Data supporting: Comparison of chilling requirements between boreal and temperate tree species in Germany and the USA

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 MN, U.S.A. (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 hours photoperiod. We additionally took twigs from the same locations on three different dates and immediately placed them into the forcing chambers.