Postglacial biogeography of Sierra lodgepole pine (Pinus contorta var. murrayana) in California

Plant macrofossils and pollen from 13 sites are used to reconstruct the biogeography of Sierra lodgepole pine (Pinus contorta Dougl. ex. Loud. var. murrayana), and its relationship to climate change, within the Sierra Nevada. During Late-Wisconsin deglaciation, lodgepole pine grew at least 500 m lower in elevation than today. Lodgepole pine was widely established within its modern elevational range between 10 500 and 9000 years BP. During the early Holocene, lodgepole pine remained an integral member of montane forest assemblages below its present elevational range. Macrofossil evidence suggests successive disappearance from most lower elevation sites between 9000 and 6750 years BP. A likely explanation suggests progressive climatic warming and drying of soils during the dry early Holocene, causing local extermination at the lower elevation sites. Within its present elevational range, a lack of macrofossils and a decline in diploxylon pollen suggests possible absence of lodgepole pine near several sites during the middle Holocene, with the tree continuing a retreat to higher, cooler elevations during this time. A return to lower elevation sites after ca 1700 years BP probably resulted from cumulative cooling during the Neoglacial. The postglacial migration pattern for Sierra lodgepole pine differs somewhat from that of the more widely distributed Rocky Mountain subspecies (P. contorta var. latifolia). While the pattern for the Rocky Mountain lodgepole was both latitudinal and elevational (only now reaching its northernmost extent in Canada [Cwynar & McDonald, 1987]), Sierra lodgepole's migration was largely elevational with little migrational lag. Additional paleo-sites south of the present distribution of ssp. murrayana will help to clarify any latitudinal movements during the late Wisconsin.