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Wälder aus der Balance: Die Auswirkungen von Dürre und Baumsterben auf den Kohlenstoff- und Wasserkreislauf

Fachliche Zuordnung Ökologie und Biodiversität der Pflanzen und Ökosysteme
Forstwissenschaften
Förderung Förderung von 2016 bis 2023
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 286857303
 
Erstellungsjahr 2024

Zusammenfassung der Projektergebnisse

Trees and forests are affected by a rapidly changing climate. The rise in frequency and severity of extreme drought events, exemplified by the conditions in Germany during the exceptionally dry years of 2018 and 2019, has led to reduced tree vitality and increased mortality. Despite the common assumption in the drought tolerance of pine trees, Scots pine experienced widespread mortality during these extreme drought years. However, whether this trend will persist, diminish, or escalate remains uncertain, primarily due to gaps in our understanding of drought resilience processes, especially concerning how stress impacts impede post-stress recovery. This project concentrated on investigating the responses of pine trees to drought and heat stress, as well as their capacity to recover from such stress events, by integrating experimental and modeling approaches. Our experimental findings emphasized the tight regulation of the carbon (C) balance in pine trees: a reduction in C gain from photosynthesis led to corresponding decreases in respiration and growth. During periods of water scarcity and low assimilation, both above- and belowground growth ceased abruptly, accompanied by a decline in starch reserves. This suggests that pine trees can swiftly respond to stress and maintain a relatively balanced C metabolism, thus avoiding C starvation. However, as drought persists, dehydration and damage to the hydraulic system intensify. Furthermore, we demonstrated that the impacts of drought stress exacerbate with increased atmospheric dryness and heat stress, and elevated CO2 levels do not ameliorate the stress response. Upon release from severe drought, Scots pine seedlings were unable to repair damage to the water transport system, delaying the recovery of photosynthesis and growth, while carbohydrate reserves accumulated. In addition to these adverse responses, we observed that stress can prompt structural adjustments, such as reductions in leaf area, which can decrease tree water loss and mitigate drought stress in the longer-term. To evaluate these processes at the forest level, we incorporated a detailed tree hydraulic description into an advanced ecosystem model. The model accurately depicted the impacts of drought by simulating a reduction in hydraulic conductance, leading to losses in leaf area and sapwood area. We successfully applied the model to a semi-arid pine site, which is welladapted to seasonal drought conditions. Our model findings aligned with observations, showing minimal loss of hydraulic functioning in the semi-arid forest. This contrasts with the severe drought responses observed in temperate forests following the 2018/2019 summer droughts, which were captured by the model through damages of leaf and sapwood area. Albeit often overlooked by models, simulating these extreme stress damages is crucial for evaluating the long-term effects of climate change on forest ecosystems.

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