Differences in carbon uptake and water use between managed and unmanaged European beech forests

Based on 23 site-years of eddy covariance measurements made above a managed beech forest and a nearby unmanaged, old-growth forest in central Germany, a comparative analysis of the carbon and energy fluxes of the two forests was carried out. Both forests are located at similar altitude and they face similar meteorological conditions. They are also similar with respect to canopy height (37 m) and mean tree age (120 years). The managed beech forest is a monospecific, even-aged stand with species-rich ground vegetation and a leaf area index of about 4, whereas the old-growth forest is beech-dominated but interspersed with ash and sycamore trees. It has a multi-layer canopy consisting of trees of various ages ranging from 0 to 260 years and its leaf area index is about 5. On average the two forests did not differ significantly in annual net carbon uptake (508 and 483 g C m-2 a-1 for the managed and the unmanaged forest, respectively), however the managed forest showed a much larger interannual variability in gross primary production than the unmanaged forest did. This trend agreed well with independent dendrometric measurements made in both forests. In contrast, ecosystem respiration did neither vary significantly between the two forests nor between different years. The total annual evapotranspiration was slightly higher at the unmanaged forest site (549 mm a-1 compared to 504 mm a-1 at the managed site), which was probably due to a higher interception loss from the denser canopy in the unmanaged forest. We discuss whether the conclusion can be drawn from this case study that common forest management activities improve the water use efficiency of European beech forests but make them more vulnerable against extreme meteorological conditions such as, for example, summer heat waves or late frosts in spring.