Seedling Recruitment of the Invasive Species Berberis Darwinii (Darwin's Barberry): What Contributes to Invasion Success?
Berberis darwinii is an invasive tree species that is considered a serious threat to indigenous ecosystems and biodiversity conservation throughout New Zealand. I examined the recruitment dynamics of this species in order to identify traits contributing to invasion success, and thus pinpoint critical stages for management. In order to do this, I measured patterns of both spatial and temporal seed dispersal, and compared rates of germination and seedling survival across a range of light environments. I also measured patterns of growth, biomass allocation, photosynthetic performance, leaf morphology, and water use efficiency across different light environments. In most of these experiments I compared the performance of B. darwinii to four ecologically similar, co-occurring native species. Berberis darwinii produced large quantities of viable seed that were widely dispersed by birds. Almost all viable seeds germinated in the spring following dispersal, indicating that B. darwinii does not form a persistent seed bank. Rates of germination and seedling survival were significantly higher in B. darwinii compared to the native species, although seedling establishment of all species was limited to high-light environments. Berberis darwinii also had approximately twice the photosynthetic capacity of the native species, but this advantage was limited to high-light environments. Berberis darwinii does not vary significantly in proportional biomass allocation across light environments, suggesting that this type of plasticity does not contribute to invasion success. Berberis darwinii was more water use efficient in sun compared to shade, but the same pattern was evident in four of the five native species. The critical stage of recruitment for B. darwinii was first-year seedling establishment. Rates of mortality were highest at this stage, and were largely associated with seedling density and low light availability. Seedling mortality was near 100% beneath the parent canopy, indicating that seed dispersal is critical to B. darwinii recruitment. These results suggest that B. darwinii is not shade-tolerant as a seedling, and that management practices should be concentrated on the removal of fruiting adult plants and seedlings growing in open sites.