Ecosystem Services on Shrub-Encroached Rangelands: Balancing Supply and Demand

Abstract

The encroachment of woody plants into grasslands and savannas has been a phenomenon widely reported across both the Southwestern US and globally. Once established, woody plants may be long-lived and highly persistent creating numerous challenges for resource managers. The upper limits of shrub cover are regionally dictated by mean annual precipitation, but topoedaphic features play a key role in determining these limits, which can vary widely at the local level. Grasslands and savannas are of high value to socioecological systems due to the diverse portfolio of ecosystem services they provide. However, the transition to shrublands and woodlands can alter both processes and functions ultimately disrupting the availability of these services. Although there is a significant body of research on woody plant encroachment, little is known about the upper limits of shrub cover for a given topoedaphic setting or how this conversion has altered important ecosystem services. Improving our understanding of these topics can enhance management of these systems and is therefore of broad geographic interest. The overarching goal of this dissertation was to explore the woody plant encroachment phenomenon through a holistic lens evaluating both the physical and social impacts of this landscape wide cover change. Within the following manuscript I will present research on the underlying topoedaphic drivers dictating rates and patterns of encroachment at the local level, the long term impacts of encroachment on key ecosystem services, as well as the social perceptions on which ecosystem services are valued highest across these systems. The first study used high spatial resolution imagery to classify cover of a model shrub (Prosopis velutina, velvet mesquite) proliferating in a Sonoran Desert grassland. The analysis also explored how the upper limits of shrub cover varies across ecological sites and topoedaphic settings. Upper limits were found to have a wide range variously dictated by elevation, slope inclination/aspect, soil texture, and rainfall re-distribution. Furthermore, this variation was not equal across ecological sites, especially between lowland and upland sites. For the second project I conducted a case study in southern Arizona and New Mexico to evaluate stakeholder perceptions of and preferences for various ecosystem services provided on semi-arid rangelands where shrub proliferation has impacted traditional livestock grazing. Perceptions of rangeland ecosystem services were elicited via a visually-based landscape interpretation while preferences were quantified using Best-Worst Scaling (BWS). Results suggest that stakeholders familiar with rangelands and their management generally perceive low shrub cover as providing a wider range of valued ecosystem services compared to rangelands with high shrub cover. Contrary to expectations, ecosystem service preferences in the context of shrub encroachment were generally uniform across all stakeholder groups surveyed (e.g. ranchers, state/federal governmental employees, non-governmental land managers, academicians, recreationists), with habitat for biodiversity and erosion control being identified as the most preferred. The third study built on the above case study by spatially modeling changes to the two highest valued ecosystem services (habitat for biodiversity and erosion control) following documented shrub encroachment across a semi-arid grassland in southern Arizona from 1936-2017. While overall shrub cover change was found to be low over this time (~5%) it was highly variable across the landscape. Changes to ecosystem services were also found to have high spatial variability with some services being increased in concert with encroachment while others were negatively impacted. Understanding where and to what extent these services have been altered will help improve planning efforts with respect to the location, type (e.g., prescribed fire), and timing of brush management.