Early vs. Asymptotic Growth Responses of Herbaceous Plants to Elevated Co2

Although many studies have examined the effects of elevated carbon dioxide on plant ‘‘growth,’’ the dynamics of growth involve at least two parameters, namely, an early rate of exponential size increase and an asymptotic size reached late in plant ontogeny. The common practice of quantifying CO2 responses as a single ‘‘response ratio’’ thus obscures two qualitatively distinct kinds of effects. The present experiment examines effects of elevated CO2 on both early and asymptotic growth parameters in eight C3 herbaceous plant species (Abutilon theophrasti, Cassia obtusifolia, Plantago major, Rumex crispus, Taraxacum officinale, Dactylis glomerata, Lolium multiflorum, and Panicum dichotomoflorum). Plants were grown for 118–172 d in a factorial design of CO2 (350 and 700 mL/L) and plant density (individually grown vs. high-density monocultures) under edaphic conditions approximating those of coastal areas in Massachusetts. For Abutilon theophrasti, intraspecific patterns of plant response were also assessed using eight genotypes randomly sampled from a natural population and propagated as inbred lines. The species and genotypes examined generally showed enhanced early relative growth rates (RGR) under elevated CO2; however, such early growth responses did not necessarily correspond to increases in asymptotic size. As a result, interspecific and genetic correlations between early RGR enhancements and asymptotic size enhancements were weak. Effects of elevated CO2 also varied systematically among the size metrics examined. The maximum leaf area attained by individually grown plants was generally reduced under elevated CO2; however, most species showed positive effects of CO2 on vegetative mass at final harvest. Final biomass responses were greatest in species showing large enhancements in root growth. In addition, CO2 responses of plants grown in dense monocultures differed from plants grown individually, showing reduced responses for both early RGR and final biomass. Average reproductive responses to elevated CO2 were positive for individually grown plants, but negative for dense monocultures. It has previously been suggested that differences in species’ growth rates may be the best single predictor of plant growth responses to CO2. Here we found that RGR (at ambient CO2) appeared to limit potential early growth responses among species, but that there was no such relationship between RGR and asymptotic size responses. We suggest that the latter may be determined in part by the degree to which large carbon sinks are formed late in plant ontogeny. The overall pattern of early and asymptotic growth responses to elevated CO2 varied widely both among and within species, and we found little evidence for the existence of discrete ‘‘functional types.’’ We argue that both early and asymptotic responses are of fundamental interest from a modeling and a physiological perspective. Both types of growth response should be incorporated in all efforts to understand and predict responses of terrestrial vegetation to rising CO2 and other aspects of global change.