Invariant scaling relations across tree-dominated communities

Organizing principles are needed to link organismal, community and ecosyste m attributes across spatial and temporal scales. Here we extend allometric theory-how attributes of organisms change with variation in their size-and test its predictions against worldwide data sets for forest communities by quantifying the relationships among tree size-frequency distributions, stan ding biomass, species number and number of individuals per unit area. As pr edicted, except for the highest latitudes, the number of individuals scales as the -2 power of basal stem diameter or as the -3/4 power of above-groun d biomass. Also as predicted, this scaling relationship varies little with species diversity, total standing biomass, latitude and geographic sampling area. A simulation model in which individuals allocate biomass to leaf, st em and reproduction, and compete for space and light obtains features ident ical to those of a community. In tandem with allometric theory, our results indicate that many macroecological features of communities may emerge from a few allometric principles operating at the level of the individual.