During the first three weeks of January 2003, 14 students joined Dr. Dave Barrington, professor of botany at UVM, for his 13th tropical botany field trip in Costa Rica. With Dr. Barrington as our guide, we set out to get a grip on the Costa Rican flora, an overwhelming experience for neophytes to tropical botany. For anyone who has struggled to key out the relatively few plant species in a woodlot in New England, the forests of Costa Rica present such diversity that it is hard to know where to begin. More than 9,000 plant species are estimated to be present in Costa Rica, a country the size of West Virginia.
Why is Costa Rica so diverse? Certainly its position in the tropics and the diversity of habitats created by a rugged topography and varied climate are big factors. But even these factors would not necessarily lead to such high diversity if it were not for the variety of sources from which plant species have come to Costa Rica. Where have the plants of Costa Rica come from?
Some of the Costa Rica’s plant immigrants came from points north. A first glance at the bark of ‘roble blanco’ might make one wonder if it is indeed the same white oak of eastern North America. Actually a different species of the same genus (Quercus), it lives in mid- to high-elevation forests in Costa Rica.
The heath family (Ericaceae) is also familiar to New Englanders. Instead of residing in bogs and barrens, like our familiar blueberries, Costa Rican members of the heath family are often epiphytes, growing on branches of forest trees. Though these habits may seem very different, they can provide similar conditions for plant growth. Leatherleaf growing on a raised bog in New England and an epiphyte growing in a cloud forest in Costa Rica both have to live in challenging conditions, without the benefit of mineral soil. Instead, they must draw all the nutrients they need from rainwater or the decaying organic matter around them. This specimen of the genus Cavendishia has a tuber that can store water and nutrients for the lean times.
As surprising as it was to see plants similar to those in North America in Costa Rica, I was just as surprised to recognize plants like those I’d seen during travels in southern latitudes. The genus Gunnera is found in the cool and wet forests of mid-elevation montane regions of Costa Rica, a climate strikingly similar to coastal southern Chile, where a different species of the genera grows. In both cases these fast growing plants colonize disturbed areas, such as landslide scars and road cuts in the mountains of Costa Rica, and sea-side bluffs and dunes in southern Chile.
The Osa Peninsula in southern Costa Rica receives its weather from South America, and many of its plant species as well. But instead of harboring species from the Andes, the rainforests of the Osa have an affinity with Amazonia. This legume of the genus Parkia, is found in the Amazon Basin and in Africa, a fact that hints at another biogeographic story. This species became established before South America split from Africa 100 million years ago and has been a canopy tree in the majestic rainforests of both continents ever since. Its migration to Costa Rica has been a much more recent event, occurring only during the past 3 million years, when Costa Rica linked up with South America.
Though too geologically young to have had a chance to develop a truly unique flora, Costa sits at a crossroads between two continents, allowing plants from North America, the Andes, and Amazonia to coexist in a small, but surprisingly diverse country.