Vascular flora of the Liebre Mountains, western Transverse Ranges, California

The Liebre Mountains form a discrete unit of the Transverse Ranges of southern California. Geographically, the range is transitional to the San Gabriel Mountains, Inner Coast Ranges, Tehachapi Mountains, and Mojave Desert. A total of 1010 vascular plant taxa was recorded from the range, representing 104 families and 400 genera. The ratio of native vs. nonnative elements of the flora is 4:1, similar to that documented in other areas of cismontane southern California. The range is noteworthy for the diversity of Quercus and oak-dominated vegetation. A total of 32 sensitive plant taxa (rare, threatened or endangered) was recorded from the range.


INTRODUCTION
The Transverse Ranges are one of southern California's most prominent physiographic features. In contrast to California's other principal cordillera-the Sierra Nevada, Coast Ranges, and Peninsular Rangeswhich are oriented north-south, the Transverse Ranges trend east-west. East-west trending ranges are uncommon in North America, and geologic mechanisms responsible for the anomalous orientation of California's Transverse Ranges are not yet fully understood (Norris and Webb 1990). Major units of the Transverse Ranges include, from east to west, the Little San Bernardino, San Bernardino, San Gabriel, Santa Monica, Liebre, Santa Susana, Topatopa, Pine Mountain, and Santa Ynez ranges. Technically, this enumeration should include the northern group of Channel Islands, which are geologically an extension of the Santa Monica Mountains. The Transverse Ranges cover a linear distance of nearly 520 km (320 mi) between their western end at Point Arguello near Santa Barbara and their eastern terminus in the Eagle Mountains near Desert Center (Sharp 1972;Norris and Webb 1990). Physical breaks between the component ranges are often obscure and a rather diverse array of names has been applied to various configurations, particularly in the western portion where circumscription of physiographic units is complicated by convergence of the Tehachapi and Inner Coast ranges.
Although the Transverse Ranges border the Los Angeles Basin, California's most densely populated region, and have been the subject of considerable botanical exploration over the years, published floristic accounts of the component ranges are surprisingly scarce. Broad-scale floristic reports for the San Gabriel Mountains include Johnston's (1919) flora of the pine belt and Peirson's (1935) handbook of trees and shrubs. Published documentation of the San Bernardino Mountains is little better, limited to Parish's (1917) enumeration of the pteridophytes and spermatophytes and McBride et al.'s (1975) checklist for the montane coniferous forest. By far the best-documented element of the Transverse Ranges has been the Santa Monica Mountains, which have been comprehensively covered by Raven et al. (1986). While the body of floristic literature for the Transverse Ranges is certainly augmented by more narrowly focused local studies (e.g., Derby andWilson 1978, 1979;Lewis and Gause 1966;Muns 1984Muns , 1985aMuns , 1985bMuns , 1986Muns , 1992Muns , 1994Parish 1890;Sawyer 1987;Swinney 1994) and unpublished technical reports and dissertations (Boyd et al. 1993;Krantz 1994;Mistretta 1995;Robinson 1953;Thome 1971Thome -1973, many segments of the range remain virtual floristic terra incognita. The threat to southern California's native flora from urbanization, agriculture, pollution, habitat fragmentation, and invasive exotic taxa is pervasive and growing. Regions such as the Transverse Ranges contain large tracts of natural habitat that are biologically diverse, relatively intact ecologically, and mostly administered in public trust; these areas are vital for meeting societal goals of preserving California's natural heritage. A critical component of any strategy for managing regional biological diversity is developing a baseline account of the resources being managed. Presently, work is ongoing to provide comprehensive documentation for both the San Gabriel Mountains (0. Mistretta, pers. comm.) and San Bernardino Mountains (A. Sanders, pers. comm.). In this paper, I present a preliminary floristic account of another important segment of the Transverse Ranges, the Liebre Mountains region. The study includes the results fieldwork conducted by myself and/or Timothy S. Ross, with various associates, as well as review of collections housed in the herbarium of Rancho Santa Ana Botanic Garden (RSA-POM) and elsewhere.

Physiography
The Liebre Mountains represent the easternmost end of what is referred to collectively as the Western Transverse Ranges (Hickman 1993), and they occupy a transitional position between the Santa Susana, Topatopa, and Pine Mountain ranges to the west and San Gabriel Range to the east. The northern base of the Liebre Mountains defines the southwestern border of the Mojave Desert. As circumscribed here, the range is a roughly triangular area bounded by the Santa Clara River on the south and southeast, California Aqueduct along the north and northeast, and Interstate 5 along the west (Fig. 1). The study area encompasses approxi-mately 1630 km 2 ( 613 mF), with elevation ranging between 1764 m (5788 ft) on Burnt Peak and 294 m (965 ft) where the Santa Clara River crosses Interstate 5.
