Oaks and Oak Woodlands

Thursday, October 20 at 10:00-11:40 am, Oak Room

Session Description: A suite of iconic oak species and oak woodlands have been enduring anthropogenic threats since the state of California was formed in 1849. Thousands of trees and acres of oak woodland habitat have been lost or modified to clear way for intense land use changes and fire suppression, and today they continue to face a mosaic of pressures – disease, drought, invasive species, and wildfire – associated with continued urban and agricultural development pressures, and a changing climate. This session will examine critical California oak woodland habitats by exploring the science, policies, cultural burning, and other practices necessary to conserve, stabilize, and regenerate critical oak woodlands ecosystems in the face of ever-increasing social, climatic, and ecological pressures.

Session Chairs: Anne Morkill (Laguna de Santa Rosa Foundation, Santa Rosa, CA, USA) and Isaiah Thalmayer (Point Blue Conservation Science, Petaluma, CA, USA)

4.1 Re-Oaking: a strategy for restoring native oak ecosystems in developed landscapes

Sean Baumgarten (San Francisco Estuary Institute, Richmond, CA, USA), Erica Spotswood (San Francisco Estuary Institute, Richmond, CA, USA), Robin Grossinger (San Francisco Estuary Institute, Richmond, CA, USA), Matthew Benjamin (San Francisco Estuary Institute, Richmond, CA, USA), Steve Hagerty (San Francisco Estuary Institute, Richmond, CA, USA), Erin Beller (San Francisco Estuary Institute, Richmond, CA, USA), April Robinson (San Francisco Estuary Institute, Richmond, CA, USA), Letitia Grenier (San Francisco Estuary Institute, Richmond, CA, USA), Micaela Bazo (San Francisco Estuary Institute, Richmond, CA, USA), Frances Knapczyk (Napa County Resource Conservation District, Napa, CA, USA)

Before 1900, native oaks were abundant in many of California’s valleys. These trees played a foundational role for a diverse array of native wildlife, reflecting a long history of coexistence. In the San Francisco Bay Area, vast expanses of oak savanna, dominated by the immense and long-lived valley oak (Quercus lobata), historically occupied the rich alluvial soils of Santa Clara Valley in the South Bay and Napa and Sonoma valleys in the North Bay. Over the past two centuries, however, much of this oak savanna has been cleared to make way for agricultural and urban development; in many areas, the loss of mature oaks exceeds 95%. Despite the significant changes these landscapes have experienced since European settlement, there are opportunities to restore oaks in settings as diverse as backyards, parks, streets, schools, office parks, farms, vineyards, and creeksides. Bringing oaks back into developed landscapes would provide huge benefits to native biodiversity along with ecosystem services such as carbon sequestration, shading, and stormwater capture. The Re-Oaking initiative was established to provide an overarching strategy to help guide and prioritize oak restoration—or re-oaking—efforts in the region and beyond, 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. Re-oaking design guidelines for Silicon Valley and the North Bay—drawing on historical ecology, urban ecology, and natural history—summarize key features of oak woodland ecosystems that enable support for biodiversity and ecological function, such as nodes of multiple large oak trees, oak corridors between nodes, and native understory vegetation with downed logs and leaf litter. In the North Bay, priority areas for re-oaking Napa and Sonoma valleys were identified by comparing present and past distributions of valley oaks and analyzing opportunities and constraints in the modern landscape.

4.2 California native oak habitat for endangered, threatened, and candidate species

Angela Moskow (The California Oaks program of California Wildlife Foundation, Berkeley, CA, USA)

California Wildlife Foundation’s California Oaks program produced a report examining the interrelationship of oaks and biodiversity, with a focus on species and subspecies that are federally and/or state designated as endangered or threatened (listed), or are candidates for federal or state Endangered Species Act designation. Vertebrate data were derived from a query of the California Wildlife Habitat Relationship information system focused on vertebrate species that utilize oak (Quercus) and tanoak (Notholithocarpus) habitat for reproduction, cover, or feeding. Plant and invertebrate lists were created first with a cross-reference of California Natural Diversity Database (CNDDB) occurrence records with the oak woodland dataset in California Department of Fish and Wildlife’s Areas of Conservation Emphasis (ACE) system. Next, a threshold was established to exclude species with a low average percentage overlap of all CNDDB occurrences with the oak woodlands layer in ACE. Thirty-three listed, candidate, and/or state fully protected terrestrial vertebrate species or subspecies were found to be dependent upon oak and tanoak habitat. The CNDDB query found 134 listed or candidate plant species and subspecies out of 839 sensitive native plant species associated with oak habitat. It includes 26 listed or candidate invertebrate species and subspecies out of 201 sensitive invertebrate species and subspecies that are associated with oak habitat. The CNDDB queries were exclusively for oak (Quercus), as CNDDB does not track tanoak.

