David Magney's Flora of Ventura County


Floristics

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Floristics is the study of the distribution of plants over a given geographic area.   This page discusses the floristics (statistics, etc.) of vascular plants found within the geographic boundaries of Ventura County, California.

The flora of Ventura County is comprised of approximately 2,741 native (indigenous) and naturalized (originated elsewhere but now growing "naturally") vascular plant taxa.   Of these, 70 taxa have been observed by are unvouchered, and another 181 taxa expected to occur but for which no vouchers are known, nor have they been observed by the author.   A total of 2,490 taxa have been vouchered or observed within Ventura County.   A total of 2,420 taxa (88.3%) are known to occur in the county that are supported by voucher specimens.   Of the 2,490 taxa known to occur in Ventura County, 1,857 taxa (74.6%) of the flora are native, and another 633 (25.4%) taxa are not native (based on a flora of 2,490 taxa - vouchered and observed/reported).   A small percentage (9.7%) of the flora taxa (242 taxa) are quite common (dominant or common [>30 populations]), on a scale ranging from dominant to rare (dominant, common, occasional, scattered, uncommon, and rare).   About half of the native plant taxa in Ventura County are uncommon (6-10 populations) to rare (1-5 populations).

If the species were distributed evenly over the total area of Ventura County, there would be about 1.4 species per square mile, which seems sparse, but is actually relatively high.   Of course, species are not so distributed.   They are grouped together in a variety of habitats, with some species occupying many different habitats and over half the species occupying only very specific habitat niches that consist of much smaller sized areas, certainly not a square mile.

The known flora is represented by 729 genera in 154 families.   The three largest families are the Asteraceae (Sunflower family), Poaceae (Grass family), and Fabaceae (Pea family), collectively representing 30.3% of the flora.   Just 26 families (16.9%) cumulatively represent a total of 74.7% of the flora (2,490 taxa).

Twenty-six Largest Families
  • Asteraceae: 350 taxa, 14.1%
  • Poaceae: 213 taxa, 8.6%
  • Fabaceae: 192 taxa, 7.7%
  • Polygonaceae: 114 taxa, 4.6%
  • Brassicaceae: 91 taxa, 3.7%
  • Polemoniaceae: 78 taxa, 3.1%
  • Onagraceae: 71 taxa, 2.9%
  • Chenopodiaceae: 66 taxa, 2.7%
  • Cyperaceae: 59 taxa, 2.4%
  • Plantaginaceae: 53 taxa, 2.1%
  • Boraginaceae: 51 taxa, 2.1%
  • Hydrophyllaceae: 50 taxa, 2.0%
  • Rosaceae: 49 taxa, 2.0%
  • Lamiaceae: 48 taxa, 1.9%
  • Apiaceae: 47 taxa, 1.9%
  • Orobanchaceae: 37 taxa, 1.5%
  • Solanaceae: 36 taxa, 1.5%
  • Caryophyllaceae: 35 taxa, 1.4%
  • Ranunculaceae: 34 taxa, 1.4%
  • Convolvulaceae: 31 taxa, 1.2%
  • Euphorbiaceae: 30 taxa, 1.2%
  • Malvaceae: 29 taxa, 1.2%
  • Rhamnaceae: 28 taxa, 1.1%
  • Juncaceae: 25 taxa, 1.0%
  • Phrymaceae: 21 taxa, 0.8%
  • Alliaceae: 21 taxa, 0.8%
The genus with the most species occurring within Ventura County is Eriogonum, of the Polygonaceae (Knotweed family), with 42 species plus 19 varieties.   The second richest genus is Lupinus, of the Fabaceae (Pea family), with 39 taxa (27 species with 4 subspecies and 21 varieties).   Phacelia is the third largest genus, of the Hydrophyllaceae (Waterleaf family), in Ventura County with 37 taxa (27 species with 14 varities).   For comparison, Kern County (211,370 ha), more than four times the size of Ventura County, has 46 taxa of Eriogonum and 32 Lupinus taxa, representing a flora relatively less rich than Ventura County.   Similarly, San Diego County (11,870 ha), over twice as large as Ventura County, has only 28 taxa of both Eriogonum and Lupinus.

Comparing Ventura County to all of California, Ventura County contains 25% of all the native taxa known to occur in the state, yet Ventura County represents only 1% of the state's landmass.   Ventura County ranks 15th in the state with the most number of native taxa, and 5th in total number of taxa (native and naturalized).   (Based on floristic data compiled by the Biota of North America Program [BONAP], April 2011.)