Physiography of the Liebre Mountains region is strongly controlled by two of Southern California's major fault systems: the San Andreas on the north and northeast, and the San Gabriel on the west and south (Dibblee 1982). The eastern boundary of the range, and its general separation from the San Gabriel Mountains, is defined by the Soledad Fault. The range can be generally divided into two physiographically and geologically discrete parts; the rugged, mountainous north and northeastern section, and a lower area of rolling hills and small erosional valleys in the west and south (Dibblee 1982).
Portal Ridge (including Ritter Ridge), separates Antelope Valley, the westernmost end of the Mojave Desert, from the rift zone of the San Andreas Fault. The steep escarpments of this narrow, northwest-trending ridge system contrast sharply with the relatively gentle relief along its crest. At its northwest end, the ridge is VOLUME 18, NUMBER 2 The massive San Andreas rift is characterized by a series of deep, elongate valleys separated from each other by low divides. From northwest to southeast these include Oakdale Canyon, Oakgrove Canyon, Pine Canyon, Leona Valley, and Anaverde Valley. The southern edge of the rift zone is marked by another series of steep escarpments comprising the Liebre-Sawmill-Sierra Pelona crest. These relatively narrow, elongate ridges, like Portal Ridge to the north, are characterized by extensive areas of gentle topography across their summits.
To the south of the Liebre-Sawmill-Sierra Pelona crest lies the body of the mountainous portion of the range. Topography is characterized by steep, rugged ridges and narrow, winding canyons. Important topographic features within this area include Del Sur Ridge, Jupiter Mountain, Tule Ridge, Red Mountain, Warm Springs Mountain, Sawtooth Mountain, Burnt Peak, and Red Rock Mountain. A series of subsidiary faults of generally northeast trend divide this block between the San Andreas and San Gabriel fault zones and are mirrored by the principal drainages of the range. These include Soledad, Mint, Bouquet, Elizabeth Lake, and San Francisquito canyons. Much of the western end of the range is drained by tributaries of Castaic Creek, while slopes on the extreme northwestern edge drain into Piru Creek. Ultimately, all drainage from the Liebre-Sawmill-Sierra Pelona crest southward drains to the Santa Clara River, and ultimately, the Pacific Ocean.
In addition to the principal drainages, some of which support year-round surface water, there are several large bodies of water within the range. Most of these represent manmade reservoirs, including Bouquet Reservoir, Castaic Lake, and Pyramid Lake. A large reservoir was once constructed in San Francisquito Canyon, but suffered a catastrophic failure of the earthen dam in 1928 (Sharp 1972). Natural permanent and seasonal lakes are restricted within the range to the valleys within the San Andreas Rift and adjacent Portal Ridge. These are fault sags and include Elizabeth Lake, Munz Lakes, Lake Hughes, and Quail Lake (the latter two now augmented by earthen dams), as well as Tweedy and Gookins lakes on Portal Ridge.

Geology
The Liebre Mountains region is geologically complex (Jennings and Strand 1969). An excellent, detailed geologic overview of the range is provided by Dibblee (1982), and I will present only a brief synopsis here. The Liebre-Sawmill-Sierra Pelona crest and adjacent uplands are eroded largely from pre-Cenozoic basement complex, as are significant portions of Portal Ridge. Liebre Mountain itself is predominantly composed of granitic rocks, while Sawmill Mountain is dominated by gneiss. An extensive area of ancient Pelona schist nearly bisects the range from Sierra Pelona and adjacent Portal Ridge southwestward to San Francisquito Canyon. The lower, hilly regions to the west, south, and southeast of the Liebre-Sawmill-Sierra Pelona crest are characterized by Cenozoic sedimentary and volcanic rocks, these often highly deformed and eroded. The area occupied by these substrates represent two ancient depositional basins, the Ridge Basin along the west, and Soledad Basin along the southeastern edges of the range. These sediments were originally deposited under largely marine conditions. Subsequently, they have been extensively uplifted and deformed, resulting in areas of striking badlands topography, as well as the unusual formations in the Vasquez Rocks area near Agua Dulce.

