ContentsPutah and Cache: Entering

Previous piece More like this

Next piece

Next chapter

 

Natural Vegetation of the Putah-Cache

Michael G. Barbour and Valerie Whitworth

The Putah-Cache bioregion is situated in the driest, innermost band of the Coast Range, where fog penetrates least often. Summer heat becomes extreme and a typical Mediterranean climate prevails. Our bioregion or watershed extends westward, from near sea level along the Sacramento River, to the crest of the innermost ridge of low mountains of the Coast Range. The Coast Range forms a fog fence, high enough to keep cool marine air from penetrating to the Great Central Valley. In summer, coastal fog typically sits offshore by night and invades the coastal strip by day. On exceptional days, when the Central Valley is hottest, fog crawls eastward through canyons, valleys, and passes in the Coast Range, and sometimes spills over the crest like the curl of some enormous wave.

The texture of vegetation covering interior foothills and valleys is a patchwork of woodland with scattered trees of gray-green pine and blue-green oak, muddy green chaparral with dense, rigid shrubs, and open golden-brown grassland. Collectively, these three vegetation types characterize most of the watershed's area from sea level to 2000 feet elevation.

While all three vegetation types occur within the same span of elevation and thus the same general climate, they vary with soil depth, soil texture, slope face, and fire history. Grassland tends to occupy the gentlest slopes with the deepest and most finely textured soil, and may burn every several years. Chaparral tends to cover the steepest slopes with the shallowest and coarsest soils and burns every 15-25 years. Foothill woodland grows on intermediate slopes and soils and has a complex fire history, with some stands burning every decade and others only once or twice a century.

The original interior grassland blanketed much of the Central Valley as well as low elevations along the central and southern coast. It covered more than 13 million acres and an additional nine to ten million acres underneath oaks in the foothills, for a total area representing one fourth of California. The interior grassland was probably dominated by half a dozen species of bunchgrasses, particularly purple and nodding needlegrasses (sp. Nasella), fescue (Festuca californica), ryegrass (Elymus glaucus), squirrel tail (Sitanion hystrix) and two species of melic grass (sp. Melica). Between the grasses were some annual and perennial herbs, very abundant and colorful in the spring.

This grassland no longer exists except in small preserves. Beginning in the 19th century, livestock were kept at high numbers, year-round, in fenced pastures. The grazing pressure and soil-surface disturbance favored exotic annuals over the native bunchgrasses. In addition, fire was controlled and weed seeds were accidentally introduced. In a dramatically short time, bunchgrass prairie was converted to an annual grassland of European grasses and forbs. The most common of these non-native species include two species of wild oats (sp. Avena), soft chess (Bromus mollis), red brome (B. rubens), ryegrass (Lolium multiflorum), and several species of filaree (sp. Erodium).

As terrain becomes steeper, generally at an elevation of above 300 feet, the grassland becomes an understory to trees. At first the trees are widely scattered or restricted to canyons where runoff is concentrated, but with increasing elevation the trees become dense enough for their canopies to touch, forming a woodland or forest.

If Californians were to legislate a state vegetation type, foothill woodland would be the logical choice because of its historic importance, the large area it covers, its wide familiarity, and its endemic tree species. This is a two-storied vegetation type with an open tree canopy shading only one third of the continuous carpet of grasses and forbs beneath. The dominant trees in the watershed are blue oak (Quercus douglasii) and interior live oak (Q. wislizenii), but common associates include coast live oak (Q. agrifolia), black oak (Q. kelloggii), valley oak (Q. lobata), gray or ghost pine (Pinus sabiniana), and buckeye (Aesculus californica). In low-lying areas, the deciduous oaks are decorated with a lace-like lichen called grandfather's beard; its presence is an indication that summer fog occurs here, because this lichen is dependent on high humidity for much of its water.

These trees are a mix of evergreen and deciduous species. The deciduous taxa, such a blue oak and valley oak, appear to be in decline. Few young trees have become established in the last century. If this pattern continues for another century, today's mature individuals will have reached the end of their natural life spans and these deciduous oak woodlands will become converted to evergreen woodlands or to grasslands. The causes of this decline may include exploding populations of root-eating pocket gophers, browsing deer and cattle, and competitive introduced annual plants-all of which can kill oak seedlings and saplings. Evidently, evergreen seedlings and saplings are either less often attacked or are more tolerant of competition.

