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The Health of Blue Elderberry (Sambucus mexicana)and Colonization by the Valley Ederberry Longhorn Beetle (Desmocerus californicus dimorphus)in Restored Riparian Habitat
ABSTRACT THE HEALTH OF BLUE ELDERBERRY (Sambucus mexicana) AND COLONIZATION BY THE VALLEY ELDERBERRY LONGHORN BEETLE (Desmocerus californicus dimorphus) IN RESTORED RIPARIAN HABITAT by Meghan Gilbart Master of Science in Biological Sciences California State University, Chico Summer 2009 Horticultural restoration of floodplains recreates riparian habitat that is critical to a diversity of wildlife, including many endemic, threatened and endangered species. This type of restoration frequently occurs on highly regulated rivers, where the natural processes that shape riparian plant communities have been modified and truncated. The truncation of river processes places importance on restoration planting designs, which must consider the heterogeneous nature of the environment, complex natural vegetation structure as well as the succession of plants adapted to fluvial systems. One of the largest riparian restoration efforts in the country is along the regulated Sacramento River in the Central Valley of California, where restoration targets imperiled xii wildlife such as the federally threatened Valley elderberry longhorn beetle (Desmocerus californicus dimorphus, VELB). The VELB is endemic to the Central Valley and specializes on facultative riparian blue elderberry shrubs. As a target species of the restoration, over 96,000 elderberry shrubs have been planted in the last 16 years in a range of planting designs to create VELB habitat. The planting designs that include elderberry range from open to closed canopy communities, yet there has been no monitoring of elderberry among the different planting designs beyond the initial three-year monitoring period. Using a factorial design, I sampled elderberry shrubs across both open and closed planting designs, and in old and young sites in 23 restoration fields of the Sacramento River National Wildlife Refuge to evaluate the current health of planted elderberry and the corresponding occupation by VELB. My results indicate that open, low cover planting designs can allow elderberry to develop into larger, more robust shrubs that will reach maturity, whereas closed canopy designs likely stress elderberry shrubs and reduce living material over time. Recent VELB occupation was observed in 78% of all fields but only 21% of all shrubs searched. Beetle occupation increased with restoration age but showed a weaker and inconsistent relationship with cover. Closed canopy planting designs may attract beetles initially through chemicals released by stressed elderberry shrubs, but in light of successional changes that will take place in the planted fields over time, open canopy planting designs provide more consistent habitat. A diversity of planting designs is therefore recommended for restoration of VELB habitat, but both elderberry health and VELB occupation should be monitored over time in these sites as plants continue to age.