Limited modelled legacy effects of drought on carbon and water fluxes in a temperate oak forest despite hydraulic impairment

Abstract. The projected increase in the intensity, frequency and duration of summer drought extremes under climate change poses a growing risk to forest ecosystems, even in mesic regions such as the UK. To accurately predict the impact of future summer droughts, it is necessary to account for both their immediate and legacy effects, as well as their influence on ecosystem sensitivity to drought over time. We introduce a stomatal optimisation scheme, Profit-Max, within the Joint UK Land Environment Simulator (JULES) land surface model (LSM) to improve predictions of plant response to drought induced water stress. We further extend the model to represent drought legacy effects through two approaches to capture hydraulic impairment of xylem. We evaluate these developments by simulating an old growth oak forest at the Alice Holt site in southern England during the 2022 UK summer (July to September), when rainfall was substantially below average (77 mm of rainfall compared with a 1999–2018 median of 140 mm) and national temperatures exceeded 40 °C for the first time. During the 2022 drought year, the stomatal optimisation scheme predicted reductions in annual GPP and LE, −26 % and −33 % respectively, relative to an average year (2011–2017). This was larger than the measured reduction in LE of −6 %. The hydraulic legacy schemes simulated percentage loss of conductance of 9 % to 40 % ,with a carry-over effect on LE flux (−1 % to −11 %). We further explored ecosystem responses under more extreme future summers, including zero rainfall in spring (similar to the meteorology conditions of 2025) and a 50 % reduction in rainfall over the remainder of the year. Despite substantial increases in the percentage loss of conductance (10 % to 55 %), the simulated reductions in GPP and LE remained modest in 2023, consistent with observations. The magnitude of these legacy effects, however, was sensitive to the representation of hydraulic impairment and recovery. Overall, our results demonstrate the feasibility of representing drought legacy effects within a LSM and highlight the importance of hydraulic impairment assumptions in shaping projected ecosystem responses to extreme drought. While the simulations suggest limited legacy impacts on carbon uptake and latent heat fluxes at Alice Holt following the 2022 drought, evaluation of the underlying plant water status and hydraulic function will require independent observations of water potential and hydraulic function across the soil-plant-atmosphere continuum, as flux tower data alone may not fully constrain longer-term impacts on growth and resilience.