Environmental heterogeneity and the spatial structure of fern species diversity in one hectare of old-growth forest

The precise relationship between species diversity and spatial heterogeneity has not often been investigated using quantitative and repeatable measures of environmental variation. In this study, we map the metre-level distribution of fern species in one hectare of old-growth forest and test for a relationship between diversity and heterogeneity of physical features and soil conditions. The nineteen species recorded in the hectare were non-randomly distributed and varied greatly in abundance and spatial aggregation. Different species distributions were not independent of one another; three groups were formed with species which occurred together significantly more often than random expectation. Physical and soil conditions were highly variable and spatially autocorrelated from the 5 m scale up to the extent of the whole hectare. Based on the sites where they grew, species differed in their preferences for soil moisture, fertility and pH. Fern diversity was highest at sites with high soil moisture and low soil fertility; however, there was no relationship between diversity and the environmental variance within quadrats. Unpredictable spatial distribution patterns produced by processes of dispersal and immigration may obscure any relationship between diversity and spatial heterogeneity at this fine scale.