Urban heat Islands shape epiphytic communities of lichens and bryophytes

Lichens and bryophytes are renowned for their use as bioindicators of environmental pollution and climate change. In urban environments, climate change may directly affect the temperature regime, thereby worsening the intensity of the urban heat island effect (UHI). Using lichens and bryophytes as bioindicators for the UHI allows urban planners and policy makers to mitigate the UHI in a targeted approach. Here, we investigated whether and how the diversity and community composition of epiphytic lichens, algae and bryophytes are influenced by the UHI gradient. We collected species and abundance data on 303 trees from the genus Tilia in three Dutch cities (Amsterdam, Leiden, and Rotterdam) along an UHI gradient. Additionally, abiotic data (relative humidity, temperature, and vapour-pressure deficit) were collected using sensors on 24 trees in Rotterdam. The results show that lichens have a peak in biodiversity in zones where the UHI is intermediate, whereas bryophyte biodiversity increases linearly along the gradient. Furthermore, the biodiversity of epiphytic lichens and bryophytes increases with the age of urban trees. We identified 23 indicators for different zones along the UHI gradient. Their suitability as indicators at the macroclimatic (UHI) level was supported by their microclimatic niche according to the sensor data. Our study, therefore, provides a novel, robust biological framework for developing or refining UHI monitoring in cities.

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