Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/1996
Publication Date: N/A
Interpretive Summary: In recent years there has been increasing interest in using native species to revegetate western rangelands. The interest is particularly high on public lands, but private landowners are also looking for good forage species. Idaho fescue is an important native grass that is widely distributed and has very good forage characteristics. We wanted to determine if prior grazing history influenced the growth pattern of this species. Plants which had not been grazed since 1937 and plants that have always received grazing pressure were planted in a common garden and compared over a 2-year period. It appears that over a 50-year period, the grazed and nongrazed populations developed different sets of adaptations. The grazed plants were adapted to maintaining high rates of photosynthesis after leaf removal, and thus regrowing leaves quickly. Selection of Idaho fescue populations for seed production and subsequent reseeding efforts will require careful consideration of prior history. The speed at which genetic change occurs was surprising.
Technical Abstract: We tested the hypothesis that prior grazing history will influence the defoliation responses of Festuca idahoensis growing in a common garden environment. Plants were collected from a grazed pasture and adjacent exclosure which had not been grazed since 1937. Individual plants were established in a common garden at 1 m spacings during spring of 1989. We compared defoliated and nondefoliated treatments within the two population during 1992 and 1993. Photosynthesis, conductance, and xylem potentials were measured about monthly, until quiescence, and carbon isotope ratio (13C) was measured on senescent leaf tissue. Defoliated plants exhibited compensatory photosynthesis and averaged an 11% increase in photosynthesis the first year, and a 35% increase during the second year compared to undefoliated plants. No differences in photosynthesis were found between collections. However, outside exclosure plants had higher stomatal conductance and more negative 13C (thus lower water use efficiency) than within exclosure plants. We suggest that the higher conductance of previously-grazed relative to nongrazed populations allows for higher photosynthetic rates in response to defoliation, and aids in the rapid reestablishment of the plant canopy. Defoliated plants with a history of grazing pressure tended to have the highest rates of photosynthesis. The sacrifice in water use efficiency may be a trade-off in the adaptation of this species to defoliation. The most surprising aspect of this study was the fact that genetic differentiation of the populations appears to have occurred over a relatively short time period.