The density dependence of plant responses to elevated CO2

1 Stands of the annual Brassica kaber were grown at a range of six densitie s in both ambient and elevated CO2 environments, and measurements of shoot growth were made from seedling emergence through to reproduction. 2 Early in stand development (21 days following emergence), CO2 enhancement (beta) for above-ground biomass was highly density-dependent, ranging from 1.41 at the lowest density (20 plants m(-2)) to 0.59 at the highest densit y (652 plants m(-2)). 3 As stands matured and total biomass exceeded a relatively low threshold l evel (< 10.0 g m(-2); c. 20% of final yield), the density-dependence of bet a disappeared. Above this shoot biomass threshold, beta-values remained rem arkably stable (beta = 0.34) across a broad range of stand biomass, indepen dent of a stand's initial density or age. 4 Average stand-level reproductive beta-values at a final harvest were very similar to biomass values (beta = 0.38) and, as with biomass values at lat er stages, showed no apparent density-dependence. 5 These results highlight the importance of considering density and the tim e-course of stand development simultaneously when assessing the potential f or CO2-induced growth enhancements in plants.