Shoot Structure, Leaf Physiology, and Daily Carbon Gain of Plant Species in a Tallgrass Meadow

We analyzed the importance of shoot structural and leaf physiological characteristics in determining interspecific differences in whole-shoot carbon gain in a tallgrass meadow. Canopy structure, aboveground mass, leaf nitrogen content, and leaf photosynthesis were determined for individual shoots of the dominant grass Miscanthus sinensis (a C4 species) and for four forb species (all C3). These data were used to calculate net daily carbon gain of individual shoots in absolute terms, per unit leaf area, and per unit aboveground biomass (Pmass). Pmass in turn, is an important component of growth per unit mass (relative growth rate, RGR). Miscanthus shoots had a higher total carbon gain and a higher Pmass than shoots of the other species. The greater Pmass of Miscanthus shoots was not because they captured more light per unit mass (Φmass), but because they achieved higher rates of photosynthesis per unit of absorbed light (Plight; Pmass = Φmass × Plight). The latter was mainly due to the higher leaf photosynthetic capacities per unit nitrogen associated with the C4 pathway of this species. Among the forbs, Potentilla freyniana had the highest Pmass, even though it was the shortest, most shaded species in the stand. This high Pmass was because its shoots had relatively high leaf-area-to-mass ratios (resulting in high Φmass) and low rates of dark respiration (resulting in high Plight). Sensitivity analysis of our model showed that relative to the other species these traits enable Potentilla to achieve high carbon gain in the shade but not at high light. It also showed that the traits of Miscanthus enable it to achieve relatively high carbon gain at high light but not at low light. Another sensitivity analysis revealed that an increase in leaf area and associated light capture of shoots, while keeping their total canopy nitrogen constant and thus reducing their leaf N content and associated photosynthetic capacities, reduced the estimated carbon gain for most of the shoots in the stand. These results indicate that interspecific differences in carbon gain per unit mass in this stand were more closely associated with differences in leaf physiology than with structural differences that determine light capture. Corresponding Editor: J. H. Richards.