The theory of metabolic ecology predicts specific relationships among tree stem diameter, biomass, height, growth and mortality. As demographic rates are important to estimates of carbon fluxes in forests, this theory might offer important insights into the global carbon budget, and deserves careful assessment. We assembled data from 10 oldgrowth tropical forests encompassing censuses of 367 ha and > 1.7 million trees to test the theory's predictions. We also developed a set of alternative predictions that retained some assumptions of metabolic ecology while also considering how availability of a key limiting resource, light, changes with tree size. Our results show that there are no universal scaling relationships of growth or mortality with size among trees in tropical forests. Observed patterns were consistent with our alternative model in the one site where we had the data necessary to evaluate it, and were inconsistent with the predictions of metabolic ecology in all forests.

Intraspecific density-dependent effects in the Barro Colorado Island (Panama) study area are far stronger, and involve far more species, than previously had been suspected. Significant effects on recruitment, many extremely strong, are seen for 67 out of the 84 most common species in the plot, including the 10 most common. Significant effects on the intrinsic rate of increase are seen in 54 of the 84 species. These effects are far more common than interspecific effects, and are predominantly of the type that should maintain tree diversity. As a result, the more diverse an area in the forest is, the higher is the overall rate of increase of the trees in that area, although sheer crowding has by itself a negative effect. These findings are consistent with, but do not prove, an important role for host-pathogen interactions (defined broadly) in the maintenance of diversity. Ways are suggested by which to test host-pathogen models and competing models.Tropical rainforests vary greatly in tree species richness, but in most of the humid tropics mature communities with diversities of up to 300 species of tree per hectare are common (1, 2). Equilibrium models (3) have been proposed to explain such high species richness and the maintenance of many different species over long periods. These require unique resource-heterogeneity and resource-utilization niches for each species (4, 5). If there is spatial partitioning (reviewed in ref. 6), or temporal partitioning of light gaps created by treefalls, this would increase the number of niches (7-11). Such habitat partitioning predicts positive density-and frequency-dependence within species. The effect of light gaps on niche partitioning has, however, recently been called into question (12).Another class of models is based on negative density-or frequency-dependence. We define density-dependence as correlations of life history parameters with total biomass at the outset of the census, and frequency-dependence as correlations of life history parameters with the numbers of trees present at the outset of the census. These measures are often, but not always, correlated.We can divide these models into two categories. The first, which we will call abiotic models, have only rarely been investigated (13). They are based on density-dependent abiotic factors in the immediate environment. Density-dependence might be the result of the removal of nutrients from the soil or of a buildup of toxic waste products. When the density is reduced through mortality, nutrients are replenished and toxins are degraded, allowing the cycle to begin anew. The second category is made up of biotic models, and deals with interactions of tree species with their enemies (pest pressure). As Gillett (14) was the first to point out, pests increase the diversity of the local environment, effectively increasing the number and diversity of ecological niches for host species in the forest. The best-known of these models is that of Janzen (15) and Connell (16), which predicts that species-specific pests will te...

Tropical forests vary substantially in the densities of trees of different sizes and thus in above-ground biomass and carbon stores. However, these tree size distributions show fundamental similarities suggestive of underlying general principles. The theory of metabolic ecology predicts that tree abundances will scale as the )2 power of diameter. Demographic equilibrium theory explains tree abundances in terms of the scaling of growth and mortality. We use demographic equilibrium theory to derive analytic predictions for tree size distributions corresponding to different growth and mortality functions. We test both sets of predictions using data from 14 large-scale tropical forest plots encompassing censuses of 473 ha and > 2 million trees. The data are uniformly inconsistent with the predictions of metabolic ecology. In most forests, size distributions are much closer to the predictions of demographic equilibrium, and thus, intersite variation in size distributions is explained partly by intersite variation in growth and mortality.

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