Urban domestic gardens (XIII): Composition of the bryophyte and lichen floras, and determinants of species richness

Private, residential gardens form a substantial proportion of the undeveloped land in urban areas. Evaluating their role in supporting biodiversity is crucial to (i) predicting which plant and animal species can persist in towns and cities, (ii) understanding the regional impacts of urbanisation, and (iii) guiding sympathetic garden management by owners. To obtain baseline information on a poorly-studied component of garden biodiversity, we measured the size and composition of the cryptogam assemblages in 61 domestic gardens in the city of Sheffield, UK. A total of 67 bryophyte and 77 lichen taxa were recorded. Bryophytes ranged from 3 to 24 species per garden, with a mean richness of 11.3 species; lichens ranged from 2 to 30, with a mean of 14.9 species. Stone substrates supported the highest lichen richness, although minor substrates contributed unique species. Just over one fifth of bryophyte species were recorded in grass lawns, and these were more widespread than those of other habitats. Most cryptogams were scarce, with around one quarter of both bryophytes and lichens occurring in single gardens, and only 10% were found in more than half of the gardens. Garden area - correlated with substrate richness - and garden altitude were the only two factors explaining variation in cryptogam richness (bryophytes 39.1%, lichens 32.4%). Positive correlations existed among bryophyte, lichen and vascular plant richness, and these were only partially mediated by the effect of garden area. Therefore the opportunity remains for garden owners to support cryptogam richness, the most effective action being to enhance substrate diversity.