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Vernal pools form in small depressions underlain by a subsurface layer that impedes drainage, so that water stands sufficiently long during the winter rainy season to prohibit terrestrial vegetation from developing. Many vernal pool plants flower in showy concentric rings as pools dry out. Standing water during the winter, followed by total desiccation during the dry Mediterranean summer, are essential for the development of vernal pool vegetation and for exclusion of strictly aquatic or strictly dryland species (Holland 1976).
Soil texture, moisture holding capacity, salinity and microrelief are important determinants of vernal pool vegetation (Holland and Jain 1988). Many vernal pool soils are very old (600,000-4,000,000 years BP) (Harden 1987) and prolonged profile development has led to formation of aquatards (commonly called duripans, hardpans or claypans) which create locally perched water-tables (Holland and Jain 1988). As the water evaporates, ionic concentrations rise in the pools and in surrounding soils, resulting in saline, alkaline or acidic conditions (Holland and Jain 1988).
Conditions for vernal pool formation occur in much of the CFP where various impediments to rapid drainage allow pooling and subsequent evaporation. Holland (1978) estimates that 16,800 kmē of vernal pool habitat originally occurred in the Great or Central Valley, which had the vast majority of such habitat within the CFP. Vernal pool vegetation is most developed on terrace soils along the east side of the Great Valley at the base of the Sierra foothills (Hoover 1937), but there were once abundant pools on terraces along the western valley margin and on the valley floor. Old marine terraces in San Diego County and adjacent Baja California support pools with endemic plant species (Holland 1976) (Map 6) Well developed pools also occur on soils derived from volcanic mudflows in the northern Great Valley, on lava-capped mesas in the foothills of the northern Sierra Nevada in Butte County, on the Modoc Plateau, and in the Peninsular ranges in western Riverside County (Holland 1976). Pools are rarer in Sierran foothill valleys and in the Coast Ranges from San Luis Obispo County north to Lake County.
Ephemeral pools supporting endemic-rich floras are found in other parts of the world, especially where a Mediterranean climate of winter rains and summer drought occurs (Zedler 1987). Perhaps the closest analogues to California's pools are found in Spain, western Australia, western South America and South Africa. Genera such as Eryngium, Lythrum, Isoetes, Myosurus and Callitriche are found in common between vernal pools in Spain and California (Zedler 1987), but these plants are cosmopolitan and broadly aquatic rather than limited to vernal pools. Stronger floristic affinities occur with vernal pools in Chile and Argentina, and involve the amphitropic distributions of Lasthenia, Downingia, Navarretia and Legenere, all genera more distinctively associated with California vernal pools.
The characteristic feature of ephemeral pools is a unique vegetation type distinguished by a preponderance of showy annuals that flower in decreasing concentric rings as the pools dry (Holland and Jain 1988). Growth form ranges from diminutive plants 3-5 cm tall in Myosurus, Crassula (=Tillaea) and Psilocarphus to 30-50 cm tall Eryngium, Navarretia and Pogogyne (Jain 1976).
Vernal pools are most common and conspicuous in open grassland, but occur under a variety of tree canopies ranging from Quercus lobata (valley oak) or Q. douglasii (blue oak) savannah to Pinus ponderosa (yellow pine) forests (Holland and Jain 1988). Pools are also known from understorey herbaceous cover in the Coast, Peninsular and Transverse ranges, from sagebrush steppe (R.F. Holland, pers. comm.) and as vernal ponds in dune slacks (Holland and Jain 1988).
Vernal pools vary in size and have been given many names, but the smallest are often called vernal swales and retain standing water only briefly or not at all; intermediate depressions are typically called vernal pools, and the largest (over 20 ha) are frequently referred to as vernal lakes (Crampton 1976; Holland 1976). Taxa found in the bottom of smaller, shallower pools often occur along the margins of deeper pools (Holland 1976), whereas other species, particularly members of the unusual grass tribe Orcuttieae (Griggs 1976; Stone et al. 1988), are most typically found only in the deepest pools or vernal lakes.
