Growth and reproduction of New Zealand Acaena (Rosaceae) species in relation to rarity and commonness

In New Zealand, as elsewhere, research on rare species has been dominated by autecological studies of individual threatened species. Limitations of this approach are that it involves no comparison with related common species which may have similar traits, and that the minimal sample size prevents generalisation about causes and consequences of rarity. We report on experimentally determined growth and reproductive traits of 10 rare and common Acaena(Rosaceae) species from two taxonomic sections (sect. Ancistrum and sect. Microphyllae). We examined the relationship between rarity or commonness and relative growth rate, mode of vegetative expansion, morphology/presentation of reproductive structures and reproductive allocation. Rarity and commonness were defined according to geographic range size, measured as the number of 10-km grid squares containing at least one record of the species. There were tendencies across both taxonomic sections for species with large range size to have higher relative growth rates and in section Microphyllae, faster lateral expansion. Among section Ancistrum species, common species tended to produce inflorescences for a shorter period and held their capitula higher above the canopy, but other reproductive attributes showed little association with range size. In section Microphyllae all reproductive traits tended to be positively associated with range size. This was mainly due to the single very common species having high fecundity. The lack of strong patterns among our results may reflect insufficient sample size or that the rare species represent different types of rarity.