Abstract: Southern California’s Santa Ana Mountains are a coastal range surrounded by a metro area of 18 million people. The vascular flora of these mountains is relatively well-studied, but gaps in our knowledge remain. Limited access to Ladd Canyon has made it a botanical black hole relative to the rest of the Santa Anas. The canyon covers 18.2 km2 in the Cleveland National Forest. It is dominated by chaparral and coastal sage scrub and includes significant knobcone pine (Pinus attenuata Lemmon) stands and riparian woodland. Features that make it an intriguing area to study include the only serpentine soil in the Santa Ana Mountains, large tracts of habitat that have no recorded fire history, steep topography that one observer has suggested funnels moist marine air through the canyon, and relatively low human impacts. To help close the gap in our knowledge of this area, I collected 692 voucher specimens over 79 field trips from 2020 to 2022 and produced a vascular plant checklist of 324 taxa, including the rare plants Lepechinia cardiophylla Epling (California Rare Plant Rank 1B.2), Calochortus weedii Alph. Wood var. intermedius Ownbey (1B.2), and Monardella hypoleuca A. Gray subsp. intermedia A.C. Sanders & Elvin (1B.3) and several watchlist taxa. To investigate the effects of abiotic and human factors on the Ladd Canyon flora, I have made use of my own recorded weather data and data from Southern California Edison, compared the occurrence of edge-of-range taxa in the area with the mountains as a whole, and compared the presence of annual and naturalized taxa with the Santa Ana Mountains as a whole and with the San Mateo Canyon Wilderness Area in the southern part of the mountain range. Weather data and edge-of-range taxa do not support increase influence of moist coastal air compared with the rest of the mountains. A relatively low proportion of annual taxa suggests an effect of long fire intervals on the canyon’s flora. A relatively low proportion of naturalized taxa suggests a difference in the flora because of lower human use and long fire intervals.

The Agua Tibia Mountains, located at the northern end of the Palomar Range, have historically been the site for very little botanical research when compared with other units of the Peninsular Ranges, such as the Santa Ana Mountains. This study reported the results of a floristic survey of the mountains that was conducted over a four-year period, from September 1994 through May 1999. This study area encompassed the drainage system of Agua Tibia Mountain and is topographically and geologically diverse. A complex mosaic of herbaceous, shrub- and tree-dominated vegetation was found in range. Vascular plant taxa reported for the mountains totaled 954, including 756 considered native and 198 considered nonnative. Among these were sensitive plant taxa. range extensions, and disjunct taxa that created a rich diversity of plant species and revealed important insights into the phytogeographic history of mountains.