University of Nevada, Reno Plant Community Response to Mowing In
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University of Nevada, Reno Plant community response to mowing in Wyoming big sagebrush habitat in Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Natural Resources and Environmental Science By Matthew Church Sherman Swanson/Thesis Advisor May 2017 Copyright by Matthew Church 2017 All Rights Reserved THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by MATTHEW CHURCH Entitled Plant Community Response to Mowing in Wyoming Big Sagebrush Habitat in Nevada be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Sherman Swanson, Phd, Advisor Elizabeth Leger, Phd, Committee Member Barry Perryman, Phd, Graduate School Representative David W. Zeh, Ph.D., Dean, Graduate School May, 2017 i Abstract Thousands of acres of Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) habitat have been mowed throughout Nevada to create fuelbreaks for wildfire control, enhance resilience and resistance to invasive plant species, increase perennial herbaceous understory, and improve wildlife habitat. To improve our understanding of how these plant communities respond to mowing and whether treatment goals are being met, an observational study of paired mowed and unmowed locations was conducted between 2010 and 2015 across the geographic extent of mowed Wyoming big sagebrush in Nevada. In 2015, foliar and ground cover, perennial herbaceous density, and soil profile data were collected at 32 locations within 16 individual mow projects of different ages. Wilcoxon signed rank tests indicated significantly different mean cover values of ground cover, functional groups and individual species between treated and untreated locations. Bare ground (p < 0.01), moss (p < 0.01), cryptogam (p = 0.01), total foliar cover (p < 0.01), non resprouting shrub (p < 0.01), Poa secunda (p = 0.01) and Phlox sp. (p = 0.04) cover were lower in mowed locations. Litter (p < 0.01), tall perennial bunchgrass (p = 0.02), Bromus tectorum (p = 0.08) and Elymus elymoides (p = 0.01) cover were greater in mowed locations. Median density values of tall perennial bunchgrasses (p = 0.01), Sphaeralcea sp. (p = 0.04), and Agropyron cristatum (p = 0.08) were greater in mowed locations, while Linanthus pungens density (p = 0.02) was lower. Generalized linear models examining the influence of abiotic variables on herbaceous perennial response showed that greater amounts of precipitation in March through June following treatment were correlated with greater increases in perennial herbaceous cover (p < 0.01). An increasing percentage of clay in the soil A horizon was correlated with ii greater perennial herbaceous density following treatment (p = 0.02). Precipitation during July through September in the year following treatment was correlated with declines in both perennial herbaceous cover (p < .01) and density (p < .05). These results indicate variable responses to mowing in Nevada. iii Table of Contents Abstract…………………………………………………………………………………….i Table of Contents…………………………………………………………………………iii List of Tables……………………………………………………………………………..iv List of Figures…………………………………………………………………………….iv Introduction………………………………………………………………………………..1 Methods……………………………………………………………………………………6 Site Selection……………………………………………………………………...............6 Experimental Design………………………………………………………………………9 Statistical Analyses………………………………………………………………………10 Results……………………………………………………………………………………14 Discussion………………………………………………………………………………..21 Management Implications………………………………………………………………..29 References………………………………………………………………………………..32 Appendix A: Mow Project Descriptions…………………………………………………37 Appendix B: Study Site Information………………………………………………….…49 Appendix C: Soil and Transect Photographs……………………………………….……51 Appendix D: Foliar Cover Data…………………………………………………...……117 Appendix E: Perennial Herbaceous Density Data……………………………...………117 Appendix F: Soil Data………………………………………………………………….117 iv List of Tables Table 1. Cover values and variability of untreated study sites…………………………14 Table 2. Results of Wilcoxon Signed Rank Tests……………………………...………15 Table 3. Correlation matrix of abiotic variables…………………………………….…19 Table 4. Study site information……………………………………………………...…49 List of Figures Figure 1. Study site locations……………………………………………………………8 Figure 2. 2015 Plot design……………………………………………………..……....10 Figure 3. Ground cover values for mowed and unmowed populations………………..16 Figure 4. Shrub cover values for mowed and unmowed populations………………….16 Figure 5. Graminoid cover values for mowed and unmowed populations………….…17 Figure 6. Forb cover values for mowed and unmowed populations………………...…17 Figure 7. Graminoid density values for mowed and unmowed populations………..