Interannual variation in UV-B and temperature effects on bud phenology and growth in Populus tremula.

Warming affects phenological processes such as spring bud break and autumnal bud set, and also growth rates of trees. Recently, it has been shown that these physiological processes also may be influenced by the ultraviolet-B (UV-B) part of the solar spectrum, and there are reasons to expect that the two environmental factors induce interactive effects when acting in concert. In this study, our aim was to elucidate how experimental enhancements in temperature and UV-B, alone and in combination, affect growth and seasonal phenology of Eurasian aspen (Populus tremula) over several growing seasons (three years). Moreover, we tested how environmentally induced changes in phenology affect the growth achieved over each season, that is, the importance of a prolonged growing season for growth yield. The plants grew in an outdoor experiment with modulated enhancements of temperature and UV-B during the growing season. Both UV-B and temperature enhancement affected bud set dates, while bud break dates were only affected by temperature enhancement. Temperature delayed bud set in all years, but gradually less over years, while UV-B yielded earlier bud set the first year but showed a delayed response the following years. Bud break was always earlier under temperature enhancement. The experimentally induced extension of the growing season in both ends had a positive effect of growth throughout the three-year period. However, the reduced responsiveness of bud set to both enhancement treatments suggest that the plants gradually acclimated to the modified climate, a finding that should also be investigated for other tree species.