Come back stronger? The response of mature beech and spruce trees to renewed drought in a long-term throughfall exclusion experiment

The Kranzberg Forest Roof Experiment (KROOF: https://www.lss.ls.tum.de/en/lsai/kroof/) is a long-term study investigating the impact of drought on about 100 mature European beech and Norway spruce trees in both pure and mixed stands. From 2014 to mid-2019, the trees were exposed to severe experimental drought conditions, after which they underwent a five-year recovery period. The main focus of the current study is to investigate how drought-naïve and drought-legacy trees respond to renewed droughts. Did the preceding drought, which caused a significant decline in physiological and morphological parameters such as photosynthesis, water consumption, growth and leaf area, weaken or strengthen the trees? Does growth in mixtures affect the response to renewed droughts?

Here, we present the results of two datasets. The first study made use of the natural summer drought in 2022. Drought-naïve trees exhibited strong negative effects of drought, such as reduced stomatal conductance and xylem sap flow, as well as reduced growth. Conversely, legacy trees recovering from the preceeding drought period showed significantly reduced drought stress. This was due to the lower water consumption of spruce trees, caused by their reduced whole-tree leaf area. Three years after the drought treatment, the leaf area of legacy spruce trees was still 30% lower than that of drought-naïve trees. Interestingly, legacy beech trees also benefited from the previous drought treatment despite not showing significant reductions in leaf area. It seems that beech trees benefited from the water saving of neighboring spruce trees, as their roots reach far into the soil under spruce.

The second study started in spring 2025 as the third phase of the KROOF experiment. Here we compare drought-naïve and legacy trees under experimentally induced drought conditions. In this phase of the KROOF experiment, the trees are exposed to extreme, potentially lethal drought conditions, with full exclusion of precipitation throughfall and stem runoff, over the whole year. Initial data support the hypothesis that legacy trees have acclimatised to the previous drought period. For instance, we observed a delayed reduction in predawn twig water potential in legacy trees compared to drought-naïve trees. While all trees have survived the extreme drought treatment thus far, we anticipate the first trees to die within the next two years, likely beginning with spruce. In the following years, we will study whether there are differences in mortality patterns between drought-naïve and legacy trees, and between growth in pure and mixed stands.