On drier, steeper, or south-facing slopes, oak woodland is replaced by chaparral, a vegetation type consisting of a single layer of impenetrable shrubs 4-8 feet tall with intricately branched, interlacing evergreen canopies. The most common shrubs in the watershed are chamise (Adenostoma fasciculatum) scrub oak (Quercus dumosa), toyon (Heteromeles arbutifolia), California coffeeberry (Rhamnus californica), and several species of manzanita (Arctostaphylos) and ceanothus or jimbush (Ceanothus). Characteristic, but less abundant, species include poison oak (Toxicodendron diversilobum), redberry (Rhamnus crocea), and chaparral pea (Pickeringia montana). Scattered trees occasionally overtop the shrubs: California bay (Umbellularia californica), gray pine (Pinus sabiniana), and sometimes clusters of closed-cone pines or cypress. The ground is nearly bare of herbs, except for the first few years following wildfire.

Chaparral covers about 10 million acres of California and ranks among the five largest vegetation types in the state. Its range and habitat make it subject to late summer and fall lightning strikes. If humidity is low and winds are high, a high-intensity wildfire is the result. Chaparral fires are hot, in contrast to grassland fires, because there may be 50 tons or more of fuel (shrubs) per acre and this biomass is rich in flammable oils. Soil surface temperatures reach the melting point of aluminum, and the enormous amount of heat generated creates a local windstorm capable of throwing flaming bits of shrubs and trees tens of yards in front of the fire, easily crossing fire breaks.

After fire, chaparral regrows in a predictable way within half a dozen years. Seeds and roots deeper than a few inches below the soil surface are not killed by the fire because soil is an effective insulator. Many chaparral shrubs are also capable of forming specialized, swollen regions where the buried stem base joins the root system. During the first winter and spring following the fire, the fire- tolerant shrubs resprout while others germinate from seed, and a rich assemblage of annual herbs germinate. These herbs are rare beneath mature chaparral. But they are abundant now because their seed coats are hard and require heat to crack. It is also possible that they are stimulated to germinate by the release of volatile chemical substances during the fire, or that they are inhibited from germinating at other times by chemicals given off by shrub leaves. In addition, small animals in unburned chaparral usually eat herbs—animals that require the protection of shrub cover. When cover is removed by fire, those herbivores are no longer present and the herbs can thrive.

Fire, then, regulates flora. Fire also has other ecological effects. For example, fire releases nutrients from shrub twigs and leaves; winter rains leach those nutrients in the form of ash into the soil, where they are taken up again by the roots of herbs and young shrubs.

Within 4-6 years, shrub canopy cover has returned to 100% and the herb cover has declined to less than 10%. The shrubs grow vigorously over the next several decades and then slow. Ceanothus shrubs, which have a shorter life span than other shrubs, begin to die and are replaced in the canopy by the expanding canopies of chamise, manzanita, and scrub oak. The chaparral community becomes less rich as it ages beyond 30-40 years.

Although chaparral stands as old as 80-100 or more years are known to exist, the probability of wildfire is such that a given acre of chaparral will typically reburn in less than 25 years. This fire return period is very similar to what it was before the arrival of Euro-American. We have been able to suppress wildfires in grasslands and in forests, but not in chaparral, probably because chaparral often grows on steep slopes in roadless areas, making it difficult for firefighters to reach, and because it burns hot and easily jumps all the but the widest of fire lines. It is the last vegetation types to still experience a natural fire regime, unchanged by humans.

Chaparral becomes noticeably more open on serpentine outcrops. Serpentine is a gray-green metamorphic rock rich in magnesium and such heavy metals as nickel. Many plant species are intolerant of serpentine but those capable of growing on it become more abundant there than in other parts of their range because of the lack of competition. A few species, for reasons we do not understand, grow only on serpentine outcrops and nowhere else. Leather oak (Quercus durata), for example, is a shrubby oak completely restricted to chaparral on serpentine soil. Gray pine, in contrast, is able to grow both on and off serpentine, but it is most abundant on serpentine. Good examples of serpentine chaparral are along Capell Creek, the Cedar Roughs (along the Berryessa-Knoxville Road), Cedar Canyon, along Route 175 as it climbs toward Cobb Mountain, and along Route 20 as it approaches Route 16, east of Clear Lake.

Serpentine chaparral is also enriched by the presence of island-like strands of three closed-cone conifer species. Perhaps the most bizarre relationship between plants and fire in the watershed is that of closed-cone conifers. The conifers are knobcone pine (Pinus attenuata), Sargent cypress (Cupressus sargentii), and MacNab cypress (Cupressus macnabiana). These plants only produce offspring after the parent generation has been killed by fire. The parent trees cling together in tight, fire-prone groves. Their dense lower limbs make it certain that any fire reaching a grove is able to ladder up into the crown and become hot enough to kill the trees.

