The impact of an invasive weed Tradescantia fluminensis on native forest regeneration

Abstract

1. Tradescantia fluminensis is an invasive weed of New Zealand, eastern Australia and Florida, where it carpets the ground in canopy-depleted native forest remnants and prevents regeneration. The aim of our study was to determine the Tradescantia biomass levels at which this occurs. 2. At two podocarp/broad-leaved forest remnants in New Zealand, we showed that Tradescantia biomass increased logistically with available light, to a maximum at 10-15% full light. The maximum Tradescantia biomass was greater at the wetter site (819 g m-2) than at the comparatively dry site (695 g m-2). 3. Native forest seedling species richness and abundance decreased exponentially with increasing Tradescantia biomass, for example from 3.4 and 81.5 m-2, respectively, in the absence of the weed, to 0.37 and 6.28 m-2 at maximum Tradescantia biomass. We attributed this to decreasing light levels beneath Tradescantia. Under approximately 500 g m-2 of Tradescantia (c. 100% weed cover), available light was reduced to < 1% full light. 4. 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 Tradescantia and non-Tradescantia habitats. 5. We estimated the response of seedlings of six commonly occurring native woody species to Tradescantia biomass. Macropiper excelsum appeared to be the least tolerant, its LD50 (the biomass of Tradescantia at which its abundance is reduced to 50% of the maximum) being approximately 12 g m-2, whereas Dysoxylum spectabile was the most tolerant, with an LD50 of 40 g m-2. 6. Dysoxylum spectabile germination and early seedling establishment could occur in dense Tradescantia, but the probability of survival over 20 months decreased logistically with increasing Tradescantia biomass. The probability of survival at 100% weed cover was 6% compared with 84% for seedlings in non-Tradescantia habitat. We estimated that emergence above the weed occurs only where cover of Tradescantia is < 200 g m-2 (70-90% cover). 7. 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 Tradescantia-affected forest remnants.