Comparing northern German Scots pine plantations with natural beech forests: Soil carbon pools and growth dynamicsDissertation
Datum der mündl. Prüfung:2022-03-24
Betreuer:Prof. Dr. Christoph Leuschner
Gutachter:Prof. Dr. Christoph Leuschner
Gutachter:Prof. Dr. Dominik Seidel
EnglischForestry in the North German Lowlands has been historically characterized by pine in particular. Apart from pine, beech is one of the most important deciduous tree species in this region and would naturally also be the predominant species. The intensive cultivation of pine in the North German Lowlands evolved due to demographic changes and beginning of industrialization in the 18th century. Due to its low site requirements and fast growth, pine was considered the optimal tree species for the already nutrient-poor sandy soils degraded by partly intensive use as forest pasture or litter raking. The large-scale cultivation has been affecting landscape structure, biodiversity, soil, macro-, and microclimate until today, and will probably do so in the future. As climate change progressed over the last decades, more and more disadvantages of monocultures became apparent. From this followed a rethinking in forestry towards more site-appropriate mixed stands. Restructuring forests is important so they can form stable ecosystems for the future climate and thus continue to perform or provide their ecosystem functions and services. Particularly important functions regarding the ongoing climate change are the storage of carbon to mitigate climate change and provision of drinking water. There has been much debate among experts about which tree species or mixed stands will form the forests of the future in northern Germany. If one follows the potential natural vegetation, it would be beech forests in large parts of the lowlands. However, with reference to climate change and the associated shift of species ranges, a rethinking of the approach is necessary. This dissertation thesis aims to contribute to the decision process between tree species for future cultivation by analyzing paired beech forests with pine plantations in terms of their soil carbon storage and their climate sensitivity and growth behavior. Data were collected in 10 pairs of pine and beech stands in the North German Lowlands along a climatic gradient. In each stand, 10 soil profiles were established for the analysis of soil data. Soil samples were taken from four soil depths, up to 60 cm depth and the organic layer. For the dendrochronological investigations, cores of 20 trees per stand were taken and analyzed in the laboratory and correlated with climate data. While the organic layer on average stored about 80 % more C under pine than beech, the pools of soil oranic carbon (SOC) and total N in the total profile (organic layer plus mineral soil measured to 60 cm and extrapolated to 100 cm) were greater under pine by about 40 % and 20 %, respectively. Besides tree species, neither precipitation nor temperature influenced the SOC pool for this data set. An extended data set (which included additional pine stands established more recently on former agricultural soil) revealed that, besides tree species identity, forest continuity is an important factor determining the SOC and nutrient pools of these stands. In the majority of beech stands, basal area increment (BAI) has lost its positive trend since the 1980s or growth declined since then, while the BAI of pine has continually increased. Long-term change in June precipitation is a more important determinant of beech growth trends in the study region than the amount of mean annual precipitation (MAP), while pine growth is largely dependent on the warmth of February/March. Yet, pine growth is also sensitive to low precipitation and high climatic aridity in midsummer, with sensitivity increasing toward low MAP. Until recently, the negative impact of increasingly warm summers on growth has been compensated by the positive effect of late-winter warming, resulting in stable (or increasing) growth trends. Climate sensitivity of growth has significantly declined since the 1980s in beech, while no such trend could be observed in pine, even in regions with declining June precipitation. In light of existing research, the results of this study suggest that neither pine nor beech forests are unconditionally sustainable forest communities for future forestry in the North German Lowlands. From the current perspective, it will probably come down to a compromise in the choice of sustainable tree species, where a balance between climate sensitivity and productivity should be the goal. In this context, further research is needed on which tree species – and, in particular, which mix of species – can meet both the demands of future forest ecosystems and show possible adaptation potential.
Keywords: Beech; Pine; Fagus sylvatica; Pinus sylvestris; Soil organic carbon; Basic cations; Tree species effect; Forest history; Dendrochronology; Drought sensitivity; June precipitation; Non-stationarity; Precipitation gradient; Sandy soil; climate sensitivity; winter warming; summer drought