Over 200 species of vascular plants (Holland 1976) in about 56 genera (Jain 1976) are known from vernal pools in California. About half of these plants are known primarily from vernal pools (Holland and Jain 1988) and more than 70% are native annuals that reproduce by seed, mostly through selfing (Zedler 1987). Introduced annuals comprise less than 7% of the species, and most of these are from South America (Holland and Jain 1988). The only perennial genera that typically occur in vernal pools in California are Isoetes, Marsilea, Eleocharis, Eryngium and Lythrum (Jain 1976). Ferns are uncommon and represent less than 3% of the species total. Most sizeable pools contain 15-25 vascular plant species (Holland 1976), but never the same ones, so no two vernal pools are alike in species composition. A list of some important genera in vernal pools is given in Table 19.
Vernal pools contain a remarkable part of the Californian flora that has resisted the invasion of primarily Mediterranean exotic plants better than most other Californian vegetation types (Holland 1976). In fact, 91% of vascular plants currently found in California vernal pools are natives (Holland 1976). This contrasts favourably with surrounding grasslands which frequently comprise 35-40% exotic annual grass and herbaceous species, in which 99% of annual biomass is contributed by exotics (R.F. Holland, pers. comm.). Undoubtedly the sharp contrasts in soil hydrology caused by cycles of inundation and desiccation promote specific adaptations that are lacking in most exotic plants, hence their inability to survive in vernal pools. Vernal pools are thus "too wet for exotics, too dry for aquatics" (R.F. Holland, pers. comm.).
The families Asteraceae, Poaceae, Campanulaceae (Lobelioideae), Limnanthaceae, Scrophulariaceae and Marsileaceae are well-represented in California's vernal pool flora (Jain 1976). Endemism is exceedingly high among California vernal pool flora; about 69% occur only in the CFP and 55% are found only in California itself (Holland 1976). For example, eight of nine members of the grass tribe Orcuttieae are restricted to vernal pools within the CFP; all are rare and endangered (Skinner and Pavlik 1994). Most species of Downingia and Limnanthes are endemic to vernal pools within the CFP.
Centres of endemism within California's vernal pool flora lie in Sonoma County and adjacent Lake and Mendocino counties. They support several endemic and endangered taxa, including Blennosperma bakeri, Eryngium constancei, Limnanthes bakeri, L. vinculans, Lasthenia burkei, Navarretia leucocephala subsp. pauciflora, N. leucocephala subsp. pleiantha, Parvisedum leiocarpum and Pogogyne douglasii subsp. parviflora. The Great Valley is isolated by the Coast Ranges and contains the bulk of vernal pool endemics, including most members of the grass tribe Orcuttieae, and endemics in Chamaesyce, Downingia, Eryngium, Gratiola, Juncus, Limnanthes, Navarretia and Orthocarpus, among others. San Diego Mesa vernal pools and adjacent Baja California are home to endangered endemics such as Eryngium aristulatum var. parishii, Muilla clevelandii, Navarretia fossalis, Orcuttia californica, Pogogyne abramsii and P. nudiuscula.
California's vernal pool flora is highly endangered: 73 vernal pool taxa, about one-third of the total, are listed in the California Native Plant Society's Inventory of Rare and Endangered Vascular Plants of California (Skinner and Pavlik 1994). Two of these are extinct, 53 are "rare, threatened, or endangered in California, and elsewhere", four are "rare, threatened, or endangered in California, but more common elsewhere", five are poorly known and more information about them is needed, and nine are plants of limited distribution.
Practical uses for vernal pool plants are virtually unknown. Artificial selection of Shippee meadowfoam (Limnanthes floccosa var. californica) oils is in progress; this oil possesses some of the lubricating properties of sperm whale oil and remains liquid at high temperatures (Gentry and Miller 1965). Orcuttia exudates may prove useful for insect pest deterrence; genes for this feature could eventually be transferred to important cereal crops (Griggs 1981). Downingia, Limnanthes and Navarettia are used occasionally as ornamentals, and there is similar potential in other groups, including Sidalcea and several genera of Asteraceae.
