What if extreme droughts occur more frequently? – Mechanisms and limits of forest adaptation in pine monocultures and mixed forests in Berlin-Brandenburg, Germany

Abstract. Forests in Eastern Germany are already experiencing the detrimental effects of droughts, exemplified by the severe conditions of the 2018 drought year. With climate change, such extreme events are expected to become more frequent and severe. Previous work suggests that mixed forests exhibit greater resilience against droughts than monocultures. Our study aims to investigate the impact of increased frequency of extreme droughts, such as those seen in 2018, on biomass, structure and traits of forests in the Eastern German federal states of Berlin and Brandenburg. Utilizing the flexible-trait Dynamic Global Vegetation Model LPJmL-FIT, we simulate the growth and competition of individual trees in both, pine monoculture forest and mixed forest. The trees belong to different plant functional types or in case of pine forest are parametrized as Pinus sylvestris. We create drought scenarios from high resolution climate input data by re-shuffling the contemporary climate with increased frequencies of the extreme drought year 2018. For each scenario, we simulated vegetation dynamics over 800 simulation years which allowed us to analyze shorter-term impacts, in the first decades of the drought scenarios, as well as the long-term adaptation of the two forest types to those new climate normals. We evaluated the resulting long-term changes in biomass, plant functional traits and forest structure to examine the new equilibrium state emerging for each scenario. Our findings revealed nuanced responses to increased drought frequency. In pine monoculture forests, increased drought frequency reduced biomass and increased biomass variance, indicating higher system instability. Conversely, in mixed forests, biomass initially declined in scenarios with increased drought frequency but eventually recovered and even exceeded baseline levels after 100–150 years. We explain recovery and increase of biomass through two forest adaptation mechanisms; first, we saw a shift in the plant community towards broadleaved trees and second, plant traits shifted towards increased average wood density, decreased average tree height and increased average tree age. However, for the most extreme scenario with drought occurring each year, the adaptive capacity of the mixed forest was exceeded and the biomass halved compared to the baseline scenario. In our study, for the first time LPJmL-FIT is used with a resolution as high as 2 by 2 km², which allows us to observe spatial heterogeneity drought impacts within the Berlin-Brandenburg area. Pine monocultures suffered, especially in the warmer urban areas and mixed forests in the central-west of Brandenburg, benefitted in the long term. This study highlights the capacity of natural mixed forests in contrast to pine monocultures to adapt to increasing drought frequency up to a certain limit. The results underscore the importance of considering biodiversity in forest management strategies, especially with regard to more frequent dry periods under climate change.