A Physiological Basis for Niche Separation Between Agropyron Smithii (C₃) and Bouteloua Gracilis (C₄)

Abstract

Agropyron smithii (a C3 species) and Bouteloua gracilis (a C4 species) occur in the same habitat of the shortgrass prairie ecosystem in northeastern Colorado. The hypothesis tested in this study was that the two species differ in their fundamental niches partly as a function of their different physiological responses to temperature and moisture gradients. Growth and gas exchange patterns were measured on plants grown in controlled—environment chambers at cool (20°/15°C) and warm (35°/15°C) temperatures. Total dry mass growth of A. smithii was approximately two times greater when grown at cool compared to warm temperatures, whereas total dry mass of B. gracilis was 2.7 times greater when grown at warm compared to cool temperatures. The temperature for maximal growth was correlated with the temperature response patterns for gas exchange of leaves and roots. A smithii had maximal rates of net photosynthesis when grown at cool temperatures, while B. gracilis had maximal rates when grown at warm temperatures. In plants not water stressed, net photosynthesis was limited primarily by stomatal aperture in B. gracilis and internal leaf factors in A. smithii. Rates of net photosynthesis of both species from both growth temperatures were very sensitive to leaf water potential and decreased exponentially with decreasing leaf water potential. Root respiration, measured simultaneously with leaf gas exchange in hydroponically grown plants, was greater in A. smithii grown at 20°/15°C and in B. gracilis grown at 35°/15°C, and decreased with decreasing solution water potential. Root respiration of warm—grown A. smithii and cool—grown B. gracilis was lower than that of plants grown at more nearly optimal temperatures and did not decrease with decreasing solution water potential. These finding suggest that the temporal displacement of growth between A. smithii and B. gracilis in their native habitat is partly a function of the differential temperature responses of photosynthesis and related metabolic processes. The differences in the temperature responses of photosynthesis appear to be a consequence of the species having different photosynthetic pathways. The similarities between the species in their physiological responses to water stress suggest that seasonal moisture gradients are not as important a parameter along which niche separation has occurred as are seasonal temperature gradients.

Community assembly and invasion: An experimental test of neutral versus niche processes

“…Second, C4 grasses are strong nitrogen competitors in our system (25) and have many traits that make them dominant competitors for nitrogen. C4 grasses have higher nutrient-use efficiencies, greater water-use efficiencies (31,32), and lower tissue nitrogen concentrations (32) than C3 grasses. At Cedar Creek Natural History Area, a comparison of two C4 and three C3 grass species showed that C4 grasses more strongly reduced midsummer soil nitrate (27), had lower tissue nitrogen concentration, greater nitrogen-use efficiency, greater net productivity, and greater proportion of biomass below ground (33).…”

“…Third, C3 grasses attain peak biomass in the cool times of year, i.e., spring and fall (32,36), whereas C4 grasses grow most actively in the hot summer months (31). These differences are attributable to their different photosynthetic pathways (31,32).…”

Local distribution of C3 and C4 grasses in sites of overlap on Mount Kenya

“…As long as there is sufficient diversity of microsites at a given locale, both C 3 and C 4 species could coexist. Temporal separations of C 3 and C 4 plants in communities have been measured and found to be significant in Wyoming (Boutton et al 1980), Colorado (Kemp and Williams 1980), and South Dakota (Ode et al 1980). These studies found C 3 species to be dominant, in either biomass or frequency, in the spring.…”

Production characteristics of the mixed prairie: constraints and potential

“…I produced about 400 kg ha-r more biomass above-ground than short-grass native range but about 100 kg ha-l less of total plant yield (Redente et al 1989). Net primary production on ungrazed Agropyron-Koeleria, Agropyron (smithii)-Stipa (viridula), and Stipa-BoutelouaAgropyron sites was about 7,000, 7 ,820, and 12,600 kg ha-r, respectively, with about 80% below ground (Sims and Coupland 1979 (Smoliak 1986 more readily and produced almost 3 times more biomass in a warm environment (Kemp and Williams 1980). The offsetting growth between the species may be a strategy for reducing interspecifi c competition (Williams 1974) and increasing community production (Lorenz and Rogler 1972)butthe most visible effect is on the community structure.…”