One main goal of forest ecology is to infer past changes and predict future changes in species composition, and examining the diameter distributions of canopy species is often used toward this end Theory indicates that higher rates of population change, λ, lead to the process of being eliminated from the forest-perhaps more steeply declining size distributions (more juveniles relative during succession-fail to reproduce and thus lack sufto adults). But other parameters also affect the size distribution: ficient advanced regeneration. ''The lack of congruence lower growth rate of juveniles and lower survival at any size pro-between overstory and understory in these stands is both duce more steeply declining size distributions as well. Empirical an indicator of change as well as a major concern for evaluation of 216 tree populations showed that juvenile growth conservation' ' (Foster et al. 1996, p. 420). In the tropics, was the strongest predictor of size distribution, in the direction the use of diameter distributions to project community predicted by theory. Size distribution did correlate with population growth, but weakly and only in understory species, not canopy change has been especially important in Africa, and sevspecies. Size distribution did not correlate with the growth rate of eral early studies suggested that dominant canopy species larger individuals nor with survival. Results suggest that static in-in African forests do not produce juveniles in the immeformation on the size distribution is not a good predictor of future diate area. This led to the idea of cyclical succession, or population trends, while demographic information is. Fast-grow-the mosaic theory of regeneration provide in-depth analyses of these ideas, using extensive quantitative data on size distributions. But the underlying assumption-that populations with low juvenile density relative to adult density are in deEcologists often use size distribution to indicate the cline-has not been explicitly tested. Here we address health of a population. If there are large numbers of ju-two ways of making this test. We first employ a theoretical approach, examining quantitative population models * E-mail: [email protected]. to determine how size distribution relates to population † E-mail: [email protected]. growth. Then we proceed with a direct, empirical test us- ‡ E-mail: [email protected]. ing records of population change over 13 yr in a tree § E-mail: [email protected]. community. In all 216 species abundant enough to ana
