COMMUNITY STRUCTURE AND FOREST INVASION BY AN EXOTIC HERB OVER 23 YEARS

Abstract

We studied the invasion of a New Zealand mountain beech (Nothofagus solandri var. cliffortioides) forest by the exotic perennial herb, Hieracium lepidulum. We used data from 250 randomly located permanent plots (400 m2) established in 1970 that sampled 9000 ha of forest. Frequency of H. lepidulum was 11%, 43%, and 57% in 1970, 1985, and 1993, respectively. For each year of measurement, invasion patterns were related to (a) distance to the forest margin as a measure of dispersal limitation, (b) community structure, (c) stem biomass dynamics indicating disturbance history, and (d) environmental characteristics. In 1970, invaded plots had more species and lower potential solar radiation, and they were closer to the forest margin; however, invaded plots were only weakly predicted by these site variables. H. lepidulum also invaded relatively species-rich subplots (0.75 m2) showing that community structure was also significant at a microsite scale. Using the same sets of variables, the ability to predict which plots were invaded in any year increased from 1970 to 1993. This supports our hypothesis that in early invasion stages, with dispersal limitation, an invader may occur in only a subset of suitable sites giving a weak relationship with site variables. By 1993, distance to the forest margin was no longer related to which plots were invaded, and invaded plots had more species, occurred at lower elevations on more sheltered topographic positions, and had more fertile soils than uninvaded plots. Even though site variables were not independent (e.g., plots on fertile soils tend to have more species), multiple logistic regression showed that, all else being equal, invaded plots still tended to have more species than those not invaded. Our study therefore questions the hypothesis that, all else being equal, species-poor habitats are more prone to invasion by exotic species.