WWETAC Projects

Project Title: Prescribed fire regime and grazing effects on understory vegetation and exotic invasive plants

Principal Investigator: Becky K. Kerns, Pacific Northwest Research Station, Western Wildland Environmental Threat Assessment Center, Prineville, OR

Collaborators: Michelle Buonopane, Oregon State University, Walt Thies, USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR (emeritus).   

Key Issues/Problems Addressed: Prescribed fire is an important tool used to manage and restore millions of acres of forests in North America. However, scientists have had little information about how prescribed fire and cattle grazing—common practices in many Western ponderosa pine forests—affect plant abundance and reproduction in the forest understory. Knowledge about the effects of fire on understory vegetation typically comes from single-burn prescribed fire or wildfire studies that exclude domestic livestock grazing, In 2002, scientists began to explore how these practices affect vegetation in a five-year study of postfire vegetation in eastern Oregon ponderosa pine forests where cattle have been routinely pastured from late June or early July through early to mid August.

Setting and Approach: Kerns and her research team took advantage of previously established prescribed-burn plots in the Malheur National Forest near Burns, Oregon, to conduct the study. Four mixed-age forest stands, about 90 to 140 acres each and located at the southern end of the Blue Mountains in the Emigrant Creek Ranger District, provided diverse sampling areas. Herbaceous richness, cover, and grass reproductive capacity were evaluated over five growing seasons following prescribed fires. 

 Key Findings:

  • Grazing effects were the same in burned and unburned areas. This allowed scientists to evaluate the outcomes of grazing and fire independently.
  • Excluding grazing for five growing seasons increased total vegetation cover, native perennial forb cover, grass height, grass flowering stem density, and the cover of some shrubs in both burned and unburned areas. No difference in the number of plant species was observed in grazed versus ungrazed areas.
  • Cattle did not seem to preferentially select either spring or fall reburned areas over unburned areas.
  • Reburning in spring or fall had little effect on native perennial plant cover or the number of species present, but reburns reduced sedge cover. Areas that were reburned in fall showed an increase in the cover and number of nonnative and early successional plant species.
  • Application of a different postfire grazing regime, other than late June/early July through mid to late August, or resting the areas after the fires, may have resulted in a different outcome.

Excluding cattle in areas that had been exposed to long-term grazing resulted in conditions more consistent with typical understory ecological restoration goals compared to the reintroduction of very frequent fire. These results are only applicable to the fire and grazing regimes conditions examined in this study. These results are “the first look” at vegetation effects; a 10-year evaluation is planned in 2012.

Kerns, B.K., Buonopane, M. B., Thies, W., and Niwa, C.  2011.  Reintroducing fire into a ponderosa pine forest with and without cattle grazing: understory vegetation response.  Ecosphere 2(5):artXX.doi:10.1890/ES10-00183.I. (PDF, 2.7 MB

Kerns, B.K., W.G. Thies, C.G. Niwa. 2006. Season and severity of prescribed burn in ponderosa pine forests: Implications for understory native and exotic plants. Ecoscience 13(1)44-55. (PDF, 1.3 MB)

WWETAC Project ID:  FY05BK3

Cheatgrass Dominance

Summary: Ponderosa pine is a major forest type in western North America and a key focus of prescribed fire and fuel reduction treatments. However, prescribed fires now interact with present environmental conditions that are very different than historical conditions. Fire is being returned to landscapes where exotic plants are often present and exotic livestock graze. Tree densities and fuel loading are typically excessive, native species may have tenuous or declining populations and native seed banks may be lacking. In addition, prescribed fires may be applied temporally outside the historical range of variability. Because species differ in the timing of peak sensitivity and response to burning, vegetation patterns in response to fire can differ by season of burn. We hypothesize that grazing will detrimentally affect understory abundance in response to the fires, but that effects would be linked to species life history characteristics and known grazing preferences. We also expect major exotic species in the study area, Bromus tectorum, (downy cheatgrass) to increase following the fires, and that exotics in general would be less abundant in areas excluded from grazing. In addition to hypothesis testing, we explore plant community patterns in relation to season and severity of prescribed fire, grazing, forest structure, substrate, and environmental heterogeneity.

Project ID: FY05BK3