Ecological and Social Aspects of Riparian Restoration and Non-Native Plant Invasions: Studies from the Pacific Northwest, U.S. and the Western Cape, South Africa

Removal of non-native plants with the goal to reestablish native communities is an important component of riparian restoration initiatives, worldwide. There is a strong need to better understand ecological and social processes pertaining to invasive non-native plants (INPs) along riparian corridors to inform the formulation of effective restoration plans. In this dissertation I present the results of field experiments conducted in the Pacific Northwest, U.S. and a stakeholder analysis conducted in the Western Cape, South Africa to examine three key issues for riparian restoration following non-native plant invasions. Specifically, I focus on (1) the responses of riparian communities to INP removals, (2) the mechanisms that underlie effects of INPs on native species, and (3) the design of incentives to engage diverse stakeholders in INP management.;Chapter one examines riparian understory response to Polygonum x bohemicum (bohemian knotweed) removal along riparian corridors in the Pacific Northwest. I used a BACI (before after control impact) experimental design to examine changes in the abundance, diversity, and composition of herbaceous and woody species one year before and two years after knotweed removal. I also quantified the role of habitat factors and species functional traits in influencing patterns of community recovery. I found that knotweed removal resulted in the rapid colonization of native broadleaved and coniferous trees, native and non-native shrubs and herbs. Two habitat factors: height-above-river at the back of the study plots and knotweed litter depth, were correlated with understory development and the responses of individual plant functional groups. These findings provide the first empirical evidence that knotweed removal can be effective in promoting native riparian recovery. However, interactions between habitat factors and plant traits were important in the pattern of community reassembly.;Chapter two compares the mechanisms that underlie knotweed's effects on native broadleaved and coniferous tree seedlings with differing life history strategies and resource requirements. Using a paired experimental design, I compared the performance of an early-seral, N-fixing species. Alnus rubra, and mid- and late-seral coniferous species, Picea sitchensis and Tsuga heterophylla in riparian plots where knotweed had been experimentally removed or retained. I found that knotweed imposed strong controls on seedling survival and growth. The mechanisms of control varied among species in ways that were consistent with their ecophysiologies. Early and mid-seral trees were primarily affected by reduced light availability (6--7% of ambient levels) beneath the knotweed canopy. The late seral and highly shade tolerant tree was sensitive to changes in below-ground resources as indicated by reduced (64% lower) ectomycorrhizal colonization and increased root:shoot ratios in the presence of knotweed. These findings suggest knotweed's suppression of diverse species is a function of its ability to modify multiple resource environments or plant mutualisms.;In chapter 3 my study location shifts to South Africa's Western Cape to examine perspectives of landowners and other key stakeholders regarding a new (2008) ecosystem service approach to clearing INP on private lands proposed by the National Working for Water Program. Data were obtained from multiple sources, including face-to-face, semi-structured interviews with private landowners in three study areas, and online questionnaires completed by local conservation professionals and Working for Water managers.;I found strong consensus among stakeholders in their preference for shared landowner and government responsibility and for a policy mix that combines all identified tools. However, landowners from the three study areas differed in the level of responsibility they were willing to assume. Stakeholders also diverged in terms of their perceptions of the proposed financial incentives. Landowner perspectives were strongly linked to ecological and social aspects of the landscapes in which they lived. For example, landowners in catchments with higher proportions of public lands expressed a reluctance to engage in structured clearing operations if neighboring public lands were not cleared. Landowners in catchments with invasive aliens concentrated along waterways perceived a disproportionate management burden placed on riverside property owners without receiving commensurate benefits. These findings emphasize the importance of understanding the ecological and social context in designing incentives for the restoration of riparian systems influenced by non-native plant invasions. (Abstract shortened by UMI.)...