Differences between four sympatric subtropical tree species in the interactive effects of three environmental cues on leaf-out phenology.

• How environmental cues interactively regulate spring phenology remains largely unstudied. • A novel method to test vegis theory, interaction of chilling and forcing, was introduced. • Vegis effect was found in two species, while effects of photoperiod were generally minor. • In the other species, the timing of leaf-out was mainly regulated by forcing. • This implies that species-specific models are needed in global change research. Climatic warming is currently changing the spring phenology of extratropical trees, and this has several important effects on the trees and ecosystems. The major climatic cues regulating the spring phenology are winter chilling, spring forcing, and photoperiod. The interactions between these three remain largely unstudied because most studies concentrate on the effects of one cue, or maximally two, at a time. We studied the effects and interactions of chilling duration, forcing temperature, and forcing photoperiod simultaneously in four subtropical tree species. The main emphasis in our experiments was on the interaction of chilling duration and forcing temperature. The existence of this interaction was suggested in the 'Vegis theory', put forward decades ago but largely forgotten since. We also introduced a novel method for testing the theory experimentally. We found support for the Vegis theory in two of the four species examined. In the other two species the leaf-out timing was largely controlled by spring forcing. The effects of photoperiod were generally minor. Our results show that there are major differences between sympatric subtropical tree species in their phenological responses to environmental cues. These differences need to be addressed in the development of process-based tree phenology models. Our results further suggest that different subtropical trees respond differently to climatic warming because of differences related to the Vegis theory. This hypothesis remains to be tested in further studies. [ABSTRACT FROM AUTHOR]