Closed-cone conifers retain their cones on branches long after the seeds have matured. Normal conifer cones open when the seeds are ripe (generally in the fall), and the cones themselves fall off by the next year. Closed-cone conifers remain closed and attached—in some cases for decades. They open only when temperatures are high enough to melt a resin that seals the cone scales shut. The required temperature to open cones for knobcone pine is 200 F, a temperature easily produced in a wildfire. For several days after the fire there is a massive rain of seeds onto the ash-rich soil surface, and these seeds germinate after the first fall rains. Thus the parent generation dies, but the site is immediately re-occupied by the next generation.

At elevations above 2000 feet, especially on north-facing aspects, chaparral and foothill woodland both gradually give way to a forest much richer in tree species and relatively empty of herb cover.

The tallest overstory trees are conifers, most typically Douglas-fir (Pseudotsuga menziesii) or ponderosa pine (Pinus ponderosa). These trees are scattered, however, and do not form a continuous canopy cover. Beneath is a more closed canopy of broadleaf trees, some evergreen and some deciduous. Evergreens include California bay, coast live oak, canyon live oak (Quercus chrysolepis), madrone (Arbutus menziesii), and tanbark oak (Lithocarpus densiflorus); winter-deciduous species include black oak, big leaf maple (Acer macrophyllum), mountain dogwood (Cornus nuttallii), and hazelnut (Corylus cornuta). Shrubs and herbs are only scattered, and the ground surface is covered with a thick layer of leaves. Unfortunately for those allergic to it, poison oak is one of the more common shrubs. Mosses grow in dense patches on exposed rock surfaces and along the lower trunks of trees.

Mixed evergreen forest is an ecotone between foothill woodland and montane conifer forest, containing some species that extend upslope from woodland and some that extend downslope from conifer forest. It is California's richest vegetation type in terms of tree species. The inner coast ranges—and thus our watershed—are too arid and too low to support montane conifer forest, and even the mixed conifer forest is relatively uncommon. Throughout California, however, this forest type covers about 3-4 million acres, and almost all of it has been seriously disturbed because of logging for Douglas fir. The hardwoods are capable of stump sprouting when cut or burned, so the result of logging is usually the creation of a denser mixed evergreen forest devoid of any deciduous trees.

The best example of mixed evergreen forest in our watershed is the area above 3000 feet elevation between Cobb Mountain and Boggs Mountain State Forest. In addition, some elements of this forest extend downslope on serpentine outcrops. For example, Douglas-fir is found along Capell Creek (mile 43) at only 2000 feet elevation, and at other places in the watershed are sugar pine (Pinus lambertiana), ponderosa pine, coast live oak, canyon live oak, and incense cedar (Calocedrus decurrens).

Another vegetation type rich in tree species is the riparian forest. It, too, occupies relatively little area within our watershed. At low elevation, however, it used to extend thousands of yards away from the widest reaches of creeks and rivers. That forest grew on sediment deposited by annual winter and spring floods—sediment that gradually built up into well-drained natural levees. In such situations, the riparian forest had (and sometimes still does have) a complex, four-layered architecture. The overstory consists of winter-deciduous, broadleaf trees such as Fremont cottonwood (Populus fremontii), white alder (Alnus rhombifolia), black walnut (Juglans hindsii), and valley oak. Beneath is a more scattered layer of shorter trees such as box elder (Acer negundo). A tall shrub layer often includes elderberry (Sambucus mexicana), coyotebrush (Baccharis pilularis), and wild rose (Rosa californica). Finally, the ground layer contains climbing or trailing blackberries, one species of which is native (Rubus ursinus), and a great variety of perennial forbs, grasses, and sedges. Vines stretch and twine from the soil throughout he canopies of shrubs and trees at all heights, adding an element of chaos. The most common vines are wild grape (Vitus californica), poison oak, and Dutchman's pipe (Aristolochia californica).

The forest's architectural complexity and richness of species no doubt explain this vegetation's high diversity of mammalian and avian species. Consequently, as humans converted this forest into orchards, farmland, artificial levees, and residential neighborhoods, many of the resident animal species became rare, threatened, or endangered. Today, only a few percent of the original riparian area remains, and in isolated patches too small to sustain viable populations of many birds and mammals. Within our watershed, the best stands occur along Putah and Cache Creeks below 600 feet elevation, but even here human trampling has reduced and simplified the herb cover to the point that most of it is from introduced European weeds; additionally there are two introduced woody plants, Chinese tree of heaven (Ailanthus altissima) and tamarisk.

