Tropical mountain cradles of dry forest diversity

摘要

Of all ecosystems in tropical America, seasonally dry forests, patchily distributed where relatively fertile soils, annual rainfall < 1,600 mm, and strong seasonal drought coincide, are perhaps the least understood and most endangered. Mesoamerican dry forest diversity peaks in southwest Mexico, where up to 16% of tree species are local endemics. Judith Becerra, in this issue of PNAS, presents a novel historical analysis of Mexican dry forests based on a time-calibrated phylogeny of the dry forest tree genus Bursera, which was the source of the sacred Mayan incense known as copal. Becerra found that peak diversification of Bursera lineages [34-17 million years ago (Ma)] in northwest Mexico followed the uplift of the Sierra Madre Occidental (34-15 Ma), whereas diversification of a southern lineage (peaking at 13.5 Ma) tracked the rise of the Neovolcanic axis (23-2.5 Ma) (Fig. 1). The rising mountains, through their influence on regional climate, permitted dry forests to take hold in Mexico and to spread into Central America.