Varying growth response of Central European tree species to the extraordinary drought period of 2018 – 2020

Abstract

Droughts have intensified in Central Europe and will likely become a major driver of forest ecosystem change in the future. Regional studies are needed to understand drought impacts on forests and to provide guidance for adaptive management. In this study, we have investigated the effects of an extraordinary drought period in 2018 – 2020 on tree growth in the State of Bavaria (Germany). We (i) assessed overall changes in tree growth compared to pre-drought conditions, (ii) mapped the spatial configuration of drought and its impact on growth across ecoregions, (iii) quantified the standardized drought effect for the most abundant tree species, and (iv) analyzed the inter- and intraspecific variability in growth responses to drought.Covering a large environmental gradient, we compared 3,462 tree basal area growth records at 200 inventory plots with the pre-drought period of 2006 – 2017. We analyzed drought impacts on tree growth using nonparametric randomization tests and linear quantile mixed models (LQMMs).Median annual tree growth declined by 41.3% during drought. Decreases in growth were statistically significant for 13 out of 20 species. Growth reductions were most distinct in northwestern ecoregions of Bavaria. After controlling for other covariates of tree growth, the most abundant conifers, Picea abies [Karst.] and Pinus sylvestris [L.], were overall more susceptible than the common broadleaves, Fagus sylvatica [L.], Quercus robur [L.], and Quercus petraea [Matt.]. Yet, all species exhibited a high intraspecific variation in drought response.Altering tree species composition is a key strategy to adapt forest ecosystems. Our study reveals high potential to mitigate future drought impacts by admixing P. abies- and P. sylvestris-dominated forests with more drought-adapted broadleaved tree species. In addition, the high intraspecific variability in drought susceptibility indicates potential to promote provenances or individuals with low drought sensitivity to reduce the local loss of tree species.