Comparing Ventura County to other states, the county has more native taxa than five states, including: New Hampshire, Alaska, Nebraska, South Dakota, Rhode Island, and North Dakota (listed in order of decreasing species richness), as well as the District of Columbia and the U.S. Virgin Islands.   When you add naturalized nonnative plants to the calculation, Ventura County beats out Vermont, Kansas, Delaware, Iowa, New Hampshire, Nebraska, South Dakota, Alaska, District of Columbia, North Dakota, and the U.S. Virgin Islands in total number of their vascular plant floras (BONAP 2011).

Growth habit/form is another way to analyze the flora.   Growth habit, as defined here, focuses on the plant's life span (indirectly), structure, and woodiness.   It includes annuals, biennials, perennials, shrubs, and trees, which are further subdivided into monocots and dicots, and spore-bearing for the herbaceous species, i.e. annual grasses and graminoids (all monocots), annual ferns and fern allies, annual herbs (dicots), annual vines, biennials, perennials (similarily subdivided), perennial vines, shrubs, and trees.   The annuals complete their life cycles within one year, usually within one or two seasons.   Biennials take about two years to complete their life cycles, and perennials live for three or more years.   Some perennials persist from year to year only in the form of bulbs, tubers, or other type of structure while dying back to the ground every year, while other perennials retain their structure yearround.   Shrubs and trees have well-defined trunks and branches that are woody and live for several to many years, some long-lived species living for over 100 years.   The statistics for these groups are listed below:

  • Annual ferns and fern allies: 1 (1 native; 0 nonnative)
  • Annual grasses and graminoids: 138 (52 native; 68 nonnative)
  • Annual herbs: 890 (629 native; 191 nonnative)
  • Annual vines: 20 (11 native; 7 nonnative)
  • Biennial herbs: 40 (20 native; 20 nonnative)
  • Perennial ferns and fern allies: 33 (33 native; 0 nonnative)
  • Perennial grasses and graminoids: 261 (199 native; 62 nonnative)
  • Perennial herbs: 590 (476 native; 114 nonnative)
  • Perennial vines: 52 (33 native; 19 nonnative)
  • Shrubs: 366 (308 native; 58 nonnative)
  • Trees: 75 (39 native; 36 nonnative)

Clearly the herbaceous plants far outnumber the woody taxa, by 5:1.   The flora contains 1,049 annuals, representing 43.4% of the flora.   The flora contains 976 herbaceous perennials (including the biennial taxa), representing 40.4% of the flora.   Woody plants (trees and shrubs combined - 441 taxa) represent just 18.3% of the flora.

Paleobiogeography

The orogeny of portions of the western Transverse Ranges, such as Dry Lakes Ridge, Nordhoff Ridge, Pine Mountain Ridge, etc., has been suggested as middle Pliocene (Axelrod 1976, 1980) and major deformation as middle Pleistocene (Gross 1958).   Few Pliocene floras have been described near Ventura County; however, inferences can be drawn from the fossil floras located to the north of the county.

A major reason for the floristic diversity and richness, besides the presence of many microhabitats, is the paleobiogeography (botanical history in geologic time) of Ventura County.   Dramatic climatic shifts have occurred locally from the Eocene through the Pleistocene to present.   Each extreme in climate change provided plants from northern (arctic) and southern (subtropical) environments to migrate into Ventura County, only to be stranded (relics) in refugia (remaining suitable habitats).   Many chaparral species, or their ancestors, originated in Central America (primarily Mexico), while most riparian species are holoarctic (circumboreal).   Riparian species have high water requirements, which requires them to remain in close proximity to permenant water sources.

The climate during the late Pliocene was apparently wetter and cooler overall than today in the Ventura County region, as indicated by the presence of Coastal Mixed Conifer Forest in the Sonoma floras in the San Francisco Bay region (Axelrod 1976) and the Mt. Reba, Turlock Lake, and Broken Hills floras (Axelrod 1980).   With the Coast Ranges (including western Transverse Ranges) not appreciably elevated during that time, the marine influence extended inland to the Sierra Nevada.   Milder winters along the western foothills of the southern Sierra Nevada were the result of this marine influence, as were higher water temperatures, about 2F higher than today (Kern 1973, Axelrod 1980).   Summer rain was apparently still prevalent during Pliocene time.   This mild climatic regime allowed Ceanothus spinosus, Karwinskia, Lyonothamnus, Malosma laurina, Persea, Robinia, Sapindus, and others to survive as far north as the San Francisco Bay region.   These plants are now restricted to coastal and insular southern California, the Southwest, or Mexico.   Of these, only Ceanothus spinosus and Malosma laurina are found in Ventura County today, and with Lyonothamnus found only on the Channel Islands (including Anacapa Islands).   The mountains of Ventura County still had not risen out of the sea, at least not appreciably.   Terrain was still low, permitting a more even distribution of precipitation.   Drought stress was lower than at present due to the cooler summers, allowing migration of some species into areas of lower precipitation than would presently support them.   Axelrod (1980) proposes that many of the taxa from various vegetation zones at this time intermingled to a greater degree than they do under the current Mediterranean climate.

