The McLaughlin Reserve lies within the California Floristic Province, and has a Mediterranean climate with hot dry summers from April to October and cool wet winters from November to March. Summer temperatures may reach as high as 40°C (104°F), while winter temperatures occasionally fall below freezing. Mean annual precipitation is roughly 75 cm (30 inches). Variability is the rule in Mediterranean climates, and annual variation in the amount and timing of rainfall and temperature translates into large differences in the composition of the springtime annual flora.
The reserve supports a striking mosaic of plant communities thanks to its rich variety of geologic substrates. Serpentine plant communities include mixed serpentine chaparral, cypress chaparral, serpentine grasslands, and serpentine seeps and riparian habitats. Non-serpentine plant communities include blue oak woodlands, annual grassland, chamise chaparral, mixed chaparral, and riparian woodlands. Both substrates support streams with rich riparian communities. There are human-created plant communities on the sites disturbed and reclaimed by Homestake. This chapter describes these communities, their major species, and the environmental factors that govern their distribution in the landscape. Click Here for a full plant species list.
The McLaughlin Reserve's abundance of serpentine and related rocks is responsible for its significant botanical features. Plants on serpentine in California have been the subject of innumerable studies and an entire book by botanist Arthur Kruckeberg.12 As described in the geology chapter, most soils derived from serpentinite tend to be highly infertile because of their extremely high levels of magnesium, chromium and nickel, low concentrations of nutrients such as calcium and nitrogen, and low water-holding capacity. Serpentine areas can generally be distinguished by their gray-green or reddish rocky soils and shrubby or stunted vegetation with small leathery leaves.
(This chapter follows the practice of many botanists in using the term "serpentine" broadly to include related ophiolitic rocks such as partly serpentinized peridotite, gabbro, and basaltic greenstone. Soils derived from all of these rocks share low calcium to magnesium ratios and unusual floras. The variation in "serpentine" plant communities caused by the differences between these materials has yet to be well explored.)
Plant responses to serpentine have been categorized by Kruckeberg as avoidance, indifference, and endemism. Avoiders include such taxa as live and blue oaks (Quercus wislizenii and Q. douglassi) that cannot grow on serpentine. Most exotic species in California are also serpentine avoiders, and therefore the flora on serpentine is as pristine as any that can be found in the state. Indifferent taxa, those that grow both on and off serpentine, include toyon (Heteromeles arbutifolia), California bay (Umbellularia californica), chamise (Adenostoma fasciculatum), and gray pine (Pinus sabiniana). (However, large pure stands of chamise are typically found on sandstone, not on serpentine.) Studies of a number of supposedly "indifferent" species have shown that populations on serpentine are actually serpentine-tolerant races, or ecotypes.
Serpentine endemism, or restriction to serpentine, has long fascinated evolutionary biologists. Two evolutionary pathways are thought to have produced serpentine endemics. So-called paleoendemics are widespread species such as leather oak (Quercus durata) and MacNab's cypress (Cupressus macnabiana), found on serpentines throughout California. These are believed to have descended from ancestral species that grew in many different habitats, until climate change caused the nonserpentine populations to become extinct. In contrast, neoendemics or "insular" taxa are species with ranges often as small as a single county or less (e.g. several Ceanothus and Arctostaphylos spp.), which are believed to have arisen from ancestors on nearby nonserpentine soils.
Experiments show that serpentine endemics will grow in non-serpentine soils if given some care, and often grow better on nonserpentine soil when grown alone. This suggests that competition with other species is the reason for their restriction to serpentine. Perhaps not surprisingly, then, the degree of endemism varies from species found 100% on serpentine to ones that are sometimes found on other rocky substrates such as volcanic outcrops. An even lesser degree of restriction is shown by so-called indicator taxa, which are restricted to serpentine only in parts of their geographic ranges (e.g. gray pine, Pinus sabiniana, also known as ghost or foothill pine). Kruckeberg estimates there are 215 true endemics and 221 serpentine indicators in California, making this one of the world's richest serpentine floras. Within the state, the North Coast Ranges are especially rich for serpentine plants.
