Persistence of Idaho fescue on degraded sagebrush-steppe

Abstract

Idaho fescue (Festuca idahoensis Elmer), a palatable native perennial bunchgrass, has persisted on degraded sagebrush-steppe despite invasion by alien plants, excessive livestock grazing, and increased density of woody vegetation due to fire suppression. Survival of these populations in the presence of competitive alien plants suggested 2 possibilities: 1) that Idaho fescue produces seedlings that successfully compete for soil resources with alien invaders, and 2) that Idaho fescue seedlings tolerate stress caused by resource uptake by alien neighbors. We compared germination and growth of Idaho fescue from an undisturbed population with that of conspecific populations from disturbed (grazed and invaded) sites to determine whether disturbed-site seedlings had greater potential for resource capture. Recruitment in Idaho fescue from degraded sites did not appear to be aided by rapid seed germination or greater tolerance of moisture stress during germination. A greater proportion of seeds from the undisturbed site germinated; they germinated faster, and were no more sensitive to water stress, than were seeds from disturbed sites. For both groups, decreasing water potential from 0 to -0.5 MPa had little effect on germination percentages but declined at -1 Mpa. Germination rates slowed with decreasing water potential. Though Idaho fescue from undisturbed and disturbed sites extended roots down the soil profile with equal speed, seedlings from the undisturbed site produced 3.5 times more root length, had 2.7 times greater root length density, and 3.4 times more leaf area than disturbed-site Idaho fescue. The higher growth rate and greater root length density in Idaho fescue from the undisturbed site translates to greater exploration and exploitation of the environment. The 2 Idaho fescue groups had equivalent specific root length, specific leaf area, and root weight ratio. Idaho fescue from disturbed sites showed strong, positive geotropic growth whereas branching and diageotropic growth were greater in Idaho fescue from the undisturbed site. Cheatgrass (Bromus tectorum L.) extended roots faster than did Idaho fescue, with 17 times the root length, 6 times the root length density, and 10.8 times the leaf area of undisturbed-site Idaho fescue. Cheatgrass and Idaho fescue had equivalent specific leaf area, but specific root length of Idaho fescue was nearly twice that of the alien. Roots accounted for about 31% and 55% of cheatgrass biomass. Competitive ability did not appear to promote recruitment in Idaho fescue populations on degraded rangelands. Idaho fescue seedlings from the undisturbed-site were better competitors than disturbed-site seedlings, but interference from neighboring cheatgrass most strongly inhibited shoot growth of both Idaho fescue and cheatgrass. Idaho fescue had little effect on cheatgrass shoot growth. Selection of stress-tolerant genotypes from original populations may best explain the continued existence of Idaho fescue on grazed and invaded sites. We suggest that tolerance of moisture stress combined with vegetative longevity, are mechanisms behind Idaho fescue's persistence.