Enhancing plant diversity and reducing wildfire risk in oak woodlands

Oak woodlands are some of California’s most important landscapes. They support high biodiversity, provide services like carbon storage and water regulation, and hold deep cultural value. But many oak woodlands are increasingly threatened by invasive annual grasses. These grasses can crowd out native wildflowers, build up dry “thatch” that carries fire, and alter how these ecosystems function.

One tool land managers are using more often is targeted grazing: bringing in animals to eat grasses and reduce fine fuels. While grazing is widely used, we still don’t have many studies that test what dry-season (summer) grazing does to plant communities, especially to native species. To help fill that gap, UC Davis researchers collaborated with the Land Trust of Napa County to study whether targeted summer grazing with sheep and goats can reduce invasive grass fuel loads and improve plant diversity in California oak woodlands. We set up paired plots, one grazed and one ungrazed, and tracked plant species richness and abundance over multiple years.

Overall, grazing had clear benefits for native wildflowers:

• Native forb richness and cover increased substantially in grazed plots.
• Non-native grass cover did not decrease overall, although we did see small increases in some non-native forb species.
• When we looked at the full plant community, grazed plots shifted in composition. That shift was driven mostly by more native forb species, with a smaller contribution from increases in some non-native forbs.
• We also identified specific “indicator” species that were more common in grazed areas which were mostly native forbs.

Why might this be happening? Grazing likely helps by reducing thick layers of dead grass, which can block light and make it hard for seedlings to establish. By thinning that thatch, grazing may create better conditions (more light and more open microsites) for native plants to germinate and grow.

This work shows that dry-season grazing with sheep and goats can be a multi-benefit land stewardship strategy. It can reduce invasive grass thatch, boost native plant diversity, and help lower fire risk in grass-invaded woodlands. At the same time, grazing by itself is unlikely to eliminate invasive grasses, and in some cases it may encourage certain non-native forbs. The takeaway is that targeted grazing can be a powerful way to support native species and improve coexistence, even if it doesn’t fully suppress invasives. If reducing non-native plants is a primary goal, grazing will likely work best as part of an integrated strategy paired with other tools and adapted over time. Incorporating targeted dry-season grazing into broader restoration planning may be a useful approach not only in California oak woodlands, but in invaded dryland ecosystems more broadly.