Some like it cold: bryophyte responses to a warmer and wetter climate

Background & Aim: Understanding the relative importance of biotic and abiotic factors on species assemblages in changing environments is critical to assess climate change effects on ecological processes and biodiversity (Lavergne et al. 2010). Bryophytes differ from flowering plants in their responses to climate and yet remain overlooked. In this study, we take a non-experimental approach to determine environmental factors structuring bryophyte communities along climatic gradients. Materials & Methods: Twelve sites were selected along a climatic grid (four levels of annual precipitation: 600, 1200, 2000, 2700 mm - three levels of mean summer temperature: 7.5, 9.5, 11.5°C) in subalpine and alpine grasslands of South-western Norway. Bryophyte species frequency, richness, biomass, height and vascular plant cover were recorded within five 25×25 cm quadrats. We also measured abiotic (temperature, precipitation, solar radiation, soil moisture, aspect, pH) and biotic (vegetation height, biomass, cover of vascular plants and litter) factors; and extracted bryophyte traits (plant and spore size, life form, Ellenberg habitat indicators) from the BRYOATT database (Hill et al. 2007). We used DCA analyses to assess pattern in vegetation-environment relations and GLM to test bryophyte responses to environmental variables. Main Results & Interpretations: We identified 61 bryophyte species. High temperature and low solar radiation were positively correlated to dense and thick layers of bryophytes, covering more area yet harbouring a lower number of species (1–2 species). Bryophyte biomass, cover and richness were negatively correlated with vascular plant cover, suggesting competitive interactions. Precipitation and moisture influence bryophyte cover, height and composition at the warmest sites only. Bryophyte response to temperature and precipitation was strongly reflected in plant traits. Warmer and wetter environments favoured larger species that were light- and acidity-tolerant, and produced smaller spores. These results highlight the importance of multiple climatic drivers and biotic interactions in determining community composition and abundance. Future warmer climates in the region might lead to shifts in bryophyte assemblages and reduced species diversity directly via physiological limitations and indirectly via increased competition with the vascular flora. Bryophyte growth being greatly variable in time and space, compositional predictions are thus difficult. Numerous studies have approached this issue through multifactorial experiments (Pedersen et al. 2001); and multiple approaches (combined natural gradient and experimental studies) remain needed to better understand these complex systems.