Quantifying drought legacy effects in a Mediterranean oak forest using eddy covariance, sap flow, and stem growth data

The frequency and intensity of droughts is expected to increase with climate change. Droughts can affect vegetation through e.g. hydraulic failure, depletion of carbon reserves and reduced growth, which in turn can influence ecosystem functioning beyond the duration of the drought. Previous studies have provided examples of drought legacy effects for example by reduced gross primary productivity (e.g. in Yu et al. 2022). It remains unclear whether legacy effects occur in drought-resistant mediterranean oak forests and to what extent those effects can be detected across water- and carbon-related vegetation variables.

This study investigates legacy effects on gross primary productivity (GPP), sap flow and stem growth of holm oak (Quercus ilex L.) using measurements from an evergreen Mediterranean forest in southern France during the time period 2000 to 2015. We jointly analyze sap flow and growth measurements from several trees located within the footprint of the eddy covariance GPP measurements. In order to isolate the legacy effects, we follow the approach of Yu et al. (2022) using a random forest regression model to predict potential sap flow or gross primary productivity based on concurrent hydro-meteorological conditions and compare potential and actual values. The same approach was applied to stem growth data, but using a linear regression model due to the fewer observations available.

Our study shows that a drought in 2006 caused comparable drought legacy signals across all considered vegetation variables. A slight positive effect, i.e. higher measured values than predicted, was diagnosed in the first post-drought year and no legacy effects were detected in the second post-drought year. The results suggest that Q. ilex shows a fast and complete recovery after the first post-drought year, which can be expected as it is a drought-adapted species. We also observe a large variability in legacy effects across individual trees, which suggests that individual tree properties and local soil characteristics might affect their drought sensitivity and resilience. 

Jointly analyzing drought legacy effects across different scales and variables opens the possibility of a holistic understanding and thus helps to improve the representation of drought effects in predictions of the future land carbon sink. 

Reference: Yu, X., Orth, R., Reichstein, M., Bahn, M., Klosterhalfen, A., Knohl, A., Koebsch, F., Migliavacca, M., Mund, M., Nelson, J. A., Stocker, B. D., Walther, S., & Bastos, A. (2022). Contrasting drought legacy effects on gross primary productivity in a mixed versus pure beech forest. Biogeosciences, 19(17), 4315–4329. https://doi.org/10.5194/bg-19-4315-2022