# Hydro‐Functional Traits and Their Dissimilarity to the Neighbourhood Buffer Tree Growth Against the 2018–2020 Central European Drought

## TL;DR

This study shows that tree growth during drought is influenced by water-use strategies and that mixing tree species with different strategies can improve forest resilience to drought.

## Contribution

The study introduces a novel hydro-functional trait-based approach to understand tree growth during drought and highlights the benefits of species mixing.

## Key findings

- Hydraulic safety and stomatal control traits significantly influence tree growth during drought.
- Trees with dissimilar hydro-functional traits to their neighbors grow better during drought.
- Certain traits beneficial under drought are less so under normal conditions, indicating a performance trade-off.

## Abstract

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 2611 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. Our results reveal that traits related to hydraulic safety and stomatal control potential were key drivers of tree growth across drought and non‐drought years. During the severe multi‐year drought, the growth of focal trees increased with either higher hydraulic safety or tighter stomatal control potential. Trait syndromes that were less advantageous under normal conditions provided beneficial effects under drought stress, reflecting a trade‐off in performance across conditions. Additionally, we found that hydro‐functional dissimilarity between a tree and its surrounding neighbors provided benefits for tree growth during drought. Therefore, our study suggests that planting tree mixtures with distinct hydro‐functional strategies can enhance resistance to future droughts.

Climate change is putting forests at risk. Our research shows that a tree's water‐use strategy largely determines its growth across years: whereas one strategy performs better in normal years, the other is advantageous during drought. Trees grow best during drought when surrounded by neighbors whose water‐use strategies differ from their own, suggesting that planting a mix of species with different water‐use strategies could help forests better withstand future droughts.