Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2003
Publication Date: N/A
Interpretive Summary: Species richness and diversity are important indicators of the health of plant communities. Russian knapweed displaces many native species along riparian areas throughout the western United States. We evaluated the effects of glyphosate, fosamine, and clopyralid plus 2, 4-D for controlling Russian knapweed and enhancing species richness and diversity. Glyphosate increased total species richness and diversity, but yielded mainly non-native grasses and forbs. This herbicide may be useful for recovering the health of plant communities, but restoring native species seems unlikely using any of these herbicides.
Technical Abstract: Species richness and diversity are important indicators of ecosystem function and may be related to plant community resistance to invasion by non-indigenous species. Knowledge about the influence of various strategies on species richness and diversity is central to making wise invasive plant management decisions during restoration. Our objective was to determine the influence of clopyralid plus 2,4-D, glyphosate, and fosamine at various application rates and timing, on the density and biomass of Russian knapweed and associated desired plant groups. In a randomized complete block design with four replications, 28 treatments (3 herbicides x 3 herbicides rates x 3 herbicide application timings, and an untreated control) were applied to two sites located along the Missouri River riparian corridor in Montana. Clopyralid plus 2,4-D (3,6-dichloropicolinic acid + 2,4-dichlorophenoxyacetic acid), glyphosate [N'(phosphonomethyl) glycine], and fosamine [ethyl hydrogen (aminocarbonyl) phosphonate] were applied in June (spring rosette stage of Russian knapweed), July (bud to bloom stage of Russian knapweed), and August (flowering stage of Russian knapweed). Herbicide rates were clopyralid plus 2,4-D at 0.08 (clopyralid) + 0.42 (2,4-D), 0.13 + 0.67, and 0.18 + 0.92 kg a.i. ha -1; glyphosate at 0.6, 1.2, and 1.8 kg a.i. ha-1; fosamine at 3.6, 7.2, and 10.8 kg a.i. ha-1. Density of each species was recorded during June and August of 2001 and 2002. Species richness and Simpson's Reciprocal Index (1/D) were calculated. By August 2002, only the glyphosate treatment (4.6 species m-2) yielded greater total richness over that of the control (3.5 species m-2). At that time, diversity after applying clopyralid plus 2,4-D remained similar to that of the control (1.4), but glyphosate (2.3) and fosamine (2.0) increased total species diversity. Non-native grasses and forbs accounted for the increases in richness and diversity. Glyphosate may be appropriate for enhancing ecosystem function and possibly niche occupation to preempt re-invasion by Russian knapweed, but restoring native species seems unlikely using any of these herbicides alone.