Applying niche theories to the evaluation of urban trees: A tripartite approach on niche breadth, species distribution models, and water use strategy

The escalating uncertainty associated with climate change, coupled with increased demands in multi-functional ecosystem services, presents a bi-fold challenge in today’s urban tree management. To address these challenges, integrating an interdisciplinary and science-based approach into conventional landscape practice is essential. In line with this effort, the thesis explores three perspectives rooted in the ecological theory focusing on the concept of niche, central to explaining biogeographic distributions of species. Each perspective introduces distinct ecological niche aspects: (1) bioclimatic niche breadths, (2) biogeographic distributions that integrate both climate and terrestrial components, and (3) biological interactions within the given geo-ecological and climatic conditions. The first two approaches employ macro environmental factors based on species native distributions while the third approach specifically compares species water use with sap flow measurements. The assessed values based on niche breadths and species distribution models were then juxtaposed with the actual mortality rates of selected Acer and Quercus species across the three major botanic gardens of the UK, areas predominantly beyond their native range. Considering that the UK has experienced more frequent droughts and heatwave stress in recent decades, analysing the mortality rates specifically during the period of 2000-2021 offered a significant opportunity to test species-specific tolerance. As a result, most ex-situ Acer species demonstrated relatively high mortality rates according to their respective niche positions below their lower limits while Quercus species in general showed relatively more resilience regardless of their niche positions. However, there are some species that showed high adaptability counterintuitively beyond their native range. Interestingly, species that are phylogenetically closer consistently exhibited more similar mortality rates than those species that are environmentally more similar. The finding indicates the presence of inherited phenotypes or genotypes that confer resilience to specific stress. This speculation is further supported by the last analysis, demonstrating how distinct physiological traits respond and interact differently under the identical environmental stress.