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.

No abstract

We analyzed seed production of mountain beech (Nothofagus solandri var. cliffortioides) forest along an elevational gradient in New Zealand from 1020 to 1370 m (treeline) for the years 1973–2002. We used seed production data from nine elevations and a site‐ and species‐specific net carbon (C) availability model from two elevations (1050 m and 1340 m) to examine how three variables (temperature, soil moisture, and net C availability) during three key periods (resource priming, flowering primordia development, and flowering) influence seed production. These three strongly interrelated variables have all been considered determinants of seed production but have not previously been tested together in a single analysis. Seed production increased over the 30‐year period, with the greatest increases at high elevations; this increase was driven by a greater frequency of intermediate‐sized seeding years. We then determined how temperature, soil moisture, and net C availability determined seeding, and examined whether temporal trends in the seeding data could be linked to similar temporal trends in temperature, soil moisture, or net C availability. High seed production was related to cool summers with high soil moisture during resource priming, warm summers during flower primordia development, and low net C availability during flowering. Positive temporal trends in temperatures during the period of flower primordia development accounted for the increase in seed production, suggesting that increasing temperatures are promoting more frequent seed production at high elevations.

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In 250 2 0 m X 20 m permanent plots in the Craigieburn Range, Canterbury, New Zealand, 1970 stem density was 2,19l/ha, basal area was 52.4 m2/ha, and stem biomass was 178.1 P. A. Harcombe is Professor in the