Drivers of leaf-out phenology and their implications for species invasions: insights from Thoreau's Concord

To elucidate climate-driven changes in leaf-out phenology and their implications for species invasions, we observed and experimentally manipulated leaf out of invasive and native woody plants in Concord, MA, USA. Using observations collected by Henry David Thoreau (1852-1860) and our own observations (2009-2013), we analyzed changes in leaf-out timing and sensitivity to temperature for 43 woody plant species. We experimentally tested winter chilling requirements of 50 species by exposing cut branches to warm indoor temperatures (22 degrees C) during the winter and spring of 2013. Woody species are now leafing out an average of 18 d earlier than they did in the 1850s, and are advancing at a rate of 5 +/- 1 d degrees C-1. Functional groups differ significantly in the duration of chilling they require to leaf out: invasive shrubs generally have weaker chilling requirements than native shrubs and leaf out faster in the laboratory and earlier in the field; native trees have the strongest chilling requirements. Our results suggest that invasive shrub species will continue to have a competitive advantage as the climate warms, because native plants are slower to respond to warming spring temperatures and, in the future, may not meet their chilling requirements.