Estimating the urban heat island effect with lichens and bryophytes: a citizen science protocol

The urban heat island effect (UHI), whereby urban areas are warmer than the surrounding rural areas, has become an increasing threat to human health and affects biodiversity. Counteracting this public health concern requires large-scale monitoring of urban heat. One cost-effective way to obtain this data is by using bioindicators, such as lichens and bryophytes. They are well-known bioindicators readily available in the urban environment, and have proven to respond to the UHI. Gathering large amounts of data in a short time span may prove difficult for many institutes or municipalities, but can be resolved by incorporating citizen science. However, lichen and bryophyte species identification with no prior experience may prove problematic for citizen scientists. Therefore, we developed a citizen science protocol that requires scoring the presence of easily recognisable morphotypes of epiphytic lichens and bryophytes, instead of individual species. Data was collected in urban environments in the Netherlands by participants at three knowledge levels: an expert in lichenology and bryology, university biology students, and citizen scientists. Using the expert dataset, we trained a baseline multiple linear model (LM) and a random forest (RF) model, which predicted local UHI with low root-mean-square errors (RMSE = 0.29 °C and 0.20 °C, respectively). Prediction accuracy declined with decreasing participants' expertise. We conclude that a simplified protocol using morphotypes of lichens and bryophytes can be an effective, low-cost way of monitoring the UHI. However, some training or experience is required to obtain sufficiently accurate observations.