Summary1 Metabolic scaling theory predicts that the rate of resource use per unit area is independent of the average mass per individual and that the slope of the log masslog density relationship should be − 4/3. 2 Data were obtained from plant communities along a natural gradient of moisture and latitude in north-west China to test the generality of this theory. 3 The allometric exponents (slopes of the log mass-log density relationship) for aboveground biomass decreased with natural moisture levels and plant cover, deviating from the predictions of the energy equivalence theory. Allometric exponents for belowground and total biomass were similar among the three sites and were much closer to the predicted value of − 4/3. 4 Metabolic scaling theory may be applicable under many growth conditions, but not when restricted to above-ground biomass under drought stress. The rate of supply of the limiting resource per unit area determines which plant parts behave according to theory.

A standardized neighbor removal experiment was conducted in 12 plant communities located on three continents to test the null hypothesis that competition intensity (CI) was independent of the amount of plant biomass present. Six plots were chosen in each community to cover the range oflocal variation in plant biomass. In each plot the relative growth rate (RGR) of transplanted Poapratensis (Poaceae) seedlings was compared in the presence and absence of neighbors. Neighbors were removed experimentally using R. J. READER ET AL. Ecology, Vol. 75, No. 6 herbicide. Removing neighbors increased RGR of transplants significantly in most plots. CI increased with an increase in the amount of neighbor biomass present in one community where the range of neighbor biomass was greater than in any other community. In contrast, CI did not change significantly with an increase in neighbor biomass in other communities where the range of neighbor biomass was smaller. For the communities combined, CI was not related to neighbor biomass in a consistent fashion. These results indicate that competition may reduce growth over a wide range of habitat productivity, but the relationship between CI and neighbor biomass may differ among communities.

When dense populations of even‐aged plant monocultures are subject to intense competition, mortality can occur in a process known as self‐thinning, in which changes in biomass are accompanied by decreases in density. On a plot of log biomass versus log density, self‐thinning populations show a linear relationship called the self‐thinning line. Variations in the fertility level of the substrate are known to affect self‐thinning in a number of ways. Populations from substrates with different fertility levels have been observed to self‐thin along the same line, or along different lines. A review of several experiments using the one species grown at different fertility levels was undertaken to look for any mechanisms that might account for the different patterns observed. It was postulated that the critical difference between whether populations followed a common or different line was the way in which competition developed in the stands as biomass accumulated. For the common‐line pattern, data on the canopy volume required to support a given biomass showed that biomass packing did not differ between fertility levels, supporting the model of a common competitive mechanism operating at all fertility levels. When different lines were observed, the development of competition differed as plants increased in size and biomass accumulated at each fertility level. Over the upper range of fertility levels, biomass packing values per plant increased as fertility declined and the position of self‐thinning lines followed predictions from biomass packing data. At the low end of the fertility scale, biomass packing values still decreased with fertility level, but the position of self‐thinning lines was not linked to the biomass packing of individual plants: root interactions were presumed to dominate competition and the trajectory of self‐thinning lines.