Correlates of vascular plant species richness in fragmented indigenous forests: assessing the role of local and regional factors

Abstract

Both local and regional factors determine local species richness. We investigated the relative role of local (13 soil and tree stand structure variables) and regional factors (19 climate, land cover and geographic location variables) in determining the richness of several vascular plant functional groups in indigenous forest fragments in southeastern New Zealand. The predictor variables explaining the largest fraction of the variation in species richness were identified using a backward and forward stepwise procedure, with adjustments for the number of variables and testing for multicollinearity. The total proportion of variation explained by local and regional factors was highest for tree species richness (54.1%) and lowest for herbaceous species richness (28.2%). We found differences between the functional groups in the extent to which species richness was explained by local vs. regional factors, but both showed some ability to explain the species richness of all functional groups. The abundance of the strongly competitive tree species Nothofagus menziesii (silver beech) had a strong negative effect on total, tree and herb species richness, but it had only minor effects on woody and fern species richness. Once the effect of this local variable was accounted for, the remaining variation in tree and woody species richness was mainly explained by regional variables. Herbaceous and fern species richness, on the other hand, was strongly correlated with local as well as regional variables. We emphasize the importance of using a fixed plot size when the relative effects of local and regional factors on patterns of species richness are to be compared and evaluated.