Ecological separation among fern species in an old-growth forest

The validity of niche-assembly rules in explaining community structure is revisited by testing for ecological separation among ferns. An intensive, fine-scale survey of fern abundance and environmental variation was done in 1-ha of old-growth forest. Three methods of detecting niche segregation (canonical correspondence analysis, detrended correspondence analysis and GIS mapping) suggested that most fern species at the site are distributed according to distinct environmental preferences. The most important gradients separating fern species are first, the amount of soil moisture and second, soil nitrate concentration. Contrary to other findings, pH had little influence on controlling fern distribution. Spatial autocorrelation, detected by partialled ordinations, obscured the presence of niche partitioning. As well, sampling grain changed the apparent location of some species on environmental gradients and their ecological similarity to other species. Finer-scaled environmental heterogeneity or dispersal-mediated processes may account for the unexplained variation in fern species abundance of this site.