Invasion by a perennial herb increases decomposition rate and alters nutrient availability in warm temperate lowland forest remnants

We determined the impact of the invasive herb, Tradescantia fluminensis Veil., on litter decomposition and nutrient availability in a remnant of New Zealand lowland podocarp-broadleaf forest. Using litter bags, we found that litter beneath mats of Tradescantia decomposed at almost twice the rate of litter placed outside the mat. Values of k (decomposition quotient) were 9.44 +/- 0.42 yrs for litter placed beneath Tradescantia and 5.42 +/- 0.42 yrs for litter placed in native, non-Tradescantia plots. The impact of Tradescantia on decomposition was evident through the smaller forest floor mass in Tradescantia plots (2.65 +/- 1.05 t ha(-1)) compared with non-Tradescantia plots (5.05 +/- 1.05 t ha(-1)). despite similar quantities of annual leaf litterfall into Tradescantia plots (6.85 +/- 0.85 t ha(-1) yr(-1)) and non-Tradescantia plots (7.45 +/- 1.05 t ha(-1) yr(-1)). Moreover, there was increased plant nitrate available, as captured on resin bags, in Tradescantia plots (25.77 +/- 8.32 cmol(-)/kg resin) compared with non-Tradescantia plots (9.55 +/- 3.72 cmol(-)/kg resin). Finally, the annual nutrient uptake by Tradescantia represented a large proportion of nutrients in litterfall (41% N. 61% P, 23% Ca, 46% Mg and 83% K), exceeded the nutrient content of the forest floor (except Ca), but was a small proportion of the topsoil nutrient pools. Taken together, our results show that Tradescantia increases litter decomposition and alters nutrient availability, effects that could influence the long-term viability of the majority of podocarp-broadleaf forest remnants affected with Tradescantia in New Zealand. These impacts are likely mostly due to Tradescantia's vegetation structure (i.e., tall, dense mats) and associated microclimate, compared with native ground covers.