Vernal pools are associations of native plants in ecological island habitats that have a high proportion of endemics and native species. Springtime vernal pool floral displays are spectacularly beautiful and stimulate great interest in the preservation of vernal pools, their component species and surrounding habitats. Several vernal pools genera, such as Limnanthes, Blennosperma, Orcuttia and Downingia, have radiated recently, some in concert with specialized or obligate bee pollinators, particularly in the genera Andrena and Panurginus (Andrenidae) (Thorp 1976). Vernal pools also support perhaps 25 endemic invertebrates, including several endangered fairy shrimps (Anostraca: Branchinecta and Linderiella) and various clam shrimps (Conchostraca) and tadpole shrimps (Notostraca) (Martin 1990).
Vernal pool habitat is widely used for cattle and sheep grazing; under less intensive use, monetary yields are about $300/acre/year (R.F. Holland, pers. comm.). This represents tens of millions of dollars per year within the CFP, but precise quantification is difficult. Vernal pools aid flood control and also provide invaluable habitat for migrating and wintering waterfowl, but the monetary values of these, too, are unknown. Vernal pools in San Diego County, the North Bay (Napa and Sonoma counties) and much of the Great Valley occur on extremely valuable real estate which is under relentless development pressure.
More than 90% of vernal pool habitat in California has been destroyed or degraded by agricultural conversion, heavy livestock grazing, unregulated gold mining and urbanization, with lesser impacts from various hydrological alterations, brush clearing for fire control and off-road vehicle activity. As much as 40% of the soils of the Great Valley and associated terraces had the potential to support vernal pools (Holland 1976), and it is thought that vernal pools were once common in most of these areas, being most numerous on younger terrace soils. Agriculture and urban development have eliminated about 95% of these (extrapolated from Holland 1978) and the best remaining pools are now found on higher, older terraces (Holland and Jain 1988).
Extensive vernal pools containing several rare endemic plants (Blennosperma bakeri, Lasthenia burkei, and Limnanthes vinculans) previously occurred in Sonoma County north of San Francisco. The U.S. Fish and Wildlife Service (1990) estimates 90% of the primary habitat for these plants has been altered or destroyed by flood control, urbanization, agricultural development or livestock grazing. In southern California, losses are estimated at 97% for San Diego Mesa vernal pool communities (Bauder 1986; Gustafson 1990).
Surprisingly, light cattle or horse grazing does not appear to be a significant threat, despite trampling (Zedler 1987). Vernal pools also have a surprising ability to recover from partial disruption such as discing (light ploughing) over the course of a few decades, though deep vehicle ruts through pool areas remain unfilled by earth for 10-15 years (Zedler 1987).
Vernal pools are among the most severely threatened vegetation types within the CFP (Wickenheiser 1990). Most of the remaining sites have no legal protection. An integrated conservation strategy for vernal pools will require several different approaches. Many taxa are quite localized geographically and different types of vernal pools occur in different areas, so preserves must be scattered throughout the CFP, that is, from southern Oregon to northern Baja California. Furthermore, the genetic diversity of a single species is exhibited over a large geographical scale and is, in part, a consequence of different environmental conditions in different areas (Griggs 1981). Therefore, preserves should encompass the maximum environmental variation experienced by a given taxon.
Vernal pools are essentially habitat islands within other communities (Jain 1976; Wickenheiser 1990), so island biogeographic theory predicts high rates of extinction in particular pools. The conservation implication of this is that preserves must contain many pools to serve as sources of propagules for colonization. Because proper floristic development hinges on precise hydrologic relationships, surrounding watershed areas for pools need to be preserved along with the pools themselves. Large unfragmented areas preserving adjacent habitats are required to support pollinators, and will incidentally protect many other rare non-pool plant species that occur in the vicinity of vernal pools (Zedler 1987).
