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Oak Woodlands

Plant Communities of Mount Diablo State Park

Excerpted from MDIA's book Plants of the East Bay Parks, by Glenn Keator, Ph. D.

Oak Woodlands

Coast Live Oak | Glenn Keator

Probably no more characteristic treelands occur in our part of California than oak woodlands, for they typify large areas of low foothills in both the Coast Ranges and Sierra Nevada. From a distance oak and foothill woodlands look similar; relative dense groupings of trees. Where these same trees are widely spaced apart, they're referred to as savannah. In both, oaks grow as broad, round umbrellas; only on closer inspection is it obvious that they are of several different species. In what is often called foothill woodland, where the terrain is steeper or higher in elevation gray pines and California buckeyes often join ranks with the oaks.

In contrast to chaparral, oak and foothill woodlands most often occupy gently sloping terrain. When the occur on steep slopes, they do so on north- or east-facing hillsides, where the hot summer sun does not linger all day. Chaparral prefers sun-drenched south- and west-facing slopes.

Oaks have developed two equally effective water-conserving strategies for the hot, dry summers they must endure: live oaks bear long-lived, leather, evergreen leaves that resist wilting and are covered with a waxy layer to prevent drying out; deciduous oaks -- called robles -- produce thin deciduous leaves that are shed when water supplies dwindle but are renewed during the peak of the wet-winder spring rains. Both kinds grow side by side, blue oaks with interior live oaks, valley oaks with coast live oaks, and canyon live oaks with California black oaks. Some, such as valley and coast live oaks, prefer canyon bottoms with a higher summer water table; others, such as blue and interior live oaks, live perched high on hilltops or along sides of rolling slopes.

Because of their sometimes prolific production of acorns -- with their attendant rich stores food -- oaks are the intermediaries of their ecosystems, creating abundant food for insect larvae, rodents and, not so long ago, Native Americans. Various midges and minute wasps also find oaks to their liking, as nurseries for their young; they lay eggs in various tissues of the oak and these grow into the multifarious galls we see commonly on oak trees. And even the parasitic mistletoes favor oak trees as places to grow and prosper; they bring with them birds that depend on their berries for food.

Woven into this web of life are the several shrubs and numerous grasses, wildflowers, and bulbs that benefit from the shaded protection of oak branches or the increased soil stability and water-holding properties of oak roots. In the more open oak woodlands, the wildflower displays can vie with those of our best grasslands.

Buckeyes and gray pines help fill out the personalities of these woodlands; while the rounded canopies of buckeyes mimic those of oak trees (but in miniature), the uneven and often double barreled spires of gray pine punctuate and contrast with these umbrella shapes. Gray pine is the picture of a conifer well-adapted to dry, drought summers; its sparse, gray needles reflect away summer sun and its stout trunks hold water reserves needed to complete the production of the oversized seed cones. Among the heaviest of all seed cones in the world, gray pine's also are armed with stout spine-tipped scales, but the offer up nutritious food. The large pine "nuts" are similar to those of the desert- and drought-adapted pinyon pines, and they are important to local animal life as yet another source of food. So too, doubtless, are the poison-laced chestnut-shaped seeds of the buckeye, for the poisons are not harmful to some animals. These seeds are also adapted for rolling, being perfectly round, and allow buckeyes to disperse their seeds downhill to the protection of shaded canyon bottoms.

The rich food reserve in the dormant trees -- oaks, buckeyes, and gray pines -- not only encourage animal dispersal of the seeds but give the seeds a head start when they germinate. Should they land in the shade of competing trees, the extra stored food allows the resulting saplings the chance to grow vigorously toward light.

While seed dispersal in oak and foothill woodlands therefore differs markedly from that in trees of riparian woodlands, pollination is another matter. All oaks and pines rely on wind to carry their pollen, just as with most riparian trees. Pollination occurs during late winter and early spring, just when winds are likely to be most reliable. The buckeye, however, uses another strategy. Its colorful candles of white flowers attract large numbers of pollinators, though the poisons in the nectar favor butterflies (which are immune to the poisons) over bees and other insects).

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