R scripts used in Sachsenmaier et al. (2025): Hydro-functional traits and their dissimilarity to the neighbourhood buffer tree growth against the 2018-2020 Central European drought

Climate change is predicted to increase the frequency and duration of drought events, threatening the functioning of Central European forests. While diverse forests often promote long-term growth stability, their performance during extreme drought events remains debated. Understanding the effects of forest diversity on tree growth during drought requires not only a consideration of tree interactions with direct neighbouring trees but also of the species’ morphological and physiological characteristics, i.e., a trait-based approach. Contrasting species-specific drought responses might be driven by hydro-functional traits, which shape a tree’s hydraulic safety and stomatal control strategy.
We investigated individual tree growth before, during, and after the unprecedented 2018-2020 Central European drought, from a hydro-functional perspective. We analysed annual growth data from 2,611 trees in a temperate tree diversity experiment (MyDiv experiment, Germany), measured 14 hydro-functional traits, and modelled individual tree growth across the years 2016-2021, considering the interaction of climatic conditions with hydro-functional trait syndromes. We analysed if the hydro-functional dissimilarity to the tree neigbhourhood can buffer tree growth against drought.

The R Scripts used for this analysis:
(1) synthesize all hydro-functional trait data measured, calculates the species mean of all 14 traits, and performs a Principal Component Analysis (PCA)
(2) uses those PC axes to predict tree growth over the years
(3) uses the trait data,calculates hydro-functional dissimilarity for each focal tree, and uses this variable as a predictor of tree growth over the years (alone, and in interaction with the hydrofunctional identity of the trees).