The Future of Arid Grasslands: Identifying Issues, Seeking Solutions

This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Factors Controlling the Structure and Function of Desert Grasslands: a Case Study from Southeastern Arizona Carl E. Bock and Jane H. Bock1 ABSTRACT Twenty-eight years1 field work at and adjacent to the Appleton Whittell Research Ranch in Santa Cruz County, Arizona, have focused on the effects of livestock grazing, fire, drought, and establishment of non-native glasses on the vegetation and wildlife of this desert grassland site. Grazing reduces grass cover and fire frequency, while encouraging shorter-stature grasses and wildlife typical of lower elevation desert sites. Fires cause temporary declines in grass cover and some shrubs, and temporary increases in herbs and seeds, while favoring birds, rodents, and insects dependent on seeds and preferring more open habitats. Droughts cause temporary declines in shrubs, certain herbs and grasses, and associated wildlife. One key grass species, plains lovegrass, appears to survive drought better if it has recently burned. The independent and interactive impacts of drought, fire, and grazing have created temporal and spatial mosaics of native plant and animal assemblages. However, the establishment of non-native African lovegrasses threatens to permanently diminish many native populations of both plants and animals. Future integrity of the site depends upon reversing the spread of these exotics. INTRODUCTION Precipitation, fire, and ungulate grazing are major factors determining the structure and function of most of the world 1S grasslands, including those of western North America (Anderson 1982, Sims 1988, McClaran and VanDevender 1995). However, grazing by large hooved mammals has been a relatively unimportant force pre-historically in the American Southwest, due to scarcity or absence of North Americals only abundant native grazer, the bison (Mc~""'onald 1981, Truett 1996). Historically, two powerful anthropogenic forces have been added purposefully to the repertoire of factors influencing arid grasslands of the Southwest: livestock grazing and exotic grasses, particularly African love grasses of the genus Eragrostis (Anable et al. 1992, Bahre 1991, 1995, Roundy and Biedenbender 1995).2 Understanding the independent and interactive effects of fire, drought, grazing, and colonizing species depends upon long-term comparisons among areas variously affected or immune from these environmental forces. The Appleton-Whittell Research Ranch is a 3,160-ha sanctuary of the National Audubon Society on the Sonoita Plain - a high desert grassland in Santa Cruz County, 1 Both at the Department of EPO Biology, University of Colorado, Boulder CO. 2 See Appendix for scientific names of all species. 33 southeastern Arizona. A former operating cattle ranch, the site has been ungrazed by livestock and generally undisturbed since 1968, and so has had the potential to seNe as a site for monitoring post-grazing ecological changes, including fuel accumulations resulting in the return of wildfire to something perhaps resembling prehistoric frequency and intensity. In addition, the Research Ranch continues to support stands of exotic African love grasses planted prior to its establishment as a sanctuary. Our objective in this paper is to review studies conducted on and adjacent to the sanctuary since 1968, and to summarize what they suggest about the ways the precipittion, fire, livestock grazing, and alien grasses may generallly affect the assemblages of native plants and animals comprising desert grassland communities. STUDY AREA AND METHODS The Research Ranch includes rolling semiarid grasslands, oak savannas, and riparian bottom lands along largely seasonal watercourses, between the Santa Rita and Huachuca Mountains, in Santa Cruz County, Arizona. Elevations range from 1400 to 1560 m. Predominant soils are gravely loams (Richardson et al. 1979). Between 1968 and 1995, mean daily low January temperature was -2.1 oc, mean daily high June temperature was 32.1 oc, and mean annual precipitation was 44 em (range 27 to 73). About 60% of annual precipitation falls during the summer monsoon season, between July and early September, which is the period of peak plant productivity. Grassland habitats dominate the northern two-thirds of the sanctuary, where most of our studies have been conducted (J. Bock and C. Bock 1986). Broad floodplains are dominated by big sacaton, a tall grass frequently growing in nearly pure stands, interspersed with patches of blue grama, vine mesquite, and native dicots. Slopes adjacent to floodplains support diverse mixtures of mid- and short stature grasses, including side oats grama, blue grama, plains love grass, purple three awn, Texas bluestem, tangle head, spruce