The intraspecific drought tolerance of Betula pendula Roth. : ecotypic aptitude as tree selection criteria for the urban landscape

Sammanfattning: The ecosystem services provided by urban trees are substantial contributors to the quality of urban living, and their environmentally regulating capabilities are increasingly regarded as integral in order to efficiently secure urban resilience, by ameliorating the human and material impacts of extreme weather events. However, current trends of urban development presents challenges of how to preserve urban greenery ecosystem functioning, particularly in light of the ongoing climate change; which also has brought forth the importance of taxonomically diverse urban forests, to mitigate risks of catastrophic tree loss associated with these conditions. The elevated temperatures and irregular water availability which generally characterizes the highly urbanized landscape thusly necessitates approaches that promotes usage of trees with corresponding adaptations which adequately facilitates survival in spite of such environmental stressors. Such adaptations have previously been studied in species native to similarly tough habitats. But granted the wide distribution of some cosmopolitan tree species, such as Betula pendula Roth., there’s opportunity for significant differential ecotypic variations due to the locally distinct selective pressures, which could be expedient from an urban tree selection perspective and leveraged to further the goal of improved urban forest resilience. To probe this opportunity, the leaf drought tolerance of 13 global B. pendula s.l. proveniences was investigated by measuring leaf osmotic potential at full turgor in order to estimate their hydrological limitations by its firm linear correlation with the turgor loss point (TLP). Local climatic selective pressure was investigated by characterization and statistical comparison of the local climates and atmospheric water balance. The group of European subspecies ssp. pendula was found to be significantly more drought tolerant than the Asiatic subspecies ssp. mandshurica and ssp. szechuanica, at a TLP of -2,75 MPa, compared with -2,28 MPa for both respectively. A linear correlation was found between the birch provenience’s TLP and the local climatic potential evapotranspiration (PET). Though the relationship couldn’t be fully established due to lack of conclusive data; the possible driving factors for this adaptive separation were however discussed, and besides currently acting pressures, the recent evolutionary history of ssp. pendula was highlighted as possibly contributing to the observed results. The modes and extent of drought adaptations in B. pendula were also discussed in the wider context of ecotypic differentiation of other deciduous temperate tree species, along the role of geography. Methods for enhancing the selection of fit ecotypes for the urban landscape were explored, e.g. by using analytical advantages provided by GIS-aided spatial analysis.