Succession in old pastures of central Amazonia: Role of soil fertility and plant litter

Abstract

The effects of neighbor vegetation during succession were studied in old-field and rain forest sites of central Amazonia. We investigated how plant litter and the availability of soil nutrients (P and K) influenced interactions between neighbor vegetation and colonizing species. We monitored the establishment and growth of seedlings of four tree species, sown in old-field and forest plots in which vegetation, plant litter, and soil P and K were experimentally manipulated. In the old field, removal of vegetation significantly decreased seedling establishment of Oenocarpus bataua, Socrathea exorrhiza, and Aspidosperma discolor (suggesting facilitation in these late successional species) but had no effect on Inga edulis (a mid successional species). In contrast, neighboring vegetation inhibited the growth of all sown species: removal significantly increased biomass. In the forest site, removal of forest vegetation had no effect on establishment but significantly increased seedling mass, suggesting inhibition. Plant litter removal significantly decreased seedling establishment of all species in the old field, and of S. exorrhiza and I. edulis in the forest, suggesting positive effects. A significant interaction between plant litter and vegetation removal for O. bataua and S. exorrhiza in the old field showed that litter may also contribute to facilitation. Seedling mass of the three late-successional species was consistently not enhanced by P and K addition. In I. edulis, however, mass increased 2.5 times after P and K addition when vegetation was removed but did not vary in intact old-field vegetation. Seedling establishment in the old field was significantly higher in S. exorrhiza and A. discolor compared to the forest, while plant biomass in S. exorrhiza, A. discolor, and I. edulis was significantly higher. Neither plant litter nor P and K addition could account for these differences. We conclude that facilitation and competition may operate during the early stages of forest succession in Amazonia, that plant litter contributes to the facilitation process, and that soil P and K do not influence the inhibition process, suggesting that light competition may be important. As succession proceeds, the effects of neighbor vegetation switch from positive to negative.