| The study examined the energy flow dynamics of the perennial grasses in a desert grassland ecosystem, located in northern Arizona, as part of an environmental impact study of the Navajo and proposed Kaiparowits coal-burning electrical generating stations. This involved field monitoring of driving variables, state variables, and certain additional productivity correlated variables of the perennial grass system. Using this information and information from the literature a computer simulation model of C(,4) and C(,3) perennial grass carbon (energy) flow was constructed for the desert grassland.; The goals of the investigation were to elucidate the energy flow dynamics of the perennial grass system, identify sensitive and vulnerable points in the system, and determine the adaptive strategies of the grasses.; The following conclusions were made from the investigation. Moisture is the primary limiting factor of grass productivity. The low tufted grasses appear to reproduce primarily by vegetative means whereas the erect bunchgrasses appear to reproduce primarily sexually. However, none of the grasses show significant sexual reproduction during unfavorable years. The C(,3) grasses concentrate their growth during the spring. The C(,4) grasses, which also conduct a significant amount of growth during the spring, exhibit more pronounced growth in response to the summer thunderstorm moisture inputs. It appears that a species specific soil moisture threshold in the top 10 cm of the soil must be surpassed for approximately three days for a summer growth pulse to occur. During periods of growth initiation, the shunting of belowground carbohydrate reserves aboveground for rapid shoot production appears to be an important growth strategy in the grasses. Reduced metabolic levels during winter dormancy and aestivation seem to be important for continued survival. Moisture is, also, the primary limiting factor for the physical weathering/decomposition of dead grass material. The simulation analysis suggests that at annual precipitation levels below 10-12 cm, grass production is substantially reduced with relatively small decreases in annual precipitation. The simulation analysis also suggests that the distribution of seasonal and intraseasonal precipitation is at least as important to annual grass production as is the amount of annual precipitation. |
