Bryophytes as bioindicators of the atmospheric environment in urban-forest landscapes

Bryophytes have been used as indicators to evaluate atmospheric conditions in urban areas. However, further research is needed for their effective application. In this study, we focused on four metrics related to atmospheric problems in urban areas (nitrogen concentration [% N], isotope ratio of nitrogen [δ15N], index of atmospheric purity [IAP], and richness of hygrophilic life-forms [RHL]). Then, using linear and generalized linear models, we examined the influence of land use on these four metrics in urban atmospheric environments, and evaluated the usefulness and limitation of them. The % N and δ15N models were well explained by the influence of nitrogen sources and/or sinks. The RHL models were significantly affected by urban and forest areas, reflecting urban heat islands. Surprisingly, IAP was higher in urban areas, but comparisons of IAP are not informative in areas with narrow pollution gradients. Land use strongly affected % N and δ15N models in smaller areas because of a point-source influence of nitrogen pollution, whereas RHL was strongly influenced by land use at larger scales owing to drought stress in urban settings. Correlations among the metrics revealed that severe drought stress tended to occur in areas with high nitrogen pollution. The nitrogen pollution sources were diverse, with no significant correlation of δ15N values with % N.