FOREST SPATIAL DYNAMICS WITH GAP EXPANSION - TOTAL GAP AREA AND GAP SIZE DISTRIBUTION

Recent studies on forest dynamics in diverse forested ecosystems sugge st that forest stands are disturbed more frequently if they are next t o existing gaps, and that gaps once formed tend to expand their area i n subsequent years. We examine total gap area and the size distributio n of gaps at equilibrium in a lattice-structured forest model. Each si te undergoes transition between two states (gaps and non-gaps), and th e disturbance rate (transition from non-gap to gap) increases with the number of gap sites in the neighborhood. Dynamics based on a mean-fie ld approximation (i.e. neglecting of spatial structure) failed to pred ict total gap area and the gap size distribution in the equilibrium fo rest. Pair approximation, which considers a closed dynamical system of average and local gap density (the conditional gap density among neig hbors of a randomly chosen gap site), can predict the total gap area, the correlation between neighbors, and the gap size distribution fairl y accurately. If the recruitment rate increases in proportion to non-g ap area in the forest, the model may show bistability. We analyse data on forest spatial dynamics in the light of the model. We conclude tha t gap size distribution can often be described using two statistics (g lobal and local gap densities) and that these in turn can be predicted by the dynamics of gap formation, gap expansion, regeneration, and ga p closure. (C) 1996 Academic Press Limited