NATURAL VIEW: A tale of two oaks

When I first moved to California, many years ago, I knew only Eastern deciduous oaks: trees with interesting leaves with cutouts that turn color and drop every fall. In the face of months of snow and freezing temperatures, those oaks grow large, thin leaves that are killed by the first hard frost and drop to the ground as mulch.

I accepted California evergreen oaks as part of the state’s balmy strangeness. Our local oak trees are those now-familiar green giants, Coast Live Oaks. Their smaller tough leaves are oval, with varying numbers of sharp points around the margins. The two kinds of oaks follow different survival strategies: winter deciduous and evergreen. And then a few years ago, I began to notice places where both types grow.

Live oaks grow mainly on the coast, where it never gets cold enough to freeze the leaves. The oaks grow toward the bottom of the canyons, just enough upslope to survive the freezing air that turns willow and sycamore leaves yellow and red. So the oaks can put their energy into building tough, long-lasting leaves.

Go inland a few miles, away from the coastal influence of cooler summers and warmer winters, and you find oaks with lobed leaves that drop in the fall. Responding to our drier conditions, their leaves are smaller and rounder than those of Eastern oaks, but they’re unmistakably oak leaves.

The Pacheco Pass connects the Central Valley to the coast, about 80 miles south of San Francisco, and 400 miles north of here. Highway 152 runs through rolling hills of grazing land and oak woodlands, and it affords another peek into the lives of the oaks.

Higher on the hillsides, above the freezing winter air that fills up the canyon in winter, grow evergreen live oaks. Down near the bottom of the valley grow oaks that are bare in winter. It’s not a perfect separation, but the distinction is there.

But east of San Francisco Bay, and across the Berkeley Hills in Lafayette, I found a west-facing hillside where the two types of oaks live together side by side: winter deciduous and evergreen. It’s enough to make a naturalist gnash her teeth.

The reality is that life is tenacious and always testing the boundaries of the possible. Acorns sprout where they fall or are carried and buried by industrious squirrels and scrub jays. If they’ve sprouted in the wrong place, if the conditions are not possible for their genetic makeup, they die. Sometimes it takes years for that extra-cold winter or severe drought to do the pruning.

I suspect that Lafayette hillside has a microclimate on the warm side for the deciduous oaks. But if they and the evergreen oaks can manage to survive next to each other, who’s to say one of them is in the wrong place?

There are 21 species of oaks in California, described in Fred Roberts’ Oaks of California. The story above is complicated by stubborn species like Engelman oaks, that have oval leaves but are deciduous. This abundance is just one more indication of the immense biodiversity in the state.

ELISABETH M. BROWN is a biologist and the president of Laguna Greenbelt Inc.