Some of the exceedingly localized vernal pool species have already been targeted and protected by conservationists. For example, Tuctoria mucronata is known from only one set of pools in Solano County, but is protected in The Nature Conservancy's (TNC) Jepson Prairie Preserve along with a carabid beetle (Elaphrus viridis) found nowhere else. TNC's Boggs Lake Preserve protects Navarretia leucocephala subsp. pleiantha and Orcuttia tenuis, among other rare plants. On the other hand, Lasthenia conjugens is now known from only two of its seven original counties and most remaining sites are threatened by agricultural or urban development.
Some examples of different types of vernal pools are currently protected. The California Department of Fish and Game (DFG) recently acquired Table Mountain in Butte County, which is one of the finest Northern Basalt Flow vernal pool sites and supports the endangered Juncus leiospermus var. leiospermus. In 1992, the DFG, California Department of Parks and Recreation, U.S. Bureau of Land Management and TNC purchased Big Table Mountain in Fresno County, another important Northern Basalt Flow vernal pool site protecting endangered Castilleja campestris subsp. succulenta. The DFG already owns Loch Lomond in Lake County, a Northern Volcanic Ash Fall vernal pool supporting the narrow endemics Eryngium constancei and Navarretia leucocephala subsp. pleiantha, and also Phoenix Field in Sacramento County, a Northern Hardpan vernal pool site with endangered Orcuttia viscida. Much of the Southern Interior Basalt Flow vernal pool habitat on the Santa Rosa Plateau in western Riverside County is protected by TNC. Several rare plants including three Orcuttieae are protected in the Northern Volcanic Mudflow vernal pools at TNC's Vina Plains Preserve in Butte County. Unfortunately, because of limited resources, management is inadequate at many of these preserves protecting vernal pool species.
Perhaps the greatest preservation priorities for California's remaining vernal pools are Sonoma County's rapidly dwindling Northern vernal pools, the nearly extinct San Diego Mesa vernal pools and, especially, the Northern vernal pools and their habitat within the Great Valley and associated terraces. Protection and regional natural resource planning in these areas is currently promoted by recent formal listings of many vernal pool plants under either the federal Endangered Species Act or its California counterpart. Twenty-nine California vernal pool plants now or will shortly benefit from such protection, including all eight California Orcuttieae and many members of Limnanthes, Navarretia, and Pogogyne.
Artificial creation of new vernal pools to compensate for destruction of existing pools during development is of concern, since there is no evidence that artificial pools retain their vernal pool plants over a long period of time. Frequently, artificial pools are established without regard for essential associated species, such as specialized pollinators. While it appears that artificial pools may initially be able to retain vernal pool plants, visual appearance and functional values, such as food chain support, do not approximate to conditions in naturally occurring pools (Ferren and Gevirtz 1990). Moreover, created pools are often intermixed with naturally occurring pools. This misguided mitigation may engender outbreeding depression (Dole and Sun 1992) or alteration of natural hydrology, and promote subsequent degradation of both natural pools and the landscape.
Map 6. Vernal Pools, California, U.S.A., and Baja California, Mexico (CPD Site NA16g)
Bauder, E.T. (1986). San Diego vernal pools: recent and projected losses; their condition; and threats to their existence. Prepared for Endangered Plant Program, California Department of Fish and Game.
Crampton, B. (1976). A historical perspective on the botany of the vernal pools in California. In Jain, S. (ed.), Vernal pools, their ecology and conservation. Institute of Ecology Publication No. 9: 5-11. (University of California, Berkeley.)
Dole, J. and Sun, M. (1992). Field and genetic survey of the endangered Butte County meadowfoam Limnanthes floccosa subsp. californica (Limnanthaceae). Conservation Biology 6(4): 549-558.
Ferren, W.R. and Gevirtz, E.M. (1990). Restoration and creation of vernal pools: cookbook recipes or complex science. In Ikeda, D.H. and Schlising, R.A. (eds), Vernal pool plants their habitat and biology. Studies from the Herbarium No. 8: 147-178. (California State University, Chico.)
