The Value of Native Understory Species in Riparian Restoration Projects
By Andrew Rayburn, Restoration Ecologist, River Partners
River Partners has developed a highly successful formula for riparian restoration that focuses on a combination of cost-effective agronomic practices, sound science, and productive relationships with landowners, public agencies, local governments, irrigation districts, and nonprofit organizations. Our restoration projects provide numerous recreational and employment opportunities for people, in addition to enhancing ecosystem services related to wildlife, pollinators, plants, water, and soil.
From a vegetation perspective, we tend to connect most deeply with the charismatic macro-habitat – the restoration of overstory forests of cottonwoods, willows, and oaks. While the 30-foot cottonwoods of a 5-year old restoration site seem very impressive to us, it turns out that the smallest components of the vegetation community – the grasses, herbs, and shrubs – may play an even more important role in wildlife recovery and restoration success than those gentle giants. Thanks to significant developments in our understanding of wildlife response to restoration, and the horticultural realities of weed infestations on floodplains, we now recognize the importance of actively restoring riparian understory communities, especially because recent research suggests native understory species are unlikely to naturally colonize restored riparian forests along California rivers (McClain et al. 2011).
River Partners’ approach to riparian restoration blends modern agricultural and horticultural techniques, restoration ecology, and adaptive management. To date, we’ve planted over 1.5 million native trees and shrubs on 8,000 acres of floodplains along streams and rivers across California. Common native grasses used by River Partners in riparian restoration projects include creeping wildrye (Elymus triticoides), blue wildrye (Elymus glaucus), saltgrass (Distichlis spicata), and purple needlegrass (Stipa pulchra) (Fig. 1). River Partners also strives to utilize native forbs (Fig. 2), such as gumplant (Grindelia camporum), mugwort (Artemisia douglasiana), and goldenrod (Euthamia occidentalis), in addition to native rushes and sedges, such as Santa Barbara sedge (Carex barbarae).
Different understory species have different soil and water requirements for effective establishment. Native grasses are often drill-seeded, while forbs and sedges are often established using broadcast seeding or plug planting. After planting, the understory is adaptively managed for at least one additional year through a combination of irrigation, mowing, and herbicide application to control exotic species.
Native understory species are now recognized as critical components of riparian restoration projects in California since they enhance the provision of ecosystem services and contribute to restoration success (Golet et al. 2008, Tjarks 2012, Rogner 2013). Native understory species are critical components of foraging patches for the endangered riparian brush rabbit (Sylvilagus bachmain riparius), a frequent target of riparian restoration efforts in the northern San Joaquin Valley. The endangered Least Bell’s Vireo (Vireo bellii pusillus) favors riparian habitat with a dense, mugwortdominated understory (Wood et al. 2006). Following the recommendations of the California Partners in Flight Riparian Bird Conservation Plan (RHJV 2004), River Partners restored such habitat in the San Joaquin River National Wildlife Refuge (SJRNWR) as part of a large-scale riparian restoration project that continues to this day. Happily, Least Bell’s Vireos were observed breeding successfully in this habitat at SJRNWR in 2005 and 2006, the first such occurrence in over 60 years.
We have also observed that nativeplanted understories may also be more resistant to invasion of problematic riparian weeds such as yellow starthistle (Centaurea solstitialis), perennial pepperweed (Lepidium latifolium), salt cedar (Tamarisk spp.) and giant reed (Arundo donax) (Tjarks 2012). Native grasses, such as creeping wildrye, are highly competitive and can form dense mats that prevent exotic weed establishment. The inclusion of fastgrowing native forbs, such as mugwort and gumplant, further contributes to invasion resistance by reducing available light and soil resources for newly germinated weed species. Monitoring data suggests both rapid and sustained replacement of exotic species by mugwort and other native understory species, which grow and spread rapidly in shady conditions beneath overstory vegetation and respond well to disturbances like fire and flood (Tjarks 2012, Fig. 2).
The diverse palette of native understory species used by River Partners also benefits pollinators that support agricultural production and provide critical ecosystem services in California landscapes. Diversity in pollen sources and diversity in the timing of nectar availability has been identified as an important restoration objective to support native pollinator populations (Harmon-Threatt 2009). With over 12 species of perennial flowering herbs and numerous shrub species that provide nectar and pollen throughout the year, we can expect that our efforts likely support native pollinator populations in meaningful ways.
The use of native understory species in riparian restoration projects has additional benefits related to forage provision and soil fertility. Native grasses (such as Stipa pulchra, George et al. 2013) and native forbs (such as Trifolium spp.) can provide high-quality forage for both livestock and wildlife, supporting restoration goals and facilitating the control of exotic species through carefully timed grazing by cattle, sheep, or goats. Leguminous forbs also fix nitrogen and have positive effects on soil fertility.
Thanks in part to River Partners’ innovative restoration approach, inclusion of native understory species has become an integral part of “state-of-the-art” riparian restoration projects in California. This is turn has helped motivate state-wide research on the weed-control benefits of native understory species, their value to wildlife and pollinator species, and their long-term response to flooding and other forms of disturbance. In the future, River Partners will continue to experiment with new understory species and planting methods to enhance the provision of ecosystem services in our riparian restoration projects.
George, M.R., S. Larson-Praplan, M. Doran, and K.W. Tate. 2013. Grazing Nassella. Rangelands 35(2):17-21.
Golet, G., T. Gardali, C.A. Howell, J. Hunt, R.A. Luster, W. Rainey, M.D. Roberts, J. Silveira, H. Swagerty, and N. Williams. 2008. Wildlife response to riparian restoration on the Sacramento River. San Francisco Estuary and Watershed Science 6(2).
Harmon-Threatt, A.N. et al. 2009. Breeding system and pollination ecology of introduced plants compared to their native relatives. American Journal of Botany (96):1544- 1550.
McClain, C.D., K.D. Holl, and D.M. Wood. 2011. Successional models as guides for restoration of riparian forest understory. Restoration Ecology 19(2):280-289.
RHJV (Riparian Habitat Joint Venture). 2004. The riparian bird conservation plan: A strategy for reversing the decline of riparian associated birds in California. California Partners in Flight. Version 2.0.
Rogner, M. 2013. Wildflower restoration at the La Barranca Unit. River Partners Journal 10:7.
Tjarks, H. 2012. Using a native understory to control weeds in riparian restoration. California Invasive Plant Council News 20(2): 8-9.
Wood, J.K., C.A. Howell, and G.R. Ceupel. 2006. Least Bell’s Vireo Breeds in Restored Riparian at San Joaquin River National Wildlife Refuge. 2005 Final Report. PRBO Contribution #1511.
The above article originally appeared in the December 2013 issue of the River Partners Journal.