Climate
The Liebre Mountains region experiences a typical Mediterranean-type climate of warm, dry summers and cool, moist winters. Under this regime, most precipitation falls as rain resulting from Pacific frontal storms during the months of November through March (Fig.  2). Winter snow, although generally light and shortlived, is frequent along the highest ridges of the Lie- Boyd ALISO bre-Sawmill-Sierra Pelona crest. Exceptionally strong, cold storms bring snow to extensive areas above 1000 m, and sometimes even lower. While precipitation patterns are relatively uniform throughout the range, there is considerable variation in average annual precipitation between different sites (Table 1 ). Topography, regional rainshadow effects, marine layer penetration, and cold air drainage all exert their effect on local microclimates, and are reflected in the distribution of various floristic elements and vegetation types.

HISTORY OF BOTANICAL EXPLORATION
Based on herbarium specimens deposited at RSA and elsewhere, I have been able to document from the range, collections made by at least 188 individual primary collectors (excluding associated collectors) covering a span of more than one century . A majority of these are limited, ad hoc efforts, frequently restricted to areas serving as the principal transportation corridors of the time. It appears that the Liebre Mountains was not an area of intense floristic interest to earlier botanists. Aside from the present study's efforts, very few collectors visited the range repeatedly over a series of years. Most notable of those who did include LeRoy Abrams, Elbert Benjarnine, Anstruther Davidson, F. Raymond Fosberg, Ralph Hoffmann, Marcus E. Jones, Philip A. Munz, Frank W. Peirson, Bonnie C. Templeton, Ernest C. Twissel- Elev. m (ft) precip mm (in)  Table 2. A summary of collecting activity by decade, as expressed by the number of specimens collected, and number of primary collectors, is presented in Fig. 3 and 4 respectively. It is readily apparent from these graphs that, exclusive of the present study, greatest interest and activity in the range was during the 1920s and 1930s. Work and travel restrictions during the war years of the 1940s clearly had considerable impact, drastically reducing botanical activity in the range. A slight renewed interest during the 1950s, 1960s, and 1970s, was followed by another decline during the 1980s. Except for the collecting efforts associated with this project, the downward trend would have continued during the present decade. VEGETATION The vegetation of the Liebre Mountains region is a complex mosaic superimposed upon a backdrop of the area's diversity of geologic substrates, topography, and microclimate. The vegetation patterns of the range are further complicated by the past history of wildland fire and other disturbance, both natural and anthropogenic.

Scrub Vegetation Series
Scrub is the most abundant and diverse kind of vegetation within the range, and is characterized by a predominance of one or more species of shrubs and subshrubs. Tree species are absent, or of only minor importance. Scrub vegetation may be relatively uniform physiognomically from stand to stand, but species composition can vary greatly depending on factors such as seral stage, exposure, slope, substrate, and moisture availability. Characteristic shrub-dominated series found in the range are presented in Table 3. For convenience, these have been grouped into four physiognomic/ecological categories--chaparral, sage and sagebrush scrub, desert scrub, and riparian scrub.
Chaparral.-The dominant components of chaparral vegetation series are hard-wooded, evergreen, sclerophyllous shrubs. The composition and relative dominance of shrub species is highly variable between different series; however, the unifying physiognomic characteristic is the relatively dense, frequently impen-  etrable overs tory of intricately branched shrubs. Chaparral vegetation series are the most common and diverse type of scrub vegetation found in the Liebre Mountains region, occurring throughout the elevationat range and on most geologic substrates.
Second in importance to charnise-dominated stands are those chaparral series where various shrub forms of Quercus species are dominant or codominant. These include Brewer oak (Quercus garryana var. breweri), canyon live oak shrub (Q. chrysolepis), interior live oak shrub (Q. wislizeni var. frutescens), interior live oak-canyon live oak shrub, interior live oak-chaparral whitethorn (with Ceanothus leucodermis), interior live oak-scrub oak, scrub oak, and scrub oak-chaparral whitethorn series, as well as the scrub oak-charnise series mentioned above. Oak-dominated series are most prevalent at mid-to upper elevations, especially across the southern flank of the Liebre-Sawmill-Sierra Pelona crest. At lower elevations, scrub oak-dominated series are restricted to relatively mesic exposures.
Series dominated by species of Arctostaphylos or Ceanothus are of more limited distribution within the range, generally appearing as localized stands adjacent to mixed charnise or oak series. Most frequent are stands of the Eastwood manzanita and wedgeleaf ceanothus series. Stands of the hoaryleaf ceanothus series are restricted to the south-central edge of the range, between the Agua Dulce area and Soledad Canyon. Conversely, stands dominated by cupleaf ceano-thus are largely restricted to the northern edge of the range, along Portal Ridge.
