Cheatgrass: Fire and Forage on the Range

Chapter 16. Wildfire on the Range

The central issue in any discussion of the ecological ramifications of cheatgrass invasion and dominance on Great Basin rangelands is the role cheatgrass plays in the stand renewal process—the process by which a given plant community perpetuates itself on a specific site. Much of what appears in the literature concerning wildfires fueled by cheatgrass is observational and not based on measured results from designed experiments. Such observations are valuable, but it is difficult to interpret them with scientific precision. Plants do not live forever. Sooner or later they die and new seedlings have to be recruited either to perpetuate the preexisting community or to introduce a new community to the site. Essentially, the stand renewal concept is based on the premise that the species composition and dominance of a given community are highly influenced by how the tenure of the previous assemblage of plants on the site was terminated. Many years ago in a paper authored with Jack Major, we recognized burning as the stand renewal process for big sagebrush communities.1 Big sagebrush stands tend to consist of plants of roughly the same age, within 10–15 years, indicating that the previous community on the site was catastrophically destroyed. The decade-plus variation in age is a reflection of sagebrush’s slow recolonization due to limited seed dispersal. Historically, the Great Basin rangelands have experienced eleven kinds of renewal processes. chapter 16 Wildfire on the Range 266 1. A dynamic—in relation to climate—fire interval that occurred on specific sites before European contact (including aboriginal uses of wildfires) 2. Vertebrate and invertebrate herbivory 3. Promiscuous burning introduced largely, but by no means entirely, by herdsmen 4. Attempts at absolute control of wildfires on rangelands 5. Substitution of mechanical control by plowing for wildfires as a stand renewal process for big sagebrush/bunchgrass plant communities 6. Substitution of herbicide applications for wildfire for sagebrush/ bunchgrass community renewal 7. Wildfires fueled by cheatgrass 8. Grazing management introduced as a substitute for mechanical or herbicidal stand renewal in sagebrush communities 9. To a limited extent, wildfires burning in native grasslands that were restored through grazing management 10. Deliberate use of prescribed burning to renew stands 11. Fires burning in annual grassland communities It is within this array of stand renewal processes that the current burning of Great Basin rangelands operates. Let us examine each of them more closely. If you searched long enough in the Great Basin, you probably could find one spot where all eleven types of stand renewal by wildfires are within the field of view. You will need very good vision to view the precontact example because it probably will be on a very steep slope at high elevation, with a cliff that served as a firebreak located below the stand. We are not suggesting that anyone put in a lot of time searching for such a spot; our point is to emphasize the diverse array of stand renewal processes and therefore successional plant communities that can occur in big sagebrush/bunchgrass potential communities in the Great Basin. Fire Intervals before European Contact. The first stand renewal process that occurred in the Great Basin is the most difficult to identify and quantify. Estimates of the interval between wildfires in sagebrush/bunchgrass communities before European contact are highly variable and often controversial . Only recently have comprehensive studies tried to reconstruct the post-Pleistocene vegetation of the Great Basin. Analysis of fossil pollen indicates that five-needle pine species (subalpine tree species such as limber w i l d f i r e o n t h e r a n g e 267 268 c h e a tg r a s s and bristlecone pines) were growing near the maximum levels (4,380 feet elevation) of the pluvial lakes of the Great Basin during the glacial maxima of the Pleistocene. Currently, five-needle pines are found at elevations from 8,000 to 10,000 feet in the Great Basin. Apparently, many low-elevation refugia existed during the pluvial lake periods. Perhaps the geological diversity associated with fault-block mountain ranges contributed to these plant refugia, which somehow existed before such diverse species as the prototype salt desert shrubs or Utah juniper.2 As the climate became warmer and drier during the early Holocene (10,000–8000 bp [years before the present]), the great pluvial lake basins became barren, wind-eroded lake plains that were gradually...