Plants on serpentine have already provided abundant research opportunities at the reserve. For example, one recent study used serpentine outcrops as a natural model system for examining the effects of habitat isolation, and found that the diversity of endemic plants was lower on small isolated outcrops than within extensive areas of serpentine.7,8 Experimental work revealed that some serpentine endemic species suffered reduced reproductive success on small outcrops, while others were absent because of a lack of suitable habitats within serpentine outcrops.13,14 Several studies on the persistence of rare plants have taken place on and around the reserve.6,10
Mixed serpentine chaparral, found on the thin rocky soils of the Henneke series, is dominated by evergreen shrubs such as leather oak (Quercus durata), white-leafed manzanita (Arctostaphylos viscida), musk brush (Ceanothus jepsonii var. albiflorus), silk-tassel bush (Garrya congdonii) and fremontia (Fremontodendron californicum). Scattered among these endemics or near-endemics are "indifferent" species such as gray pine, toyon, bay laurel and to some extent chamise. Understory herbs, which are typically sparse, may include oniongrass (Melica californica), squirreltail (Elymus elymoides), jewelflowers (Streptanthus spp.), dwarf wild flax (Hesperolinon spp.), coast range morning glory (Calystegia collina), sickle-leaved onion (Allium falcifolium), Woolly paintbrush (Castelleja foliolosa) and many others.
Almost 20% of the reserve consists of serpentine mixed chaparral. A good place to examine this community is on Research Hill. Much serpentine mixed chaparral on the Knoxville BLM lands burned in the fire of October 1999.
McNab's cypress (Cupressus macnabiana) may be found within mixed serpentine chaparral, or may form nearly pure stands. Sargent's cypress (Cupressus sargentii) is found primarily along serpentine riparian areas, but extensive stands may also occur on hillsides (these are more common in the Knoxville BLM lands than on the reserve). Large stands of either species are sometimes referred to as "northern interior cypress forest", but we refer to them as "cypress chaparral" because of their ecological affinities. The two species can be distinguished by the short, wide stature and light green leaves of McNab's cypress, in contrast to the darker, taller, and more treelike Sargent's cypress. The cones of McNab's cypress also have conspicuous bumps on their scales.
The transition between mixed serpentine chaparral and cypress chaparral may relate to subtle differences in soil chemistry and texture within Henneke-series soils. For example, Sargent's cypress is found on soils with especially low calcium levels and high water-holding capacity.11 Both cypress species are often found in even-aged stands whose age reflects the time since the last fire. A study in the Knoxville BLM lands found most Sargent's cypress stands to be 80-120 years old.11
Cypress chaparral is the least diverse of the Reserve's plant communities. Understory herbs include slender bird's beak (Cordylanthus tenius ssp. brunneus), phlox-leafed bedstraw (Galium andrewsii), dwarf flax (Hesperolinon disjunctum), Indian warrior (Pedicularis densiflora), and California milkwort (Polygala californica).
Cypress chaparral forms 7% of the reserve. A stand of McNab's cypress can be seen on the west side of Research Hill, and both species are abundant on the BLM land along the Knoxville-Devilhead Road (where much cypress chaparral burned in October 1999).
Soils formed partly or entirely from serpentine or related ophiolitic rocks, classified in the Henneke, Montara and Okiota soil series, are nutrient-poor and have a low calcium to magnesium ratio. These harsh soils typically support grasslands with a high complement of native species. Herbs such as clarkia (Clarkia purpurea and C. gracilis), birds-eye gilia (Gilia tricolor), goldfields (Lasthenia californica) and mariposa lily (Calochortus luteus, C. superbus and C. vestae), may form spectacular displays in meadows along Morgan Valley Road and on Little Blue Ridge. There are also native perennial bunchgrasses that have become rare elsewhere in California, such as purple needlegrass (Nassella pulchra), squirreltail (Elymus elymoides), bluegrass (Poa secunda), and onion grass (Melica spp.).
These "serpentine grasslands" tend to be substantially less invaded by exotic annual grasses and forbs than nonserpentine grasslands. One study at the reserve estimated the proportion of native species as 80% in serpentine versus 40% in nonserpentine grasslands; within the serpentine grasslands, the proportion of native species increased as the calcium to magnesium ratio declined.9 These meadows are truly special places in a California landscape in which other grasslands have been almost entirely taken over by Mediterranean species.
Springs and streams in serpentine areas often provide flows that last into late summer, in an otherwise summer-dry environment. This combination of serpentine soils and summer water forms the habitat for a diverse array of late-flowering plants, many of which are localized endemics. Five serpentine seep specialists were rare enough to be included in D'Appolonia's pre-mine environmental survey: serpentine sunflower (Helianthus exilis), Cleveland's butterweed (Senecio clevelandii), Cleveland's milkvetch (Astragalus clevelandii), swamp larkspur (Delphinium uliginosum), and bare monkey flower (Mimulus nudatus).
Along larger streams on serpentine, the riparian vegetation includes Sargent's cypress (Cupressus sargentii) and Brewer's willow (Salix breweri) and azalea (Rhododendron occidentale). A stand of Sargent's cypress can be found along the old county road about 100 meters west of the Core Shed. Serpentine riparian habitat may be seen along Hunting Creek south of the county line, as well as around the Hunting Creek campground on the Knoxville BLM land. However, the banksof Hunting Creek north of the county line are dominated by woody species typical of non-serpentine riparian zones, such as valley oaks, probably because of the admixture of non-serpentine and serpentine sediments.
