Dynamics in CO2 uptake, growth, and mortality of an old-growth temperate forest under drought stress

Understanding drought stress responses and adaptation mechanisms in forest ecosystems towards climate extremes is crucial. This knowledge aids in assessing adaptive capacities and developing supportive management measures. Here, a comprehensive long-term data set obtained in the Hainich National Park, an old-growth mixed-beech forest in Central Germany (DE-Hai), gives the opportunity to investigate stress effects and transformation processes caused by the 2018 and 2019 summer droughts on tree and stand scale. The forest displays a near-natural, diverse system with a range of tree age classes and species (main species: Fagus sylvatica, Fraxinus excelsior, Acer pseudoplatanus).

In this study, we combined long-term observations of stand-level CO₂ exchange obtained with the eddy covariance method with annual growth records and structural indices of 80 trees obtained from dendrometer bands and terrestrial laser scans. Further, drone and satellite imagery provided estimates of tree mortality through canopy gap dynamics. Based on this multi-scale data set, we strive to better understand the link between forest CO₂ uptake and tree response dynamics under the influence of a severe drought.

During the drought events in 2018 and 2019, we observed that the forest remained a net CO₂ sink, but the CO₂ uptake strength was considerably diminished (up to -30%) in comparison to the reference period of the previous 17 years. Moreover, the reduction in CO₂ uptake extended beyond the duration of the droughts, which implies significant changes in the mechanisms and dynamics of the forest. Further, an increase in the canopy gap fraction by more than 50% in 2021 indicated a significant increase in tree mortality. Surviving trees were affected differently by the droughts depending on species-specific stress response strategies and a tree’s role as competitor or suppressed individual. In particular, the growth of older and larger trees, mostly Fraxinus excelsior, was impaired during and after the drought period. However, approximately half of the observed trees, mostly suppressed, vital Fagus sylvatica, showed a positive growth trend during and after the drought period.

The structural diversity of the old-growth mixed forest could buffer the drought-induced outage in the CO₂ uptake strength, though the increased growth of a large cohort of surviving, suppressed Fagus sylvatica could not compensate for the diminished CO₂ uptake by a few dying dominant trees. The natural succession dynamics in the forest ecosystem seem to be accelerated due to drought events. A continuous and consistent long-term monitoring of forest ecosystems is needed to further investigate the initiated transformation processes, the stand and tree resilience, and additionally the impact of legacy effects.