Positive legacy effects after experimental drought in a beech-spruce forest

This contribution reports on the effects of drought legacies on mature trees after five years of experimental summer drought in a mixed forest in southern Germany. The study objects are c. 70-80 year old European beech (Fagus sylvatica) and Norway spruce (Picea abies) trees growing in either monospecific or mixed species interactions; a total of about 100 trees in 12 plots under experimental drought (Kranzberg Forest Roof Project: KROOF). For five consecutive years, trees were subjected to complete throughfall exclusion during the growing season. This resulted in up to 80% reduction in physiological and morphological parameters such as reduced photosynthesis, growth and leaf area development. In general, the drought effect was much stronger in the more isohydric spruce compared to the more anisohydric beech. After 5 years of throughfall exclusion, drought release was initiated in the early summer of 2019, resulting in faster recovery in beech compared to spruce.

This presentation will focus on the response of previously drought-stressed trees to a natural summer drought in 2022, three years after the start of drought release, with significant legacy effects under the renewed drought. Previously drought-stressed trees showed increased resistance to renewed drought, with higher stomatal conductance, pre-dawn twig water potential and soil water availability, resulting from lower water use of spruce. We interpret this as a positive legacy effect resulting from leaf area acclimation to the preceding drought. At the end of the five years of experimental drought, spruce reduced its leaf area by more than 60%. After three years of drought recovery, spruce leaf area was still reduced by 1/3 compared to unstressed spruce.

Interestingly, drought stress was also reduced in neighboring beech trees in the absence of leaf area reduction. A ²H-labeling experiment of soil water on the experimental plots showed that beech root water uptake reaches far into the spruce-dominated soil area. These results are supported by analyses of the distribution of beech and spruce roots in the plots. We conclude that during drought, the more anisohydric beech effectively accesses water in the soil under the more isohydric spruce, and thus benefits from the water-saving strategy of spruce even years after previous drought events.