Gap-Dependent Recruitment, Realized Vital Rates, and Size Distributions of Tropical Trees

In closed-canopy forests, plant morphology and physiology determine shade tolerance and potential growth and mortality rates; potential vital rates and ongoing gap dependence determine realized vital rates; and realized vital rates determine individual size distributions. This hypothesis was evaluated for the 73 most abundant canopy tree species from Barro Colorado Island, Panama. The percentage of recruits located in tree-fall gaps (P), sapling growth (G), and mortality (M) rates, and the coefficient of skewness of size distributions (g1were determined from censuses of all individuals >10 mm dbh in a 50-ha plot. Seven qualitative, bivariate predictions relating g1, G, M, P, and wood density (W) were evaluated. Six of the seven predictions were substantiated in pairwise analyses. A path analysis integrated all seven predictions and explained 51% of the interspecific variation in g1. Size distributions with many large individuals and a long tail of relatively rare, small individuals ($g_1 < 0$) characterized gap-dependent species with large fecundity, seed mortality, seedling mortality, G, M, and P. Size distributions with many small individuals and a long tail of relatively rare, large individuals ($g_1 > 0$) characterized shade-tolerant species with the opposite traits. The percentage of tropical tree species that require tree-fall gaps to regenerate has been estimated to range from 70% for old-growth forests. Our analyses suggest that there are not large numbers of functionally equivalent species at either extreme of the regeneration continuum. Rather, there are very few extremely shade-tolerant and extremely light-demanding species. Most species have intermediate light requirements and lifestyles.