Early Autumn: Drought Accelerates Leaf Senescence in Temperate Tree Species.

Climate warming has led to an increase of drought events with consequences on physiological processes in trees and implications for carbon sequestration. Drought may lead to changes in the timing of autumn phenology and premature senescence. However, mechanisms linking physiological responses to drought stress and changes in phenological events in tree species remain poorly understood. We investigated how drought impacts autumn phenology and the timing of leaf senescence in three temperate tree species, along with the associated physiological responses. For this purpose, we designed a factorial field experiment with the evergreen species white pine, and two deciduous species, red maple and white oak, and used rainout structures to reduce ~50% of precipitation in drought-stressed plots compared to watered control plots. To assess drought impacts on autumn phenology and senescence we measured growth, photosynthesis, and leaf spectral reflectance. Drought-stressed deciduous seedlings exhibited premature senescence compared to controls. Water deficit during the growing season also decreased photosynthetic rates, height growth and biomass accumulation. The photochemical reflectance index effectively captured drought-induced changes in leaf physiology and phenology in white pine and the chlorophyll carotenoid index tracked the seasonality in deciduous trees, demonstrating that carotenoid-based vegetation indices are promising tools for monitoring drought stress responses in temperate forests.