Warming-induced changes in leaf phenology could amplify the effects of spring drought on tree seedlings

Droughts are not only becoming more frequent and intense but are also increasingly occurring at different times of the year. Seasonal shifts in water availability, including changes in early growing-season precipitation, could lead to more frequent spring droughts in temperate ecosystems. However, the ecological effects of warming combined with water shortages at the start of the vegetation period remain incompletely understood. We conducted a climate chamber experiment to examine how warming temperatures (current, + 2 °C and + 4 °C) and the timing of drought (spring vs. summer) interact to affect seedling survival and growth of four temperate tree species ( Picea abies, Pinus sylvestris, Fagus sylvatica, Sorbus aria ) with contrasting leaf habit (deciduous vs. evergreen) and drought tolerance (low vs. high). Using high temporal resolution data from regional weather stations we simulated realistic seasonal changes in weather and drought conditions at the submontane-montane ecotone of the northern Alps. We found that spring drought impacted seedling survival more strongly than summer drought. Under warmer climate, these effects were potentially amplified via two pathways: (i) higher atmospheric water demand and (ii) shifts in phenology that, when not matched by shifts in drought timing, expose seedlings to drought during particularly vulnerable development stages. Our results highlight that warming-induced advances in leaf phenology may compound the effects of drought. The simultaneously reduced efficiency of species adaptations to drought under higher temperatures suggests that hotter droughts could increasingly challenge tree regeneration in temperate forest ecosystems in a warming world.