Restriction of a C3grass to dry ridges in a semiarid grassland

摘要

Contrary to generalizations about differences in the adaptive significance of C3and C4photosynthesis,Stipa neomexicana, a C3grass, was restricted to the driest sites along a topographic and soil moisture gradient in a semiarid grassland in Arizona. The distribution ofStipa neomexicanawas inversely related to the abundance of C4grasses. The total biomass of C4grasses increased from low values on ridge crests to maximum values on lower, wetter sites. Ordination analysis confirmed that the topographic and soil moisture gradient was the factor most closely associated both with grass species distributions and with total biomass and cover. Other factors, such as soil fertility, did not appear to be closely related to vegetation patterns. The data suggest thatStipa neomexicanawas excluded from sites with greater soil moisture by competition with C4neighbors. While it is likely that other factors besides photosynthetic type determine the ability to survive on the driest sites in this system, C4photosynthesis appears to confer an advantage in competitive ability in microhabitats with greater amounts of soil moisture.