The impact of an invasive weed Tradescantia fluminensis on native forest regeneration.
T. fluminensis, an invasive weed of New Zealand, eastern Australia and Florida, carpets the ground in canopy-depleted native forest remnants and prevents regeneration. In this study, T. fluminensis biomass levels at which this occurs were determined. At two podocarp/broad-leaved forest remnants in New Zealand, we showed that T. fluminensis biomass increased logistically with available light (maximum at 10-15% full light). The maximum T. fluminensis biomass was greater at the wet site (819 g/m2) than at the comparatively dry site (695 g/m2). Native forest seedling species richness and abundance decreased exponentially with increasing T. fluminensis biomass. We attributed this to decreasing light levels beneath T. fluminensis. Under ∼500 g/m2 of T. fluminensis (∼100% weed cover), available light was reduced to <1% full light. The compositions of the extant vegetation, seed rain and seed bank were consistent with our interpretation that light availability drives the differences in seedling species richness and abundance between T. fluminensis and non-T. fluminensis habitats. We estimated the response of seedlings of six commonly occurring native woody species to T. fluminensis biomass. Macropiper excelsum [Piper excelsum] appeared to be the least tolerant, its LD50 (the biomass of T. fluminensis at which its abundance is reduced to 50% of the maximum) being ∼12 g/m2, whereas Dysoxylum spectabile was the most tolerant, with an LD50 of 40 g/m2. D. spectabile germination and early seedling establishment could occur in dense T. fluminensis, but the probability of survival over 20 months decreased logistically with increasing T. fluminensis biomass. The probability of survival at 100% weed cover was 6% compared with 84% for seedlings in non-T. fluminensis habitat. We estimated that emergence above the weed occurs only where cover of T. fluminensis is <200 g/m2 (70-90% cover). We predicted that increases in native species richness and abundance will accumulate with increasing suppression of the weed. Imposing shade by planting trees to improve the canopy cover is a potentially useful tool for restoration of T. fluminensis-affected forest remnants.