The duration of high spring light for understorey plants: Contrasting responses to spatial and temporal temperature variation

Abstract

In temperate deciduous forests, the short period of high light in spring, prior to canopy closure, is crucial for annual carbon gain of spring understorey plants. However, if the vegetative phenology of deciduous trees and spring herbs respond differently to climate warming, the duration of this high light period might be altered. Here, we present the first field-based study of the impact of spatial and inter-annual climate variation on the duration of the high light period for herbaceous spring understorey plants based on direct measurements of leaf phenology. For the tree canopy and two understorey species, Trillium erectum and Erythronium americanum, we estimated phenology using automated cameras from 2017 to 2023 along an elevational gradient in Québec, Canada. Direct observations of leaf phenology allowed us to avoid an unlikely assumption in some previous studies (which were based on flowering time): temperature independence of the delay between leaf-out and flowering. For both understorey species and the tree canopy, leaf phenology was sensitive to spatial and temporal variation in spring temperature; earlier leaf expansion occurred at warmer sites and in warmer years. Temporal variation in spring temperature had a stronger effect on understorey than canopy phenology, such that warmer temperatures were associated with an increase in the duration of high spring light, especially at high elevation. Across the spatial (elevational) temperature gradient, temperature sensitivity of one understorey species (T. erectum) was weaker than that for the canopy. Thus, the combination of warmer conditions and different canopy tree species at low elevation was associated with a shorter duration of high spring light. Synthesis. Effects of climate warming on phenological escape of spring understorey plants are likely to be small or positive. Results for temporal temperature suggest that over years or decades (while tree canopy composition remains constant), understorey plants should benefit from a warming-induced prolongation of the high light period. In contrast, results for spatial temperature suggest that, at high elevation, long-term warming and associated shifts in canopy composition might shrink the time window of high light, although an increase in deciduous tree cover would increase the magnitude of light availability.