Impact of Recurrent Droughts on the Water Use and Growth Dynamics of Larch and Spruce: insights from a long-term experiment in the Austrian Alps

In the face of climate change, droughts are becoming more frequent and intense, exposing trees to ever-increasing physiological stress. Despite extensive research, the effects of recurrent droughts on tree carbon and water relations remain poorly understood, particularly in mountain forests. At a subalpine Long-Term Ecological Research (LTER) site in the Austrian Alps, we investigated the impacts of eight years of recurrent summer droughts on two conifer species—larch (Larix decidua) and spruce (Picea abies). Using comprehensive dendrometer and xylem sap flow data from 2021–2024, encompassing three years of drought followed by one year of recovery, we tested the following hypotheses: (i) recurrent droughts amplify drought responses of radial tree growth dynamics and water use, (ii) drought history causes lagged responses on growth dynamics and water use during a recovery year, with larch exhibiting greater resilience. Our preliminary findings reveal significant drought-induced reductions in sap flow, as well as in mean and maximum growth rates for both species during the treatment years. Yet, contrary to our expectations, multiple recurrent droughts did not significantly amplify the growth and water use sensitivity of trees at this subalpine site. During the recovery year, sap flow did not show legacy effects for either species; however, growth rates remained consistently suppressed, most notably in larch. Thus, our results suggest that although recurrent summer droughts do not have any lagged effects on water use dynamics in a recovery year, legacies lead to major reductions in growth, particularly in larch, which may be less resilient than expected.