Dendroclimatic investigation of European beech growth dynamics along a precipitation gradient in northern Germany
by Bat-Enerel Banzragch
Date of Examination:2025-04-29
Date of issue:2025-05-22
Advisor:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Markus Hauck
Persistent Address:
http://dx.doi.org/10.53846/goediss-11259
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Abstract
English
This thorough study of European beech (Fagus sylvatica) growth limits in Northern Germany shows the major difficulties caused by climate change and global warming. Drawing on the findings of three main chapters, we arrive at several significant conclusions on the elements affecting the beech growth surface: rising temperatures, changing precipitation patterns, drought, and the increasing influence of extreme weather events. Rising Temperatures and Heat Stress: European beech is particularly vulnerable to rising temperatures in important growth months, including April, June, and September. Heat stress in beech populations has grown as the number of hot days with temperatures above 30 °C rises dramatically since the 1980s (Chapter 4). Summer's higher vapor pressure deficits (VPD) lower stomatal conductance, so restricting photosynthesis and growth. Although warming has lengthened the Growing Season Length (GSL), the higher frequency of heat stress offsets the advantage of a longer season. Extended GSL over 150 days has produced a minimal increase in growth, especially in drier areas; rather, it has caused further decreases due to continuous heat stress (Chapter 4). This shows how, whereas a longer growing season used to foster growth, it is now causing stress-induced decreases in growth. Another major limitation for European beech is drought sensitivity and water stress. The trees are highly dependent on water availability during the summer, especially in June, when maximum stem development occurs. In places where growing season precipitation is less than 360 mm, European beech falls dramatically (Chapter 3). The combination of low precipitation and a declining climatic water balance (CWB) intensifies water stress, particularly during the months of April and June (Chapter 2). A negative CWB causes water shortage, hydraulic failure, and carbon hunger—where evapotranspiration exceeds precipitation. Particularly in the southeast and drier areas of Northern Germany, where water limitations are more clear-cut, these processes reduce growth and increase death rates. The declining CWB has mostly affected the drier parts of Northern Germany, resulting in increased squeezing of beech populations. While water balance varies somewhat in some coastal regions, most inland and southeast places have a negative CWB, which increases drought stress (Chapter 2). This negative water balance prevents beech trees from receiving enough moisture, causing harm to their physiological processes and increasing their chances of growth reduction and death. Soil conditions and stand structure greatly affect European beech's sensitivity to drought and stress brought on by climate change. Drought-induced growth decreases are more likely in beech populations that thrive on sandy soils with low water retention capacity (Chapter 3). Similarly, drought exacerbates the competition for nutrients and water in deep forests with high tree density. Researchers have demonstrated that thinning, a common forest management technique, lessens these effects by lowering competition and increasing resource availability, thereby strengthening beech tree resilience during dry times (Chapter 3). This emphasizes the need for controlling stand density and enhancing soil conditions to allow beech plants to withstand future environmental challenges. Regional Growth Declines: Climate change causes variations in European beech growth over Northern Germany. Drier and more continental sites are experiencing larger growth losses due to their higher heat stress and drought sensitivity. In these places, trees often react to environmental problems such as drought, which causes a general decrease in growth (Chapter 3). Chapter 2 illustrates that wetter coastal areas have more consistent growth patterns, even if rising VPD and heat stress during hot summers are impacting these locations. These geographical differences stress the need for creating customized forest management plans considering the particular meteorological and environmental conditions of every region. Rising temperatures, combined with a prolonged drought, present a twofold threat to European beech from heat and drought. Severe physiological stress brought on by high temperatures and water shortages throughout the growing season lowers growth rates and raises death risk. Particularly sensitive to these combined stressors, beech populations in dry regions aggravate the consequences of heat and water shortages (Chapter 3). This emphasizes the need for coordinated management methods addressing heat and drought to promote beech forest long-term survival in the face of increasing temperatures.
Keywords: dendrochronology, climate
