Drought tolerance differs between urban tree species but is not affected by traffic pollution

Urban trees provide multiple benefits such as shading and cooling, which become more and more important due to the increasing frequency and severity of heat and drought periods. It is thus necessary to identify tree species, which are able to tolerate such extreme conditions in urban areas. We studied the resistance and resilience of stem diameter growth of five deciduous tree species in an urban area in Southwest Germany to three exceptional drought periods for differences between and within species, especially considering the intensity of traffic emissions (NOx). We further investigated the stable isotopic composition of carbon (δ13C) and oxygen (δ18O) in the tree rings. By analysing the stable isotopic composition of nitrogen in the wood, we aimed to find out if δ15N may serve as an indicator of NOx emissions from traffic.

Stem diameter growth in all species was strongly limited by low water availability in spring, as was also reflected in elevated δ13C and δ18O values in Acer platanoides and Tilia cordata, which were particularly sensitive to drought. In contrast, growth of Platanus × hispanica and Quercus robur was less affected by drought, and resistance of Carpinus betulus ranged in between. Across species, δ15N was higher in trees experiencing higher NOx traffic emissions and being located closer to roads. These conditions, however, unexpectedly did not significantly affect drought resistance and resilience. Our study demonstrates both the large potential and the associated interpretative challenges of coupled dendroecological and multi-isotopic analyses. It also indicates clear species-specific differences in drought tolerance and thus helps to identify suitable tree species for urban areas.