Impact of the Eocene on the Evolution of Pinus L.

Pinus evolved in middle latitudes of the Northern Hemisphere in the middle Mesozoic. By the late Cretaceous pines had spread east and west throughout Laurasia, attaining high diversity in eastern Asia, the eastern United States, and western Europe, but having little representation at high northern latitudes. Changing climates in the early Tertiary established warm and humid tropical/subtropical conditions in a broad zone to 70°N throughout middle latitudes. Pines and their relatives disappeared from many middle-latitude areas during this time and were replaced by diverse angiosperm taxa of the boreotropical flora, which were adapted to the equable, tropical climate. The effect of this climate change and spread of boreotropical flora was to displace pines from their former habitats. A hypothesis is defended that pines shifted, during the three warm periods of the Eocene, into three major refugial areas in the Northern Hemisphere: high latitudes, low latitudes, and upland regions of middle latitudes, especially in western North America. Some of these refugial areas (e.g., Mexico/Central America) underwent active volcanism and mountainbuilding in the Eocene and became secondary centers of pine diversity. Many phylogenetic patterns within Pinus can be traced to this fragmentation, isolation, and evolution in Eocene refugia. Subsections Oocarpae and Sabinianae appear to have originated from refugia in Mexico and Central America. Older subsections such as Sylvestres, Ponderosae, Contortae, and Strobi were distributed over several refugia; subsections Leiophyllae, Australes, and Cernbroides evolved in southern refugia in North America; and Canarienses evolved in southern refugia along the Tethys seaway in Eurasia. Following the cooling and drying of the climate at the end of the Eocene, many angiosperm taxa of the boreotropical flora became extinct and pines recolonized middle latitudes, a zone they have occupied to the present. Migration out of refugia provided additional opportunities for hybridization and introgression, as formerly isolated lineages expanded and met.