Proliferating poplars: the leading edge of landscape change in an Alaskan subalpine chronosequence

We remeasured a classic chronosequence study in the subalpine zone of the Alaska Range to evaluate how plant community attributes have changed across a set of different-aged terraces over a 54-yr period (1958–2012). Our work focused on whether the tempo and trajectory of successional development described in the original study have changed over this period during which summer temperatures warmed by approximately 2°C. Our work revealed a rapid increase in the distribution, stature, and abundance of balsam poplar trees that was unanticipated in the original successional model alongside evidence that established late-successional plant communities have changed relatively little over the same time period. The spatial distribution of poplar expansion was both directional and highly variable, with greater expansion occurring in sites that were young surfaces in 1958, or else were disturbed during the intervening period. We present evidence that early successional environments in this region may be particularly susceptible to rapid alteration stimulated by climate warming that has allowed tree establishment and growth in subalpine areas. Sparsely vegetated sites allow for invasion or expansion of some species to be quickly realized because there is less resistance from competition with established vegetation, including mosses that insulate and paludify the soil. We suggest that established vegetation communities may have physical characteristics (such as cold and/or acidic soil profiles) that are inimical to the establishment of balsam poplar and may also be a source of competitive inertia, conferring a measure of resistance to directional changes in the landscape mosaic. However, when an early successional species has traits that allow it to persist and fundamentally alter the vegetation mosaic over time, as is the case with balsam poplar, it may serve as the leading edge of compositional changes with profound consequences. Our results highlight the capacity of a single species to catalyze the changes that may eventually lead to the altering of an entire landscape mosaic.