Late tertiary paleoaltitudes and vegetational zonation in Mexico and Central America

During the middle Pliocene seven principal arborescent plant communities occupied the zone between sea level and the highest elevations of the eastern Transvolcanic Belt—manglar (mangrove swamps; Rhizophora), bosque caducifolio (deciduous or Quercus/ Liquidambar woods), bosque de encino (oak woods; Quercus, both lowland tropical and upland temperate types), bosque de pino (pine woods; Pinus), bosque de pino y encino, and bosque de oyamel (high-altitude Abies/Pinus woods). The presence of Picea, now confined in Mexico to the northern mountains 1000 km distant from the fossil locality in south-eastern Veracruz state, and the poor representation of the selva alta perennifolia (tropical rain forest) suggest cooler mean annual temperatures estimated at 2-3°C below the present. A downward shift in ecotones of about 500-1000 m is suggested, with a maximum altitude in the region of about 2500 m (the present maximum is 5650 m; present altitudinal range of Abies in eastern Mexico is 2400-3600 m). In southern Central America (Costa Rica, Panama) all modern anologues of taxa recently reported from late Eocene and early Miocene palynofloras can be accommodated in a zone between sea level and 1200-1500 m (current maximum elevations c. 3500-4000 m). Tertiary temperature fluctuations were muted by the insular environment of proto-Central America, and altitudinal shifts in ecotones are not evident in the fossil floras.