Patterns of gray rubber rabbitbrush occurrence in burned sagebrush-grasslands, Missouri River Breaks, Montana
Owings, Glenn Curtiss
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Sagebrush-grasslands represent a large portion of the plant communities within the arid plains of the western United States. Grasses, forbs, and shrubs exist as subdominants to sagebrush that vary in density according to disturbances such as fire, wind, and defoliation. Fire is an important modifier of succession in sagebrush-grasslands, and shrub response to fire may be altered by browsing. The fire response of gray rubber rabbitbrush (Chrysothamnus nauseosus (Pallas ex Pursh) Nesom & Baird ssp. nauseosus) has not been well documented. Furthermore, it is heavily browsed in central and northeastern Montana and is an important winter forage for ungulates. This study investigated how it responds to fire and browsing. Fifteen sites (9 burned and 6 adjacent non-burned) in the Missouri River Breaks, Montana were analyzed to investigate relationships between shrub density, time since fire, shrub age, and browsing level (n=15). Density was counted in one 625m ² macroplot per site. Gray rubber rabbitbrush age (seedling, juvenile, mature) was estimated using basal stem diameter. A qualitative browsing level was assigned based on growth form characteristics (low, moderate, high). Time since fire was not significantly correlated with differences in gray rubber rabbitbrush (P=0.701, R-sq=0.00%) or sagebrush (P=0.391, R-sq=0.00%) density as a percentage of the shrub community (n=9). As a percentage of the shrub community, big sagebrush density at a site was a good indicator of gray rubber rabbitbrush density (n=15, P<.001, R-sq=68.45). Rabbitbrush decreases within the shrub community as sagebrush dominance increases. Gray rubber rabbitbrush composed a higher percentage of the shrub community in burned than non-burned sites (n=6, P=0.005). Big sagebrush composed a higher percentage of the shrub community in non-burned sites than burned sites (n=6, P=0.001). The percentage of gray rubber rabbitbrush shrubs in the "high" browse class was not a statistically significant predictor of density as a proportion of the shrub community (n=13, P-value=0.161, R-sq=9.49%). Results indicate that gray rubber rabbitbrush responds to fire by increasing shortly after disturbance, falling out of the community at some threshold as sagebrush is re-established. Browsing did not affect the ability of gray rubber rabbitbrush to dominate for a period after fire.