Ludwig Maximilian University of Munich (LMU) biologist Susanne Renner and her research group have now looked at the effects of this warming trend on the timing of leaf emergence (leaf-out) in a broad range of shrubs and trees.

"It is widely believed that warmer temperatures will extend the growing season and that leaf-out in our flora will occur at progressively earlier times in the year," said Renner.

"However, whether air temperature or day-length is the dominant factor determining the date of leaf-out is actually known for very few of the thousands of species of trees and shrubs," he added.

Temperature and day-length are the primary triggers for leaf development, and selective forces during the course of evolution have determined which signal is actually used in a given species.

The results of the study showed that in many plants that thrive in warmer southern climes, day-length acts as a safety barrier so that these trees do not risk having their leaves damaged by late frosts - and increased temperature does not override this barrier.

This strategy is the better bet for thermophilic species because they are unable to cope with frost damage and are therefore susceptible to late frosts.

"This evolutionary adaptation is particularly striking in the case of beech trees, in which leaf-out occurs relatively late in the year," Renner added.

"Southern species develop their first leaves up to a month later than plants from our temperate climate," Renner noted.

"And the more of these thermophilic species expand their range northwards as temperatures rise, the more late flushers we may gain," she added.