Fossil evidence of the Mt. Reba, Broken Hills, Turlock Lake, Oakdale, and Mulholland floras of central California supports the idea that the topography of southern and central California was not substantially elevated until the Pleistocene.   The Mt. Reba flora (7 million years BP), today at 2,675 m (8,700 ft.) in Alpine County, apparently was growing under a mean annual temperature of 56F and at a maximum elevation of 923 m (3,000 ft.).   This is based on inferences from modern relations of vegetation and climate in the Sierra Nevada.   The flora is a combination of taxa similar to those of modern Douglas-fir and Mixed Evergreen Forests in the 615-923 m (2,000-3,000 ft.) foothill belt of the Sierra Nevada.   Elements of this flora found on Dry Lakes Ridge and Pine Mountain Ridge today include Pinus ponderosa, Quercus chrysolepis, and Salix lasiolepis.   The estimated difference in the mean annual temperature of 22F between the fossil site and that now at Mt. Reba implies that the area has been elevated about 1,846 m (6,000 ft.) in the last 5-7 million years.

The Broken Hills flora (Axelrod 1980), from approximately 5 million years BP, near Kettleman City in the San Joaquin Valley supported Oak Woodland vegetation like that now found on the interior slopes of the Santa Lucia Mountains.   The mild, warm climate of the fossil site allowed Magnolia, Persea, Populus, Sapindus, and Ulmus to grow.   Of these, only Populus balsamifera ssp/var. trichocarpa is found today in Ventura County.   The fossil area is now at a height of 220 m (715 ft.) above sea level, which indicates the relief of this locality has changed very little in the past 5+ million years and supports semi-desert vegetation.

The Turlock Lake flora (Axelrod 1980), now at an elevation of 74 m (240 ft.) and dated at 4.5 million years BP, is located on the lower foothills of the Sierra Nevada, east of Fresno.   The fossil flora contained species now in the Oak Woodland belt, close to Mixed Evergreen Forest, apparently indicating a slight rise in precipitation due possibly to the build-up in the icecaps in the boreal region.   The increase is suggested by the comparisons with the older, more xeric, Oakdale flora (6 million years BP) located a few miles northwest of the Turlock Lake flora (Axelrod 1944a).

Chaparral taxa became most diverse during the Middle Hemphillian (=late Miocene time, 6 million years BP), which was the driest part of the Tertiary Epochs in this region.   Chaparral components migrated in large numbers northward into the San Francisco Bay Area at this time, as shown by the Mulholland flora (Axelrod 1944b, 1980).   Axelrod's (1980) comparison of the Hemphillian floras of central California suggested a distributional gradient of the vegetation from Closed-Cone Forest along the coastal strip that was replaced inland by (Live) Oak Woodland, and by Broad-leaved Evergreen (Sclerophyll) Forest on higher, cooler levels of the coastal zone.   Oak Woodland/Cismontane Native Grasslands occupied areas of the Central Valley north of the marine embayment, up the Sierra piedmont.   Between 307-461 m (1,000-1,500 ft.), broad-leaved sclerophyll vegetation dominated the landscape, intermingling with Lower Montane Coniferous Forests near 769-923 m (2,500-3,000 ft.), which extended eastward over the Sierran summit divide.   Oak-Juniper Woodland took over in the lowlands in the lee of the Sierra Nevada.   This migration episode brought components of several vegetation types to lower elevations than occurs today and, therefore, allowed various units to occupy areas such as Dry Lakes Ridge that include Hemphilian chaparral, coniferous forest, oak woodland, etc.

With the onset of glaciation during the Pleistocene Epoch, accompanied by the rapid orogeny of the Sierra Nevada and the Coast, Transverse, and Peninsular Ranges, many shifts in the florule units occurred relatively rapidly.   The late Wisconsin climate was wetter and cooler than at present or during the Upper Pliocene.   The presence of Pseudotsuga and/or Sequoia in the Carpinteria (Chaney and Mason 1934), Tomales (Mason 1934), and Willow Creek (Santa Cruz Island) (Chaney 1934) floras indicates a southward shift of these coastal forests 240-320 km (150-200 miles) south of their Pliocene and present distribution (Axelrod 1981).

The Carpinteria flora (Chaney and Mason 1934) contains many elements of the present Closed-Cone Pine Forest that occurs 320 km (200 miles) farther north on the Monterey Peninsula.   Some of the taxa contained in the flora are Cupressus goveniana, Juniperus californica, Myrica californica, Pinus muricata, P. radiata, P. sabiniana, Pseudotsuga menziesii, and Ceanothus thyrsiflorus, none of which now occurs as far south as Carpinteria.   Of these, only Juniperus californica is found naturally in Ventura County today.

