Increased exposure to chilling advances the time to budburst in North American tree species

The phenology of trees is highly susceptible to changing global temperatures. Leaf budburst advances with increasing spring temperatures, but can also be delayed when warmer winters reduce chilling exposure. Results from long-term observations show that increasing temperatures have triggered advanced budburst in the past decades, but some studies also show that budburst advance has slowed recently. Here, we conducted an experiment with five temperate deciduous tree species (Acer rubrum L., Larix laricina (Du Rois) K. Koch, Populus tremuloides Michx., Quercus ellipsoidalis E. j. Hill, Betula papyrifera Marsh.) and one invasive species (Rhamnus cathartica L.) in Minnesota, USA, to assess the impact of chilling on the timing of leaf budburst. We collected twigs over two winter seasons (2011/2012 and 2012/2013) on a biweekly basis and exposed them to spring-like temperatures of 21 °C/16 °C day and night, long day photoperiod (16 h). We found a significant relationship of advanced budbreak with increased chilling for all species tested (P < 0.001) and significant differences in the timing to budburst among all species (P < 0.001). Acer rubrum responded strongly to chilling, showing a very steep linear decline in days to budburst with increased exposure to chilling. On the other end of the spectrum, L. laricina responded least to increases in chilling. These results suggest that rising global temperatures will likely have diverse impacts on tree species with potential implications for species interactions such as competition.