Quantifying current and potential future impacts of balsam woolly adelgid infestation on forest biomass

Balsam woolly adelgid (Adelges piceae; BWA) is an invasive forest pest in the US whose infestation in fir forests can cause widespread tree mortality. A growing body of evidence suggests that the severity of BWA's effects is linked to climatic conditions, where sites featuring seasonally warmer temperatures tend to demonstrate higher degrees of insect-induced forest degradation. Thus, a warming climate may promote the expansion of BWA-driven damage, particularly in highly susceptible subalpine fir (Abies lasiocarpa) forests. The degree to which climate change may foster BWA infestation and the relationship between current and future infestation damage severity are largely unknown. To understand BWA's potential climate-driven impacts on subalpine fir forests now and in the future, we built a field-validated, spatially explicit predictive model that estimates BWA infestation severity as a function of locally downscaled temperature variables. Using only four seasonal temperature variables, our model was able to explain over 78% of variance in measured severity. We applied that model to the prediction of severity for five different time periods (2020, 2040, 2060, 2080, and 2100), with the 2040 and onward predictions being driven by four separate climatic shared socioeconomic pathways (SSP126, SSP245, SSP370, and SSP585). These high resolution (similar to 30 m) maps were compared to spatially coincident modeled estimates of subalpine fir aboveground biomass, the results of which provide valuable insight into the degree to which BWA may drive forest degradation, loss of ecosystem services, and increase in available fuels. In northern Utah, the southeasternmost extent of BWA infestation in the western US, the two major mountain ranges, Wasatch and Uinta, demonstrated very different climatic susceptibility to infestation damage, with the former being highly exposed to BWA-induced degradation under even moderate climate projections, and the latter being fairly robust against BWA's effects in all but the most extreme climate scenarios. In 2020, 41% of the study area's subalpine fir biomass is climatically exposed to some level of damage from BWA. By 2100, even under moderate climate projections (SSP245), 79% will be exposed, with 37% predicted to feature relatively high severity. By placing our results within the context of the Resist, Accept, Direct land management framework, we offer spatially explicit guidance to inform current and future silvicultural practices to mitigate future damage to subalpine fir forests at the leading edge of BWA expansion in the western US.