The wild diversity of Sonoma County's oak trees

Where is your favorite spot to enjoy Sonoma County’s magnificent oak woodlands? Is it a stretch along your morning commute, a park you often visit, or perhaps the trees in your yard?

Oak trees are perhaps the single most iconic element of our landscape. So what do we know about them?

For starters, there are 10 different species in Sonoma County, and in many places the trees are blended together in an unusual way. While other counties boast high numbers of blue oak woodlands or Oregon oak woodlands, here scientists just scratch their heads and label many of our forest stands “mixed oak.” This diversity can be seen in the patchwork of greens in the forest canopy around places like Spring Lake.

Where they do carve out their own single-type stands, oaks can teach you something about the microclimate and soil type you are in.

The majestic valley oaks love the deep soils in our valley bottoms. Hike through the Laguna de Santa Rosa to get a good look at them. Their leaves are dull green, without sharp tips and deeply lobed. Blue oaks endure relatively hot and rocky locations. Look for them on inland ridges like those in Sonoma Valley above the town of Glen Ellen. Their leaves are blue-green, no sharp tips and shallowly lobed.

Black oaks are water lovers and enjoy shady canyons like those in Hood Mountain and Sugarloaf Ridge. Their leaves are large and sharply tipped, with deep lobes.

Oregon oaks take up where the valley oaks leave off on hillsides with fertile soils. They can be seen with the coast live oak rimming the meadows at places like Taylor Mountain. Leaves are bright green, lobed and not sharply tipped.

And finally there is our most ubiquitous oak, the coast live oak, which dominates the western portion of the county while holding its own in almost any setting. Its leaves are dark green, curved, with sharp little spines.

To make matters more interesting, many of Sonoma County’s oaks hybridize, creating another 12 versions of oak to inspire your appreciation for the wonders of nature. Annadel State Park is a good example of rampant hybridization between blue oak and Oregon oak.

And we have shrubby oaks such as leather oak and scrub oak, beautiful examples of plants adapted to drought and inhospitable soils.

So why are oak forests important? Many would say they are a huge part of our county’s scenic landscape, but they also provide valuable habitat for more than 300 wildlife species and as many as 5,000 insect species. Oaks provide an abundant food source as well as cavities that house birds like the oak titmouse, and fallen logs and branches used by ants, beetles, salamanders and even frogs. Oak woodlands are simply teaming with life.

While each of our oaks has its own story to tell, together they share some common challenges. The first is surviving development and some forms of agriculture. While most residents cherish their oaks, the trees sometimes are removed to make room for housing, commercial development, row crops and vineyards.

The majority of our oaks are not directly threatened by development, but they still face challenges. Historically, oaks were managed with fire by Native Americans. Many of the mature trees we now enjoy got their start in an environment in which frequent ground fires kept shade-loving trees like Douglas Fir and Bay trees to a minimum. Oaks, madrone and manzanita are fire tolerant, but in many cases they are actually fire dependent. They need that regular burning to maintain their presence.

When oaks have little or no help from fire, they become stressed by increasing competition and are even more vulnerable to insects and diseases like Sudden Oak Death. Since 1995, nearly 10 percent of our region’s forest land has been affected, including coast live oak, black oak and tan oak trees. The diversity of our oak forests helps buffer us from the impact of this introduced pathogen.

In addition to challenges to mature trees, oaks have trouble growing the next generation. Take a good look at our oak forests, and you will see a distinct lack of seedlings, saplings and young trees. Some say that grazing has kept acorns and young sprouts out of the forest’s understory. Others blame an over-abundance of deer, the lack of fire or a shifting climate. Still others point to the competition of introduced grassland species in oak forests. As with most ecological issues, the answer is probably all of the above.

As land managers, we are keenly interested in promoting the health and vigor of Sonoma County’s oak woodlands. One unique challenge in our smaller parks is that of oaks being loved to death. The constant camping, picnicking and playing in their shade can compact their soils and prevent them from accessing enough water and nutrients from leaf litter.

Moving forward, land managers will be doing a number of things to help oaks on public lands. We are monitoring and mapping the impacts of sudden oak disease, managing grazing animals in such a way that acorns and seedlings can persist, exploring the feasibility of prescribed fire in specific settings, and fencing people and animals out of critical areas to decrease compaction and increase regeneration.

And we are always looking to learn more. Bouverie Preserve in the Sonoma Valley has a demonstration project that shows how easy it is to grow oaks from acorns.

We also are helping to celebrate the oaks of Sonoma County through interpretive programs and robust public outreach. Sonoma County Regional Parks host thousands of school children for a program called “Acorns to Oaks” that teaches kids about the cultural and ecological importance of these forests. Oak woodlands are central to the character of the Sonoma County. Hopefully we’ll work together to protect and promote their future.

Melanie Parker is the Natural Resource Manager for Sonoma County Regional Parks.