PhD Thesis

Forest response to climate warming and drought in Europe

Mirela Beloiu Schwenke (09/2017-08/2021)

Support: Carl Beierkuhnlein, Jörg Ewald, Cyrus Samimi, Sigrid Liede-Schumann

Global surface temperature is rising at an alarming rate. The combined forces of global warming and drought are threatening ecosystems around the world. In recent years, the Northern Hemisphere has been hit by a combination of drought and heatwave - with devastating environmental and economic consequences. These disasters, which were once considered purely rare events, have increased significantly in frequency, intensity, and duration in recent years. Forests are of great ecological and economic importance for the proper functioning of natural and human systems. Hence, forest observed susceptibility to increasing warming and droughts is of great concern. Given that the future of forests is uncertain, there is an urgent need to assess forest response to these pressing climate change issues. This dissertation aims to expand current understanding and knowledge of forest response to climate warming and drought. To achieve this aim, this dissertation focuses on two major objectives (1) to investigate species dynamics under climate change and (2) to assess the impacts of drought on saplings and mature tree species in Europe. Through empirical research and the application of various methodological approaches, I seek to provide international leaders, forest managers, and practitioners with practical information to support their decision-making, policies, and actions. High temperatures and altered precipitation patterns are expected to lead to shifts in climate zones and thus large-scale shifts in vegetation. As a result, species ranges are expected to shift to higher elevations or higher latitudes as their climatic optimum shifts. Yet trees that cannot cope with these changes risk being affected by climate change-induced stress. Despite extensive research, it is still unclear whether tree species can cope with global warming. An ideal model system to answer these questions is represented by mountain treelines. Mountain treelines are considered sensors of climate change, meaning that they are expected to respond quickly to climatic warming. Therefore, in Manuscripts 1 and 2, I contribute to the current understanding of how tree species cope with climate warming by researching tree populations from remote mountain regions. In Manuscript 1, I investigated treeline dynamics based on in-situ measurements of Swiss stone pine (Pinus cembra L.) trees from two different protected areas in the Carpathian Mountains. Using spatial statistics, similarities and differences in the spatial structure were identified between the two Pinus cembra populations. In Manuscript 2, I assessed treeline dynamics in the Samaria National Park, on the semi-arid Mediterranean island of Crete. Using historical and recent high-resolution aerial imagery, I assessed the spatio-temporal tree dynamics over the past 70 years. In contrast to the Carpathians, where results indicated a shift of trees to higher elevations in the area protected since 1935, no shift of trees was observed in the Crete Mountains. Accordingly, the absence of climate-driven migration should raise concerns about the threats associated with future warming, drought stress, and wildfire. Therefore, conservation managers should consider options and needs to support adaptive management. In addition to vegetation shifts, climate warming and drought periods are directly affecting the forest ecosystems of Europe. The current forests were established in the much colder climate of the 18th and 19th centuries, while the current seedlings and saplings are established in warmer conditions. Hence, germination and establishment took place under different climatic conditions. Evidence suggests that the increase in frequency and intensity of droughts will lead to abrupt changes in species composition and forest functioning. To assess species-specific responses to drought, European temperate forests are regarded as an optimal ecosystem due to their high susceptibility to drought compared to other temperate forest ecosystems. Thus, in Manuscripts 3 and 4, I presented a comprehensive quantification of the impact of the 2018 and 2019 summer drought on sapling species in temperate forests. The results suggested that drought damaged trees regardless of size, but saplings recovered faster than mature trees. Moreover, slow sapling recovery led to their mortality. Mortality increased from Quercus spp. to minor broadleaved species (e.g., pedunculate oak (Quercus robur) (0%), sessile oak (Quercus petraea) (4%), sycamore (Acer pseudoplatanus) (5%), European beech (Fagus sylvatica) (6%), silver birch (Betula pendula) (6%), European hornbeam (Carpinus betulus) (8%), field maple (Acer campestre) (12%), ash (Fraxinus excelsior) (12%), elder (Sambucus nigra) (16%), and rowan (Sorbus aucuparia) (17%)). Species-specific responses to drought are key to understanding which species are more capable of coping with climate warming and anticipated drought events. These results are essential for developing and implementing adaptive forest management strategies to mitigate the impacts of climate change. This dissertation provides one of the first assessments of tree dynamics in the remote protected areas of the Carpathians and Crete Mountains, along with a comprehensive quantification of species-specific response to drought in Central Europe. Hence, these four studies provide conservationists, forest managers, stakeholders, and private property owners with practical information on tree dynamics and species-species response to climate warming and drought.