Riparian Vegetation Structure and Composition in the Fire-Prone Ecosystem of Eastern Washington

Abstract

The objectives of this study were to compare riparian and upland areas, summarize the range of vegetation conditions present in the second half of the 20th century, and correlate vegetation with processes on the landscape. Matching research to management, the spatial extent of the study was the area managed by the Tapash Sustainable Forest Collaborative. To meet study objectives, this research used Interior Columbia Basin Ecosystem Management Project photo-interpreted 1949 and 1992 resource aerial photos, GIS, Permutational Multivariate Analysis of Variance (PERMANOVA), and Nonmetric Multidimensional Scaling (NMDS). Measures of vegetation structure included total crown cover, overstory crown cover, number of canopy layers, and structural stage. Measures of vegetation composition included dominant overstory species, dominant understory species, and riparian-wetland designation. Riparian areas, as defined by the Northwest Forest Plan, were not found to be significantly different than their upland counterparts for any measure of vegetation structure or composition in either time period. Composed of more Douglas-fir and less ponderosa pine, the Tapash was structurally similar, but was compositionally unique from the rest of eastern Washington. Elevation was the most important predictor of vegetation with higher elevations tending to be denser and composed of true firs, hemlocks, spruce, and redcedar. Conservation status did not have clear effects on vegetation. Logging promoted early structural stages. Indicating a mixed-severity fire history in the second half of the 20th century, greater fire prevalence was associated with zero and one canopy layer and bare understory. Fire prevalence was also positively correlated with riparian-wetland areas, suggesting fire reduced coniferous encroachment or directly promoted fire-adapted riparian vegetation. In outlining both the legacy of the landscape and the relative importance of variables influencing vegetation, this research offers agencies working in the Tapash the local science needed for effective management.