Influence of forest tree species composition on bryophyte diversity in mixed and pure pine and oak stands.

Summary The effects of mixed stands on biodiversity are increasingly being studied. Nevertheless, among the few studies dealing with epiphyte taxa, even fewer compare mixed stands with the pure stands of each species. We evaluated the diversity of tree-dwelling bryophyte communities in mixed and pure oak-pine stands (Quercus petraea (Matt.) Liebl., Pinus sylvestris L.) in French lowland forests. The main explanatory variables were the phorophyte species and the stand type (pure versus mixed) at tree level, the stand composition (pure oak, pure pine and mixed) at plot level. In addition, we investigated the role of the chemistry of the bryophyte substrates (soil, bark) and water supplies (stemflows, throughfalls), the basal area and interfering plant cover. Data were analyzed using Generalized Linear Models under Bayesian statistics. Richness and abundance of bryophytes were significantly and strongly higher on oak. Pine bryophyte richness at tree level was higher in mixed compared to pure stands, whereas mixed stands did not enhance oak bryophyte richness. Three species had a clearly higher probability to be found on pine, whereas seven species occurred clearly more frequently on oaks. Some species had a higher probability to be associated to pine in mixed stand than in pure stands, and one species was more prone to be found on pine within pure stands. As a conclusion, bryophyte biodiversity at the landscape level should benefit from the simultaneously presence of the three stand types: pure oak, pure pine and mixed. Exploratory models suggested strong effects of water supply chemistry which could explain the phorophyte species effect on bryophyte diversity. Basal area had a strong quadratic effect on plot epiphytic richness, and the best models for bryophyte diversity per tree always promoted the basal area effect in addition to (or interaction with) the main species and composition effects.