Ecological Aspects of the Distribution of C 4 Grasses in Selected Habitats of Costa Rica

Abstract

A distributional study of 65 grass species in seven habitats of Costa Rica was done to determine the differential distribution of C3 and C4 grasses and their adaptations to climatic conditions of rainfall, temperature, and sunlight. The highest proportion of C4 grasses were found in lowland areas with prevailing conditions of high temperatures, high insolation, and water stress, whereas the majority of C3 species were found in high attitude environments with conditions of lower temperatures, lower atmospheric 02 concentration, and no water stress. C4 PLANTS HAVE BEEN SHOWN to differ significantly from C3 plants in their physiology, anatomy, ultrastructure, and environmental requirements (Black 1971; Black, Chen, and Brown 1969; Laetsch 1974; Purohit and Tregunna 1974; Smith and Epstein 1971). Limiting climatic factors and ecological adaptations also differ between these two photosynthetic types (Black 1971, Cooper and Tainton 1968, Downes and Hesketh 1968, McWilliams and Mison 1974, Purohit and Tregunna 1974). C4 plants are generally characterized by a low atmospheric CO2 compensation point, low to negligible rates of photorespiration, a high optimum temperature range, a well-developed bundle-sheath containing chloroplasts, and a high light-saturation point; whereas C,3 plants have a higher CO2 compensation point, high rates of photorespiration, an optimum temperature range of 10-25?C, a poorly developed bundle-sheath generally without chloroplasts, and a light saturation point much lower than C4 plants (Black 1971; Black, Chen, and Brown 1969; Bidwell 1974; Cooper and Tainton 1968). Photosynthesis in C4 plants is unaffected by high or low atmospheric 02 concentrations whereas photosynthesis of C3 plants is inhibited at high 02 concentrations and enhanced at low 02 concentrations (Downes and Hesketh 1968). C4 plants have a higher C02 fixed/ water-use ratio and are therefore better adapted to arid environments (Black 1971, Laetsch 1974, Downes 1969). Thus, C4 plants have a competitive advantage over C3 plants in areas of high light intensity, high temperatures, high atmospheric 02 concentrations, and low CO2 concentrations (Bjorkman 1976, Cooper and Tainton 1968, Downes and Hesketh 1968), and generally experience a selective disadvantage in cool, moist environments where low atmospheric 02 concentrations and high C02 concentrations enhance photosynthesis of C3 plants (Downes and Hesketh 1968, Teeri and Stowe 1976). Not all C4 plants have a high optimum-temperature range as evidenced by the C4 species Atriplex sabulosa, the photosynthesis of which is completely inhibited, as occurs with most C.3 species, at temperatures of 44-45?C. These temperatures are considered optimal for most CG species (Bjorkman 1976). The objective of this study was to gather field data relating the differential occurrence and distribution of C4 and C3 grasses in selected habitats of Costa Rica to certain environmental and climatic conditions, taking into account the physiological and anatomical differences inherent in the two pathways. A similar study has been done with grass species in North America (Teeri and Stowe 1976) to determine the relative importance of several climatic variables in the distribution and occurrence of C4 species. In this way, both the ecological distribution of C4 grass species and possible evolutionary selective pressures can be studied from environmental and climatic