Late Cenozoic Evolution of the Sierran Bigtree Forest

Axelrod, Daniel

The Late Cenozoic history of the bigtree community has been analyzed in terms of the changing spatial and floristic relations of an ancestral forest of diverse origins. Most species in the modern community are of Arcto-Tertiary derivation, but Madro-Tertiary plants are associated with it, chiefly at lower altitudes. More numerous species of both geofloras were associated with the forest during Miocene and Pliocene times. Occupying western Nevada during the Late Miocene, the ancestral bigtree forest migrated to the windward slopes of the Sierra Nevada as rainfall decreased during the Pliocene. During and following this migration, the selective influence of changing climate (disappearance of summer rain, increasing winter precipitation, greater ranges and extremes of temperature), acting on the varying ranges of tolerance of the species and their ecotypes, greatly modified the composition of the community. Species eliminated from it include (1) Arcto-Tertiary plants whose nearest relatives survive now (a) in the Sierra-Cascade forest at either higher or lower altitudes, (b) in the Coast Forest, (c) in the Rocky Mountain forest, and (d) in the hardwood deciduous forests of eastern Asia and eastern America, and (2) Madro-Tertiary plants whose modern counterparts occur chiefly (a) in woodland country at lower levels in the Sierra Nevada or to southward, (d) in the conifer woodland of the Great Basin and areas to the east, and (c) in southwestern North America. Those that survived to form the living community were either preadapted to a summer-dry climate, or evolved ecologic races that tolerate these conditions. Conifers, which had gradually been increasing in numbers during the Pliocene when plants requiring summer rain were being eliminated, finally assumed dominance in the Quaternary. Differences between the northern and southern parts of the bigtree community, and at its upper and lower margins, developed chiefly in the Quaternary in response to the temperature, as well as moisture, relations of the component species that were segregated in response to these factors. Glaciation in the Sierra Nevada apparently disrupted a more nearly continuous bigtree forest to give rise to the present scattered groves that occur between the major glaciated canyons.

Late Cenozoic Evolution of the Sierran Bigtree Forest

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