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Browsing by Author "Stevens, Glen N."

Now showing 1 - 4 of 4
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    Fine Root Dynamics in a Pinus palustris Mill. Ecosystem: The Role of Sampling Interval and the Soil Environment
    Stevens, Glen N. (Virginia Tech, 2001-03-09)
    Chapter 1 Abstract: We examined the impact of sampling interval on fine root production and mortality estimates by comparing data from a weekly minirhizotron sampling regimen to subsets of the same data representing biweekly, monthly, bimonthly, and quarterly sampling regimens. We also investigated possible sources of error involved in the root tracing technique and estimated root herbivory using the full weekly sampling regimen. Data were collected for eleven months from a Pinus palustris Miller woodland in southwest Georgia. As sampling interval increased, estimates of production and mortality declined, while estimates of mean fine root lifespan increased. Annual production values ranged from a maximum of 1.26 mm/cm2 for weekly sampling to 0.83 mm/cm2 for quarterly sampling. Total mortality varied from 0.97 mm/cm2 to 0.53 mm/cm2. Bias increased at a decreasing rate when sample interval was increased from weekly to monthly. The root tracing protocol added some small, random error to growth measurements; re-measuring roots returned values 0.16% smaller than initial measures. We also observed a root mortality and regrowth phenomenon that may be measurement error or short-term fluctuation in root length. Herbivory accounted for greater than 20% of fine root biomass produced. Our study suggests that increases in sampling frequency from monthly to weekly can provide substantial gains in accuracy for estimates of root dynamics. Chapter 2 Abstract: We examined the impact of soil environmental variables (soil temperature, moisture, and available nitrate (NO3-) and ammonium (NH4+)) on the production, mortality, standing crop, turnover, and lifespan of Pinus palustris Miller fine roots using the minirhizotron technique. Data were collected for a full year from a P. palustris woodland in southwest Georgia. Mean soil temperatures appeared to have little influence on root processes, while temperature variance had a strong effect. More thermally variable microsites had increased root turnover and reduced root lifespans. Soil resources had a significant impact on demography; in particular, soil moisture and nitrate stimulated production, mortality, and turnover. High levels of soil resource availability also significantly reduced lifespan. Root lifespan was variable among individual roots based on root width, depth in the soil volume, and season of root production. Soil moisture had the strongest overall influence on root demography. This may result from the nature of our ecosystem (deep sands and subtropical climate); in addition, severe drought during our study may have enhanced the role of soil moisture, allowing environmental controls to increase in strength relative to within-plant controls on root demography.
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    Patterns in soil fertility and root herbivory interact to influence fine-root dynamics
    Stevens, Glen N.; Jones, R. H. (Ecological Society of America, 2006-03)
    Fine-scale soil nutrient enrichment typically stimulates root growth, but it may also increase root herbivory, resulting in trade-offs for plant species and potentially influencing carbon cycling patterns. We used root ingrowth cores to investigate the effects of microsite fertility and root herbivory on root biomass in an aggrading upland forest in the coastal plain of South Carolina, USA. Treatments were randomly assigned to cores from a factorial combination of fertilizer and insecticide. Soil, soil fauna, and roots were removed from the cores at the end of the experiment (8-9 mo), and roots were separated at harvest into three diameter classes. Each diameter class responded differently to fertilizer and insecticide treatments. The finest roots (< 1.0 mm diameter), which comprised well over half of all root biomass, were the only ones to respond significantly to both treatments, increasing when fertilizer and when insecticide were added (each P < 0.0001), with maximum biomass found where the treatments were combined (interaction term significant, P < 0.001). These results Suggest that root-feeding insects have a strong influence on root standing crop with stronger herbivore impacts on finer roots and within more fertile microsites. Thus, increased vulnerability to root herbivory is a potentially significant,cost of root foraging in nutrient-rich patches.
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    Preparing the Way for a Biofuels Industry in Virginia: Assessment of the Feasibility of the Agricultural, Energy, and Conservation Communities to Implement a Sustainable Energy Industry
    Waldon, Jefferson; Convery, Ken; Cushwa, Charles T.; Klopfer, Scott D.; Stevens, Glen N. (Virginia Tech, 2009-12-18)
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    Trophic dynamics in the fine-root based food web: integrating resource heterogeneity, root herbivores, and root foraging
    Stevens, Glen N. (Virginia Tech, 2005-05-24)
    Resources in the soil are heterogeneously distributed. We know that plant species differ in their root responses to nutrient patches and that these differences in foraging can influence plant competition. However, most studies of root-resource interactions overlook the potential top-down influence of root herbivores. While root herbivores can influence plant community structure, the extent to which they influence ecosystem-scale factors such as net primary production is unclear. In addition, little is known regarding root herbivore foraging behaviors and, more importantly, whether these foraging behaviors can actually influence species interactions. In this dissertation, I present a conceptual model of soil-root-herbivore interactions in which soil resource heterogeneity structures both root dynamics and the abundance and influence of root herbivores. I conducted two field and one greenhouse experiment examining this proposed model. The dissertation includes an introductory chapter (Chapter 1), a field study examining root responses to manipulations of soil fertility and root herbivory (Chapter 2), a greenhouse study that used plant species responses to heterogeneity to develop predictions about the role of root herbivores in mixed-species neighborhoods (Chapter 3), and a field study of planted communities examining soil fertility and fauna effects on above- and belowground structure and function (Chapter 4). In all cases, there were significant effects of root herbivores on community structure and components of net primary production. Resource distribution had a strong effect in studies conducted in sandy, nutrient-poor soils (Chapter 2 and 3), but had a reduced effect in the study conducted at Kentland Farm in loamy soils (Chapter 4). Interactions between resource availability and root herbivory were common. These results support the theory that the potential benefit of resource-rich patches may be constrained by root herbivores. This research complements recent findings that demonstrate other potential costs of species foraging behaviors (such as exposure to soil anoxia and increased drought stress), as well as potential effects of root herbivores and other soil fauna on plant diversity.