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Re-Oaking North Bay: A Strategy for Restoring Native Oak Ecosystems, Focusing on Napa and Sonoma Valleys

Sean Baumgarten, San Francisco Estuary Institute

Robin Grossinger, San Francisco Estuary Institute; Matthew Benjamin, San Francisco Estuary Institute; Micaela Bazo, San Francisco Estuary Institute; Frances Knapczyk, Napa County Resource Conservation District

Vast expanses of oak savanna historically occupied the rich alluvial soils of Napa and Sonoma valleys (Dawson 2008, Grossinger et al. 2012). The immense and long-lived valley oak (Quercus lobata) dominated these savannas, accounting for more than 65% of trees. Over the past two centuries, however, much of this oak savanna has been cleared to make way for orchards, vineyards, and towns.

The Re-Oaking North Bay initiative was established to provide an overarching strategy to help guide and prioritize oak restoration -- or re-oaking -- efforts in the region, with the ultimate goal of restoring our native oak communities in places where they could once again thrive and benefit our landscapes into the future. Priority areas for re-oaking Napa and Sonoma valley floors were identified by comparing present and past distributions of valley oaks, analyzing opportunities in the modern landscape, and demarcating potential areas to exclude based on valley oaks’ physiological constraints.

The re-oaking spatial strategy map highlights various priority areas for restoring valley oaks on the Napa and Sonoma valley floors, including around existing oak nodes, along riparian corridors or other linear features, and in areas of recent and historical oak loss. Groups, or nodes, of multiple oak trees in relatively close proximity allow trees to cross-pollinate (Pluess et al. 2009) are generally more beneficial to wildlife than widely spaced trees. For instance oak nodes 15-20 acres in size and with at least 20 trees are likely necessary to support acorn woodpecker colonies (Spotswood et al. 2017). Within oak nodes, individual oak trees should be spaced close enough together to facilitate wildlife movement and ensure that trees can cross-pollinate. In order for successful pollination to occur, oaks within nodes should be spaced no more than 500 feet apart (Sork et al. 2002, F. Davis pers. comm.).

Continuous corridors of oak trees facilitate connectivity between oak populations, and provide opportunities for wildlife to move both across the valleys (i.e., between oak nodes) and into adjacent upland habitats. Such connections are critical for supporting biodiversity in developed areas and maintaining plant, bird, and other wildlife populations that are resilient to disturbances like fires and floods, and to future climate changes (Tewksbury et al. 2002, Beninde et al. 2015). Tributaries of the Napa River and Sonoma Creek are particularly high priority areas for creating oak corridors, as these waterways provide natural pathways between upland oak populations on either side of the valley (Gray et al. 2018). In addition to tributaries, other linear features like highways, city streets, farm roads, margins of agricultural fields, or property boundaries can provide good opportunities for creation of oak corridors. Overly narrow corridors will exclude many wildlife species (Holmes et al. 1999, Hilty and Merenlender 2004, Collins et al. 2006), and thus widening existing corridors through re-oaking in surrounding areas can greatly increase the ecological value of these oak corridors.

Areas of recent or historical oak loss can also be prime locations for oak restoration. Though many aspects of the landscape have been altered in Napa and Sonoma valleys, the historical presence of oaks is often a strong indicator that the physical conditions of a particular area may still be suitable for oaks, at least at a coarse scale. Alluvial fans, for example, historically supported high densities of valley oaks (Griffin & Critchfield 1972). Areas of oak loss that overlap with potential oak nodes and corridors meanwhile offer opportunities to maximize the benefits of oaks.

Areas with soil types, groundwater levels, or other physical conditions that limit oak growth may be unsuitable for oak restoration, and are indicated as potential areas to exclude on the spatial strategy map. Valley oaks require well drained soils and are unlikely to tolerate clay-rich soils. In addition, valley oak root systems are not adapted to withstand very shallow water tables (Cooper 1926), while water tables deeper than 60-80 feet may be inaccessible to oak taproots (Lewis and Burgy 1964, Brown and Davis 1991). These sites may be unsuitable for oak restoration in the absence of irrigation or other active management.

