple stressors may reasonably be expected to cause additional, unforeseen changes to forest structure, integrity, and the distribution of dominant species, including the possibility of instigating rapid, nonlinear changes once a “tipping‐point” threshold is reached (Chapin et al.…”

“…, , Cahoon et al. ), engendering different expectations of the potential consequences. The three primary (and interrelated) drivers of vegetation change from warming climates that have been the focus of this literature are described in the three subsections that follow.…”

“…However, substantial variation among species responses to warming have emerged from recent research (Cahoon et al. , Trugman et al. ), and some studies have noted, in particular, the importance of interactions between site‐level habitat variation and climate (Nicklen et al.…”