Effects of species richness and elevated carbon dioxide on biomass accumulation: a synthesis using meta-analysis

Magnitude of growth enhancement by elevated CO2 in a plant assemblage is dependent on a number of biotic and abiotic factors, including species richness. In this meta-analysis, we examined effects of elevated CO2 on plant biomass accumulation in single- (populations) and multi-species (communities) assemblages. The primary objectives were to statistically synthesize the voluminous CO2 studies conducted so far and to assess the collective response of plant growth to elevated CO2 as affected by species richness. Our analysis showed that biomass enhancement by higher CO2 was consistently lower in communities than in populations. For example, total plant biomass (W-T) increased only 13% in communities compared to 30% in populations in response to elevated CO2 across all studies included in this synthesis. Above- and below-ground biomass responded similarly as WT to elevated CO2 and species richness. Smaller growth enhancement by CO2 was found in communities consisting of species of different growth forms (woody vs. herbaceous species) or functional groups (legumes vs. non-legumes). This pattern was consistent across three major classes of facilities (closed, semi-open and open systems) used to manipulate CO2 concentrations. An analysis of free-air CO2 enrichment studies revealed that the population-community difference in growth enhancement by higher CO2 was also dependent on the rate of N addition. Populations responded more than communities only when soil was amended with N. From the CO2 studies synthesized in this meta-analysis, it is obvious that the collective growth responsiveness to elevated CO2 will be lower in communities than in populations. We hypothesize that resource usurpation, i.e., competitive compartmentation of growth-limiting resources by less responsive species, may be important in determining growth response to elevated CO2 in a community and is one of the reasons responsible for the lower biomass enhancement by elevated CO2 in communities, as found in this synthesis.