Impacts of native grasses and cheatgrass on Great Basin forb development

Abstract

Land managers need more information on native forb growth and interactions between forbs and grasses to improve degraded sagebrush steppe habitats in the Great Basin, and to increase the diversity of revegetation seed mixes. This is especially important in areas infested with Bromus tectorum (cheatgrass), an annual grass present in more than 100 million acres of the Great Basin. To gather information on forb growth and measure the effects of both native grasses and B. tectorum on forbs, I conducted a greenhouse experiment with 5 native forbs: Lomatium sp., Eriogonum umbellatum, Machaeranthera canescens, Penstemon speciosus, Sphaeralcea munroana; two native grasses: Elymus elymoides and Poa sandbergii; and B. tectorum. Forbs were grown alone or with a grass, and were harvested after 6, 9 or 12 weeks of growth. Excluding Lomatium, which became dormant before week 12, forbs did not differ in shoot relative growth rate when growing alone, but the root relative growth rate of P. speciosus was 50% greater. Neither native grass reduced the biomass of any forb, but growth rate was reduced for two forbs. In contrast, B. tectorum reduced the biomass and growth rates of all forbs, between 50 and 96%. In a second experiment, to test the ability of native forbs to establish and grow with B. tectorum, 4 forb species were grown in plots seeded with densities ranging from 45-360 B. tectorum plants m -² at two locations in the Snake River Plain. Water content declined with increasing densities of B. tectorum and forb seedling survival was reduced for 2 species when B. tectorum densities were greater than 150 plants m -². Reductions in forb biomass greater than 90% occurred for three species when B. tectorum densities were less than 100 plants m-². This study demonstrated that sites with low expected densities of B. tectorum should be selected when seeding forbs, but that native forbs can establish in sites with B. tectorum densities less than 150 plants m -², that native grasses and forbs can be used together in seed mixes, and that phonological and morphological characteristics can be used to optimize the diversity of seed mixes.