top grama, and curly mesquite. Level to rolling uplands are dominated by stands of blue grama, plains love grass, wolf tail, purple three awn, hairy grama, spruce top grama, and curly mesquite. Uplands also support scattered mesquite trees and various shrubs, including burro weed, yerbe de pasmo, and cat claw mimosa. Research on the sanctuary has been carried out using standard field and analytical methods for quantifying and comparing abundances of vegetation and animal populations among areas with known differences in land-use history. Vegetation line-point intercept transects and quadrants, bird transects and point counts, rodent live-trapping, and grasshopper hoop counts are among the field methods that have been employed (Onsager and Henry 1978, Verner 1985, Krebs 1989). Readers are referred to the REFERENCES below for details of these procedures, but in each case an attempt was made to compare areas differing only in terms of the variable of interest. These have included comparable mesa tops transected by boundary fences separating the sanctuary from adjacent grazed lands, grassland communities sampled before and after wildfire, stands dominated by exotic love grasses vs. adjacent topographically similar areas without the exotics, etc. LIVESTOCK GRAZING Understanding the effects of livestock grazing in arid grasslands depends upon the existence of ungrazed control areas for comparison. No such exclosures were established at the time livestock were introduced into the Southwest, although the impacts of grazing during the drought years of the 18901S doubtless were extreme (Bahre 1995). Ecological changes on exclosures established since 1900 cannot tell us with certainty what desert grasslands looked like prior to the time of Coronado, but they can help us to rank species in terms of their sensitivity to livestock grazing. 34 LIVESTOCK /GRArNG~ . Shift to Short- Reduced Litter Reduced F1re Stature Grasses and Grass Cover Frequency ~ Alteredtildlife/ Community INCRE~~SE WITH GRAZING DECREASE WITH GRAZING Merriam's Kangaroo Rat Hispid Pocket Mouse Western Harvest Mouse Grasshopper Mouse Fulvous Cotton Rat Scaled Quail Montezuma Quail Mourning Dove Botteri's Sparrow Homed Lark Cassin's Sparrow Lark Sparrow Grasshopper Sparrow Bunchgrass Lizard Fall Grasshoppers Summer Grasshoppers Curly Mesquite Grass Three-awn Grasses Black Grama Wolftail Grass Sprucetop Grama Sideoats Grama Hairy Grama Cane Beardgrass Plains Lovegrass Figure 1. Effects of livestock grazing on native flora and fauna of the Appleton Whitten Research Ranch Sanctuary in southeastern Arizona. Cross-fence comparisons were not conducted on the Research Ranch when livestock were first removed in 1968, although a vegetation analysis of the sanctuary did occur in 1969-1971 (Bonham 1972). Because the land-use history of the sanctuary was similar to that of the Sonoita Plain as a whole prior to 1968 (Bahre 1977), it is reasonable to assume that subsequent cross-fence differences between the sanctuary and adjacent operating cattle ranches have been the result of livestock exclusion. Brady et al. (1989) sampled vegetation canopy cover in 1984 on some parts of the sanctuary originally inventoried by Bonham (1972), and coupled this with cross fence comparisons. Their results showed approximate fivefold increases in the canopy cover of mid-height grasses, both cross-fence and over time within the sanctuary, while other components of vegetation were not significantly different. We compared grass canopy cover across fence lines on eight sites around the perimeter of the sanctuary in 1990, 22 years after livestock removal (C. E. Bock and J. H. Bock 1993). Total grass canopy cover was significantly higher on the ungrazed portion of each site, due largely to relative increases in abundances of taller bunch grasses. There was a strong positive 35 correlation between flowering height and amount of positive response to release from grazing among the ten most common grass species. Taller species such as plains love grass, cane beard grass, and side oats grarna were much more abundant inside the exclosure, while shorter-stature grasses such as blue grama, spruce top grama, black grama, and curly mesquite did not differ across fence lines, or were more common in grazed areas (Figure 1). Woody plants have responded variously to livestock removal. Kenney et al. (1986) found that one of the two most abundant shrubs in the region, yerbe de pasmo, was more abundant on than off the sanctuary, apparently because livestock will browse it in winter. However, mesquite and burro weed did not differ across fence lines (C. Bock et al. 1984). Animals responding positively to livestock exclusion have been species generally characteristic of

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