For the past ten years, PRBO partners U.S. Fish and Wildlife Service (USFWS), The Nature Conservancy (TNC), and River Partners have been refining cost-effective, large-scale riparian restoration strategies, emphasizing mixed riparian and valley oak riparian forest types, with input from PRBO. Over this time they have restored approximately 4,800 acres of land, mainly within river meander zones and oxbow islands, from aging flood-prone orchards to young riparian forests. This year marked a full decade of PRBO avian research and monitoring on the Sacramento River, making this project one of our longest cooperative efforts. During this period, restoration sites have grown from knee-high saplings to young forests hosting a myriad of breeding, migrating, and wintering songbirds. In many places, young native vegetation now fills the large gaps between stands of tall, aged riparian forest as far as the eye can see.
|A restored Sacramento River site that was planted with seedling trees and shrubs in 1991. Photo by Stacy Small.|
The characteristics of Sacramento River restoration sites change so rapidly that visitors to the oldest restoration sites, planted 12-14 years ago, have difficulty believing that these towering riparian forests were planted in rows of tiny shrubs and saplings less than a decade-and-a-half ago. Fremont cottonwoods planted on Phelan Island have grown rapidly; some have snapped in high winds, providing snags for cavity-nesters like the Downy Woodpecker and Ash-throated Flycatcher. California blackberry is slowly creeping out of the neighboring remnant riparian forest into the shadiest depths of this restoration site, to blur the transition between old, mature forest and cultivated restoration.
Advancing our knowledge of how to do restoration has required time and close interaction between PRBO and the implementation teams. In the early years of restoration on the Sacramento River, the great challenge for pioneering restoration ecologists was to develop effective techniques to grow riparian shrubs and trees under severely altered ecological conditions. They faced a disrupted hydrologic regime and a fierce weed seed bank that, without intensive maintenance, could easily outcompete native restoration plantings for water in the first growing season.
Within the first several years of PRBO's riparian bird monitoring, it became apparent that Sacramento Valley bird populations had taken a hard hit in the last half-century. Species that once nested in the region were absent or dwindling as breeders, and several open-cup nesters that continued to breed in the region were suffering low reproductive success, due to nest predation and cowbird parasitism. Successful habitat restoration includes creating conditions for successful colonization and reproduction by key riparian bird species.
In 1999, PRBO offered its collaborators a series of restoration recommendations intended to increase available nesting habitat for open-cup nesters (such as Common Yellowthroat, Yellow-breasted Chat, Lazuli Bunting, Yellow Warbler, and Spotted Towhee). Working together, we developed several approaches to the problem of "complexifying" riparian understory on restoration sites. At the USFWS Pine Creek Unit, we worked with TNC and USFWS to design shrub patches interspersed with tree/shrub clumps. The goal here was to provide riparian forest with a semi-open canopy, to support nesting Yellow-billed Cuckoos, and a dense understory to support nesting Yellow Warblers, Lazuli Buntings, and Black-headed Grosbeaks. Within four years of planting, Black-headed Grosbeaks, in addition to other species, were nesting and fledging young at the site.
One of the most challenging recommendations for our practitioner partners to implement was to incorporate a native forest understory (herbaceous and low shrub) layer into restoration plans. Meeting this challenge required developing propagation and planting techniques for native species with different water and light requirements; some of these had never been planted previously in large-scale riparian restoration projects. Since adding the understory component to their restoration strategy, TNC has more than doubled (to 37) the total number of species planted at their restoration sites. New understory species recently added include California blackberry, mugwort, goldenrod, Santa Barbara sedge, and hoary nettle. Planting an understory also requires novel restoration designs. For instance, TNC has recently shifted to planting in clusters of two and three species, a tree and one or two understory species such as a shrub, grass, or vining plant (pipevine, wild grape, Clematis).
Measures of Success
We use several types of monitoring data to measure a restored habitat's suitability to birds. As an initial step, along the Sacramento River, we examined the relationship of riparian bird diversity to age of the restoration project. Figure 1 shows how, as the age of the restoration project increases, so does the diversity of songbirds using it. In this example, we used a refined list of birds to calculate diversity, thereby eliminating the chance that an increase in diversity could be due to the presence of non-native species such as House Sparrow or European Starling. A more diverse bird community, however, may not necessarily equate to a "healthy" one in terms of reproductive performance and survival. Measuring nest success at restored and remnant forest sites showed that for Lazuli Bunting and Spotted Towhee success was similar, and for Black-headed Grosbeak success was higher on the restored plots. These results are evidence that our partnership approach on the Sacramento River is working well for birds.
Together with colleagues at PRBO, I am currently analyzing data on reproductive success, nest predation, and nestling growth rates, as they relate to microhabitat and landscape features at both restoration and forest sites. Learning more about how songbirds fare in natural forest habitats on the river will further contribute to the success of riparian restoration programs.