The Transverse Ranges, with their east-west orientation, are one of southern California's most prominent physiographic features, and contain large tracts of intact, biologically diverse, public trust lands. Areas such as these are critical for meeting societal goals of preserving California's natural heritage, but long-term conservation requires development of a baseline account of the resources being managed. Although much of the Transverse Ranges border the Los Angeles Basin, California's most densely populated region, published floristic accounts of the component ranges are surprisingly scarce. This study represents a preliminary floristic account of the Liebre Mountains region, and includes the results fieldwork conducted over a nine year period (1990-1999), as well as review of collections housed in the herbarium of Rancho Santa Ana Botanic Garden (RSA-POM) and elsewhere. The Liebre Mountains form a discrete unit within the western portion of the Transverse Ranges, and are geographically and floristically transitional to the San Gabriel Mountains, Inner Coast Ranges, Tehachapi Mountains, and Mojave Desert. A total of 1,010 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 taxa and associated oak-dominated vegetation, and represents the southern limit of the foothill woodland flora characteristic of ranges bordering California's Central Valley. A total of 32 sensitive plant taxa (rare, threatened or endangered) was recorded from the range.

Key words: flora, Liebre Mountains, sensitive plants, southern California, Transverse Ranges