Seedling growth rate and survival do not predict invasiveness in naturalized woody plants in New Zealand

We tested if plant species invasiveness (measured as rate of geographic spread in New Zealand) was positively associated with seedling relative growth rate (RGR) and survival, among 33 naturalized woody plant species in four families (Fabaceae, Mimosaceae, Pinaceae, Rosaceae). Eight disturbance and fertilization treatments were applied to seedlings in pots to mimic conditions the species are likely to encounter in the wild. We tested if seedlings of more invasive species have higher RGR and greater survival under these conditions. We also tested whether more invasive species had wider tolerance of the conditions to which they were subjected. There were no significant relationships between seedling RGR or survival and invasiveness for any of the treatments. Measures of tolerance (a species' relative performance across all treatments), based on both RGR and survival, also failed to explain differences in invasiveness. Our findings contrast with those from a recent study by Grotkopp et al. (2002) showing a positive correlation between seedling RGR and invasiveness in Pinus species, even though our study included 12 Pinus species. The findings of the two studies may differ because they focus on different stages in the invasion process. Our study tested whether RGR (and survival) was associated with the rate of spread of invasive species following naturalization, whereas Grotkopp et al. primarily tested whether RGR was associated with the probability of naturalization. We caution that relationships between RGR and measures of invasiveness are likely to be confounded with human introduction effort. Among pines, species with high RGR are more likely to have been widely cultivated, for purposes such as timber production, providing more opportunities for naturalization and spread. Associations between RGR, invasiveness and human introduction effort need to be partitioned out before we can reliably infer causal relationships.