SEED AND FOREST DYNAMICS - A THEORETICAL FRAMEWORK AND AN EXAMPLE FROM THE NEOTROPICS

Models that combine seed demography and forest dynamics to predict seed densities in forest patches of different disturbance ages are proposed. Two models are derived from a generalized Lefkovitch model with immigration. The first model assumes constant seed rain; the second one considers seed rain as a function of the forest mosaic configuration that changes according to a Markov-chain process. This theoretical framework was used to analyze data for the pioneer tree Cecropia obtusifolia at Los Tuxtlas rain forest. Our first model suggests that despite low soil-seed survival rates, high seed densities in all patch types in a forest mosaic may be explained by the abundant seed rain typical of pioneer species. We found that an abundant seed rain to all patches enables the regeneration of this species despite high seed predation and pathogen attack rates in the soil. Analysis of the second model suggests that increased gap formation in sites 2-35 yr since disturbance seems to alter seed availability of pioneers and patch dynamics less than an equal increase in gap formation in mature patches (> 35 yr since disturbed). Extensive deforestation that alters seed-rain regimes will prevent regeneration of secondary species such as C. obtusifolia that have fast seed-bank turnover rates. A comparison with previous works calls for more quantitative studies on the input and loss dynamics of soil seeds in different patches for particular tropical species.