Lichens as ecological indicators in urban areas: beyond the effects of pollutants

Summary 1. In this work, we hypothesized that in urban areas with reduced pollution levels, local climatic conditions can be an important driver of lichen diversity. Thus, lichen functional groups could be used to study the effects of the urban heat island, characterized by higher temperature and lower humidity in urban than in peri-urban areas. To test this hypothesis, we sampled functional groups of epiphytic lichens in 48 forest remnants of a Mediterranean urban area (Almada, Portugal). 2. Of all tested functional groups (including those most closely associated with air pollution), groups related to water requirements showed the most significant correlations with surrounding artificial areas, roads and forest. These results suggest that the current major driver of lichen diversity is the climate within the urban area rather than air pollution, as frequently assumed. 3. The analysis of local climate using meteorological data of temperature and humidity confirmed the existence of urban heat island in the study area. 4. Synthesis and applications. The analysis of lichen functional groups gave an integrated response to the climatic modifications occurring in urban areas, namely to the urban heat island, suggesting that lichens can be used as a tool to evaluate the impact of urban areas on local climate and the effectiveness of mitigation and adaptation strategies if pollution levels are low.