The Willow Creek or Santa Cruz Island flora (Chaney 1934), dated at 14,000 years BP, still contained Pseudotsuga menziesii, Pinus remorata, Cupressus goveniana, and others, indicating the continued effects of glaciation to the northeast.   A drying trend was indicated in the San Bruno flora of the San Francisco Bay Area at this time (Potbury 1934, Axelrod 1981).

Woodrat middens from the Mojave Desert (Wells and Berger 1967) indicate a downward shift of Pinyon-Juniper Woodland by as much as 553 m (1,800 ft.) during Pleistocene glaciation.   Radiocarbon dates of 7,400 to 19,500 and 40,000 years BP firmly affix ages of the middens.

It can be reasonably assumed from the above evidence that the Transverse Ranges, which were mountainous by Pleistocene time, were invaded by Mixed Conifer Forests that periodically extended to lower elevations (around 923 m [3,000 ft.]) than equivalents are found today.

During the Holocene, the climate became warmer and drier, peaking in the Xerothermic (=Hypsithermal) between 8,000-4,000 years BP (Johnson 1977, Axelrod 1981).   This is supported by Heusser (1978) in her analysis of 12,000 years of pollen deposited in the Santa Barbara Basin.   The pollen record indicated a decrease in conifers and an increase in Quercus and more xeric taxa with time.   The coniferous forests, which had previously migrated southward, now retreated northward, leaving disjunct occurrences in the more favorable localities such as the Purisima Hills for Pseudotsuga menziesii (Smith 1976) and Dry Lakes Ridge for Pinus ponderosa (Magney 1986), and Santa Paula Peak for P. lambertiana.   Xeric taxa moved northward and coastward during this time only to be restricted to the driest sites as the post-Xerothermic climate became moister.   P. ponderosa and P. lambertiana migrated upslope when possible, and became relictual (isolated) at a few suitable sites.   The modern result is a highly complex flora in Ventura County, formed under varying climatic and topographic conditions with contributions from several earlier floras.

It seems most probable that Pinus ponderosa and its associates on Dry Lakes Ridge are relicts of the Pleistocene (Magney 1986).   Although climatic conditions today may be favorable for a more widespread distribution, competition with chaparral species probably keeps from expanding to poorer sites.

Similarly, the occurrence of Artemisia tridentata and associates is probably relictual, persisting either from the Xerothermic period or from a dry phase of the Pleistocene, as described above.

References Cited
  • Axelrod, D.   1944a.   The Oakdale Flora.   Carnegie Institute of Washington Publication 553:147-166.
  • Axelrod, D.   1944b.   The Mulholland Flora.   Carnegie Institute of Washington Publication 553.
  • Axelrod, D.   1976.   History of the Coniferous Forest, California and Nevada.   University of California Publications in Botany 70.
  • Axelrod, D.   1980.   Contributions to the Neogene Paleobotany of Central California.   University of California Publications in Geological Sciences 121.
  • Axelrod, D.   1981.   Holocene Climatic Changes in Relation to Vegetation Disjunction and Speciation.   American Naturalist 117:847-870.
  • Chaney, R.W.   1934.   A Pleistocene Flora from Santa Cruz Island, California.   Carnegie Institute of Washington Publication 425.
  • Chaney, R.W., and H.L. Mason.   1934.   A Pleistocene Flora from the Asphalt Deposits at Carpenteria, California.   Carnegie Institute of Washington Publication 415.
  • Gross, D.J.   1958.   Geology of Ortega Area, Ventura Co.   University of California Publications in Geological Sciences 4.
  • Heusser, L.   1978.   Pollen in Santa Barbara Basin, California: a 12,000 Yr. Record.   Geological Society of America Bulletin 89.
  • Johnson, D.L.   1977.   The Late Quaternary Climate of Coastal California: Evidence for an Ice Age Refugium.   University of Washington Publications, Seattle, Washington.
  • Kern, J.   1973.   Early Pliocene Marine Climate and Environment of the Eastern Ventura Basin, Southern California.   University of California Publications in Geological Sciences 96.
  • Magney, D.L.   1986.   A Flora of Dry Lakes Ridge, Ventura County, California.   (Publication Number 5.)   The Herbarium, Department of Biological Sciences, University of California, Santa Barbara.
  • Mason, H.L.   1934.   Pleistocene Flora of the Tomales Region.   Carnegie Institute of Washington Publication 415.
  • Potbury, S.   1934.   A Pleistocene Flora from San Bruno, San Mateo County, California.   Carnegie Institute of Washington Publication 415.
  • Wells, P., and R. Berger.   1967.   Late Pleistocene History of Conifer Woodland in the Mojave Desert.   Science 195:1640-1647.



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This page was created on 20 December 2008 and last updated 4 January 2017.