Species richness in boreal swamp forests of SE Norway: The role of surface microtopography

Question: What is the relative importance of environmental gradients and surface microtopography ( variation in vertical level within sampling units) for fine-scale plant species richness in Picea abies swamp forests? Location: 11 swamp forests in SE Norway. Methods: We recorded species richness ( number of species of vascular plants, mosses, Sphagnum and hepatics), depth to water table, soil base status and vertical range ( microtopographic relief) in 2400 microplots, (each 1/16 m(2)), in 150 1-m(2) plots in the 11 swamp forests. Relationships between species richness and environmental predictors were modelled by GLMM. Results: Moss and hepatic species richness increased with increasing microtopographic relief, most strongly for wet acid sites, in which similar trends were also found for Sphagnum. Relief was a poor predictor of vascular plant species richness. Conclusions: Microtopographic relief is a good predictor of local species richness in Picea abies swamp forests, partly because larger vertical variability means higher within-plot habitat diversity with respect to the wet-dry gradient, and partly because qualitatively new microhabitats associated with steep slopes are added in drier sites. The relationship between species richness and microtopographic relief is context dependent, differing in complex ways among species groups and among sites with different environmental conditions.