Plant Communities of Mount Diablo State Park
Redwood forests are represented in our area by isolated fragments found in the deepest, most protected canyon bottoms with reach of summer fogs. Although historically we know that other stands of redwoods once clothed coast-facing slopes in Oakland and Berkeley hills, redwood forest was never a major plant community in this part of California.
Redwoods are remarkable trees that extend back in time to the beginnings of the cone-bearing trees called conifers. Once, great forests of various kinds of redwoods covered large tracts in North America, Europe, and Asia. Now they exist only in protected pockets as relics from a time when the climate was more uniformly wet and had moderate year-round temperatures. The present distribution of coast redwood -- canyon bottoms and slopes in the fog belt of coastal central and northern California -- reminds us of how these trees are prevented from growing elsewhere. To the north, winters become too cold; to the south, summers are too hot and winters have too little rain; to the west, heavy salt-laden winds thwart growth next to the ocean; and to the east, summers lack fog and are too hot. Redwoods are also restricted from climbing higher than two- to three- thousand feet, owing to cold winter temperatures.
Redwoods -- if allowed to grow unhindered for hundreds of years -- exclude other trees by their tall, needle-covered branches that effectively shade out everything else. Where virgin redwood forest grows unimpeded, few smaller trees or shrubs live happily under the deep shade that their branches create. Yet redwood forest habitat is full of berry-producing shrubs. These favor forest edges and streamsides, especially in second-growth forests, where immature trees have not completed their overshadowing canopies.
In the deep shade of mature redwood forests live several smaller, herbaceous plants, such as various ferns, and sword fern (Polystichum munitum) in particular. Also in this shade are redwood sorrel (a ground-cover-forming oxalis); various violets; inside-out flower; various members of the lily family (such as trillium; false Solomon's seal, fetid adder's tongue and bead lily); wild ginger; and several saxifrages (sugar scoops, fringe-cups, piggyback plant). All must do with short, periodic bursts of sunlight, and all take advantage of their locales by vegetative means of increasing their territory. Many of these forest denizens are limited in abundance in areas with minimal winter rainfall or only periodic summer fogs; to see the redwood forest understory at its best, journey to Humboldt and Del Norte counties in the northwestern extreme of our state.
Two fascinating aspects of redwood forest plants include the abundance of fleshy-fruited, berry-producing shrubs along streams, where birds depend on them for food and so help in their dispersal; and the many ant-dispersed seeds in the shade of mature redwoods. Ant-dispersed seeds have easily-seen white elaisomes (oil bodies) appended to the main seed body. Ants are attracted by them, carry the seeds away, eat the elaiosomes, and discard the main seed with its embryo. Unrelated plants -- trillium, fetid adder's tongue, western bleeding heart, smooth yellow violet and inside-out-flower -- have hit upon this strategy as the best bet for moving their seeds.
Because deep shade creates cool, moist conditions most of the year, redwood-forest-floor plants have broad, water-wasteful leaves with maximum surface area t trap as much of the sun's light energy as possible. Trail plant (Adenocaulon bicolor), western coltsfoot (Petasites palmatus), and redwood sorrel all show thin, broad leaves that wilt easily in strong summer sun, yet that manage to remain turgid and healthy in the refreshing shade of forest aisles. Many plants here even have highly divided, fernlike leaves for efficient trapping of light energy; western bleeding heart, inside-out-flower, and baneberry are examples.
Despite the fact that redwoods create a very special niche for low-growing herbaceous plants -- cool, moist, acid soils -- these plants are seldom exclusive to redwood forests. Many other coastal forests provide the same cool, moist conditions. So although closed-cone ping and Douglas fir forests, for example, are missing from our area, they are home to the same array of plants.
Fire and flooding have helped to maintain redwood forests where otherwise redwoods might be outcompeted by other kinds of trees. Redwood bark resists burning, since it lacks pitch and sap; mature trees also recover their fire wounds efficiently. Flooding may uproot old redwoods, but seeds are adapted to germinate in litter-free, sun drenched soils such as those left behind after floods. And trees not uprooted by floods may send roots upward toward the surface or build a whole new set of roots near the surface even when the trunk and roots have been deeply buried under silt.
Redwoods are also efficient at replacing themselves when they're burned to the ground or, in the case of human intervention, at growing after being felled by logging. Dormant buds at the base of each tree are the secret; the grow into stump sprouts every year but are inhibited from growing more than a few feet tall by hormones produced by the top crown of the parent trunk. Once that source of hormones has been eliminated, the stump sprouts are free to grow, and grow they do. Circles of these sprouts become rings of mature trees in relatively short time. This ability to regrow is what has saved many now-protected redwood forests that have been logged one or more times.
Excerpted from MDIA's book Plants of the East Bay Parks by Glenn Keator, Ph. D.
Redwood National Forest, Humboldt County
Sword fern, Polystichum munitum, Prairie Creek Redwoods State Park, Humboldt County
Download Common Ferns of Mitchell Canyon (pdf)