KROOF 3: Legacy effects and limits of drought tolerance of beech/spruce stands Part A: Impact of repeated drought and drought stress recovery on the growth of pure and mixed stands of beech and spruce – Analysis across temporal and spatial scales

• Applicants: Dr. Torben Hilmers; Professor Dr. Thomas Rötzer
• Subject Area: Forestry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541961221

The "Kranzberg Forest Roof Project" (KROOF) has been funded by the DFG since 2013. The expertise of the participating groups covers forest growth and yield science, ecophysiology, and rhizosphere biology. The University of Innsbruck is involved through the Weave program. In the first funding period (KROOF 1), a throughfall exclusion experiment with about 100 trees was initiated in a mature beech/spruce forest in southern Germany to study the effects of repeated summer drought on trees and their ectomycorrhiza. In comparison, adaptation to long-term drought was studied at five sites along a precipitation gradient. In the second funding period (KROOF 2), the focus was on processes of stand recovery. We are now applying for a third funding period, in which the drought pre-treatment will be used to expose trees to a renewed, potentially lethal drought. The aim is to determine whether the trees have acclimated during the previous drought and are less susceptible to renewed drought or whether the pre-treatment has caused pre-damage and the trees reach their drought tolerance limits earlier and suffer lethal damage. In this way, drought resistance mechanisms and tree death processes will be elucidated. The following hypotheses are of central interest:"Mixing" hypothesis: Mixed stands benefit from structural heterogeneity and asynchronous resource use under drought more than monospecific stands."Weakest link" hypothesis: Acclimations to water shortage are overridden under extreme drought by the break of the weakest link in the soil-plant-atmosphere continuum."Legacy" hypothesis: Tree-soil systems with a drought legacy cope better with renewed drought than tree-soil systems experiencing severe drought for the first time.The present proposal aims to investigate the multifaceted responses of pure and mixed stands of spruce and beech to repeated drought events. It utilizes data from KROOF 1 and KROOF 2 and is supplemented by new measurements. The project sheds light on acclimatization mechanisms from the organ to the stand level and analyzes the consequences of repeated drought on tree growth and mortality. The goal is to integrate empirical results into the process-oriented individual tree growth model BALANCE. This enables precise simulations and studies on various spatial and temporal scales. Crucial insights into the dynamic interplay between forests and their changing environment can be provided, benefiting both science and sustainable forest management.

• DFG Programme: Research Grants