Impacts of warmer temperatures on the above- and belowground system of four economically important timber species
by Lena Enderle
Date of Examination:2025-01-15
Date of issue:2025-12-17
Advisor:Dr. Dietrich Hertel
Referee:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Dirk Hölscher
Persistent Address:
http://dx.doi.org/10.53846/goediss-11712
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Abstract
English
Recent decades of climate warming, along with more frequent heatwaves, have profoundly affected European forests, causing vitality loss and increased mortality in many tree species. Given the essential role forests play as carbon sinks in mitigating climate change, it is crucial to deepen our understanding of species-specific responses to warming aiming to inform forest management and preserve ecosystem functionality. While most existing studies focus on the impacts of drought, there has been comparatively less attention on the effects of elevated temperatures. This thesis investigates how four major timber species—European beech (Fagus sylvatica), sessile oak (Quercus petraea), Scots pine (Pinus sylvestris), and Douglas fir (Pseudotsuga menziesii)—respond to a warmer climate. The study compares forest stands in two regions of Germany, the northern German lowlands and the Upper Rhine Valley. In the Rhine Valley, mean annual temperatures are approximately 2°C higher, reflecting conditions expected in large parts of Germany by the middle to end of the century. The research examines both aboveground growth using dendroecological methods and belowground responses through analyses of fine root traits. Radial growth data from the past three decades was analyzed to discern trends since the significant onset of climate warming and to identify climatic variables that limit growth in both study regions. Findings reveal that a higher mean annual temperature of 2°C, alongside more frequent hot days, increases the sensitivity of beech and oak to water stress, while conifers in both regions are sensitive. Moreover, Douglas fir and Scots pine exhibited a stronger negative response to summer heat. Over recent decades, basal area increment (BAI) has declined for Scots pine and Douglas fir across both regions. Beech exhibited similar declines in the warmer Upper Rhine Valley, while oak showed a positive growth trend. Negative pointer years were closely associated with dry conditions. Although a radial growth decrease resulting from climatic stress was observed, all species recovered within one to three years, which highlights their potential resilience to short-term climate fluctuations. Belowground responses were assessed through a fine root inventory and the use of ingrowth cores. In the warmer region, fine root biomass was significantly lower for beech and Douglas fir, while cumulative fine root surface area decreased for pine, Douglas fir, and beech. Oak, however, remained unaffected. Root productivity and longevity also declined for Douglas fir and pine in the warmer region, reflecting their vulnerability to warming. Beech displayed a slight, non-significant reduction in productivity, but its root longevity increased marginally. Oak, in contrast, demonstrated tendencies toward both increased productivity and longevity, suggesting highest resilience across species. These findings reveal significant differences in belowground vulnerability to warming among Central Europe's major timber species in favor of broadleaved species. The results emphasize the complex and species-specific responses of Central Europe's major timber species to climate warming, emphasizing the importance of considering both aboveground and belowground processes. The studies underscore the elevated vulnerability of conifers compared to broadleaved species, with sessile oak standing out as the most resilient. The findings align with previous research highlighting oak as a critical component of future management strategies to promote forest resilience under climate change. However, long-term effects of more frequent and severe heatwaves in combination with droughts remain uncertain, emphasizing the need for further studies on physiological and ecological mechanisms underlying the species-specific responses. This research provides valuable insights that can are beneficial for adaptive management strategies in climate- smart forestry. By integrating more heat- and drought-tolerant species, enhancing forest structural diversity, and prioritizing site-specific approaches, we can improve forest resilience and vitality, thus protecting their role as carbon sinks in a rapidly warming world.
Keywords: dendroecology; climate warming; thermal acclimation; fine roots; forestry; tree species