The understory of chaparral series is equally variable with respect to the composition and abundance of annuals, perennial herbs, and suffruticose species. Sage and sagebrush scrub.-Compared with chaparral series, the sage and sagebrush scrub series are a lower-statured vegetation, being dominated by relatively soft-wooded, malacophyllous, facultatively drought-deciduous shrubs and subshrubs. Within the Liebre Mountains region, these scrub series exhibit a patchy distribution, often in close association with areas of chaparral. Best-developed stands are found at lower elevations at the southwestern end of the range. The Santa Clarita Valley area, including Newhall, Saugus, Valencia, and Agua Dulce, probably supported the region's most extensive development of sage and sagebrush scrub prior to urbanization.
Five shrub species, in various combinations of presence and relative dominance, characterize the majority of the sage and sagebrush scrub series represented in the range (Table 3). These include Salvia mellifera (black sage), Eriogonum fasciculatum (California buckwheat), Artemisia californica (California sagebrush), Salvia leucophylla (purple sage), and Salvia apiana (white sage). The most common and widespread series are those where Eriogonum fasciculatum is dominant or codominant. These include the California buckwheat, California buckwheat-white sage, and California sagebrush-California buckwheat series. Stands of the California buckwheat series are especially prevalent on the southern flank of Liebre Mountain, and are associated with areas of deep, loose decomposed granite. Stands of the purple sage series are well developed on sedimentary substrates at the westem edge of the range, especially about Castaic Lake.
The sage and sagebrush series discussed above are largely characterized by shrub taxa of cismontane Californian affinity. Two other important members of the sage and sagebrush series are dominated by shrubs of more interior, Great Basin affinity. These are the big sagebrush and rubber rabbitbrush series. The big sagebrush series is dominated by Artemisia tridentata and exhibits a scattered, patchy distribution across the northern edge of the range. Stands are often encountered in areas of deeper soil with cold air drainage. This series is particularly common on the northern flank of Sierra Pelona, and valleys of the San Andreas Rift zone. The rubber rabbitbrush series is dominated by Chrysothamnus nauseosus, often with more than one variety of the species being present within a given stand. This series is also best developed along the northern edge of the range, especially at the margin of the Antelope Valley.
The understory within sage and sagebrush scrub series is generally better developed than in the various chaparral series, although overall species composition is similar.
Desert scrub.-Two shrub-dominated vegetation types characteristic of the Mojave Desert, the creosote bush and Joshua tree series, are represented within the Liebre Mountains region by small outlier stands ( Scattered stands of Yucca brevifolia occur across the northern edge of the range, where locally present within other vegetation series. Most of the~e small stands would probably not warrant mapping as Joshua tree series, however. An unusual manifestation of the Joshua tree series is found at the extreme northwestern end of the range. Here, Yucca brevifolia is strongly clonal, forming dense, impenetrable thickets. Even the largest "individuals" have poorly branched crowns. These plants represent the variety herbertii. Associated 102 Boyd ALISO shrubs are scarce within the thickets, but surrounding brush and rubber rabbitbrush series. · ---------------------

Conifer woodland Riparian woodland
Understory taxa within the Joshua tree series, especially in the eastern stands, are similar to those of the creosote bush series.
Riparian scrub.-Vegetation associated with moist to wet soils of drainage courses, springs, and fluctuating lake margins, includes both shrub-and tree-dominated series. These are distributed across a variety of environmental gradients, including the nature and frequency of past flooding, fire, and other disturbance; the duration and reliability of surface water; the texture of alluvial overburden and depth to bedrock; and the stream gradient. As a general rule, areas with greater water availability, and less disturbance, tend to support tree-dominated vegetation. These will be addressed later in the context of woodland vegetation series. Portions of drainages with less reliable supplies of water, areas subject to more frequent scouring floods, and heavily or periodically disturbed situations are characterized by various shrub-dominated series, collectively treated here as riparian scrub.
Important kinds of riparian scrub in the Liebre Mountains region include the mulefat, narrowleaf willow, and scalebroom series ( Table 3). The mulefat series, characterized by dense to open stands of Baccharis salicifolia (mulefat), is the most common and widespread of the three. It is common in the periodically flooded areas about the margins of lakes and reservoirs, frequently scoured wet areas in the larger drainages, and in minor drainages throughout the range. The narrowleaf willow series, characterized by Salix exigua (narrowleaf willow), is typically associated with locations with reliable sources of water near the soil surface, as about springs and along sluggish streams.
