Nemoral European Beech and Oak Forests under Climate Change
Cumulative thesis
Date of Examination:2022-06-23
Date of issue:2023-05-16
Advisor:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Dirk Hoelscher
Referee:Prof. Dr. Christian Ammer
Referee:Prof. Dr. Erwin Bergmeier
Referee:Prof. Dr. Dominik Seidel
Referee:Prof. Dr. Helge Walentowski
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Abstract
English
Global warming and increasing drought severity are exposing temperate forests to higher stress levels, challenging forest management in the 21st century. With a projected warming by 2–3 K until 2070, silvicultural adaptation measures and natural succession might lead to the replacement of European beech forests by thermophilic oak forests in drought- and heat-affected regions of Central Europe. According to RCP scenarios (RCP2.6, RCP4.5 and RCP8.5), parts of western Romania, where beech naturally occurs at its dry-warm distribution limit, are climatically analogue to predictions for large regions of Central Europe. In a “space for time approach” we investigate impacts on ecosystem carbon storage and tree vitality for a climatically driven shift in forest structure. Therefore, we systematically sampled soils and forests over natural beech–oak ecotones, quantifying storage changes in above ground biomass carbon (AGC) and soil organic carbon (SOC) between beech (Fagus sylvatica) dominated forests and oak (Quercus petraea, Q. frainetto, Q. cerris) dominated forests. Precise predictions of climate change impacts on forests also require a better species-specific and site-specific understanding of how tree growth and tree climate relationships are affected. We assessed tree vitality in these beech–oak ecotones, by analyzing tree-ring records and investigating long-term growth-trends, resilience of radial growth to drought, growth climate sensitivity, spatiotemporal patterns of climatic sensitivity and growth synchronicity for beech populations, the three oak species and silver linden (Tilia tomentosa), a further dominant species. Our results show that a climate-warming related replacement of beech by oak forests in the course of natural forest succession or silvicultural decisions may considerably reduce ecosystem carbon storage of central European woodlands. From the cooler, more humid beech forests to the warmer, more xeric oak forests, which are 1–2 K warmer, AGC and SOC pools decrease by about 22 % (40 Mg C ha-1) and 20 % (17 Mg C ha-1), respectively. Tree-growth-climate analysis show, that radial growth of all species is positively influenced by summer precipitation and low drought intensity, and negatively by high summer temperatures. Basal area increment (BAI) of beech and linden declined in the last 10–20 years in coherence with climate warming and a deterioration of the summer water balance, while the three oak species maintained stable growth rates, though at lower BAI levels, suggesting a negative relationship between mean BAI and drought resistance among the five species. Spatiotemporal patterns of climatic sensitivity show that the importance of summer precipitation increased after the onset of climate warming (⁓ 1980), while other climate factors in spring and summer became less important. Accordingly, growth synchronicity, as a measure of common climatic stress among tree individuals, increased or remained constant for the drought sensitive beech and linden and decreased in the past decades for the oak species. The differences in growth synchrony during recent climate warming indicate a better drought adaption of oak species, a conclusion which is supported by the results for the long-term growth dynamics, showing enhanced BAI for oak in comparison to beech and linden in the last decades. Our results demonstrate that choosing stress-tolerant oaks instead of more productive timber species such as beech is a relatively safe option for Central European forestry in a warmer climate. However, if drought- and heat-affected beech forests in Central Europe are replaced by thermophilic oak forests in future, this will lead to carbon losses of ~ 50–60 Mg ha-1, thus reducing ecosystem carbon storage substantially.
Keywords: climate turning point; above ground carbon; soil carbon; dendrochronology; beech-oak ecotone; climate change; climate sensitivity; Fagus sylvatica; soil nutrient pools; basal area increment; drought resilience; growth decline; climate-growth relationship; climate warming; radial growth; Romania; tree rings; silver lime; European beech; forest management; Quercus cerris; Quercus frainetto; Quercus petraea