The chilling requirement of subtropical trees is fulfilled by high temperatures: A generalized hypothesis for tree endodormancy release and a method for testing it

ABSTRACT Spring phenology is a key phenomenon mediating the effects of climate change on terrestrial plants and ecosystems, but in regard to subtropical trees, the dormancy mechanisms that regulate spring phenology are still poorly understood. It has been suggested recently that similarly to temperate and boreal trees, subtropical tree species also show endodormancy and a chilling requirement of endodormancy release. However, there are no previous experimental results on the chilling temperature range that is effective for endodormancy release in subtropical trees. We studied endodormancy and the chilling requirement in four subtropical tree species experimentally. In addition to chilling in natural conditions, we applied controlled chilling at several constant temperatures ranging from -2 to +15 °C. Our results show endodormancy and a chilling requirement in the tree species studied and reveal several differences among the four species in the manifestation and depth of endodormancy. Most importantly, our findings indicate that contrary to the prevailing mainline conception that chilling temperatures are generally restricted to those below +10 °C, higher temperatures of up to +15 °C are also effective for endodormancy release in the subtropical tree species examined. An exact upper threshold of +10.4 °C has been experimentally established for boreal Betula pubescens. We hypothesized that this difference would be explained by differences in the occurrence of low autumn temperatures between the two respective climates. We developed a method for testing the hypothesis by analysing long-term climatic records in relation to the experimental findings. Tentative results supported our hypothesis. On the basis of this novel result, we put forward the testable generalized hypothesis that in any climatic conditions where trees show endodormancy, the range of temperatures physiologically effective in endodormancy release represents the range of typical autumn air temperatures occurring in those particular climatic conditions.