Long-term dynamics of CO2 fluxes over a managed and an unmanaged beech forest

Forests are important natural carbon sinks and can help mitigate climate change. The drought and heat waves of recent years have severely affected forests in Germany, resulting in reduced net CO₂ uptake. How forest management, age and species composition moderate the negative impacts of weather extremes on net CO₂ uptake or its recovery is still unknown.

For around 25 years, gross primary production, ecosystem respiration and net ecosystem exchange as well as evapotranspiration have been studied in a unmanaged, uneven-aged and mixed beech stand in the Hainich National Park (DE-Hai) and a managed, even-aged and pure beech stand near Leinefelde (DE-Lnf) in Thuringia, Germany.

Both forest stands were and are a substantial CO₂ sink (DE-Hai: 512±89 gC m^-2 a^-1; DE-Lnf: 590±190 gC m^-2 a^-1), whereby the annual CO₂ uptake of the managed stand varied significantly more than that of the unmanaged stand. The CO₂ sink function of both stands persisted even in the extremely dry and hot year 2018, though the annual CO₂ uptake was reduced by 27% (DE-Hai) and 64% (DE-Lnf) compared to the long-term average from 2002-2017. A reduction in CO₂ uptake was also evident in the following year, which can mainly be attributed to persistently low soil water availability. In addition, a loss of tree vitality was observed, which affected the CO₂ balance in the following years. In contrast to the unmanaged stand, however, the managed stand already reached higher uptake rates again in 2020. The differences between the stands can mainly be explained by differences in tree age and stand structure. With a mean age of about 130 years, the managed stand consists almost exclusively of vigorous beech trees (optimal phase), whereas the unmanaged stand comprises all age classes and developmental stages, in particular a high proportion of very old trees (> 180 years), which were particularly badly damaged by the drought.

Long-term flux measurement covering 25 years revealed divergent responses of the two differently managed and structured forest stands to drought. In a next step, more sites covering a range of management strategies, species and ages need to be included.