The influence of scale and species pool on the relationship between vascular plant species richness and cover in an alpine area in Norway

Recent studies emphasise the potential importance of scale and species pool on the humped-back or unimodal relationship between species richness and productivity. We use a classic phytosociological data-set from Rondane, central south Norway, to evaluate the relative importance of these factors in an alpine area. The effect of species pool is assessed using plot scores from a Correspondence Analysis (CA) of the data. Generalised Additive Models (GAM) are used to relate vascular plant species richness to cover of vascular plants, CA plot scores, and plot area in different combinations. Species richness of vascular plants is unimodally related to total vascular plant cover. Plot scores of the first three CA axes (representing the effect of species pool) have a complex relationship with species richness, but explain a large fraction of the total deviance in richness. A humped relationship between richness and cover remains after accounting for CA plot scores in the model, i.e. the relationship is independent of species pool. The results suggest that the relationship between richness and cover changes from one vegetation type to another, as evaluated statistically through the importance of the interaction between cover and CA scores in explaining variation in richness. Plot area also influences the relationship. A unimodal relationship is only evident when small plot sizes are used, whereas a monotonically increasing relationship is found at large plot sizes. Plot area has the strongest effect on the unimodal relationship between richness and cover, whereas vegetation type has only a minor effect on this relationship. This revised version was published online in June 2006 with corrections to the Cover Date.