Longitudinal- and transverse-scale environmental influences on riparian vegetation across multiple levels of ecological organization
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Riparian vegetation is distinct from adjacent upland terrestrial vegetation and its distribution is affected by various environmental controls operating at the longitudinal scale (along the river) or transverse scale (perpendicular to the river). Although several studies have shown how the relative importance of transverse or longitudinal influences varies with the scale of observation, few have examined how the influences of the two scales vary with the level of ecological organization. We modeled vegetation-environment relationships at three hierarchically nested levels of ecological organization: species, plant community, and vegetation type. Our hierarchically structured analyses differentiated the spatial extent of riparian zones from adjacent upland vegetation, the distribution of plant community types within the riparian zone, and the distribution of plant species within community types. Longitudinal gradients associated with climate and elevation exerted stronger effects at the species level than at the community level. Transverse gradients related to lateral surface water flux and groundwater availability distinguished riparian and upland vegetation types, although longitudinal gradients of variation better predicted species composition within either riparian or upland communities. We concur with other studies of riparian landscape ecology that the relative predictive power of environmental controls for modeling patterns of biodiversity is confounded with the spatial extent of the study area and sampling scheme. A hierarchical approach to spatial modeling of vegetation-environment relationships will yield substantial insights on riparian landscape patterns.
KeywordsGradient analysis Scale Hierarchy Great Basin Riparian vegetation Ordination Random Forests
Funding for Jian Yang, Thomas Dilts, and Peter Weisberg was provided by the U.S. Bureau of Reclamation (Cooperative Agreement 06FC204044). Funding for Otis Bay Ecological Consultants was provided through the USFWS—Lahontan National Fish Hatchery Complex using Desert Terminal Lakes Funds (Public Law 109-103, Sect. 208(c) administered through the BOR). LiDAR was provided by the USFWS—Lahontan National Fish Hatchery Complex using Desert Terminal Lakes Funds (Public Law 109-103, Sect. 208(c) administered through the BOR). Stephanie Kilburn, Gina Jones, Shwetha Bayya, Randy Goetz, Serena Rogers and Kurt Sable at Otis Bay Ecological Consultants provided assistance in the field or in the office. Blake Engelhardt, Joy Giffin, Dr. Jianguo (Jingle) Wu, and two anonymous reviewers provided comments on earlier drafts.
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