Trends in phenology of Betula pubescens across the boreal zone in Finland.

Timing of plant phenophases is a useful biological indicator which shows how nature responds to the variation in climate. Thus, long phenological observation series help to estimate the impact of changing climate on forest plants. We investigated whether phenological patterns of downy birch Betula pubescens respond to warming climate and whether the intensity of the responses varies among phytogeographical zones. We studied data collected by the Finnish National Phenological Network from 30 observation sites across Finland during 1997-2006. The advancement in the timing of the earliest phenophase, bud burst, ranged from 0.7 days/year in southern boreal zone to 1.4 days/year in middle and northern boreal zones. Timing of bud burst was most clearly dependent on mean May temperatures. The intensity of the response to temperature increased from south to north. The advancement of bud burst resulted into a significant lengthening of the growth period by 1.2-1.6 days per year in northern and middle boreal zones, respectively, whereas the lengthening was not significant in the southern boreal zone. No trend was observed in the timing of autumn phenophases.