Urban Environment, Drought Events and Climate Change Strongly Affect the Growth of Common Urban Tree Species in a Temperate City

Trees' ecosystem services help reducing the urban heat island intensity, but are most effective when trees are big and healthy. During the last decades, the frequency of heat waves and drought events in Central Europe has increased, exposing urban trees to even greater stress under already challenging urban growth conditions. To understand the extent to which the urban environment affects tree growth and how trees respond to drought, dendrochronological analyses were conducted on 176 urban trees of six commonly planted tree species: Tilia cordata, Acer platanoides, Robinia pseudoacacia, Platanus x acerifolia, Fagus sylvatica and Quercus robur growing within the temperate city of Munich, Germany. The impacts of climate change were studied comparing tree growth between the periods 1980-1999 and 2000-2019, as well as the effect of the urban heat island intensity, comparing urban and suburban zones. Additionally, a drought-tolerance analysis was conducted to investigate the recovery following different dry years. Species-specific trends showed the highest average basal area increment (BAI) for P. x acerifolia (44.9 cm2/year) and the lowest for T. cordata (18.4 cm2/year). In terms of the urbanisation impact on growth, significantly higher BAI was recorded for A. platanoides and T. cordata in the suburban areas, in contrast to F. sylvatica. Analysing the complete sample, the results showed growth reduction since the year 2000 (-28%). Higher drought tolerance was found for R. pseudoacacia, P. x acerifolia and Q. robur. Accumulated drought stress showed significant negative responses for A. platanoides and T. cordata. With the predicted climate change scenarios, the present study can deepen the knowledge about drought tolerance of urban trees in temperate climate and improve species selection. In particular, R. pseudoacacia and P. x acerifolia could be further planted in cities as were less affected by high urbanisation degree and grew the most despite drought and climate change conditions.