Stands of the scalebroom series, dominated by Lepidospartum squamatum (scalebroom), are generally restricted to relatively broad, low-gradient washes which are sandy and frequently scoured by seasonal floods. Historically, this was likely the predominant vegetation along portions of the Santa Clara River and the lower reaches of the Castaic, San Francisquito, Bouquet, Elizabeth Lake Canyon, and Mint Canyon drainages. Much of this habitat has been lost or radically altered by urbanization, sand and gravel mining, channelization, and agriculture. The best-developed remnants are now found in lower San Francisquito Canyon, and along the Santa Clara River downstream from the mouth of Soledad Canyon.
The scalebroom series is the most floristically diverse of the three basic kinds of riparian scrub found in the range.

Woodland Vegetation Series
Vegetation series dominated by arborescent species, although much less extensive in areal coverage than various types of scrub, nevertheless form an important part of the landscape throughout the Liebre Mountains region. Characteristic tree-dominated series found in the range are presented in Table 4. For convenience of VOLUME 18, NUMBER 2 Liebre Mountains Flora 103 discussion, these may be grouped into three broad types--oak woodland, conifer woodland, and riparian woodland. I have excluded from the discussion of woodland series, those situations where conifers have been established in artificial plantations by the National Forest.
Oak woodland.-The Liebre Mountains are noteworthy in the diversity of oak-dominated series found in the range ( Stands of the blue oak series are limited to the northwestern end of the range, especially at the western end of Portal Ridge and in the vicinity of Sandberg on the northwestern foot of Liebre Mountain. Within the study area, this series is typically found on mesic exposures of gentle to moderately sloping hills and ridges, and is relatively open and savannalike. In addition to Quercus douglasii, the dominant tree, scattered individuals of Q. lobata and Pinus sabiniana are frequently present.
Most of the best-developed stands of the blue oak series are found on private ranch lands that were not accessible during the course of this study. Although similar overall to that of black oak woodlands, the herbaceous understory of the blue oak series remains poorly sampled within the range. Common shrub associates include Juniperus califomica, Aesculus califomica, Artemisia tridentata, Chrysothamnus nauseasus, and Quercus john-tuckeri.
Those stands of the blue oak series found in the range are among the most southerly known in California. I suspect the understory may support a number of additional taxa common to this vegetation association further north, but otherwise absent from the study area. Trees and arborescent shrubs suggesting intergradation between Quercus douglasii, Q. john-tuckeri, and Q. lobata are not uncommon at the northwestern end of the range, and were included in studies by Benson et al. (1967) of hybrid swarms in oaks.
The canyon live oak series is dominated by tree forms of Quercus chrysolepis, and is generally found within the Liebre Mountains at elevations above 1000 m. Best development within the range is on steep slopes with mesic exposures, particularly across the northern flank of Liebre-Sawmill-Sierra Pelona crest, and in upper Cold Canyon at the western end of Liebre Mountain. Within the range, the canyon live oak series is typically part of a complex vegetation mosaic which includes the black oak and big-cone Douglas fir series on mesic exposures, and various chaparral series on xeric slopes. Boundaries between the different vegetation assemblages are indistinct with considerable overlap in component species. Physiognomy of these woodland series is controlled by the relative abundance of the three principal tree species, Quercus chrysolepis, Q. kelloggii, and Pseudotsuga macrocarpa.
The coast live oak series is dominated by dense to open stands of Quercus agrifolia. Examples of this series may be found scattered across the southwestern quarter of the range, particularly in the larger, broader drainages, such as San Francisquito, Bouquet, and Mint canyons. The best-developed examples of the coast live oak series are most often found on deeper alluvial soils, at elevations below 1000 m. In many areas, trees have been thinned by cutting or clearing to produce open, parklike stands. These are often the sites of rural residences. Other areas still support dense stands with nearly continuous crown cover. Depending on the frequency and intensity of past and present disturbance, the understory may be relatively depauperate and weedy, or support a rich assemblage of understory shrubs, perennial herbs, and annuals.
