A Physiological Basis for Niche Separation Between Agropyron smithii (C"3) and Bouteloua gracilis (C"4)

Kemp, Paul; Williams, George

Agropyron smithii (a C"3 species) and Bouteloua gracilis (a C"4 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.

A Physiological Basis for Niche Separation Between Agropyron smithii (C"3) and Bouteloua gracilis (C"4)

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