Comparison of chilling requirements of boreal and temperate tree species in Germany and North America.

Chilling is an important cue in the spring phenology of boreal and temperate tree species. It is well established that increased chilling reduces the days to bud break (DTB), but the effectiveness of different cold temperatures for chilling accumulation remains unknown for most species. Depending on this effectiveness, future warmer winters could either reduce or increase chilling accumulation for different tree species, resulting in delayed or advanced bud break. This could alter primary productivity and ecological interactions. We investigated chilling effects on DTB experimentally, using twigs of boreal and temperate tree species in Minnesota, USA. (8 species), and Bavaria, Germany (6 species). We collected twigs and applied artificial chilling in cooling chambers at three different temperatures (-7/-6.5 °C; 1.5/2°C; 4.5/4°C - USA/Germany) and with two different lengths (4/8 weeks), before placing them into forcing chambers at 21 °C/16°C, 16 h photoperiod. We additionally took twigs from the same locations on three different dates and immediately placed them into the forcing chambers. In both experiments, we observed DTB. Both experiments showed that longer chilling exposure reduced DTB and that temperatures below freezing contributed to chilling accumulation. Changing the chilling temperatures had a significant effect on DTB for 8 out of the 14 species. For most species, higher chilling temperatures more effectively reduced DTB than colder temperatures. With few exceptions, species growing in Germany required less chilling, and boreal species broke bud before temperate species. Our study confirms the need to understand species-specific chilling requirements since generalizations seem inappropriate when predicting future leaf out.