Drivers of Vegetation Response to Interactive Effects of Disturbance in a Sagebrush Steppe

Download or read book Drivers of Vegetation Response to Interactive Effects of Disturbance in a Sagebrush Steppe written by Lauren Cathleen Connell and published by . This book was released on 2018 with total page 124 pages. Available in PDF, EPUB and Kindle. Book excerpt: Globally, vegetation structure and patch variability in grasslands and savannas are strongly driven by natural disturbance regimes. These disturbances influence height and cover of herbaceous and woody plants, and often within a variable spatio-temporally regime that results in a heterogeneous landscape. In North America, semi-arid rangelands include grasslands and sagebrush (Artemisia spp.)-dominated shrublands that evolved with spatially and temporally variable disturbance regimes of wildfire, large ungulate herbivory, and colonial burrowing mammals. Moreover, interactions among multiple disturbances, including wildfire, herbivory by wild and domestic ungulates and colonial burrowing mammals, are driving forces of plant community structure and composition. The effects of these multiple, interactive disturbances are particularly less understood in shrubland-grassland ecotone regions, where divergent climate regimes, disturbance-sensitive vegetation communities, and historic disturbance regimes are juxtaposed and interact to create unique ecosystem responses. My study objectives were thus designed to investigate the effects of multiple, interactive disturbances and their implications for livestock and wildlife management. I addressed these topics in the Thunder Basin National Grassland in northeast Wyoming, U.S.A. In Chapter 1, I investigate the separate and interactive effects of livestock, native ungulates, fire, and small mammals on vegetation structure through a three-tiered, large-scale manipulative experiment. I used nested grazing exclosures to isolate the effects of herbivory from livestock, wild ungulates, or small mammals within areas affected by either historical wildfire, black-tailed prairie dog (Cynomys ludovicianus) colonies, or neither disturbance. I replicated this sampling design four times. I evaluated the interactive effects of herbivory and historical disturbance on vegetation structure by quantifying vegetation height, visual obstruction, shrub density, shrub canopy, and shrub leader growth. The exclusion of wild ungulates and lightly-to-moderately stocked livestock for two years did not affect herbaceous vegetation structure, shrub density, or shrub canopy cover. Maximum vegetation height, visual obstruction, heights of grasses and forbs, and shrub density were all negatively affected by prairie dogs. Both wildfire and black-tailed prairie dogs had lower canopy cover of shrubs and Wyoming big sagebrush, when compared to undisturbed sites. Shrub leaders experienced over 3-times more browsing on prairie dog colonies, when compared to undisturbed areas and the combined presence of livestock and native ungulates on prairie dog colonies caused significantly more leader browsing than in the presence of native ungulates alone. In Chapter 2, I assessed the effects of prairie dog herbivory on forage in a northern mixed-grass prairie. Black-tailed prairie dogs have high dietary overlap with livestock, which can cause forage-centric conflicts between agriculture and conservation. Research suggests prairie dogs can enhance forage quality, but it remains unclear how the strength of trade-offs between quality and quantity varies throughout the growing season, or the degree to which increased forage quality is caused by altered species composition versus altered plant physiology. I collected samples on prairie dog colonies and at sites without prairie dogs during June, July, and August 2016 – 2017 for forage quality, and August 2015 – 2017 for biomass. I collected both composite samples of all herbaceous species and also samples of western wheatgrass ( Pascopyrum smithii [Rydb.] Á. Löve) to isolate mechanisms affecting forage quality. Across years and plant sample types, crude protein, phosphorus, and fat were greater and neutral detergent fiber was lower on prairie dog colonies than at sites without prairie dogs. The effects of prairie dogs on forage quality persisted throughout the season for western wheatgrass samples. Across years, aboveground biomass did not differ significantly between prairie dog colonies and sites without prairie dogs and the effects of prairie dogs on herbaceous biomass were significantly influenced by spring precipitation. My results demonstrate season-long enhanced forage quality on prairie dog colonies due to both compositional and phenological shifts associated with prairie dog herbivory. Across years, enhanced forage quality may help to offset reductions in forage quantity for agricultural producers. In Chapter 3, I evaluated the use of conspecific acoustic signals as a potential management tool for prairie dogs. Black-tailed prairie dogs are a major driver of vegetation structure and heterogeneity in northeastern Wyoming, in addition to being highly influential on forage quality and production. The management of prairie dogs in this region is a great priority by the U.S. Forest Service and private landowners and thus I sought to explore the influence of acoustic signals on prairie dog behavior and its fitness implications. Researchers have demonstrated cues of conspecifics including acoustic signals can be successfully used in the conservation and management of avian species but it has rarely, if ever, been applied to free-roaming small mammals. The black-tailed prairie dog is a colonial, small mammal whose gregarious vocalizations create fitness benefits of group vigilance against predation and increased foraging time.