_Adenostoma fasciculatum_
_Rhamnus crocea_
_Quercus dumosa_
_Cercocarpus betuloides_
_Yucca Whipplei_
_Prunus ilicifolia_
_Ceanothus sp._
_Arctostaphylos sp._
_Umbellularia californica_
This association is characteristic of the Pacific slope of the San
Gabriels and extends from roughly 2000 feet elevation to 5000 or 6000
feet elevation. The ecotone between the chaparral and yellow pine forest
associations covers a broad elevational belt, with chaparral following
dry slopes up into coniferous forests, and conifers extending down north
slopes surrounded by chaparral.
The chaparral association is characterized by tracts of dense brushy
plants. These plants are from three to ten feet tall, their interlacing
branches often forming nearly impenetrable thickets. Typically little
herbaceous growth is present beneath the chaparral, the ground being
covered with varying amounts of mull.
The effects of fire, slope, exposure, and elevation, make the chaparral
association extremely varied with regard to habitats or plant
formations. There are nearly pure stands of greasewood on the lower arid
slopes; scrub oak, sumac, and lilac clothe less dry exposures; scrub
oak and bay trees occur commonly amid granite talus; and locally groves
of bigcone-spruce are found. Because of the many habitats present, and
the difficulty of collecting in the chaparral, less was learned of the
ecology of the mammals in this association than of those occurring
elsewhere. The distribution of several chaparral-inhabiting mammals
seems to be influenced by the distribution of locally characteristic
plants, for example oak and bay woodland, or greasewood chaparral.
Several habitats within the chaparral community support few species of
mammals and few individuals. Possibly the compact, rocky nature of the
soil limits burrowing rodents, and the lack of herbaceous growth limits
the food supply. Steep rocky slopes in San Antonio Canyon grown to
mountain-mahogany and scrub oak were sparsely populated by _Peromyscus
boylii rowleyi_, _Peromyscus californicus insignis_, and _Neotoma
fuscipes macrotis_. Fifty traps set on such a slope for one night caught
only three _Peromyscus_. Traps set in tracts of greasewood brush on dry
south slopes at the head of Cow Canyon produced only California mice,
_Peromyscus californicus insignis_ Rhoads.
Following is a list of the mammals taken in the course of approximately
600 trap nights in the lower parts of the chaparral belt. All of the
traps were set on slopes in San Antonio Canyon below 4000 feet
elevation. The list gives a general indication of the relative numbers
of rodents inhabiting one chaparral habitat: the arid greasewood-covered
south slopes of the lower chaparral belt.