Impacts of climate and insect herbivory on productivity and physiology of trembling aspen (Populus tremuloides) in Alaskan boreal forests
Climate change is impacting forested ecosystemsworldwide, particularly in theNorthernHemisphere wherewarming has increased at a faster rate than the rest of the globe. As climate warms, trembling aspen (Populus tremuloides) is expected to becomemore successful in northern boreal forests because of its current presence in drier areas ofNorthAmerica. However, large-scale productivity decline of aspen has recently been documented throughout theUnited States andCanada as a result of drought and insect outbreaks.We used tree ringmeasurements (basal area increment (BAI) and stable carbon isotopes (δC)) and remote sensing indices of vegetation productivity (NDVI) to study the impact of climate and damage by the aspen epidermal leafminer (Phyllocnistis populiella) on aspen productivity and physiology in interior Alaska.We found that productivity decreasedwith greater leafmining and was not sensitive to growing season (GS)moisture availability. Although productivity decreased during high leafmining years, it recovered to pre-outbreak levels during years of low insect damage, suggesting a degree of resilience toP. populiellamining. Climate and leafmining interacted to influence tree ring δC, with greater leafmining resulting in decreased δCwhenGSmoisture availability was low.We also found thatNDVIwas negatively associatedwith leafmining, and positively correlatedwith BAI and the δC decrease corresponding tomining. This suggests that NDVI is capturing not only variations in productivity, but also changes in physiology associatedwith P. populiella. Overall, thesefindings indicate that the indirect effects ofP. populiellamining have a larger impact on aspen productivity and physiology than climate under current conditions, and is essential to consider when assessing growth, physiology andNDVI trends in interior Alaska.