Prof. Dr. Julia Laube

Chilling is an important cue in the spring phenology of boreal and temperate tree species. It is well established thatincreased chilling reduces the days to bud break (DTB), but the effectiveness of different cold temperatures for chillingaccumulation remains unknown for most species. Depending on this effectiveness, future warmer winters could eitherreduce or increase chilling accumulation for different tree species, resulting in delayed or advanced bud break. This couldalter primary productivity and ecological interactions. We investigated chilling effects on DTB experimentally, using twigsof boreal and temperate tree species in Minnesota, USA. (8 species), and Bavaria, Germany (6 species). We collectedtwigs 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 placedthem into the forcing chambers. In both experiments, we observed DTB. Both experiments showed that longer chillingexposure reduced DTB and that temperatures below freezing contributed to chilling accumulation. Changing the chillingtemperatures had a significant effect on DTB for 8 out of the 14 species. For most species, higher chilling temperaturesmore effectively reduced DTB than colder temperatures. With few exceptions, species growing in Germany required lesschilling, and boreal species broke bud before temperate species. Our study confirms the need to understand species-specificchilling requirements since generalizations seem inappropriate when predicting future leaf out.