The non-serpentine plant communities of the reserve have been the subject of relatively little research as yet. Blue oak woodlands and annual grasslands tend to occur on the mudstone and clay soils of the Knoxville formation, while chamise chaparral and mixed chaparral occur on excessively drained, nutrient-poor soils derived from sandstones of the Great Valley Sequence (see geology). Fire ecology is one promising future area for research; most of the 720 hectares of the reserve that burned in October 1999 consisted of blue oak woodland and non-serpentine chaparral.
The reserve is home to three deciduous oaks and four evergreen oaks. The deciduous species include blue oaks (Quercus douglassi), which have shallowly lobed, blue-gray leaves and scaly light gray bark; black oaks (Quercus kelloggi), with deeply furrowed, dark gray-brown bark and very large, deeply lobed and toothed leaves; and valley oaks, which can be very large in stature and have small, dark green, deeply lobed leaves. Evergreen species include canyon live oak (Quercus chrysolepis), leather oak (Quercus durata), scrub oak (Quercus berberidifolia) and interior live oak (Quercus wizlizenii), which are described in other sections.
Blue oak woodland covers slightly over 20% of the reserve, forming a partial canopy on the fine-textured soils of the Knoxville formation, especially on cool north-facing slopes. Large stands are found on the west edge of Davis Creek Reservoir and in Clover Valley. Blue oaks may co-occur with gray pines, with California black oaks in cooler and wetter sites, and with valley oaks on streambanks. On drier and gentler slopes, oak woodlands may grade into savannas. The October 1999 fire burned Clover Valley, but appears to have killed few oaks.
Soils are especially fertile beneath oak canopies, where litter and leachate form nutrient-rich islands. This "crown effect"can last for decades after trees die or are removed. The understory in drier sites is dominated by exotic annual grasses such as soft chess (Bromus hordeaceus), rip-gut brome (Bromus diandrus) and medusa head (Taeniatherum caput-medusae). On cooler slopes and under dense oak cover there are more native grasses, and forbs such as serrated onion (Allium serratum), four-spotted godetia (Clarkia purpurea ssp. quadrivulnera), bicolored linanthus (Linanthus bicolor), downy navarretia (Navarretia pubescens), and buttercup (Ranunculus californicus).
Blue oaks are believed to be in danger of disappearing from California's hillsides. In many areas, blue oak regeneration appears to have all but ceased during the past century. Experiments show that oak seedlings and saplings experience enormous depredation by rodents, deer and livestock. Competition for water and light between oak seedlings and exotic annual grasses may also play a role. Researchers are continuing to study the decline in oak regeneration and to search for effective restoration techniques.
Grasslands on non-serpentine soils can be seen in valley floors along parts of Davis, Knoxville, and Hunting Creeks. They are dominated by Mediterranean annual grasses such as soft chess (Bromus hordeaceus), medusa head (Taeniatherum caput-medusae), oat grass (Avena fatua and Avena barbata), wild rye (Lolium multiflorum), and rattail fescue (Vulpia myuros). Forbs include natives such as annual blue lupine (Lupinus bicolor) and rusty popcorn flower (Plagiobothrys nothofulvus) and aliens such as yellow starthistle (Centaurea solstitialis), filaree (Erodium cicutarium and Erodium botrys), black mustard (Brassica nigra), and bur clover (Medicago polymorpha). In spite of extensive invasions by exotic species, these grasslands are surprisingly diverse; the D'Appolonia survey recorded over 170 species.
In the Morgan Valley and Knoxville area, the replacement of native bunchgrasses by exotic annuals probably began in earnest in the 1860's with the influx of mercury miners and homesteaders. Livestock and crops soon became common on relatively flat sites with favorable soils, and vineyards were even attempted in the southwest portion of the reserve, where relic cultivated grapes (Vitis vinifera) can still be found. Settlers cut down trees on a massive scale to fuel the mercury furnaces. In the twentieth century, as mining and farming subsided, livestock grazing became the primary use of the grasslands. All of these disturbances undoubtedly speeded the takeover of the grasslands by exotic plants.
It is important to remember that in many areas of the reserve, especially grasslands, vegetation patterns still reflect long-ago land uses. For example, the large flat meadow between the Homestake entrance and the Core Shed was once a hayfield, and the lines of old irrigation ditches can still be seen from the air. Typically for California grasslands, this meadow did not revert to native species after cultivation ceased. As another example, it is unknown how much the distribution of grassland versus blue oak woodland was affected by woodcutting in the past century.
