Community response to use of prescribed grazing and tebuthiuron herbicide for restoration of sand shinnery oak communities
The sand shinnery oak (Quercus havardii) mixed-grass community is an isolated, relict habitat located within short-grass prairie of the Southern High Plains. With the introduction of center-pivot agriculture, unmanaged grazing, oil and gas exploration and suppression of the natural fire regime, the vegetation composition of the shinnery oak community has changed during the past century. While some areas have become dominated by a monoculture of shinnery oak, the absolute amount of shinnery oak has been drastically reduced. This is particularly disconcerting not only because native biodiversity is lost, but also because this area is home to a number of species of conservation concern including the lesser prairie-chicken (Tympanuchus pallidicinctus), sand dunes lizard (Sceloporus arenicolus), and Cassin’s sparrow (Aimophia cassinii). Land managers have used herbicides (e.g., tebuthiuron) and a variety of grazing systems as tools to manage shinnery oak. However, very little research has been done to test how these tools can be used to restore an altered shinnery oak-grass community to pre-settlement standards of species composition. This thesis tests the community response in terms of abiotic factors; plant composition, structure and production; and mammal, herptile, and invertebrate community responses in terms of abundance, dominance, and diversity to a designed restoration effort from 2000-2011. The primary objectives were to (1) determine the variable response to tebuthiuron and grazing treatments, (2) assess the temporal response of the variables to the use of four treatment combinations over a 12-year period, and (3) compare resultant vegetation composition to historical standards. In Roosevelt Co, New Mexico, on the Southern High Plains, 532 ha of private land were treated with tebuthiuron (rate of 0.60 kg/ha with dune avoidance) in 2000. The state of New Mexico owned 518 ha of adjacent land, representing extant shinnery oak community (experimental control). This application rate was approximately 50% of previously recommended rates because the goal was to reduce shinnery oak to historical levels, not to eliminate it. A moderate grazing treatment was designed to take a maximum of 50% of the annual herbaceous production. To allow for inference beyond the study site, the experimental design was a combined completely randomized design with a systematic application of treatments following random assignment of initial treatment combination. The four treatments were treated/grazed, treated/not grazed, not treated/grazed, and not treated/not grazed. There were four replicates for the four treatments, totaling 16 plots. To remove the variation of year-to-year precipitation on the effect of tebuthiuron and grazing, I first ranked precipitation variables (based on two winter, growing and annual precipitation indices) and then used an analysis of variance with precipitation as a covariate. Plant and abiotic variables include soil moisture; line intercept data that was used for percent composition of shrubs, grasses, forbs, litter, bare ground, percent shinnery oak and sand bluestem (Andropogon gerardii) grass; Robel pole heights to measure visual obstruction; angle of obstruction to measure overhead obstruction; herbaceous production; and seed production for dropseed species (Sporobolus spp.), bluestems (Andropogon spp.), sand paspalum (Paspalum maritimum), and gramas (Bouteloua spp.). Small mammals and herptiles were trapped from May to September with 800 trap nights and 960 trap nights per plot per year from 2002-2010. Mammals were trapped with Sherman live traps and herptiles were trapped with pitfall and funnel traps. Abundance, dominance, diversity and the three most abundant mammals - kangaroo rat (Dipidomis ordii), spotted ground squirrel (Spermophilus spilosoma), and pocket mouse species (Perognathus spp.) - and herptiles (prairie lizard (Sceloporus lecontei), Great Plains skink (Eumeces obsoletus), and coachwhip (Masticophis flagellum)) were variables used in analyses. Invertebrates were collected in April and June with a terrestrial vacuum sampler. Invertebrate abundance, biomass, and number of taxonomic families as well as biomass of the three most abundant (grasshoppers (Acrididae), treehoppers (Acrididae), and caterpillars (Lepidoptera larvae) were variables used in analysis. My results show that at relatively low levels of tebuthiuron (0.60 kg/ha) and subsequent moderate grazing system, sand shinnery oak can be reduced and maintained at near historical levels without reapplying tebuthiuron because the tested management approach allowed grasses to remain competitive in the system. Over the ten years, there was 91% less shinnery oak in untreated areas. The removal of shinnery oak made environmental soil moisture more available for grasses and forbs to germinate and grow. Indeed, grasses increased by 149% and forbs increased by 257% in treated areas as compared to untreated areas throughout the study period. In terms of visual obstruction, there was both an herbicide and grazing effect in April such that visual obstruction increased by 30% in treated areas as compared to untreated and decreased by 6.5% in grazed areas as compared to non-grazed areas. Similarly, there was an herbicide and grazing interaction effect such that treated areas had a 14% decrease in overhead obstruction in grazed areas and non-treated areas had only 10% less in grazed areas as compared to non-grazed areas. These changes in plant composition and structure increase biodiversity such that there are more available niches to fill. The results from mammal, herptile, and invertebrate data indicated that species react on an individual basis to herbicide and grazing combinations, so the treatments yielded mixed results. I found no significant herbicide effect of overall abundance of small mammals. However, there was a significant grazing effect such that there was 23% more abundance of small mammals in grazed areas as compared to non-grazed areas, which was likely driven by kangaroo rats. In terms of herptile abundance, there was an interaction effect such that more lizards were found in treated/non-grazed and not treated/grazed areas. This mixed result indicates that species act individually in response to herbicide and grazing. Invertebrates, for the most part, responded positively to herbicide treatment and negligibly in terms of grazing, presumably due to increases in forbs. Areas that were treated with tebuthiuron and had moderate grazing statistically reached historical standards only during one year, but showed trends that were comparable to historical standards throughout the study compared to other treatment combinations. The largest difference between treated areas and historical standards was that treated areas had more forbs. This may not necessarily be interpreted as a bad thing because increase in invertebrates due to forb presence indicates good habitat quality and increases food sources for animals higher on the food chain. With low rates of tebuthiuron (0.60 kg/ha) and moderate levels of grazing, shinnery oak communities can approach historical standards and provide the most plant composition diversity. With moderate levels of grazing, shinnery oak will not need to be re-treated as grasses remain competitive in the system. With the removal of shinnery oak, there is increased soil moisture such that grasses and forbs can establish. The change from a shrub monoculture to a mixed-grass prairie changes the plant composition and structure and provides more niches for invertebrates, mammals and herptiles to fill. Treatment with tebuthiuron at low doses is the first step in managing shinnery oak communities; however, given the incredible, absolute loss of shinnery oak managers should be cautious about treating all shinnery-oak dominated areas.