Biogeomorphic effects of Ligustrum sinense (privet) invasion on the Oconee River floodplain, north Georgia
Ward, Ronald William
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Species invasions are a component of global change, but few studies document specific effects of invasive species on native ecosystems. Ligustrum sinense (privet), an ornamental shrub, escapes cultivation and invades floodplains in the southeastern US. This study combined landscape-scale historical aerial photograph analysis with local-scale analysis of environmental factors influencing privet distribution, as well as the influence of privet on forest regeneration and sedimentation rates on the floodplains of the Upper Oconee River, North Georgia. Historical aerial photographs were used to determine the extent and timing of privet invasion and particular land-uses associated with invasion at three sub-reaches along a 43-km reach of river. Sampling transects located on invaded and noninvaded floodplains were used to collect data on vegetation and geomorphologic parameters, and average annual rates of sedimentation. Sites were compared to identify conditions associated with privet invasion, and to determine whether invasion is associated with decreasing regeneration of native trees and alteration of sedimentation rates.|Privet was well established by1951, when 51% of the floodplain had already been invaded. By 1999 privet had spread to 59% of the floodplain. Increases in privet extent over the study period were primarily due to invasion of abandoned agricultural fields and pastures. Regression analysis indicated watertable depth and percent silt in topsoil account for 53% of the variation in privet density in invaded sites. Regression models explaining tree regeneration indicated greater reproductive success among trees in areas where privet is sparse or absent. Ordination analysis showed similar compositions of mature forest at all sites and fluvial landforms, but diverging sapling compositions among invaded and noninvaded areas. Dendrogeomorphologic analysis indicated a link between invasion, accelerated sedimentation rates, geomorphologic factors, and broad-scale disturbance alterations related to historical cotton cultivation on uplands. Feedback modeling revealed an association between sedimentation around privet thickets and raised levees, which likely provide improved conditions for invasion. Flood simulations showed privet thickets decrease flow velocity within the range for coarse-sand deposition. These findings are compared to invasive tamarisk (Tamarix spp.), broad-scale disturbance alterations, and channel narrowing of rivers in the American Southwest.