Plant Communities of Mount Diablo State Park
Glenn Keator, Ph. D. (excerpted from MDIA's book, Plants of the East Bay Parks)
Few of California's grasslands and open places have been untouched by humans. Many were, before settlement, quite different communities that were altered deliberately or accidentally, in many cases by removal of shrubs and trees. The most altered open places are where western "civilization" has deliberately created agricultural lands for grazing, production of hay, and nonnative vegetables and fruits, and also for housing tracts. Although few such altered lands are directly considered, many parklands have areas whose past histories often reflect such treatment. Many of these are in the process of being reclaimed by native species yet retain an unnatural appearance.
Such open places are what we call "disturbed", meaning there is little natural vegetation. Plowing, tilling, grazing, and burning have so altered the original vegetation that few native plants have survived. Instead these areas are home to plants we call weeds, escapes, and aliens. Weeds are natural here because they're designed to be opportunists, waiting for the chance to move in when land has been cleared.
Our original grassland communities were dominated by perennial, native bunchgrasses -- clumped grasses that go dormant in summer but do not die. Hundreds of different kinds of annual, perennial, and bulb-bearing wildflowers occur between bunchgrasses, providing a magic carpet of ever-changing color from March through early June. Today, only a few such areas remain to remind us of the original splendor of these grasslands.
The changes that most grasslands have undergone are dramatic. Whether grazed or ungrazed, managed or unmanaged, the majority of grasslands show the effects of the introduction of weedy, nonnative grasses and forbs. Some of these alien grasses and flowers were brought in by design, and others were introduced by accident -- often as contaminants of crop seeds (wild oats with cultivated oats, for example); as useful hay crops (alfalfa; sweet clover; red clover), as possible food plants (cardoon, chicory, fennel), or in ballast or bricks.
Native bunchgrasses were generally more palatable than nonnative grasses; so as overgrazing progressed, the demise of these bunchgrasses was inevitable. Bunchgrasses were quickly replaced by annual European and Mid-Eastern grasses, including wild oats (Avena spp.), foxtails (Hordeum spp.), Italian rye (Lolium perenne), bromes (bromus spp.), and fescue (Festuca spp.). Meanwhile, flowers with weedy characteristics and long-range dispersal strategies began to fill the spaces between grasses. The greater the grazing pressure, the more the "armed" weeds such as cardoons and thistles took over. Today, much rangeland has been degraded by pernicious, spiny plants like star thistle (Cantaurea solstialis) and milk thistle (Silybum marianum) or by poisonous plants like Klamath weed (Hypericum perforatum).
Most grasslands in our parks are at some stage between the extremes of weedlots and natural meadow; many are recovering to bunchgrasses and native wildflowers. But few will ever be completely free of the interlopers. New evidence suggests that light grazing may actually promote better wildflower displays by removing overshadowing grasses at the time of year when wildflowers are actively growing. Much remains to be discovered.
One exception to our grassland story is grasslands on serpentine soils. Serpentine rock -- California's slick, soft, shiny bluish-green rock of metamorphic origin -- is notorious for its barren, nutrient-poor soils. Serpentine soils are low in essential calcium, high in toxic heavy metals such as molybdenum and nickel, and overly rich in magnesium, a needed nutrient that is nonetheless toxic in large quantities. Consequently, only certain specialized native flowers and grasses evolving over the eons, have managed to adapt to serpentine soils. Alien weeds and grasses are unable to grow here, so serpentine grasslands give us fine examples of bunchgrasslands in their near-original state.
Meanwhile myriad species of wildflowers, including annuals, summer-dormant perennials, and bulbs, light up our grasslands in spring. Following abundant winter rains and the long, warm days of spring, floral displays explode upon the scene,wherever wildflowers can find a space between grasses. Some years the nonnative grasses get a head start, and wildflowers end up stunted; other years, wildflowers begin growth with or before the grasses and appear in vividly colored masses.
Annuals adapt to California's summer-dry regime by dying when soils dry. Before this, however, they leave behind thousands of summer-dormant seeds. Perennials and bulbs use another ploy; they simply put their extra food and water into safe, underground roots or bulbs until the rains return. Since these subterranean structures are often several inches below the soil surface, they remain cool even during the hottest summers.
Wildflowers belong to numerous families and come in many shapes and sizes, but most are white, blue, purple, or yellow: "bee" colors. (Bee eyes do not perceive orange and red.). Bees are our most abundant, prolific pollinators of open space.