Age and light effects on seedling growth in two alternative secondary successions in central Amazonia

Background : In central Amazonia, previous low intensity land use engenders succession dominated by Cecropia spp. which proceeds at high rates; however, at higher intensity of use succession is arrested and dominated by Vismia spp. over the long-term. Factors driving these two successional pathways are unknown. Aims : We aimed to elucidate seedling growth under the two alternative successional pathways. Methods : We experimentally determined the effects of successional age and photosynthetically active radiation (PAR) on relative height growth (RHG) of nine species of shade-tolerant tree seedlings in secondary forests dominated by Cecropia and Vismia, varying in age from 1–20 years. Results : In Cecropia-dominated successions, seedling RHG decreased with increasing successional age and with associated decreasing PAR. In Vismia-dominated successions, RHG was independent of successional age and PAR, and PAR did not change with successional age, being always higher than in Cecropia stands. The RHG of seedlings was lower in Vismia- than in Cecropia-dominated stands for similar PAR levels. Conclusions : Successional age and light availability affect seedlings growth differently in the two successional pathways. Unlike in Cecropia-dominated successions, in Vismia-dominated secondary forests seedling growth is limited by factors other than light. In a scenario of increasing land use intensity, constraints to seedling development in secondary forests can reduce species diversity in human-altered landscapes