Urban heat island and Tree phenology: A spatial relationship

The urban heat island (UHI), a nocturnal phenomenon characterized by a temperature difference between urban and rural areas in temperate climates, is well-documented in large cities (>500,000 inhabitants) but also occurs in medium-sized cities (20,000–500,000 inhabitants) and small towns (<20,000 inhabitants). While urban vegetation is often considered a solution to mitigate the UHI, it is also affected by it, particularly through an earlier onset of vegetative growth in spring. However, most studies focus on temporal differences or comparisons between urban and rural areas, without thoroughly analyzing intra-urban variations.

With the growing availability of intra-urban temperature monitoring networks, more detailed research on this phenomenon is becoming possible. In the Rennes basin (France), such a network has been deployed in the medium-sized city of Rennes and its surrounding small towns (RUN).

We aim to establish the relationship between the UHI and the spring phenology of tree species. To this end, the vegetative onset of seven species was monitored through in situ observations during the spring of 2021. The responses of the sweet cherry, common maple, and pedunculate oak follow a linear trend, becoming earlier as the percentage of urbanized areas increases. The tree species responded significantly to intra-urban minimal temperature variations, as the UHI was particularly intense during the spring of 2021. The oak showed the strongest response, with its vegetative onset advancing by more than two weeks in Rennes and by over one week in the surrounding towns.

These findings raise questions about the vegetation's cooling capacity earlier in spring and during summer, when water reserves may be reduced due to earlier vegetative activity. They also highlight concerns about the ability of species to sustain activity in an environment with increasingly stressful future conditions in terms of temperature and drought.