In the most-disturbed woodlands, especially those subjected to intense grazing, the understory is densely invaded by introduced annual grasses and forbs, es- pecially  The composition and diversity of the herbaceous understory is equally variable, but may be quite rich in less-disturbed woodlands on mesic slopes. Valley oak woodland, like blue oak woodland, is best developed at the northwestern end of the study area, although historically well-developed stands ranged southward along the western edge of the range into the Valencia and Saugus area. Typically, stands dominated by Quercus lobata are characterized by gentle relief and deep, often alluvial soils. The herbaceous and shrub understory of valley oak woodland is virtually identical to blue oak woodland, and to a lesser extent, black oak woodland.
Conifer woodland.-Relative to woodland series characterized by species of Quercus, natural vegetation dominated by arborescent conifers is limited in both areal and geographic extent within the range. Nevertheless, several distinctive series are locally important components in the Liebre Mountains vegetation mosaic (Table 4). The important coniferous trees include Pseudotsuga macrocarpa (big cone Douglas fir), Juniperus califomica (California juniper), Pinus sabiniana (foothill pine), P. ponderosa (ponderosa pine), and P. monophylla (singleleaf pinyon).
Woodlands of the big cone Douglas fir and big cone Douglas fir-canyon live oak series are best developed in the steep, moist canyons draining the northern flank of Liebre and Sawmill mountains, but occur at scattered sites throughout the range in areas of similar habitat. In addition to Pseudotsuga, and to varying degrees, Quercus chrysolepis, other trees which are sometimes present include Pinus sabiniana, P. ponderosa, Q. kelloggii, and Acer macrophyllum. Across the northern flank of Liebre and Sawmill mountains, these woodlands are intimately associated with the canyon live oak and black oak series at the upper elevations, and the valley oak series near the foot of the slopes. Understory composition is virtually identical to that found in canyon live oak woodlands.
The California juniper series is found in the southeastern end of the Liebre Mountains region, but is best developed in the region between Agua Dulce and Acton. The dominant overstory species, Juniperus califomica, is most often found as an arborescent shrub in the study area. It is discussed here, vs. with other shrub-dominated series, only because Sawyer and Keeler-Wolf (1995)  The California juniper series seems to be especially / vulnerable to repeated fires with short return intervals. Most of the areas supporting this vegetation series within the study area occur outside of the National Forest boundaries and are being rapidly degraded by fragmentation and fires associated by low-density semirural development.
The Foothill pine series, like the floristically related blue oak and valley oak series, reaches the southern limit of its distribution within the Liebre Mountains region. Vegetation dominated by Pinus sabiniana is confined to the northern edge of the range, along Portal Ridge and the northerly flank of Liebre and Sawmill mountains. In well-developed stands, the overstory may be dense to relatively open, but rarely forming the deeply shaded conditions seen in big cone Douglas fir and canyon live oak woodlands. Understory composition of the foothill pine series within the range is virtually identical to that of the blue oak and valley oak series.
The most limited conifer-dominated vegetation type in the range is the ponderosa pine series. Compared with the higher San Gabriel Mountains to the southeast, and Mount Pinos region to the northwest, native stands of Pinus ponderosa in the Liebre Mountains are small and floristically depauperate. Within the study area, this vegetation series is limited to the highest portions of the Sawmill Mountain summit, surrounded by more extensive stands of the black oak and big cone Douglas fir-canyon live oak series. Understory shrubs include Chrysothamnus nauseosus, Ribes roezlii, Rhamnus tomentella, Eriogonum umbellatum, E. wrightii var. subscaposum, and E. fasciculatum. The herbaceous understory is similar to that of the adjacent black oak series.
Singleleaf pinyon, Pinus monophylla, is found in two widely separated areas of the Liebre Mountains region, the northerly flank of Sierra Pelona south of Palmdale, and the steep, sedimentary hills at the northwestern comer of the range. Vegetation dominated by singleleaf pinyon is only found at the northwestern area, however. In addition to Pinus monophylla, common associated arborescent and shrub species include Juniperus califomica, Quercus john-tuckeri, Arctostaphylos glauca, Artemisia tridentata, Ephedra viridis, Salvia dorrii, Yucca whipplei, Y. brevifolia, and Cercocarpus betuloides.  (Table 4). As with riparian scrub, the riparian woodland series occur across a variety of environmental gradients. The floristic composition and relative dominance of component taxa may be correlated with the nature and frequency of past flood events and the duration and reliability of surface water, as well as the effects of fire and past anthropogenic disturbance. Collectively, the best development of riparian woodland series in the range may be found in the larger drainage systems, such as Castaic, Elizabeth Lake, San Francisquito, Bouquet, and Soledad canyons and their major tributaries. Riparian vegetation remains relatively intact within the portion of the range administered by the Angeles National Forest, but at lower elevations, especially near the confluence of the principal drainages with the Santa Clara River, there has been extensive clearing and channelization with subsequent loss of woodland.

