Restoration and pollinator conservation: strategies for controlling invasive annual grasses on degraded tribal grasslands

Abstract

Invasive annual grasses (IAG) threaten Intermountain foothill grasslands in northwestern Montana, and these grasslands are integral to the cultural and ecological well-being of the Selis, Ksanka, and Ql'ispe Tribes. This project aimed to increase native vegetation cover and test the tolerance of native grasses and forbs to herbicides commonly used to control IAG. Three components were evaluated: a field study at two sites on Tribal grasslands, a greenhouse study of species tolerance, and a baseline bee survey at the two field sites. In the field study, six herbicides and nine species were tested, with herbicide treatments applied fall 2021 through summer 2022, depending on label recommended timings, and seeding occurring spring 2021. Canopy cover of all species was measured in 2022 and 2023. Rimsulfuron, sulfosulfuron, and indaziflam effectively reduced IAG (up to 96% control), while propoxycarbazone, imazapic, and glyphosate were less effective. Drill seeding largely failed. Geranium maculatum, Cleomella serrulata, and Gaillardia aristata were present in low numbers in 2022 but absent in 2023. The greenhouse study evaluated tolerance of 12 plant species to four herbicides, focusing on seedling emergence, biomass, and injury symptoms. Herbicides reduced emergence and reduced biomass for the majority of species compared to the non-sprayed check. All forbs showed high injury ranks except Lupinus argenteus, which ranked moderate across treatments. All grasses had high injury ranks across herbicides except Thinopyrum intermedium, which had the least injury of all species under sulfosulfuron. Although T. intermedium and G. aristata showed some emergence in indaziflam-treated soil, indaziflam prevented the emergence of all other species. A baseline bee survey conducted over three summer months in 2023 at the two field sites identified 42 bee species from 14 genera and five families. Species richness analysis predicted 48 species at one site and 74 species at the other site, indicating a relatively even distribution of species. To effectively manage IAG, rimsulfuron, sulfosulfuron, and indaziflam can be prioritized, but should be integrated with planting of specific native species to improve establishment and minimize non-target impacts. Restoration efforts should incorporate pollinator-friendly species and engage stakeholders to align management with cultural and ecological goals.