Few mammals are resident in the typical yellow pine forest as characterized by dense coniferous timber and little herbaceous or brushy growth. Here most of the species recorded actually find optimal conditions in an adjacent habitat. The forest probably harbors surplus individuals from adjacent preferred habitats, or, as in the case of chipmunks and ground squirrels, the forest often serves as forage ground while nearby brushy areas are utilized for breeding and shelter. The abundance of birds in the timber contrasts strikingly with the paucity of mammals there. The lack of a seed-producing understory, and the open duff-covered stretches of ground on which rodents would be extremely vulnerable to predation, probably in part account for the scarcity of rodents. Within the general area encompassed by the yellow pine forest there are two major habitats, namely coniferous forest and chaparral. The species of plants comprising the chaparral of the Transition Life-zone are different from those comprising the chaparral of the Upper Sonoran Life-zone on the Pacific slope. In the chaparral of the Transition Life-zone, basin sagebrush and snowbrush grow in extensive patches in clearings in the timber. Dense thickets of choke cherry cover many damp hollows, and these thickets harbor the houses of _Neotoma fuscipes_. The food and shelter afforded by these chaparral areas importantly influence the local distribution of rodents: for example, _Dipodomys agilis_ and _Perognathus californicus_ in the yellow pine area are found only in association with chaparral, being completely absent from wooded areas. The severe winter weather in this association must force many of the mammals into periods of inactivity. Probably during the long periods in the winter when snow covers the ground the heteromyids and sciurids remain below ground. Pinyon-Juniper Woodland Association MAJOR PLANTS _Pinus monophylla_ _Juniperus californica_ _Quercus dumosa var. turbinella_ _Purshia glandulosa_ _Fremontia californica_ _Cercocarpus ledifolius_ _Yucca Whipplei_ In the San Gabriel Mountains this association is limited to the desert slope and reaches its lower limit at the bases of the foothills and extends up to the lower edge of the yellow pine forests. The altitudinal extent of the pinyon-juniper association is from roughly 4000 to 6000 feet elevation.