Increasing soil water deficit negatively impacts European beech radial growth: a case study combining long-term monitoring (1996-2020) and modeling approaches.

Beech (Fagus sylvatica L.), is one of the most socio-economically valuable and widely distributed broadleaved trees in Europe. European beech favorable edaphic and climatic conditions are sufficient moisture in summer and mild temperatures in winter. This highly competitive species is also known to be drought-sensitive and thus may become more vulnerable to expected increasing number of heat waves and drought spells. A better understanding of beech response to soil drought is therefore crucial for forestry planning and forest management in a future warmer world. In this study, we investigate the water balance and radial growth dynamics of a beech stand in an ICOS site in Belgium (BE-Vielsalm), with a special focus on soil drought impacts, based on about three decades of observations (1996-2020). The continuous decreasing trend of beech radial growth (dendrochronological time series) coupled with recent crown defoliation raised concern about the vitality of this stand. On the basis of this site-specific data set, the calibration and the validation of the model HETEROFOR were performed in order to i) simulate the water balance and fluxes at the stand level; ii) track the occurrence and intensity of soil-induced transpiration deficit during the whole study period; iii) characterize the tree water uptake patterns according to soil depth for contrasted years (dry, intermediate and wet) and iv) evaluate the current and lag effects of spring and summer transpiration deficit on the observed annual radial growth of beech trees obtained from dendrochronological analysis. A good agreement between predicted and observed evapotranspiration and extractable water was obtained and showed the robustness of the model. The predicted transpiration deficit revealed an increasing trend, especially after 2010. We observed a negative effect of the spring transpiration deficit on tree radial growth during the current year as well as a carryover effect (i.e., a negative effect of the summer transpiration deficit of the previous year). This study will enrich the state of knowledge about the ongoing debate on the vulnerability of beech trees to drought in Europe.

Keywords: Fagus sylvatica, radial growth, soil water balance, forest evapotranspiration, drought, process-based modeling, eddy covariance.