Forest management in the interior western U.S. is now largely focused on wildfire mitigation, fuel reduction, and restoration. As a result, reintroducing fire and prescribed fire use is now a top priority. But prescribed fire guidelines reflect many untested assumptions and numerous questions remain. Are early or late-season burns better for restoring forest conditions? Which species are most sensitive to variations of prescribed fire regimes? While historic data is often used to answer these questions, there is increasing skepticism about using the past to establish such guidelines considering global change and present day perturbations. For example, few studies have examined forest understory response in the presence of livestock grazing, which occurs on about 235 million acres (primarily sheep and cattle) on public land in the US. To address this issue, we examine the effect of repeated spring and fall prescribed burning and cattle grazing on understory plant abundance, richness, reproduction, and species composition in four eastern Oregon ponderosa pine stands. We measured vegetation prior to a 5-year reburn and the subsequent exclusion of cattle, and five growing seasons thereafter. We used a repeat measure mixed ANOVA model in SAS to test differences in plant abundance and richness, and the MRBP, indicator species, and ordination compositional modules in PC ORD.
Results/Conclusions
Total plant cover and plant functional groups showed significant but varied responses to treatments. Reburning did not change total plant cover, but grazing exclusion increased plant cover by over 22%. This response was driven by perennial grass and forbs. Although grazing exclusion did not change grass cover, it increased grass height and number of flowering stalks. Fall reburning increased exotic and native annual species cover, a pattern that was evident from the original burns. Fall reburning also resulted in significant compositional changes, and native disturbance increasers, short-lived annuals, and exotics were key indicators. Compositional changes due to spring reburning were weaker, but perennial grasses and forbs were key indicators. By 2007, grazing exclusion resulted in significant species composition change, but indicator species were weak. Results from our study indicate that the fall prescribed reburns perpetuated early successional communities, including exotic species problems, and reduced native perennials such as sedges. Cattle grazing reduced plant cover and changes in composition were detectable but weakly developed. However, we anticipate that composition changes due to grazing exclusion will become more apparent the longer cattle are excluded.
