Managing Open Space in Support of Net Zero

Encompassing 39,000 acres in the East Bay, the Alameda Watershed is one of the largest public land holdings in the San Francisco Bay Area. The site of several water supply reservoirs, the watershed is managed by the San Francisco Public Utilities Commission (SFPUC) for both water resources protection as well as biodiversity conservation, fire risk reduction, and other goals. The landscape is characteristic of the region, with rugged topography and vegetation dominated by grasslands, shrublands, and oak woodland habitats, along with riparian forests and wetlands (Fig. 1). Carbon sequestration in the watershed’s ecosystems has received increasing attention as a potential nature-based strategy toward San Francisco’s net zero GHG emissions goal.

In total, ecosystems within the Alameda Watershed store an estimated 2.5 million metric tons of carbon (MMT C), including 0.54 MMT C in vegetation and 1.9 MMT C in soil (Table 1). Vegetation carbon includes carbon stored in aboveground and belowground living plant tissues as well as dead plant tissues such as standing and downed wood, leaf litter, and duff, while soil carbon includes carbon stored in soil organic matter.

Soil carbon is by far the biggest carbon pool (~4x greater than vegetation carbon), though the proportion of carbon stored in soil varies from approximately 50% in riparian forest to 97% in grassland (Fig. 2). Vegetation carbon storage, which is more variable than soil carbon, is lowest in grasslands and highest in ecosystems with dense woody vegetation such as riparian forest and oak woodlands; shrublands (coastal scrub and chaparral) and oak savanna have low to moderate levels of vegetation carbon.

While per-acre carbon storage is greatest in riparian forests, these forests only occupy 2% of the watershed area, and thus account for only 3% of total ecosystem carbon storage in the watershed. In contrast, grasslands account for 24% of total ecosystem carbon storage in spite of relatively low per-acre carbon storage, due to their extensive spatial coverage across the watershed.

Wildfire is common in Mediterranean-type ecosystems. Fires convert carbon stored in vegetation and surface soils into CO₂, methane, and other climate pollutants. In the Alameda Watershed, for instance, the 2020 SCU Lightning Complex fires burned 10,370 acres of watershed lands, resulting in an estimated loss of 33,100 MT C (~8% of total vegetation carbon; Fig. 3). This carbon may be recovered in the coming decades through vegetation regrowth, but the pace and overall magnitude of recovery depends on regeneration success, vegetation succession, and potential future fires or other disturbances. California’s 2020 wildfire season highlights the vulnerability of carbon sequestered in fire-prone ecosystems. If climate change increases wildfire frequency and severity, as has been predicted for California, carbon gains due to vegetation growth and regrowth may not be able to keep pace with wildfire-related losses.  

Land managers have a range of tools to increase carbon storage on natural and working lands, depending on the landscape setting and vegetation type. These carbon sequestration strategies vary considerably in their feasibility and potential to provide carbon and GHG benefits. For land managers focused on supporting healthy ecosystems, it is critical to assess whether carbon management strategies are aligned with other management goals such as biodiversity conservation and protection of water resources. A thorough consideration of co-benefits and tradeoffs is essential when managing for multiple ecosystem functions.

This study evaluated six potential carbon management strategies in the context of the Alameda Watershed, including compost application on rangelands, riparian forest restoration, silvopasture, cattle exclusion, native grassland restoration, and open space conservation. The following table summarizes the key considerations associated with each strategy, including carbon and GHG benefits, co-benefits, tradeoffs, and feasibility.