Life on the Watershed: The Woodlands of Sky Oaks

By Michele Liapes

The following is an essay about the ecology of oak woodlands, written as part of an Ecology course at the College of Marin.

An oak woodland with its rich plant and animal diversity is one of the most productive of all native California habitats. It thrives on coastal foothill slopes, such as much of the Mt. Tamalpais Watershed with sunny exposure and well drained soils. The watershed’s Sky Oaks area is actually a mix of two different habitats: the savannah consists of scattered single trees surrounded by grassland; the actual woodlands are clusters or forests, where the canopies of neighboring trees touch and different layers of shrubs, vines and other understory plants thrive in the mix of sun and shade below. Both are home to entire communities of native plants and animals that interact in countless ways.

Sky Oaks is so-called “Black oak” country, dominated by two oak species in the darker bark, or “black,” group. They are the evergreen Coastal live oak with its dense clusters of small oblong leaves and the deciduous Black oak with large, prominently lobed leaves. They mingle with Douglas fir, Toyon (the wintertime “Christmas berry” tree), California bay, Madrone with its thin outer layer of bright rust bark curling away from the gray underneath, and others. Together they provide food and shelter to a diversity of other life. The common goal is to survive.

Generous canopies and thick understory are refuge from predators as well as day to day shelter for nesting, roosting, grazing and napping. The combination of acorns, berries, seeds, tender young leaves, and other produce feeds multitudes of insects, birds, and mammals of all sizes. Some of those animal consumers disperse their plant producer’s seeds as they feed, cache for future use, or pollinate, and in the process facilitate the propagation of other young plants elsewhere. Meanwhile, the trees, shrubs or flowers have their own strategies for attracting a target disperser. These producer-consumer relationships are “mutualistic,” with each species somehow benefitting the other. Other insects, birds, and other animals are strictly hunters — the predators. Others have evolved adaptations for protecting themselves from predators. And some, like fungi, bacteria, and banana slugs, are assigned to decompose dead matter into the ground, where the broken-down nutrients are recycled for new life to start again. Whatever the function, every living thing is here because it has established its role in the community and, in one or more ways, makes life work for itself and at least some of the neighbors.

Still, a daytime visitor may not witness much of what’s going on. “We’re not morning people, but the other day we were here at sunrise, and the wildlife was incredible,” a pair of Sky Oaks regulars said recently. True, much of the animal life is “crepuscular” — more active and more easily seen at dawn or dusk. But even in the full light of mid-day, the signs of resident life are everywhere. Look for horizontal rows of neatly spaced tiny holes circling a trunk or limb. These are the work of the sapsucker, a type of woodpecker that drills deep into the wood for sap and the insects that feed on it too. The wells also allow hummingbirds and other organisms with long bills and tongues to access the leftover bounty afterward. On the ground, the dome-shaped piles of twigs and small branches nestled in the underbrush have been painstakingly collected and assembled by the dusky-footed woodrat. The haphazard-looking exterior conceals an elaborate network of compartments and tunnels that keeps growing with successive generations of the family. Middens can last for a hundred years or more, and also provide refuge for other small mammals, reptiles or amphibians. These are just two examples of nature’s various calling cards, and there are many more.

With its diversity of species and beneficial interactions, the oak woodland seems tailored for perpetuation. However, both Coastal live oaks and Black oaks are susceptible to a certain mold that causes the disease sudden oak death. Marin Water teams continually monitor and enhance watershed vitality. Fast-spreading invasive plants that can crowd out native growth are weeded out. So is dead, fire-prone understory. Newly installed raised boardwalks protect fragile seasonal wetlands from wheels and feet, and some of the off-trail “social trails” cutting through sensitive groundcover or other growth have been removed. Restoration projects promote the growth of healthy young seedlings. Relentless vigilance and action will be key to a lasting future.

Michele Liapes is a naturalist in the San Francisco Bay Area. This year she will complete her Natural History Certificate of Achievement from the College of Marin, with courses in Ecology, Botany, Physical Geography, Marine Ecology and others. Michele currently works as a volunteer interpreter at Audubon Canyon Ranch, near Stinson Beach, and at the San Francisco Botanical Garden.