Release from root competition promotes tree seedling survival and growth following transplantation into human-induced grasslands in Sri Lanka

The density of colonising tree seedlings is often very low in degraded human-induced tropical grasslands. To investigate the mechanisms that constrain seedling establishment in grasslands on former tea plantations in Sri Lanka we planted seedlings of the native tree species Dimocarpus longan Lour., Macaranga indica Wight, Symplocos cochinchinensis (Lour.) S. Moore and Syzygium spathulatum Thw. and examined effects of vertebrate herbivory, and above- and below-ground competition exerted by the grass sward on seedling growth and survival over 28 months. Seedlings of the same species were also planted in remnant patches of lower montane rain forest to determine the effects of habitat on seedling growth and survival. Less than 40% of seedlings survived to 28 months post-transplantation. The highest survival was recorded for Symplocos cochinchinensis in both grassland and forest, while Macaranga indica seedlings had the highest relative growth rate of height (RGRh) in both habitats. Root competition reduced survival of Macaranga indica and the RGRh of Macaranga indica, Symplocos cochinchinensis and Syzygium spathulatum in the grassland, while above-ground competition and exclusion of vertebrate herbivores had no effects on seedling establishment. These results suggest that Symplocos cochinchinensis would be suitable for re-establishing forest cover on degraded grasslands, although Macaranga indica would be more likely to catalyse succession because it possesses animal-dispersed fruit. Measures that overcome or restrict the effects of root competition from grasses would enhance tree seedling growth and survival more than manipulation of the light environment or protection from vertebrate herbivores.

► Native tree species can be used to accelerate succession on degraded grasslands.
► We examined the mechanisms limiting tree seedling growth and survival.
► Root competition exerted by grass inhibited seedling performance.
► Symplocos cochinchinensis and Macaranga indica can be used to accelerate succession.