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Long-Term and Broad Scale Change in Wooded Shrublands A

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Long-Term and Broad Scale Change in Wooded Shrublands A University of Nevada, Reno Plant Community Dynamics in the Great Basin: Long-Term and Broad Scale Change in Wooded Shrublands A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Natural Resources and Environmental Science by Cody Ernst-Brock Dr. Elizabeth A. Leger/Thesis Advisor December 2018 THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by CODY ERNST-BROCK Entitled Plant Community Dynamics in the Great Basin: Long-Term and Broad Scale Change in Wooded Shrublands be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Elizabeth A. Leger, Ph.D., Advisor Benjamin W. Sullivan, Ph.D., Committee Member Lee Turner, Ph.D., Committee Member Elizabeth G. Pringle, Ph.D. , Graduate School Representative David W. Zeh, Ph.D., Dean, Graduate School December, 2018 i Abstract Expansion of native pinyon-juniper (Pinus monophylla-Juniperus osteosperma) woodlands can decrease shrub and herbaceous cover in the Intermountain West, affecting habitat quality and biodiversity. Changes in the range and cover of other plant communities, including forbs and invasive grasses, are also occurring. Removing woodlands in former sagebrush ecosystems has a long management history, with interest in understory plant community responses. We revisited a restoration site in western Nevada, 32 years after tree thinning treatments had occurred, and conducted vegetation measurements within historic treatment plots. Our findings suggest tree thinning and removal can increase shrub and perennial grass cover, but tree recolonization over the long-term is possible. We also used vegetation data repeatedly collected at unmanipulated monitoring plots to calculate change in foliar and litter cover during 2011-2017. Increases in pinyon-juniper dominance influenced decreases in foliar cover of shrubs and sage- grouse preferred forbs, and litter cover of all types increased significantly. ii Acknowledgements I would like to thank my advisor, Beth Leger, for her unwavering guidance, support, and expertise. Beth always pushed me to do my best work, and was a great traveling partner, mentor, and friend. Thank you to Lee Turner and the Nevada Department of Wildlife, for their support, collaboration, and confidence in my skills and abilities. I would also like thank my other committee members, Ben Sullivan and Beth Pringle, and my co-author, Robin Tausch, for invaluable feedback on my writing and work. Thank you to Lynn Zimmerman, my first mentor, and the Great Basin Institute, for teaching me so much and giving me endless opportunities for professional development and personal growth. I would also like to thank our Vegetation/Habitat Assessment field crews for their hard work and travel to remote locations across Nevada to collect the vegetation data that made this work possible. I thank my family: Chelsa and Marcello Rostagni, Kimberly Brock, Bill Ernst, and Beverly Ernst, for their steady encouragement throughout my time in graduate school, and their enthusiastic celebration of higher education. Lastly, my partner and love, Todd Granberry, for the pep talks, dinners, feedback, understanding, and laughter. iii Table of Contents Abstract…………………………………………………………………………………….i Acknowledgements………………………………………………………………………..ii Tables of Contents……………………………………………………………………..…iii List of Tables and Figures Chapter 1………………………..…..………………………...iv List of Tables and Figures Chapter 2………………..…..………………………..………..v Background………………………..…..…………………………………..…………..…..1 Chapter 1: Long-term vegetation responses to pinyon-juniper reduction treatments in shrublands of Nevada, USA………………………..…..………………………….............7 Chapter 2: Changes in plant community composition and ground cover in sagebrush steppe: effects of pinyon-juniper cover and environmental variables……………..….....51 Chapter 1 Appendix..……………..…..………………………..…..…………………...109 Chapter 2 Appendix…………...………………………..…..…………………………...131 iv Chapter 1: List of Tables Table 1. Seed mix components and kilograms (kg) per hectare that were broadcast seeded at the Wellington Hills woodland reduction field site (western Nevada, U.S.) in 1984. Table 2. Results from mixed models using the restricted maximum likelihood method to determine overall model significance. Responses are: average foliar cover and density of target species, plant functional groups, and litter categories calculated via line-point intercept and density belt sampling; average gap lengths (in binned length categories) between perennial species, calculated via canopy gap measurements; total, native/introduced, and invasive species richness calculated via timed species inventories. Chapter 1: List of Figures Figure 1. Imagery of Wellington Hills