<p>  </p> <p>Giant ragweed (<em>Ambrosia trifida</em> L.) is an annual broadleaf plant capable of emergence throughout the cropping season, opportune colonization of disturbed soil, rapid biomass accumulation, and a propensity to evolve mutations that endow resistance to herbicides, all of which contribute to giant ragweed being one of the most challenging weeds to control in row-crop production. Many soybean growers rely on acetolactate synthase (ALS)-inhibiting herbicides such as cloransulam for control of giant ragweed prior to its emergence, though the spread of biotypes resistant to ALS inhibitors can render these herbicides largely ineffective. Mesotrione inhibits the 4-hydroxyphenylpyruvate dioxygenase (HPPD) enzyme, and applications of this herbicide have recently been approved for use in mesotrione-resistant soybean varieties. Field experiments demonstrated that preemergence applications of mesotrione resulted in greater control of giant ragweed populations segregating for ALS-inhibitor resistance than several other commonly used herbicide combinations. Where mesotrione was applied, giant ragweed biomass was reduced by an average of 84% relative to the nontreated, while treatments without mesotrione increased biomass by an average of 34% by suppressing competition from other weed species. Additionally, both soil- and agar-based bioassays demonstrated that combinations of mesotrione and metribuzin can be synergistic for control of giant ragweed. </p> <p>Cloransulam was shown to result in strong selection for giant ragweed individuals with ALS-inhibitor resistance, increasing the proportion of resistant plants that emerged at one field site from 15% to greater than 90% after a single preemergence application. This selection pressure was reduced when mesotrione was co-applied with cloransulam. However, no herbicide combination, including sequential applications of non-ALS-inhibiting herbicides, consistently resulted in a resistance frequency similar to the baseline if an ALS inhibitor was applied preemergence. Resistance to cloransulam and other ALS inhibitors is expressed in giant ragweed plants possessing at least one mutant (Trp574Leu) <em>ALS</em> allele. The distribution of this allele in one field violated the Hardy-Weinberg Equilibrium, despite the fact that <em>ALS</em> is a nuclear gene and the Trp574Leu mutation does not incur a fitness penalty. We suspected that the inheritance of this mutation may be linked with a gene or genes responsible for self-incompatibility (SI) in giant ragweed, and that linkage drag was disrupting pollination in resistant plants. This research provided evidence that giant ragweed does possess SI, as greater pollen retention, pollen tube growth, and seed set were observed in cross-pollinated plants compared with self-pollinated plants. Non-Mendelian inheritance of the Trp574Leu mutation was documented in crosses between plants from three different giant ragweed populations, indicating that the mutant <em>ALS</em> allele may be linked with an SI allele common to many plants because of a shared resistant ancestor.  In crosses between plants from one population, production of resistant F1 seeds was 33% greater on average compared with the expectation under Mendelian inheritance. </p> <p>Collectively, this research demonstrated that mesotrione may become a highly effective tool for control of giant ragweed in soybean. Applications of mesotrione can also reduce the selection for an increased frequency of ALS inhibitor-resistant biotypes induced by cloransulam, though a more robust weed management strategy may be necessary to maintain the long-term viability of ALS inhibitors.  The need for sound weed management practices is underscored by the impact of the linkage of SI and <em>ALS</em> genes, which may be encouraging a more rapid spread of herbicide-resistance than was previously anticipated.</p>

Switchgrass (Panicum virgatum L.) is a cellulosic feedstock for alternative energy production that could grow well between planted pines (Pinus spp.). Southeastern planted pine occupies 15.8 million hectares and thus, switchgrass intercropping could affect biodiversity if broadly implemented. Therefore, I evaluated effects of intercropping switchgrass in loblolly pine (P. taeda L.) plantations on plant community diversity, plant biomass production, and white-tailed deer (Odocoileus virginianus Zimmerman) nutritional carrying capacity. In a randomized complete block design, I assigned three treatments (switchgrass intercropped, switchgrass monoculture, and a “control” of traditional pine management) to 4 replicates of 10-ha experimental units in Kemper County, Mississippi during 2014-2015. I detected 246 different plant species. Switchgrass intercropping reduced plant species richness and diversity but maintained evenness. I observed reduced forb and high-use deer forage biomass but only in intercropped alleys (interbeds). Soil micronutrient interactions affected forage protein of deer plants. White-tailed deer nutritional carrying capacity remained unaffected.

