OOS 11-6 - How local adaptation plays a key role to secure urban forests in a changing climate

Urban forests provide multiple environmental and socio-economic services to people around the world. These include heat mitigation, biodiversity conservation, connection to nature, social integration, and human well-being. However, the delivery of these services relies on healthy and functional urban forests. Climate change threatens the health of urban forests and thus will impact the delivery of services. Further, urban trees often face harsh environmental conditions (e.g., droughts and heatwaves) exacerbated by the urban heat island effect, which may cause dieback, mortality, and reduced tree vitality. Therefore, the long-term stability of urban forests depends on the identification of species that are resilient to climate change. For this, we need to understand how urban trees respond and adapt to local environmental conditions by determining the climate sensitivity of urban tree growth. However, studies of the effects of climate change on trees growing in urban environments are rare and, to date, research on the climate sensitivity of urban tree growth has been conducted at the local level, limiting broad assessments of species responses to climate. This study aimed to (1) evaluate the local adaptation of urban tree growth and performance by determining spatial and temporal patterns of growth in relation to local climates and (2) identify urban tree species that have high and low tolerance to drought and high temperatures. Using urban tree inventories, we selected ten common deciduous urban tree species known to produce distinct annual growth rings. Individual trees of each species were located in seven cities across the Australian continent; these cities are distributed along temperature and precipitation gradients. The selected species were pre-identified as vulnerable or resilient to climate based on bioclimatic modelling of climate risk. We collected tree rings of street trees, sampling more than 550 individual trees across all cities. We found urban tree species differed in their growth across city climates and showed species-specific responses reflecting local environmental conditions in each city and probably management actions. Celtis australis, for example, showed a greater variability across individuals compared to Gleditsia triacanthos, which presented less variability. These results were particularly notable in dry cities. Our results can guide species selection to maximise services delivered by urban trees under future climates and minimise environmental and socio-economic losses.