…18 Figure 8. Forb density values for mowed and unmowed populations…………………18 Figure 9. Best GLM: perennial herbaceous cover response to mowing………….……20 Figure 10. Best GLM: perennial herbaceous density response to mowing………….….21 Figure 11. Photos showing the increase in cover of Chrysothamnus viscidiflorus……...25 Figure 12. Cover and density distributions of the difference between mowed and unmowed locations……………………………………………………….…26 Figure 13. Photos showing the increase in cover of Ephedra nevadensis…………...….27 Figure 14. Summary of management implications………………………………….…..31 1 Introduction Invasive annual grasses are a global concern, and are particularly problematic in the arid and semiarid western United States (D’Antonio and Vitousek 1992). Where the introduced annual grass, cheatgrass (Bromus tectorum L.), establishes in these ecosystems, it increases fire frequency, size, duration, and rate of spread (Balch et al. 2013) by increasing fine fuel continuity, abundance, and flammability (Davies and Nafus 2013). Following fire, cheatgrass commonly increases in abundance more than native species (Stewart and Hull 1949, Agbalog 2010), resulting from its increased seed dispersal distances (Monty et al. 2013), greater ability to exploit available soil resources (Monaco et al. 2003, Vasquez et al. 2008), faster growth rates (Arredondo et al. 1998) and greater seed production (Humphrey and Schupp 2001). Cheatgrass acts as an ecosystem engineer, altering soil nutrient distributions and further excluding native species (Blank and Morgan 2013). This fits the invasive plant-fire regime cycle described by Brooks et al. (2004) by altering fuel structures and fire regimes, adversely impacting native plant communities, and providing a positive feedback for its own proliferation. Cheatgrass dominates approximately 40,000 km2 of land in the Great Basin of the western United States (Balch et al. 2013). This large-scale conversion of native shrub and bunchgrass plant communities to annual grasslands adversely effects wildlife (Crawford et al. 2004, Hall 2012, Knick et al. 2003), domestic livestock production (Maher et al. 2013, Young and Clements 2009), recreation (Eiswerth et al. 2005) and public health (Yao et al. 2013). Much of this land would have previously supported Wyoming big sagebrush (Artemesia tridentata Nutt. subsp. wyomingensis Beetle and Young) with associated perennial grasses and forbs (Chambers et al. 2007). The high invasibility of 2 this particular plant community results in part from a relatively low abundance of native perennial species (Chambers et al. 2007), warm soil, low and annually variable precipitation, historic and currently inappropriate domestic livestock grazing, and ongoing anthropogenic disturbance. These same factors also contribute to difficulties restoring native vegetation in invaded Wyoming big sagebrush plant communities (Davies et al. 2011b; Miller et al. 2011). Because of the great cost and difficulty of restoration, a strategy of prevention may be appropriate (Pellant 1990, Davies and Sheley 2007, Rangeland Fire Task Force 2015). Two strategies for preventing cheatgrass proliferation are: 1) preventing fire in plant communities at risk of cheatgrass dominance following fire, and 2) increasing resistance to invasion by increasing the abundance of perennial herbaceous plants (Brooks and Chambers 2011; Davies et al. 2011b). A management option with potential to accomplish both strategies is the mechanical control of sagebrush and associated shrubs by mowing. Mowed fuelbreaks are commonly implemented in sagebrush ecosystems to temporarily reduce woody fuel foliar cover, height and volume. These treatments are designed to reduce wildfire rate of spread, severity, and size, and to decrease resource competition with perennial herbaceous plants in order to increase their abundance (Roundy et al. 2014). The use of mowing to accomplish these goals in Wyoming big sagebrush plant communities has been evaluated in recent years, particularly in eastern Oregon and by the Sagebrush Steppe Treatment Evaluation Project (SageSTEP) (see McIver and Brunson 2014). Studying eight locations in southeastern Oregon, Davies et al. (2011a) found greater concentrations of nitrate and ammonium in mowed soils, suggesting that mowing 3 does increase nutrient availability for herbaceous species. Unfortunately, they found less large perennial grass foliar cover, but more annual forb foliar cover and density and greater cheatgrass cover in mowed locations. This suggests that invasive annual species are better able to use the newly available nitrogen. This is consistent with Vasquez et al. (2008), who demonstrated the greater competitive ability of cheatgrass with increasing soil nitrogen concentrations, and a general theory of invasibility proposed