Effects of heat and drought stress on the health status of six urban street tree species in Leipzig, Germany

Trees are one of the most important elements of green infrastructure in cities. Climate change is specifically affecting trees in many European cities. Trees are experiencing negative impacts from the increase in heat waves and droughts, both of which begin, in some cases, early in the year and continue through the growing season. Current studies on the regionalization of climate change indicate that important water reservoirs such as soil and tree canopies have been drying out for years/decades, and these impacts can be observed in various parts of Europe. Trees react to stress as they age through mechanisms such as crown defoliation, early wilting, shedding of branches and, ultimately, lowered resistance to pests. As a result, massive tree death, both in park trees and street trees, can be observed in many cities. The present study provides a current inventory of street tree damage caused by heat and drought in the city of Leipzig, Germany, in 2020, the third extreme dry year after 2018 and 2019. The field maps focus on different age groups of Quercus, Tilia, Aesculus, Platanus, Fraxinus and Acer along a periurban-urban gradient. The results are clear: significant damage was found in all tree species. Older trees and newly planted trees are most likely to die as a result of extreme conditions, while younger trees with narrow trunks and crowns that have not yet expanded cope better with both heat and drought. Four out of five mapped street trees showed recognizable damage, indicating severe impacts of climate change on important elements of green infrastructure in cities.