Vegetation responses following wildfire on grazed and ungrazed sagebrush semi-desert

Abstract

A 20-year set of cover data on sagebrush semi-desert plant communities responding to wildfire and livestock grazing near Mills in central Utah provided an opportunity to compare the assumptions and adaptability of classical and state-and-transition models for describing secondary succession. Cover data were organized and analyzed by plant species, growth forms, and other ground cover classes. Graphical analysis, ordination (employing semi-strong hybrid multi-dimensional scaling), regression, and analysis-of-variance were used to determine whether the patterns observed were best described as community change (tightly linked species) or individualistic change (each species acting independently). Distinct differences in total plant cover, growth form, and species composition were found between burned (both grazed and ungrazed) and the unburned and grazed treatments. Conventional graphical and statistical analyses of burned and ungrazed plots showed greater and earlier expansion of perennial grasses and then relatively less coverweighted compositional change in recent years compared to the other treatments. Vegetation on none of the treatments appears to have stabilized toward either the pre-burn sagebrush semidesert, a new state or the potential natural community for the site involved. Pathways of change reflected in the ordinations have been complex in all treatments. The only obvious trends in responses of individual species were to fire and the inverse relationship of cheatgrass to total perennial vegetational cover. All this evidence points to few tight linkages between species or growth form groups and thus favors viewing these patterns individualistically. While the state-and-transition model allows greater flexibility than the classical model in the depiction of plant community/individual species changes consequent to any management action, it doesn't apply readily everywhere, as exemplified by this case study.

Summary of science, activities, programs, and policies that influence the rangewide conservation of Greater Sage-Grouse (Centrocercus urophasianus)

“…Moderate grazing after perennial grass dormancy (that is, late season) in the first two summers after fire is not likely to reduce the recovery ability of herbaceous communities in sagebrush-steppe (Bates and others, 2009) when rest during the growing season is permitted. Differences in herbaceous cover among burn-ungrazed and burn-grazed areas were not observed during the first 6 years after fire, but between 7 and 18 years post-fire, perennial grass cover in grazed areas was less than cover in ungrazed areas (West and Yorks, 2002), so long-term post-treatment monitoring may be important. Treated areas may draw grazing pressure from all herbivores; thus, treatment designs that consider the possibility of an unplanned escalation of use by wild horses or elk (Cagney and others, 2010) when significant populations of these species are present have better chances of meeting productivity and habitat targets.…”

“…As this optimal colonist species established, chronic grazing by cattle, sheep, and horses combined with drought and fire to increase the distribution and frequency of disturbance and further optimize this region for dominance by an annual grass (Knapp, 1996). Importantly, research in sagebrush ecosystems has revealed an inverse relation between cheatgrass dominance and native perennial herbs, especially grasses (West and Yorks, 2002). Further, the post-disturbance response of sagebrush communities to fire and similar disturbances is strongly affected by the condition and composition before disturbance, the presence of propagules, and sprouting of native species (West and Yorks, 2002;Beck and others, 2009;Epanchin-Niell and others, 2009;Condon and others, 2011).…”

“…Importantly, research in sagebrush ecosystems has revealed an inverse relation between cheatgrass dominance and native perennial herbs, especially grasses (West and Yorks, 2002). Further, the post-disturbance response of sagebrush communities to fire and similar disturbances is strongly affected by the condition and composition before disturbance, the presence of propagules, and sprouting of native species (West and Yorks, 2002;Beck and others, 2009;Epanchin-Niell and others, 2009;Condon and others, 2011). Cheatgrass competes with native grasses and forbs that are important components of sagegrouse habitat.…”

“…arbuscula). These values capture differences among sagebrush types and provide approximate time frames that support the juxtaposition of disturbance (fire) and recovery (in this case, re-colonization by sagebrush); additional information on fire and fire-return intervals, especially relating to particular ecological types and (or) conditions, is available in the literature (Nelle and others, 2000;Miller, 2001;West and Yorks, 2002;Mensing and others, 2006;Lesica and others, 2007;Miller and Heyerdahl, 2008), and consideration of these and other local details may be necessary for comprehensive planning and mitigation.…”

“…In Wyoming big sagebrush types, especially in the Great Basin (all or part of MZs III, IV, and V), the invasion by exotic annuals has resulted in dramatic increases in number and frequency of fires with widespread, detrimental effects on habitat conditions (Young and Evans, 1978;West and Young, 2000;West and Yorks, 2002;Connelly and others, 2004). For example, big sagebrush communities invaded by cheatgrass have estimated mean fire-return intervals of less than 10 years in many areas (Connelly and others, 2004), whereas the natural regime is estimated (conservatively) to be 10 to 20 times longer.…”