The geographic range of a species is a fundamental unit of ecology. The rapidly growing availability of species occurrence data presents opportunities to readily estimate species’ ranges. Importantly, ranges are dynamic in time and space. Current efforts to understand and document this dynamism suggest non-uniform and multi-directional range shifts across taxa, which runs counter to predictions that species will migrate uniformly northward in latitude and upward in elevation as climate warms. Currently missing from the literature is a systematic evaluation of the variability in range shift dynamics in a large-scale, multi-species context. Aiming to fill this gap, we map the geographic ranges of all U.S. tree species partitioned into their distinct life-stages (seedling, sapling, adult), from which we measure range shifts as a change in the size, position, and/or direction of the range between life-stage transitions. Considering a species’ range divided into its life-stage components presents the opportunity to quantify and track range shifts by snapshotting the range through time, using life-stage as a proxy for time. We operationalize this life-stage-range-mapping approach using data from the U.S. Forest Inventory and Analysis database to understand, first, whether contemporary tree range shifts are occurring and, next, how these dynamics vary across species.
Results/Conclusions
Results suggest that tree species are shifting their ranges in contemporary environments, with considerable variation. Trees do not substantially change the size of their range between life-stage transitions. They do, however, move rather significant distances between generations. Notably, these movements are multi-directional. These data do not support traditional ecological theory’s prediction of uniform northward/upward migrations. Considering where juveniles exist in relation to conspecific adults presents an opportunity to evaluate incipient range shifts, moving beyond the predictive models and historical inferences that currently ground much of this literature to instead query real-time empirical data. This work deepens our understanding of a fundamental ecological concept, the geographic range, while offering key insight into how species shift their spatial distributions on policy-relevant timescales.
