Assessing carbon fluxes following non-invasive and clear-cut management responses to widespread drought mortality of a Scots Pine plantation

In recent years, frequent dry and hot summer periods in Central Europe have caused irreversible damages to many forest ecosystems. The consequences are widespread tree mortality, including forest ecosystems in the Upper Rhine Valley. We compare two management responses to a highly impacted, mature Scots Pine (Pinus sylvestris) plantation in the Upper Rhine Valley at the ICOS Site DE-Har by assessing annual and seasonal carbon fluxes in the first seven years following the management response.

At the non-invasively managed site, >60% of all former Pinus sylvestris trees died since 2018 and consequently the canopy opened up considerably. Dead and fallen trees were generally not removed. The site has undergone a significant regime change in which increased sunlight under the damaged/missing tree crowns has accelerated growth of a deciduous understory (mainly Tilia cordata, Carpinus betulus, and Fagus sylvatica among others). At the clear-cut site, all Pinus sylvestris trees were fully removed in autumn 2017, and new saplings consisting of various broad-leaf trees, more suited for hot and dry weather conditions (including Acer platanoids, Corlyus colurna, Carpinus betulus), were planted in spring 2018 and 2019. Due to extreme drought, almost all of the saplings died shortly after they were planted and the area now consists of grasses, shrubs and a few deciduous trees.

We use concurrent eddy covariance measurements at the non-invasively managed site since 2019 and at the clear-cut site since 2021 to quantify the effect of the two management responses on net CO₂ fluxes and partitioned gross primary productivity (GPP) and ecosystem respiration (R_eco). On average over the period from 2019 to 2024, the non-invasively managed site has been a small CO₂ source (NEE = +75 g C m^-2 year^-1), compared to 20 years ago, when the mostly healthy forest was still a considerable CO₂ sink.  Typically, the non-invasively managed site is a CO₂ source during winter and autumn and a CO₂ sink in spring and summer, except for the hot and dry summer of 2022. On average over the period from 2021 to 2024, the clear-cut site has been a substantial CO₂ source (NEE = +460 g C m^-2 year^-1), mainly because of higher values of R_eco. The NEE data of the clear-cut site also show a yearly cycle, with higher values in winter and autumn and lower values in spring and summer, nevertheless the clear-cut site was a CO₂ source in all seasons during the last four years. The highest annual NEE values at both sites can be found in the hot and dry year 2022. Seven years after the clear-cut, both sites are still CO₂ sources and it is uncertain whether and when either of these sites will become a CO₂ sink.