Climate warming prolongs the time interval between leaf‐out and flowering in temperate trees: Effects of chilling, forcing and photoperiod

Leaf‐out and flowering are two key phenological events of plants, denoting the respective onsets of visible vegetative growth and reproduction during the year. For each species, the schedule of vegetative growth and reproduction is crucial to the maximization of its fitness. Warming‐induced advances of leaf‐out and flowering have been reported frequently; however, it is unclear whether the responses of the two events are equal for any given species. Using long‐term phenological records in Europe, we examined simultaneously the responses of both leaf‐out and flowering of four common temperate tree species to climate warming and further examined the effects of winter chilling, spring forcing and photoperiod on the responses of the two events. We found that regardless whether flowering or leaf‐out occurred first, the first event advanced more than the second during 1950–2013, resulting in a prolonged time interval between the two events. The temporal changes were also supported by a similar geographical trend that the time interval between the two events increased from cold to warm sites. Due to the warming‐induced reduction in chilling, the spring forcing accumulated until the second event was increased more than the forcing accumulated until the first event, and that reduced the temperature sensitivity of the second event. In addition to the effect of chilling, the shorter photoperiod, associated with the advanced spring phenology, was also likely to substantially increase the spring forcing accumulated until the second event, which thus slowed down its advance, compared to the advance of the first event. The relative contributions of chilling and photoperiod to the increased forcing varied between species and events, with chilling mostly outweighing photoperiod. Synthesis. This study provides the large‐scale empirical evidence of prolonged time interval between leaf‐out and flowering with climate warming. The unequal advances of the two events may alter the partition of resources between vegetative growth and reproduction and cause different changes of spring frost damage to vegetative and reproductive tissues, which may alter species fitness and further affect ecosystem structure and function. This study provides the large‐scale empirical evidence of prolonged time interval between leaf‐out and flowering with climate warming. The unequal advances of the two events may alter the partition of resources between vegetative growth and reproduction and cause di