In the lowest part of our watershed, such as well within the Sacramento Valley, near the Sacramento River and its riparian forest, used to occur a wetland vegetation type called tule marsh. Tule marsh occupied wide depressions between grassland to the west and riparian forest to the east. Early maps of our watershed show that the western edge of tule marsh ran north and south just east of Davis and Woodland. Tule marsh is dominated by 8-10 foot tall reed-like plants called "tule" or "bulrush," species of the genus Scirpus. They are rooted in soil that remains saturated with water for at least 6 months a year; as a result, the soil is gray from undecomposed organic material and non-oxidized iron. Although tules remain, the extensive marshes they once dominated are almost all gone-drained and replaced with agricultural crops.

At higher elevations and in steeper terrain, the riparian forest has always been restricted to a much narrower corridor, only tens of feet wide. Here the architecture is much simpler and the species richness much lower than in low-elevation riparian forest. The most characteristic riparian tree is big leaf maple. Vines and shrubs are no longer abundant, and the herbaceous understory features grasses, ferns, and mosses. This kind of riparian forest has not been converted to agriculture or suburbs, although it has been subjected to livestock grazing and trampling-activities that sometimes lead to eroded streambanks and higher loads of nutrients and sediment in the water.

Pre-contact Native Californians managed vegetation for thousands of years before the arrival of Euro-Americans. They managed and manipulated portions of it so subtly that Euro-Americans often described California as park-like, orchard-like, or garden-like without realizing that humans before them had made it so. Sometimes the scale of impact was extensive and regional. Susan Bicknell of the USDA Forest Service found grass phytoliths (fossilized plants) in soils that today lie beneath forests. She concluded that the entire northern coastal prairie is anthropogenic and was more extensive in the time before European contact, maintained by frequently set fires. Our inability to acknowledge Native Californians' impact on the land has led to wide gaps in our knowledge about species composition and vegetation in the pre-contact landscape. The first non-indigenous explorers, settlers, and ethnologists did not understand that the keepers of the knowledge were Indian women and allowed their prejudices to blind them to the extensive botanical knowledge available to them. The subsequent extermination, forced emigration, and suppression of Indian cultural activities occurred so rapidly and thoroughly that descendants have been able to retain or relearn only a small fraction of the past knowledge of management practices and their extent. Before we can add the indigenous human factor into our reconstruction of the pre-contact landscape, there must be research in a new branch of science called "ethnoecology," which extrapolates what is known about cultural uses of plants to the impact of such uses on patterns of vegetation at the regional scale.

Two centuries of Euro-American occupation have changed the Putah-Cache bioregion in both obvious and subtle ways. The disappearance of bunchgrass prairie and of riparian forest are obvious changes. The lack of successful blue oak and valley oak seedling establishment during this century is a subtle change, not yet noticeable because the overstory trees have not yet reached their natural life expectancy. But if the trend continues for another century, there will be a dramatic decline in foothill woodland acreage. The relative distribution limits of grassland, chaparral, and woodland have also changed subtly, no doubt because of fire suppression practices. In the absence of frequent fire, chaparral and woodland are capable of invading grassland, even to the extent of completely replacing small patches of grassland.

From a distance, as from a jet flying over the watershed at great elevation and speed, the vegetation might appear continuous and healthy. But on closer inspection the plant mosaic has changed and there has been a loss of species richness. To reclaim the natural vegetation, we need to expand our attempts at restoration and enhancement. This requires an understanding of the plant communities that goes beyond how they currently exist and embraces how they once existed. Already there has been a start in this direction, thanks to members of such organizations as The Nature Conservancy, California Native Plant Society, Society for Environmental Restoration, and the California Native Grass Association. Their activities are all promising, but they need to be expanded and modified to improve their success and their extent. Until recently scholars did not recognize the impact of Native American practices on the land. Other factors help explain the difficulty we face when trying to reconstruct the pre-contact landscape: a lack of expertise and familiarity in the early explorers and settler; rapid changes within floristic and vegetational landscapes that led to a substantially different landscape within only a century; the lack of a single "then," as the Sierra Nevada Ecosystem Project concluded in 1996. However, our best efforts must emphasize the importance of the indigenous peoples' practices. We must open cross-cultural communication with descendants of the original landscape. These people—repositories of knowledge—have often been overlooked or underutilized, due to suspicion about the accuracy of information passed down in an oral tradition. We still have a long way to go to reclaim the natural landscape.

Previous piece
Touring Putah-Cache Country
More like this
Vegetation
Next piece
Geology of Putah-Cache
Next chapter
Davis