Grassland Vegetation Series
Within the Liebre Mountains, there is less diversity of vegetation dominated by grasses and other herbaceous taxa as compared with the scrub and woodland series discussed above. Only four series are of regional importance in the range, the California annual grassland, nodding needle grass, common reed, and giant reed series.
California annual grassland is a floristically heterogeneous series, characterized by the physiognomic prevalence of annual grasses, especially introduced species. This broad category doubtless includes both natural, herb-dominated vegetation, and stands resulting from anthropogenic degradation of other scrub and woodland vegetation.
The most extensive development of California annual grassland within the range is found along the northern border of the study area, at the southern edge of the Antelope Valley, on Portal Ridge, Bald Mountain, and across the summits of the Liebre-Sawmill-Sierra Pelona crest. Although introduced taxa, such as A vena barbata, A. fatua, Bromus hordeaceus, B. rubens, Schismus barbatus, and Erodium cicutarium, are important in these stands, there is a noteworthy diversity of native taxa. In contrast, the more anthropogeniqtlly disturbed stands of California annual grassland are concentrated in the southern half of the range, particularly in the Santa Clarita Valley. Here, only the most hardy and vigorous of the native elements, such as Lupinus hicolor, Eschscholzia califomica, Cryptantha muricata, and Dichelostemma pulchellum are able to compete favorably with the aggressive exotics. Boyd ALISO Grasslands dominated by nodding needle grass, Stipa cemua, are small and uncommon within the range. The most intact examples are found at the western edge of the study area, in Osito Canyon. Here they are developed on deposits of locally weathered heavy soil within a broader matrix of better-drained sandstonederived soils 'Yhich support various chaparral and sage scrub series. Although exotic annual grasses have invaded these areas, as elsewhere, Stipa cemua is still dominant. Common herbaceous elements associated with these grasslands include Plantago erecta, Lotus wrangelianus, Cryptantha microstachys, Hemizonia fasciculata, Microseris lindleyi, Dichelostemma pulchellum, Astragalus gambelianus, Ancistrocarphus .filagineus, Poa secunda, and Calystegia peirsonii.
The common reed series, characterized by nearly pure stands of Phragmites australis, is restricted to low-gradient portions of the Apple Canyon drainage, just east of Interstate 15. This area is periodically inundated when Pyramid Lake is filled to capacity, perhaps accounting for the abundance of this coarse grass at the site. The giant reed series, characterized by nearly pure stands of Arundo donax, is limited to the floodplain of the Santa Clara River at the eastern edge of the study area.

Numerical summary and phytogeography
The inventory of the Liebre Mountains flora presented here is limited to "naturally" occurring taxa, that is, indigenous natives and those nonnatives thought to be growing and reproducing without direct, conscious human intervention (i.e., outside of cultivation). In this context, I have excluded long-persistent plantings of trees and shrubs, unless there was a clear indication that there has been subsequent adventive establishment. On the other hand, waifs of exotic annuals and short-lived perennial herbs were included, although I recognize that some will no doubt be transitory participants in the dynamic floristic diversity of the region.
A complete listing of the taxa is presented below in the annotated catalogue. Several taxa were excluded from the present enumeration of the flora, although voucher specimens at RSA suggest they were collected within the boundaries of the Liebre Mountains study area. In all instances, I questioned the veracity of the records because the locality information on the specimen is vague and the characteristic habitat of the taxa involved is different from that inferred by the purported collection station. Nevertheless, I have included references to these excluded taxa as an addendum to the annotated catalogue so their status may be re-examined should the taxa be encountered during future floristic work in the range.
The flora of the Liebre Mountains region is comparable to other areas of southern California in exhibiting a ratio of native to nonnative taxa of approximately 4: 1 (Table 6). Although the percentage of natives is the highest of those areas compared, this may reflect a certain sampling bias, as field efforts of the present study were largely focused on relatively lessdisturbed public lands. It is likely that further floristic documentation of habitats in the wildland-urban interface within the Liebre Mountains region will increase the number of adventive and naturalized exotic taxa.
ly higher than documented in either the Santa Ana or Santa Monica mountains. In part this may reflect greater the climatic diversity of the Liebre Mountains region which includes considerably more xeric conditions along the southwestern margin of the Mojave Desert.
