Quantifying the spatio-temporal dynamics of woody plant encroachment using an integrative remote sensing, GIS, and spatial modeling approach.

Despite a longstanding universal concern about and intensive research into woody plant encroachment (WPE)---the replacement of grasslands by shrub- and woodlands---our accumulated understanding of the process has either not been translated into sustainable rangeland management strategies or with only limited success. In order to increase our scientific insights into WPE, move us one step closer toward the sustainable management of rangelands affected by or vulnerable to the process, and identify needs for a future global research agenda, this dissertation presents an unprecedented critical, qualitative and quantitative assessment of the existing literature on the topic and evaluates the utility of an integrative remote sensing, GIS, and spatial modeling approach for quantifying the spatio-temporal dynamics of WPE.In sum, this dissertation demonstrates that integrative remote sensing, GIS, and spatial modeling approaches have enormous potential for addressing questions relevant to both rangelands research and management. However, it also suggests that much work remains to be done before we can translate our understanding of WPE into sustainable rangeland management strategies. In particular, we need to more fully explore the limitations and potentials of currently available data and techniques for quantifying WPE; build structures for data sharing and integration; develop a set of relevant standards; more actively engage in collaborative research efforts; and foster cross-cutting dialogues among researchers, managers, and communities.Specifically, this research demonstrates that the application of cutting-edge remote sensing techniques (Multiple Endmember Spectral Mixture Analysis, fuzzy logic-based change detection) to conventional medium spatial and spectral resolution imagery (Landsat Thematic Mapper, Landsat Enhanced Thematic Mapper Plus, ASTER) can be used to generate spatially explicit estimates of temporal changes in the abundance of woody plants and other surface materials. The research also shows that spatial models (Geographically Weighted Regression, Weights of Evidence, Weighted Logistic Regression) integrating this timely remotely sensed information with readily available GIS data can yield reasonably accurate estimates of an area's relative vulnerability to WPE and of the importance of anthropogenic and geoecological variables influencing the process. Such models may also be used for the testing of existing and generation of new scientific hypotheses about WPE, for evaluating the impact of natural or human-induced modifications of a landscape on the landscape's vulnerability to WPE, and for identifying target areas for conservation, restoration, or other management objectives.Findings from this research suggest that gaps in our current understanding of WPE and difficulties in devising sustainable rangeland management strategies are in part due to the complex spatio-temporal web of interactions between geoecological and anthropogenic variables involved in the process as well as limitations of presently available data and techniques. However, an in-depth analysis of the published literature also reveals that aforementioned problems are caused by two further crucial factors: the absence of information acquisition and reporting standards and the relative lack of long-term, large-scale, multi-disciplinary research efforts. The methodological framework proposed in this dissertation yields data that are easily standardized according to various criteria and facilitates the integration of spatially explicit data generated by a variety of studies. This framework may thus provide one common ground for scientists from a diversity of fields. Also, it has utility for both research and management

Effects of local and landscape processes on animal distribution and abundance

Investigations of processes that drive animal distribution and abundance are often approached at one of two different scales and therefore focus on different processes. At local scales, animals are thought to select home ranges or territory patches in an ideal manner by occupying them in order of their fitness potential, but a variety of processes can decouple choices from their fitness consequences and create non-ideal patterns of distribution. At landscape scales, the spatial arrangement of habitat patches and their size and isolation are thought to influence distribution patterns because extinction probability declines with increasing patch area and colonization probability declines with increasing patch isolation. Although understanding the relative effects of local and landscape processes on distribution is essential for conservation, very few studies have explicitly considered the fitness potential or quality of habitat when doing so, especially at small scales relevant to the behavioral choices of individuals. I integrated behavioral and landscape approaches for understanding distribution by assessing the relative and combined effects of habitat quality at territory-specific scales and the effects of habitat amount, habitat configuration, and matrix structure at landscape scales on long-term occupancy dynamics of Ferruginous Pygmy-Owls over 12 years. To quantify habitat quality, I considered the estimated additive and interactive effects of habitat resources, stochastic factors (e.g., weather), and conspecific density on reproductive output based on extensive demographic monitoring over 10 years in the same territory patches. Habitat resources explained a much greater proportion of variation in reproductive output than weather or conspecifics, but realized habitat quality was best described by the interactive effects of all these factors. High-quality habitats buffered the negative effects of conspecifics and amplified the benefits of favorable weather, but did not buffer the disadvantages of harsh weather. The positive, density-independent effects of favorable weather at low conspecific densities were offset by intraspecific competition at high densities. Habitat quality had greater effects than landscape processes on patch occupancy dynamics, and its effects were best described by interactions among habitat resources, weather, and conspecifics. Nonetheless landscape factors also had important effects: habitat amount had greater effects than habitat fragmentation or matrix structure, effects that were either positive or negative depending on local habitat quality. Although metapopulation theory is the dominant paradigm upon which many conservation strategies are based, improving local habitat quality may yield greater returns, especially when the surrounding landscape context is considered

