Microclimatic effects of the loss of a foundation species from New England forests

Foundation species have a major impact on biotic and abiotic processes and create a stable environment for many other species. Eastern hemlock ( Tsuga canadensis ), a foundation tree species native to North America, is currently declining due to infestation by an invasive insect, the hemlock woolly adelgid ( Adelges tsugae ). Loss of hemlock canopies can greatly alter the dark, cool, and damp microclimate of hemlock forests. We studied five years of microclimatic changes following logging or girdling (to simulate physical effects of adelgid) of hemlocks in a multi‐hectare‐scale experiment in a New England forest. Both logging and girdling of hemlocks caused large changes in light availability, air and soil temperature, and soil moisture. Even though the impact of logging was more rapid than the effect of gradual hemlock mortality after girdling, the microclimatic changes in these two canopy treatments converged over time. The microclimate in hardwood control plots, which represent the predicted forest composition 50 years after hemlock loss, was intermediate between the two canopy treatments and the hemlock control plots. Our fine‐scale results were generally consistent with average microclimatic effects observed in comparative studies but revealed additional changes in variance and seasonal rhythms, and the importance of stochastic events such as ice storms. The variance in air temperature, but not in soil temperature, greatly increased after loss of hemlock. We also observed a striking saw‐tooth pattern, consisting of a small peak before bud‐break in temperature differentials between hemlock control and the two canopy treatments—likely due to the insulating hemlock canopy preventing snow from melting—followed by a larger difference in temperatures after bud‐break. We expect the ongoing decline of eastern hemlock—due to both infestation and pre‐emptive salvage logging—to greatly impact the microclimate of hemlock forests, as well as the many taxa that are associated with it.