Evaluation of riparian enhancement actions in the Columbia River Basin

Riparian enhancement is a common restoration technique in the Columbia River Basin (CRB) and the Pacific Northwest. However, relatively few studies have evaluated its success and even fewer studies include long-term monitoring. Forty-one riparian planting projects located in the CRB, each with a paired treatment and control reach, were evaluated in the summer of 2018 and 2019 using an extensive post-treatment (EPT) design. At each reach, we quantified woody plant abundance, richness, diversity, and vegetation and canopy cover as riparian response variables, and measured terrace height and documented individuals with bud browse, deceased, and with predator protection as potential explanatory variables. Species richness, woody plant abundance (all height classes combined and within shrub height class), and the proportion of woody plants with bud browse or deceased were all higher in treatment than control reaches. However, no significant improvements were observed for any other riparian response or explanatory variables. Implementation techniques, including invasive species removal efforts, follow-up planting, watering, and the type of restoration project (floodplain restoration and planting, in-stream restoration and planting, or solely planting) were also considered as potential explanatory variables and were found to have significant impacts on restoration response for abundance and cover variables. Our results suggest that planting implementation methods, site level physical factors, and time since planting all influence the success of riparian planting projects and if not addressed in restoration design and implementation can contribute to a lack of detectable response.

Automated summary

Riparian enhancement is commonly used in the Columbia River Basin and the Pacific Northwest for restoration. However, few studies have evaluated its success, especially with long-term monitoring. This study evaluated 41 riparian planting projects in the CRB and found that woody plant abundance, richness, and bud browse or deceased rates were higher in treatment reaches compared to control reaches. However, no significant improvements were observed in other riparian variables. Implementation techniques and restoration project types were also found to have significant impacts on restoration response.