Vegetation communities and land cover in the mixedgrass prairie: an analysis of contemporary and historic data sources

This dissertation used both quantitative and qualitative applications to ascertain historic and contemporary vegetation cover, and determine if and to what extent vegetation change has occurred. In Chapter 1, I examined the effects of woody vegetation on the herbaceous plant community in three woody plant associations in the mixedgrass prairie; sand sage shrublands (Artemisia filifolia/Sporobolus cryptandrus - Schizachyrium scoparium shrubland association), shinnery oak shrublands (Quercus havardii /Sporobolus cryptandrus - Schizachyrium scoparium shrubland association), and honey mesquite shrublands (Prosopis glandulosa/Bouteloua gracilis-Buchloe dactyloides shrubland association). Although it was believed that increasing woody plant cover would be inversely related to herbaceous plant species richness and cover, I found that this wasn’t the case for all three of the woody plants. In Chapter 2, I examined General Land Office Public Land Survey records from 1873-1875 to determine composition and extent of woody and grassland communities in pre-settlement western Oklahoma. These records helped me determine the most common trees encountered by the land surveyors (usually marked as bearing trees), as well as the most common trees encountered along water courses, general descriptions of each township surveyed, and a mapped plat image that depicts the land cover mentioned in the surveyor notes. Through this, I was able to determine an approximate depiction of land cover prior to significant European settlement. In Chapter 3, I compared aerial photographs from either 1937 or 1941 (depending on the study area) and 2005, and created land cover maps from both periods, to determine if and to what extent four woody plant species (Artemisia filifolia, Juniperus pinchotii, Prosopis glandulosa, and Quercus havardii) have changed in abundance in the mixedgrass prairie. I found that the woody vegetation wasn’t expanding as was initially believed, and in some cases had actually decreased. In fact, mixed grasses and riparian vegetation had experienced the greatest increase over the 64-68 year period. This dissertation project used historic and contemporary data sources to determine pre-Euro-settlement land cover, followed by an examination of land cover change over the last 64-68 years, and an analysis of contemporary vegetation patterns and associations. This project used quantitative and qualitative data sources, and applied concepts from the fields of biogeography, plant ecology, landscape ecology, historical ecology, and geographic information science. Knowledge gained from this dissertation will help land use managers monitor vegetation on their properties

Interactive Effects of Black-Tailed Prairie Dogs and Cattle on Shrub Encroachment in a Desert Grassland Ecosystem

<div><p>The widespread encroachment of woody plants throughout the semi-arid grasslands in North America has largely resulted from overgrazing by domestic livestock, fire suppression, and loss of native large and small mammalian herbivores. Burrowing-herbivorous mammals, such as prairie dogs (<i>Cynomys</i> spp.), help control shrub encroachment through clipping of shrubs and consumption of their seedlings, but little is known about how this important ecological role interacts with and may be influenced by co-existing large herbivores, especially domestic livestock. Here, we established a long-term manipulative experiment using a 2 × 2 factorial design to assess the independent and interactive effects of black-tailed prairie dogs (<i>Cynomys ludovicianus</i>) and cattle (<i>Bos taurus</i>) on honey mesquite (<i>Prosopis glandulosa</i>) abundance and structure. We found that, after five years, mesquite abundance was three to five times greater in plots where prairie dogs were removed compared to plots where they occurred together or alone, respectively. While both prairie dogs and cattle reduced mesquite cover, the effect of prairie dogs on reducing mesquite abundance, cover, and height was significantly greater than that by cattle. Surprisingly, cattle grazing enhanced prairie dog abundance, which, in turn, magnified the effects of prairie dogs on mesquite shrubs. Mesquite canopy cover per hectare was three to five times greater where prairie dogs and cattle were absent compared to where they occurred together or by themselves; whereas, cumulative mesquite height was two times lower on sites where prairie dog and cattle occurred together compared to where they occurred alone or where neither occurred. Data from our experimental study demonstrate that prairie dogs and moderate grazing by cattle can suppress mesquite growth, and, when their populations are properly managed, they may interact synergistically to significantly limit mesquite encroachment in desert grasslands.</p></div