Environmental control of spring phenology in mature temperate trees

Warmer spring temperatures, as caused by climate warming, led to an earlier bud burst in spring and an extended growing season in many temperate and boreal tree species. But to which extent will the phenology of these species keep tracking rising temperatures in future? Temperature, photoperiod and degree of winter chilling are the main environmental factors controlling tree spring phenology. Contrary to the high inter-annual variation of spring temperatures, photoperiod is an astronomical, weather independent environmental cue for the progression of the season. Photoperiodic control of spring development prevents trees from flushing too early under mild winter and early spring temperatures, before the period of potentially fatal freezing damage is over. Photoperiod sensitive species are therefore likely to stop tracking climate warming the closer the temperature modulated bud break is shifted toward the genetically fixed photoperiod threshold. I assessed the photoperiod sensitivity of spring bud swelling and bud burst in several common temperate tree species using growth chamber experiments with variable photoperiod × temperature interaction and evaluated commonly used phenological models to represent these interactions. Short photoperiods delayed bud burst in late successional species to various degrees, whereas no distinct photoperiod sensitivity of bud burst was observed in early successional and exotic (ornamental) species, which are thus mostly temperature controlled. Climate warming will thus not necessarily lead to progressively longer growing seasons in late successional species, as is often assumed.