Field studies were conducted to test three application methods for applying antagonistic herbicide combinations: 1) tank mix (TMX), 2) mix-in-line (MIL), and 3) separate boom (SPB). Sethoxydim applied with bentazon, glyphosate applied with dicamba or 2,4-D, and clethodim applied with dicamba or 2,4-D had higher efficacy when applied using the SPB method. Antagonism of all the herbicide combinations above was observed when applied using the TMX and MIL methods. In some cases, antagonism was avoided when using the SPB method. Three application methods tested in greenhouse studies were 1) TMX, 2) synthetic auxin applied first (AAF), and 3) synthetic auxin applied second (AAS). The AAS application method resulted in higher weed control than the TMX and AAF methods. Analysis done through liquid chromatography mass spectrometry supported the greenhouse results with higher rates of glyphosate detected with the AAS method.

To improve habitat quality for wildlife, habitat managers prescribe various disturbances. Habitat management techniques alter the vegetation structure, composition, and quality, changing food and cover resources and availability. To investigate how habitat management and vegetation heterogeneity affect space use by wildlife species, I deployed 81 camera-traps and collected fine scale vegetation data across a variety of treatments (i.e., canopy reduction, prescribed fire, and herbicide application) in a managed loblolly pine forest. I created a new method for accounting for imperfect detection and error in camera count data. My method provided better inference about the effects of variables on animal use. Species responded differently to different functional groups of plants, but predicted use from generalized additive models showed higher use in the most intensively managed pine stands, indicating that management promotes animal use by improving habitat quality. Animals used every treatment, indicating the need for heterogeneity in resources when managing wildlife.

Bare-root Nuttall oak, Shumard oak, and swamp chestnut oak seedlings were planted on former agricultural fields in Mississippi to evaluate growth and survival following treatment by different combinations of mechanical site preparation and pre-emergent Oust XP® applications. Mechanical treatments included: (1) controls, (2) subsoiling treatment, (3) bedding treatment, and (4) combination plowing. Pre-emergent Oust XP® treatments included: (1) one-year Oust XP® applications and (2) two-year Oust XP® applications. Evaluations and measurements were performed over the course of three years. Results within this dissertation should be considered in the context of high quality former agricultural sites with silt loam soils. Seedling height and GLD increased each growing season. Height and GLD of seedlings were greater in bedded and combination plowed areas compared to control and subsoiled areas. Height and GLD were not different by Oust XP® application. However, seedling survival in two-year Oust XP® application areas was reduced compared to one-year Oust XP® application areas. Nuttall oak seedlings planted in bedded and combination plowed areas exhibited greater stem biomass compared to those planted in control or subsoiled areas. Seedlings planted in two-year Oust XP® areas exhibited greater stem and root biomass compared to those planted in areas receiving the one-year treatment. Better growth and biomass production in bedded and combination plowed areas would promote their use for site preparation on similar sites. While survival was lower in two-year Oust XP® treatment areas compared to one-year treatment areas, the difference would not warrant change in management strategy. Additionally, woody biomass was much greater in two-year Oust XP® areas. Thus, if cost is not prohibitive, treatment with two years of Oust XP® is recommended. The objective of the mixed species study was to establish baseline growth and survival data for mixed Nuttall oak and green ash plantations on former agricultural areas. Height and GLD for both species increased each growing season. Third-year survival was excellent at 96.0 percent. In conclusion, the mixture performed satisfactorily. Growth and survival were excellent, which indicates this mixture would be useful in afforesting similar former agriculture sites.

To understand the flow of water as a factor that influences aquatic vegetation communities and aquatic plant dispersal, custom-made Global Positioning System (GPS) drones were used to monitor the movement of water in Aliceville Lake, Columbus Lake, and Ross Barnett Reservoir, MS. In each reservoir, the drones drifted in the wind-generated surface current. Analysis of wind speeds suggests that a certain wind speed may be necessary to overcome gradient flow. Wind direction and wind speed should be incorporated in future spatial simulation models for aquatic plant dispersal and distribution. An herbicide evaluation on Cuban bulrush (Oxycaryum cubenese) was conducted to determine what herbicides would effectively control the invasive species. Applications made prelowering were more successful than postlowering applications for all herbicides tested with glyphosate, 2,4-D, triclopyr, diquat, imazamox, and imazapyr resulting in 100% mean biomass reduction. For postlowering applications, glyphosate, triclopyr, and diquat are recommended.

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