The role of spring frost events in controlling the expansion of temperate tree species into the boreal forest

Abstract

Increasing global temperatures are driving the migration of temperate tree species into boreal regions. This study utilized elevational climate and composition gradients and tree-rings at Mount Carleton Provincial Park, New Brunswick, to evaluate the relative influence of winter and spring temperatures on the leading-edge of temperate tree species. We found that low spring temperatures impact the relative abundance of red and sugar maple more than winter temperatures toward their elevational limits. Further, we found that annual growth for all species responded more to minimum May temperatures than to winter temperatures. These results suggest that minimum winter temperatures do not explain the upper elevational limits of these species and may not be the main constraint on the northward expansion of these species, as traditionally thought. Instead, minimum spring temperatures post-budburst may be a stronger influence. Our study calls for a reassessment of the traditional assumption that winter temperatures constrain these species.