Senescence in Temperate Broadleaf Trees Exhibits Species-Specific Dependence on Photoperiod Versus Thermal Forcing

Incomplete understanding of the processes controlling senescence limits our ability to forecast how the timing of leaf senescence will change in coming decades. In this study, we use a hierarchical Bayesian model (HBM) in association with a 27+ year record of field observations for 12 temperate deciduous tree species collected at Harvard Forest in central Massachusetts to examine how variability in bioclimatic controls affects the timing of leaf senescence. To test how general and extensible our results are over a broader biogeographic range, we used a multi-year record of land surface phenology derived from remote sensing encompassing all forested lands in New England. Results from the HBM showed that while air temperature is an important factor that influences the timing of leaf senescence, photoperiod uniformly exerts the strongest control across all 12 species. Species exhibiting the strongest dependence on photoperiod, particularly sub-species of Acer , showed low inter-annual variation and no long-term trends in the timing of leaf senescence. In contrast, species with greater dependence on air temperature, particularly sub-species of Quercus , showed statistically significant trends toward later senescence dates in response to long-term warming. Results from analyses conducted at regional scale across all of New England using data derived from remote sensing corroborated results obtained at Harvard Forest. Specifically, relative to ecoregions dominated by sub-species of Quercus, the timing of leaf senescence in ecoregions dominated by sub-species of Acer exhibited lower interannual variability and lower correlation with year-to-year variation in late-season air temperatures. These results suggest that forecasting how the timing of leaf senescence in temperate forests will change in the future requires species-specific understanding of how bioclimatic forcing controls the timing of leaf senescence