Restoring oak communities in priority areas indicated on the spatial strategy map would provide a range of benefits, from native biodiversity support to carbon sequestration to shade and temperature regulation. The re-oaking spatial strategy is intended to serve as a model for similar strategies for other valleys in the North Bay and elsewhere.

References

Beninde, J., M. Veith, and A. Hochkirch. 2015. Biodiversity in cities needs space: a meta?analysis of factors determining intra?urban biodiversity variation. Ecology Letters 18, no. 6: 581-592.

Brown R. W., F. D. Davis. 1991. Historical Mortality of Valley Oak (Quercus lobata,

Nee) in the Santa Ynez Valley, Santa Barbara County, 1938-1989. USDA Forest Service Gen. Tech. Rep. PSW-126: 202-207.

Collins, J. N., Odaya, M., Sutula, M., et al. 2006. Comparison of methods to map California riparian areas. SFEI Contribution No. 522.

Cooper, W. S. 1926. Vegetational development upon alluvial fans in the vicinity of Palo Alto, California. Ecology 7, no. 1: 1-30.

Dawson, A. 2008. Oaks through time: Reconstructing historical change in oak landscapes. Proceedings of the Sixth Symposium on Oak Woodlands: Today’s Challenges, Tomorrow’s Opportunities, October 9-12, 2006, Rohnert Park, CA.

Gray, M., Comendant, T., Micheli, L., Merenlender, A. M. 2018. Building landscape connectivity for climate adaptation: Mayacamas to Berryessa Connectivity Network (M2B). Pepperwood Preserve, Santa Rosa, CA.

Griffin, J. R., W. B. Critchfield. 1972. The distribution of forest trees in California. USDA Forest Service Research Paper PSW-82/1972. Berkeley, CA. US Department of Agriculture, Forest Service. pp. 36.

Grossinger, R. M., Askevold, R. A., Beagle, J., et al. 2012. Napa Valley historical ecology atlas: Exploring a hidden landscape of transformation and resilience. UC Press: Berkeley. p 223.

Hilty, J. A., and A. M. Merenlender. 2004. Use of riparian corridors and vineyards by mammalian predators in northern California. Conservation Biology 18, no. 1: 126-135.

Holmes, A. L., D. L. Humple, T. Gardali, and G. R. Geupel. 1999. Songbird habitat associations and response to disturbance in the Point Reyes National Seashore and Golden Gate National Recreation Area. Point Reyes Bird Observatory, Stinson Beach, CA.

Lewis D. C., R. H. Burgy. The relationship between oak tree roots and groundwater in fractured rock as determined by tritium tracing. Journal of Geophysical Research 69(12):2579-2588.

Pluess, A. R., V. L. Sork, B. Dolan, et al. 2009. Short distance pollen movement in a wind-pollinated tree, Quercus lobata (Fagaceae). Forest Ecology and Management 258, no. 5: 735-744.

Spotswood, E.; Grossinger, R. M.; Hagerty, S.; Beller, E. E.; Grenier, J. Letitia; Askevold, R. A. 2017. Re-Oaking Silicon Valley: Building Vibrant Cities with Nature. SFEI Contribution No. 825. San Francisco Estuary Institute: Richmond, CA.

Sork, V. L., F. W. Davis, P. E. Smouse, et al. 2002. Pollen movement in declining populations of California Valley oak, Quercus lobata: where have all the fathers gone?. Molecular Ecology 11, no. 9: 1657-1668.

Tewksbury, J. J., D. J. Levey, N. M. Haddad, et al. 2002. Corridors affect plants, animals, and their interactions in fragmented landscapes. Proceedings of the National Academy of Sciences 99, no. 20: 12923-12926.