The Similarity Index (SI) of Soerensen (SI = 2C/A + B X 100%; where A= number of taxa in one area, B = number of taxa in a second area, and C = number of taxa common to both areas), provides a simple method of comparing relative floristic similarity of two areas (Balgooy 1971 ). Two southern California regions suitable for comparison with the Liebre Mountains are the Santa Ana and Santa Monica mountains. These ranges are of generally similar size and topographic diversity to the Liebre Mountains region, and have relatively well-documented floras (Lathrop andThome 1978, 1985;Boyd et al. 1995, Boyd, Ross, and Roberts 1995, Raven et al. 1986Wishner 1997;Ross 1996). The total floras of the Liebre and Santa Ana mountains exhibit a SI of 56.5%. A comparison of both the native and nonnative elements of these areas exhibit similar SI values, 56.6% and 56.1% respectively. Comparison of the Liebre Mountains with the Santa Monica Mountains gives a SI of 53.3% for the total flora, 53.7% for the native component, and 51.8% for nonnative elements. As a point of further comparison, SI calculated for the Santa Ana vs. Santa Monica mountains is higher than for either range compared with the Liebre Mountains (SI = 64.4% for total flora; 67.9% for native flora).
As with life-forms, climatic diversity likely plays an important role in shaping the similarities and differences observed among the floras of these three areas. The Santa Ana and Santa Monica ranges are similar in having climatically disparate coastal and interior cismontane slopes (Lathrop and Thome 1978;Raven et al. 1986). Although the drier interior slopes support a number of "desert" taxa, both ranges lack the strong Mojave Desert influence present in the Liebre Mountains region. Conversely, the Liebre Mountains lack the lowland, coastal influences found in the Santa Ana, and especially, Santa Monica mountains.
A major impetus for selecting the Liebre Mountains region for study was its proximity to the borders of several major physiographic and phytogeographic units of southern California. The area is situated at the nexus of the Transverse Ranges, Coast Ranges, Sierra Nevada, Mojave Desert, and coastal plains (Hickman 1993;McLaughlin 1992). Ecotone areas such as this are often characterized by higher biological diversity than similar-sized areas within the core of a physiographic region. This pattern seems to hold for the Liebre Mountains region when compared with the Santa Ana or Santa Monica ranges, especially with respect to the native elements (Table 6).
A logical extension of the floristic analysis discussed above would be comparison of the Liebre Mountains with the somewhat larger, but closely juxtaposed San Gabriel, Pine Mountain, and Tehachapi ranges. Unfortunately, such analysis must await formal enumeration of the flora of those regions.

Sensitive taxa
Another goal of this study was to provide better documentation of plant taxa of conservation concern within the Liebre Mountains region. Much of the upland portions of the range are public lands managed by the Angeles National Forest. A baseline account of the resources being managed is required for establishing effective, long-term management plans. Large, ecologically intact units such as the Liebre Mountain uplands represent critical refugia for long-term conservation of southern California's low-and mid-elevation native flora. Areas which exhibit exceptional biological diversity are of even greater importance in this context.
is a broad reference to all plant taxa inventoried by CDFG-NDDB, without regard to their legal or protection status (CDFG-NDDB 1998). Other common terms used to refer to such plants (e.g., rare, threatened, endangered, or sensitive) have taken on specific legal connotations in the highly politicized and litigious arena of species conservation. In the subsequent discussion, my use of the term "sensitive plants" should be interpreted as equivalent to CDFG-NDDB's "special plants" as opposed to other narrower definitions. A total of 32 sensitive plant taxa have been documented within the Liebre Mountains region (  (Boyd and Sanders, in press).

ANNOTATED CATALOGUE OF THE VASCULAR FLORA
The following list includes all vascular plant taxa documented during fieldwork in the Liebre Mountains region, and through herbarium specimens deposited at RSA-POM and elsewhere. A representative voucher specimen is cited for each taxon listed, including collector name(s), number, and collection date. Unless otherwise cited, voucher specimens are deposited at RSA-POM. Herbarium acronyms follow Index Herbariorum, 8th ed. (Holmgren, et al. 1990).
An alphabetical arrangement has been followed for families within subdivisions, classes, or subclasses, as well as for genera within families, and species within genera. Nomenclature used in this list largely follows Hickman (1993). Family nomenclature is that of Thorne VOLUME 18, NUMBER 2 Liebre Mountains Flora 109 (1992) for the flowering plants, and Crabbe, et al. (1975)