Gentry, H.S. and Miller, R.W. (1965). The search for new industrial crops. IV: prospectus of Limnanthes. Economic Botany 19: 25-32.
Griggs, F.T. (1976). Life history strategies of the genus Orcuttia (Gramineae). In Jain, S. (ed.), Vernal pools, their ecology and conservation. Institute of Ecology Publication No. 9: 57-63. (University of California, Davis.)
Griggs, F.T. (1981). A strategy for the conservation of the genus Orcuttia. In Jain, S.K. and Moyle, P.B. (eds), Vernal pools and intermittent streams. Institute of Ecology Publication No. 28: 255-262. (University of California, Davis.)
Gustafson, S.S. (1990). Ephemeral edens. Pacific Discovery, Spring 1990: 23-32.
Harden, J.W. (1987). Soils developed in granitic alluvium near Merced, California. In Harden, J.W. (ed.), A series of soil chronosequences in the western United States. United States Geological Survey Bulletin 1590-A.
Holland, R.F. (1976). The vegetation of vernal pools: a survey. In Jain, S. (ed.), Vernal pools, their ecology and conservation. Institute of Ecology Publication No. 9: 11-15. (University of California, Davis.)
Holland, R.F. (1978). Geographic and edaphic distribution of vernal pools in the Great Central Valley, California. California Native Plant Society Special Publication No 4. Berkeley, California.
Holland, R.F. and Jain, S.K. (1988). Vernal pools. In Barbour, M.G. and Major, J. (eds), Terrestrial vegetation of California. California Native Plant Society Special Publication No. 9: 515-531. Sacramento.
Hoover, R.F. (1937). Endemism in the flora of the Great Valley of California. Ph.D. dissertation, University of California, Berkeley.
Jain, S. (1976). Some biogeographic aspects of plant communities in vernal pools. In Jain, S. (ed.), Vernal pools, their ecology and conservation. Institute of Ecology Publication No. 9: 15-22. (University of California, Davis.)
Martin, G. (1990). Spring fever. Discover, March 1990: 70-74.
Skinner, M.W. and Pavlik, B.M. (1994). Inventory of rare and endangered vascular plants of California. California Native Plant Society Special Publication No. 1, 5th Edition. Sacramento. 338 pp. + vi.
Stone, R.D., Davilla, W.B., Taylor, D.W., Clifton, G.L. and Stebbins, J.C. (1988). Status survey of the grass tribe Orcuttieae and Chamaesyce hooveri in the Central Valley of California. Prepared for the U.S. Fish and Wildlife Survey, Biosystems Analysis, Santa Cruz.
Thorne, R.F. (1981). Are California's vernal pools unique? In Jain, S.K. and Moyle, P.B. (eds), Vernal pools and intermittent streams. Institute of Ecology Publication No. 28: 1-8. (University of California, Davis.)
Thorp, R.W. (1976). Insect pollination of vernal pool flowers. In Jain, S. (ed.), Vernal pools, their ecology and conservation. Institute of Ecology Publication No. 9: 36-40. (University of California, Davis.)
U.S. Fish and Wildlife Service (1990). Endangered and threatened wildlife and plants; proposed endangered status for the plants Blennosperma bakeri (Baker's sticky seed), Limnanthes vinculans (Sebastopol meadowfoam), and Lasthenia burkei (Burke's goldfields). Federal Register 55(109): 23109-23115.
Wickenheiser, L.P. (1990). Vernal pools springtime treasure of California. Outdoor California, MarchApril: 11-13.
Zedler, P.H. (1987). The ecology of southern California vernal pools: a community profile. U.S. Fish and Wildlife Service Biological Report 85 (7.11).
This Data Sheet was written by Dr Mark W. Skinner (USDA, NRCS, National Plant Data
Center, P.O. Box 74490, Baton Rouge, Louisiana 70874, U.S.A.).
Dr Skinner thanks Ms Roxanne Bittman, Mr Tim Messick and especially Dr Robert F. Holland for their helpful comments.
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