Ecological impacts of ground cover weeds in New Zealand lowland forests

Ecological impacts of three weed species of similar life form, Asparagus scandens, Plectranthus ciliatus and Tradescantia fluminensis, were investigated in six lowland forest remnants in New Zealand. All three species form dense, ground-covering mats of vegetation, and are tolerant of a broad range of light environments. Relationships between canopy openness, weed volume, native plant abundance and native species richness were investigated. Volume of all three weed species increased as canopy openness increased. Tradescantia fluminensis appeared to be most detrimental to native vegetation, with both native abundance and native species richness decreasing sharply as weed volume increased. Plectranthus ciliatus and Asparagus scandens were also associated with declines in native abundance and native species richness, but the correlations were less pronounced and were inconsistent across sites. Regression tree analyses on data from individual sites suggested a potential threshold of weed volume for Tradescantia fluminensis, beyond which both native abundance and native species richness declined abruptly. A threshold was also evident when data from all sites were analysed together. Where native species richness did decline in association with increasing weed volume, there did not appear to be any particular native species that were more likely to be excluded than others. All three ground cover weed species are associated with declines in native plant abundance and native species richness, particularly under high light conditions where the weeds are most abundant.