Plant invasiveness and target plant density: high densities of native Schima wallichii seedlings reduce negative effects of invasive Ageratina adenophora.
Economically feasible strategies to cope with invasive species are urgently needed. Plant density can be increased to reduce competitive effects on target plants. This study indicates that increasing native plant density can be used to reduce the effect of invasive Ageratina adenophora. Seedlings of an indigenous tree species, Schima wallichii, were grown in pots containing uninvaded or invaded soil, with or without A. adenophora leaf litter on the soil surface. Schima wallichii seedlings were also grown at four densities under four levels of A. adenophora leaf litter. Root and shoot biomass and length were measured as response parameters in both bioassays. Schima wallichii growth was inhibited by A. adenophora leaf litter and invaded soil. High litter levels reduced S. wallichii root length and dry weight at low plant densities. The inhibition disappeared at high S. wallichii plant densities. As A. adenophora did not inhibit S. wallichii growth at high plant densities, adjustments of seedling density should be studied as a possible management strategy for invasion by A. adenophora and potentially by other exotic plant species. As density-dependent growth inhibition is the key characteristic of chemical interference, we propose that phytotoxins contribute to A. adenophora invasion particularly at low densities of native seedlings.