A Vascular Flora of Ladd Canyon, Santa Ana Mountains, Orange and Riverside Counties, California

The Santa Ana Mountains are a coastal range on the border of Orange and Riverside counties in southern California. Ladd Canyon, covering 18.2 km2 in the Cleveland National Forest, is a botanical black hole relative to the rest of the Santa Ana Mountains because of limited access. It is dominated by chaparral and coastal sage scrub and includes significant knobcone pine (Pinus attenuata) 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 and relatively low human impacts. To help close the gap in our knowledge of this area, we collected 802 voucher specimens over more than 80 field trips from 2017 to 2022 and produced a vascular plant checklist of 329 taxa, including the rare plants Allium marvinii, Calochortus weedii var. intermedius, Lepechinia cardiophylla, Monardella macrantha subsp. hallii, Monardella hypoleuca subsp. intermedia, Phacelia keckii and several watchlist taxa. We found that knobcone pine stands are closely associated with serpentine outcrops. A relatively low proportion of annual taxa suggests an effect of long fire intervals on the flora of the canyon, and relatively few naturalized taxa are likely the result of infrequent human use and long fire intervals. Oaks in the area can be difficult to key to species, and we discuss their identification and nomenclature. The vascular plant checklist for Ladd Canyon provided here and the herbarium specimens that support it are valuable resources for the study of the flora and ecology of the Santa Ana Mountains.