Observed street tree vitality along a climate gradient is not related to vulnerability estimated by climate envelope analysis

Increasing heat and drought stress under climate change will likely decrease the health and vitality of some urban tree species. Methods currently used to estimate the ‘vulnerability’ of tree species to climate change are often based on climate envelopes derived from global occurrence data. However, these methods are yet to be validated against measures of tree performance, e.g. vitality. In this study we analysed street tree populations in four cities along an aridity gradient in south-eastern Australia to determine whether: i) species composition and vitality changed with increasing aridity and ii) vulnerability estimated from climate envelopes was related with tree vitality. The species composition of street tree populations differed along the gradient. Less arid cities were dominated by native, evergreen species whereas more arid cities were composed primarily of exotic, deciduous, ring-porous species. More arid cities had a higher proportion of trees originating from warmer, drier environments and mean tree vitality did not decrease with increasing aridity. These results suggest that species selection has maintained a healthy urban forest along this aridity gradient. Although there were some significant relationships between climate envelope metrics and vitality, they were weak (all R 2 <0.12) and at times counter intuitive. Thus, there is little evidence that climate envelope methods in isolation can identify vulnerable tree species in urban forests. Assessing species composition and vitality across climate gradients may assist species selection in future urban forests.