Climate warming advances phenological sequences of Aesculus hippocastanum

Abstract

Warming-induced shifts in plant phenology have been frequently reported in recent decades. Previous studies primarily focused on isolated phenological events; however, the entire phenological sequences of a given species were rarely examined simultaneously, limiting our knowledge about aggregate life history response to climate warming. Here, using 210,000 in situ phenological observations across 2493 European sites during 1980–2015, we examined the responses of four typical phenological events (leaf unfolding, flowering, fruiting, and leaf coloring) of Aesculus hippocastanum (horse chestnut), a common temperate tree species, to climate warming. We found that all four phenophases of A. hippocastanum advanced, i.e., the entire life cycle during the year shifted earlier with climate warming. However, the early-season phenophases (leaf unfolding and flowering) advanced faster than late-season phenophases (leaf coloring and fruiting), leading to the extension of both growing season (from leaf unfolding to leaf coloring) and reproductive season (from flowering to fruiting). The extension rate of the reproductive season was slightly greater than that of the growing season. This implied that under warming climate, more time was devoted to both vegetative growth and reproduction, particularly the latter one, in A. hippocastanum. In addition, significantly positive correlations were observed among four phenological events, particularly for pairwise consecutive events, indicating that the responses of phenological sequences to warming were highly correlated. By simultaneously studying the responses of four key phenological events to climate warming, our findings demonstrate the differences in the phenological plasticity among phenological sequences in A. hippocastanum, deepening our understanding about the impact of climate warming on life history rather than examining individual phenological events in isolation.