field site, located in western Nevada, U.S., showing blocks and numbered plots. The area was treated in 1984 with three different woodland reduction treatments and one control. Figure 2. Foliar cover of target species and functional groups at the Wellington Hills field site in western Nevada, U.S. Figure 3. Overall average species foliar cover at the Wellington Hills field site in western Nevada, U.S., for each treatment. Figure 4. Total P. monophylla density (average trees/hectare) for 4 size classes (depicted in legend) and standing dead trees summed across all blocks in each treatment. v Figure 5. Foliar cover of seeded grasses at the Wellington Hills field site in western Nevada, U.S. Native seeded species: P. secunda. Introduced seeded species: T. intermedium, A. cristatum, and B. inermis. Figure 6. Average perennial canopy gaps (binned into different cm gap length categories) at the Wellington Hills field site in western Nevada, U.S. Chapter 2: List of Tables Table 1. Metadata for vegetation monitoring sites, including: broad monitoring time frame (earliest and latest years monitored), the number of plots within site, and average elevation, precipitation, minimum temperature, and maximum temperature for each site. Table 2. Vegetation functional groups, litter types, and target grass species used in all models Table 3a. Variables and covariates used in broad models Table 3b. Variables and covariates used in seasonal models Table 4a. % Foliar cover change values for all monitoring plots, regardless of vegetation type Table 4b. % Foliar cover change values for sagebrush vegetation monitoring plots Table 4c. % Foliar cover change values for Phase I vegetation monitoring plots Table 4d. % Foliar cover change values for Phase II vegetation monitoring plots Table 4e. % Foliar cover change values for Phase III vegetation monitoring plots vi Table 5a. Broad model, vegetation functional groups Table 5a continued. Broad model, vegetation functional groups, starting cover of functional groups/species Table 5b. Broad model, target grass species Table 6a. Seasonal model, vegetation functional groups Chapter 2: List of Figures Figure 1. Map of project monitoring plot locations across Nevada and eastern California. Figure 2. Vegetation monitoring plot layout. Three 50-m transect tapes are extended from plot center, typically oriented at 0°, 120°, and 240°. Total area of each monitoring plot is 9510-m2. Figure 3. Histograms of range of foliar cover change for selected vegetation functional groups. Figure 4. Litter % foliar cover change among vegetation types. Figure 5. Forbs % foliar cover change among vegetation types. Figure 6. Shrubs % foliar cover change among vegetation types. Figure 7. Trees, woody, nonwoody % foliar cover change among vegetation types. Figure 8. Bromus tectorum % foliar cover change among vegetation types. Figure 9. Perennial grass species % foliar cover change among vegetation types. vii Figure 10. Perennial Grasses % foliar cover change among vegetation types. Figure 11. Sagebrush cover and minimum annual temperature Figure 12. Litter cover and annual precipitation Figure 13. Shrub cover and minimum fall temperature Figure 14. Forb cover and summer maximum temperature Figure 15. Litter cover and winter precipitation 1 Background The Intermountain West has experienced landscape-scale changes in vegetation distribution, range, composition, and cover over the past century (Allen and Breshears, 1998; Bertrand et al., 2011; Langley et al., 2018; Vitt et al., 2010), and shifts within the vegetation understory of wooded shrublands have been attributed, in part, to the expansion in density and range of native singleleaf pinyon pine (Pinus monophylla Torr. & Frém) and Utah juniper (Juniperus osteosperma [Torr.] Little) (Miller and Tausch, 2001; Tausch et al., 2009; Weisberg et al., 2007). Proposed explanations for the expansion of pinyon-juniper woodlands include: increased grazing pressure, changes in climate and C02 levels, shifts in fire size and frequency, and recovery after human harvest, (Barger et al., 2009; Breshears et al., 2005; Brockway et al., 2002; Chambers and Pellant, 2008; Miller and Tausch, 2001; Miller and Wigand, 1994; Romme et al., 2009). Although these native woodlands provide habitat and cover for certain species (Balda and Kamil, 1998; Watkins et al., 2007), their expansion into former sagebrush (Artemisia tridentata) steppe can negatively affect sagebrush-obligate species including the greater sage-grouse (Centrocercus urophasianus), that rely on sagebrush habitat for nesting, cover, and forage (Bates et al., 2017; Coates et al., 2016, 2017; Davies et al., 2011; Prochazka et al., 2017). Pinyon-juniper expansion also affects understory vegetation, partly due to the ability of these woodlands to exploit significant
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