In the 19 th century, North American prairie grasses were plowed under for agricultural expansion. Nearly two hundred years later, in the 21st century, these same grasses have become key species in ecological restoration and appear likely to become equally important as novel bioenergy crops that provide ecosystem services. In these new roles, these plants' traits are being changed through selective pressures, both intentionally and inadvertently. At the same time, human activities have fundamentally altered the landscapes in which these species are now found. To understand how propagation and selection pressures alter native prairie grasses, I conducted a suite of field and greenhouse experiments to test plant interactions with abiotic and biotic factors. Since the use of marginal lands is a major focus of sustainable bioenergy development, I established common gardens of both wild-collected and propagated populations of Andropogon gerardii (big bluestem) and Schizachyrium scoparium (little bluestem) in fertile loamy soils and marginal sandy soils to measure growth traits. To assess biofuel-valuable traits in Panicum virgatum (switchgrass), I examined relationships between growth rates and biomass recalcitrance to digestion in five populations, ranging from near-wildtype to highly selected cultivars, grown in a common garden. I then evaluated relationships between these traits and P. virgatum susceptibility to a widely distributed group of aphid-transmitted Poaceae viruses (Luteoviridae: Barley and cereal yellow dwarf viruses , B/CYDVs), which cause major yield losses in cereal crops. Significant trait differences were evident among populations of all grasses studied. Propagated populations of A. gerardii exhibited a stronger plastic response to growth on fertile soils in both biomass production and reproductive investment than did wild-collected populations. In P. virgatum , fast growth in highly selected populations was further associated with increased susceptibility to virus infection. These findings highlight the need to evaluate potential selection trade-offs between fast growth and other plant traits, particularly as native grasses are further modified for biofuel-valuable traits. Interactions of perennial grasses with pathogens are particularly important because these grasses can serve as reservoirs of pathogens that spill-over into other landscape compartments. To assess the importance of community and landscape context on pathogen interactions with novel biofuel crops, I examined B/CYDV and aphid vector pressure in two potential crops comprised of native grasses—P. virgatum and mixed prairie—and a current biofuel crop—maize. In collaboration with a team of researchers, I evaluated a suite of community, landscape, and environmental variables for associations with vector abundance and virus incidence. Greater landscape diversity around study fields was associated with reduced aphid incidence, and consistent regional patterns of aphid and virus pressure were evident based on several measures. Landscape changes associated with biofuel production have the potential to either amplify or moderate aphid and virus pressures through effects on landscape diversity and land cover types. Human influence on native grasses is exerted across a broad range of scales. At the population scale, particular plant traits are favored over others, and at a larger scale, human land use practices directly and indirectly affect biotic and abiotic interactions with these grasses. These influences have important consequences, especially when major restructuring of landscapes is a likely outcome of biofuel development and when generalist pathogens, such as B/CYDVs, are involved.
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