Chamise (Adenostoma fasciculatum) forms nearly pure stands on well-drained sandstone and gabbro soils on south and west-facing slopes and ridgetops. Chamise chaparral covers about 7% of the reserve. The fire of October 1999 burned vast areas of chamise chaparral to the south and east of the reserve.
Chamise can be identified by its 1 to 3 meter tall, many-branched canopy, small white flowers, and stiff needle-like leaves that turn burnt orange in fall. The lack of herbs in its understory may be due not only to the dense canopy cover, but also to allelopathic chemicals that accumulate in the soil from fallen chamise leaves. Chamise communities are fire-dependent, with typical fire return intervals of 15-20 years; shrubs quickly resprout from their root crowns and from their seeds. For several years after a fire, the pulse of nutrients and light plus the depletion of allelopathic chemicals from the soil allow the establishment of fire-following species including yerba santa (Eriodictyon californicum), common rush-rose (Helianthemum scoparium var. vulgare), pitcher sage (Lepechinia calycina), and deerweed (Lotus scoparius).
California mixed chaparral occurs in slightly moister habitats than those dominated by chamise chaparral, but is still found on dry, rocky slopes and shallow sandstone soils. About 7% of the reserve supports this community. A large area of California mixed chaparral, including much that burned in October 1999, is found on the north and east sides of Davis Creek Reservoir.
This community is characterized by a continuous canopy 2-3 meters in height and a sparse ground cover. Shrubs are mostly evergreen with small thick leaves. Dominant shrubs are common buckbrush (Ceanothus cuneatus), toyon (Heteromeles arbutifolia), chaparral pea (Pickeringia montana), scrub oak (Quercus berberidifolia) and a shrub form of interior live oak (Quercus wislizenii var. frutecens). (The two oaks can be distinguished by their furrowed and checkered versus smooth bark, respectively.) These shrubs are well adapted to fire and drought, with most species able to quickly resprout from their crowns following fire.
Riparian zones at the reserve can be found along Davis, Knoxville, and Hunting Creeks. They support broad-leaved deciduous trees including valley oak (Quercus lobata), Fremont's cottonwood (Populus fremontii), Oregon ash (Fraxinus latifolia), California buckeye (Aesculus californica), redbud (Cercis occidentalis), and red willow (Salix laevigata). Beneath the canopy are shrubs such as squaw bush (Rhus trilobata) and California wild rose (Rosa californica),. Herbs may include dogbane (Apocynum cannabinum), mugwort (Artemisia douglasiana), white sweetclover (Melilotus indica), and false lupine (Thermopsis macrophylla).
On the reach of Davis Creek above the reservoir is a strip of riparian forest denoted as "mixed evergreen forest" on the map. This contains several tree species characteristic of cooler and wetter climates, notably canyon live oak (Quercus chrysolepis) and California nutmeg (Torreya californica). Nearby on the western edge of the reservoir are a few yellow pines (Pinus ponderosa). This is an example of a community near its southern range limit, perhaps a remnant of wetter Pleistocene times.
Riparian woodlands at the reserve are important habitats for wildlife, especially songbirds and raptors. Some of the reserve's riparian woodland have been damaged by livestock grazing, which reduces vegetative cover and destabilizes streambanks. Restoration efforts are now underway along the upper reach of Hunting Creek. Another important restoration goal is the removal of tamarisk (Tamarix pentandra), an aggressive invader that has taken over some heavily grazed areas along Knoxville Creek.
About 20% of the reserve consists of human-created plant communities on Homestake's revegetated waste rock piles, dam abutments, roadsides, and other disturbed areas. Reclamation began in 1985 and continues today, following a plan that was approved by the three counties. The species used for reclamation are non-natives chosen because they are readily available, fast-growing, and not considered to be invasive. In both serpentine and non-serpentine habitats, the seed mix includes Blando brome (Bromus hordeaceus), Wilton rose clover (Trifolium hirtum), tall fescue (Festuca arundinaceae), and pubescent wheatgrass (Agropyron tricophorum). Red brome (Bromus rubens) is planted only on serpentine substrates, while orchard grass (Dactylis glomerata) and annual ryegrass (Lolium multiflorum) are used only on non-serpentine substrates.
Eventual colonization of these areas by native plants and animals will be an interesting subject for research. The results of the annual monitoring of the reclaimed areas can be found in the Annual Report. In future phases of reclamation, the aim will be to create structurally diverse plant communities with native trees and shrubs. An odd-looking "community" can be seen on the west side of Berryessa-Knoxville Road, south of the truck shop entrance, where many woody species have been planted in a test plot.