4.3 Inoculant-supported restoration of Quercus agrifolia and Quercus lobata increases survival and restoration success

Chelsea J. Carey (Point Blue Conservation Science, Petaluma, CA, USA), Kristy Dybala (Point Blue Conservation Science, Petaluma, CA, USA), Tom Gardali (Audubon Canyon Ranch, Stinson Beach, CA, USA), Bridget Hilbig (Weber State University, Ogden, UT, USA), Isaiah Thalmayer (Point Blue Conservation Science, Petaluma, CA, USA)

Over the past century, up to 98% of riparian habitat in California has been lost or degraded, making riparian restoration critical. Oaks constitute an important part of plant communities along riparian areas, but unlike many of the other co-occurring woody species, oaks form relationships with ectomycorrhizal fungi (EMF) that may be absent from degraded sites or areas far from mature donor forests. The absence of EMF may reduce establishment and vigor, and increase mortality of oak species as the climate warms and droughts become more extreme and frequent. In the fall of 2019, we established a field experiment to assess the effects of EMF inoculation on the establishment and near-term success of Quercus agrifolia and Quercus lobata individuals in Sonoma County. We compared two different inoculum sources to a control: whole-soil inoculum sourced from beneath mature Q. agrifolia and Q. lobata trees in 1) reference riparian areas currently experiencing a similar climate to the restoration sites (“local”) and 2) reference riparian areas currently experiencing warmer and drier conditions compared to the restoration sites (“heat adapted”). Results from 2 years of growth and monitoring indicate that, when grown with the heat-adapted inoculum, Q. agrifolia had higher rates of germination, survival, and EMF colonization compared to uninoculated controls. In contrast, Q. lobata showed a mixed response to inoculum source, with strong support for higher germination rates, but only weak support for higher EMF colonization, and no support for higher survival associated with the heat-adapted inoculum. There was also weak support for higher rates of germination, survival, and EMF colonization associated with the local inoculum for Q. lobata. Implementation of this approach at 3 restoration sites in Sonoma County suggests that whole soil reference site inocula from warmer, drier climates can increase oak restoration success.

4.4 Principles and best practices for vegetation management and fire fuel reduction in California Oak Woodlands.

Jason M. Mills (WRA Landscape Restoration, San Rafael, Petaluma, Emeryville, CA, USA) 

In this era of increasing climate driven fire regimes our approach to managing vegetation may be more important now than ever before if we are to protect the environment and communities of California. The principles which guide vegetation management practices with the intention of suppressing fuels loads should vary according to the specific ecological community where implementation efforts are being focused in order to reach objectives over the long term. As support for this work increases and resources are being actively allocated throughout the state many of the natural resources within these ecosystems are facing a greater threat including Oak Woodlands. Misinformed or poorly planned fuel reduction efforts have resulted in damages to habitat on an increasingly massive scale. Forestry practices which are typically applied to coniferous forests with timber production value have little to no application for the hardwood forests of the coast range. As is the case with many landscapes within wildland urban interface (WUI) areas where fuel work is often focused contain monocultures of invasive species which has led to a proliferation of the concentration of both flashy fine and woody ladder fuels. The disturbance generated from large equipment often associated with fuel reduction efforts have been shown to both expand existing invasive populations as well as introduce and spread new emergent populations into wildlands. We have seen many invasive populations rapidly expand and increase fuels following a series of catastrophic wildfires in Sonoma County. Pre and post fire conditions allow for critical windows of time to take action and shift trajectories through orchestrating properly timed control efforts on a watershed scale to build towards a more resilient forest and future. Well informed approaches such as establishing shaded fuel breaks and management practices that consider the ecological composition of the site and place an emphasis on addressing aggressive invasive species provide the opportunity to both lower fuels loads and maintain biodiversity.

4.5 Panel of Four Speakers

Sean Baumgarten (San Francisco Estuary Institute, Richmond, CA, USA), Chelsea J. Carey (Point Blue Conservation Science, Petaluma, CA, USA), Jason M. Mills (WRA Landscape Restoration, San Rafael, Petaluma, Emeryville, CA, USA) Angela Moskow (The California Oaks program of California Wildlife Foundation, Berkeley, CA, USA)