FLOODPLAIN VEGETATION AND SOILS ALONG THE UPPER SANTA ANA RIVER, SAN BERNARDINO COUNTY, CALIFORNIA

The patterns of vegetation and soils were documented in an approximately 20 km2 area of fluvial terraces adjacent to the Santa Ana River in southwestern San Bernardino County, California. Within this area there are three terraces of differing elevations that were last disturbed during major flood events. The oldest terrace surfaces probably were last disturbed during the Agua Mansa flood of 1862, which disturbed or created many current geomorphic features of the Santa Ana River basin. The most recent disturbance of two other terraces was identified based on photographs of flooding events in 1938 and 1969. Principal component analysis identified three assemblages of species whose distribution corresponded to the three terraces that differed in elevation, soil texture, and age since last disturbance by flooding. Canonical correspondence analysis showed that the assemblages identified by PCA were highly correlated with changes in soil texture and organic matter. The most reliable indicator species were Heterotheca sessiliflora and Lepidospartum squamatum for the first assemblage (associated with the lowest terrace, and corresponding to early successional assemblages identified by other researchers); Salvia apiana and Senecio flaccidus for the second assemblage (associated with the intermediate terrace); and Artemisia californica, Opuntia parryi, and Stephanomeria pauciflora for the third assemblage (associated with the highest terrace). Eriastrum densifolium subsp. sanctorum, the rare Santa Ana River Woolly Star, was associated with the earlier phases of succession. The most important soil factor distinguishing these assemblages was the silt/clay content of the soil.