Shifting with climate? Evidence for recent changes in tree species distribution at high latitudes

Warming has been particularly strong at high latitudes in recent decades and bioclimatic models predict northern shifts in optimal conditions for most species. Climate is a strong predictor of site occupancy for trees at broad spatial scales and interacts with other drivers of forest dynamics. Recent changes in distribution and occupancy patterns should therefore provide the best evidence of a tree species' potential to shift in the direction predicted by bioclimatic models. Studies examining recent distribution changes for plants, however, have mostly done so along altitudinal gradients or have used the latitudinal position of juvenile trees relative to adult ones to infer range dynamics. This study provides rare evidence of latitudinal shifts for 11 northern tree species by assessing recent changes in distribution using globally significant inventories from 1970 to 2002. It also compares observed trends with those inferred from the position of juveniles relative to trees in a single survey. Samplings cover 6456 forest plots in temperate and boreal forests up to treeline in eastern North America. The average overall latitudinal shift was 3.07 +/- 4.37 km northward although responses were species-specific. Shifts were detected more for juvenile than for adult trees and significant northward ones were detected more at northern range limits than at the median. All species demonstrated increased frequency of plot occupancy for saplings while occupancy generally decreased for adult trees. Five out of the 11 species examined (Acer rubrum, Acer saccharum, Betula papyrifera, Fagus grandifolia, and Populus tremuloides) showed significant distributional shifts consistent with northward migration. Saplings of Abies balsamea, Picea glauca, and Picea mariana, on the other hand, showed southward shifting trends. Natural and human disturbances undoubtedly interact with climate to determine forest dynamics; this study shows whether their combined effect can shift distribution in the direction predicted by bioclimatic models. Only continued monitoring will reveal whether these observations are just transient dynamics or indicative of shifting range in this century. Our study provides a benchmark against which to assess future observations of latitudinal shifts for trees.