Radiation use efficiency and leaf CO2 exchange for diverse C4 grasses

Biomass accumulation of different grass species can be quantified by leaf area index (LAI) development, the Beer–Lambert light interception function, and a species-specific radiation-use efficiency (RUE). The object of this field study was to compare RUE values and leaf CO2 exchange rates (CER) for four C4 grasses. Biomass, LAI, and fraction of photosynthetically active radiation (PAR) intercepted were measured during three growing seasons. CER was measured on several dates and at several positions in the canopies. Switchgrass (Panicum virgatum L.) had the greatest RUE whereas sideoats grama [Bouteloua curtipendula (Michaux) Torrey] had the lowest. Big bluestem (Andropogon gerardii Vitman) and eastern gamagrass [Tripsacum dactyloides (L.) L.] values were intermediate. The two species with the greatest differences in RUE, switchgrass and sideoats grama, had similar relative amounts partitioned to roots. Likewise differences among species in the accumulation of soil carbon showed trends similar to total shoot biomass production. The light extinction coefficients (k) of switchgrass, big bluestem, and eastern gamagrass were smaller than for sideoats grama, implying that light was more effectively scattered down into the leaf canopy of the first three grasses. Whole canopy CER values were calculated with a stratified canopy approach, using LAI values, the Beer–Lambert formula with appropriate extinction coefficients, and CER light response curves. Differences among species in RUE were similar to these values for estimated whole-canopy CER divided by the fraction of light that was intercepted. High LAI along with low k contributed to higher RUE in switchgrass, in spite of lower values for single-leaf CER.