Ecological Differentiation in Some Congeneric Species of Costa Rican Flowering Plants

Abstract

Initial work for a new flora of Costa Rica disclosed a number of difficult taxonomic problems in the Piperaceae, Chloranthaceae, and Moraceae families. Herbarium studies indicated that there were very closely related taxa in a number of genera. These closely related taxa could be interpreted either as species complexes or as single variable species. Field work in Costa Rica showed that many of these closely related taxa do not grow together. They may grow in the same general geographic area, but these closely related populations are usually found in different habitats or at different altitudinal levels. These observations have been very important in making taxonomic decisions, but they may also be important in recognizing processes of population differentiation and speciation in the wet tropics. Delimitation of taxa in the neotropics is often based on rather few herbarium collections with little biological or ecological data. Such is the case in this study where estimates of affinity are based primarily on similarity or dissimilarity in vegetative and floral characters. Palynological, cytological, or biochemical data are not presently available for these species. The data are based on plants collected in Costa Rica and the adjacent provinces of western Panama. While this may seem to be a very small area from which to make general speculations, it is rather well sampled when compared to other wet tropical areas. Not only does the area of Costa Rica and westernmost Panama have the benefits of decades of botanical exploration, but it also represents an area of isolated highlands with considerable endemism. This area is a minor but natural phytogeographic region, though its lowland species are often widespread. Despite the small area there is a great altitudinal range (0-3800 m) and the patterns of rainfall are very different in different parts of the region. The deciduous forest formations of the northern Pacific lowlands can have less than 20 mm of rainfall during the dry season (December through April), while on the Caribbean side of the mountains, as little as 30 km away, the rainfall averages over 50 mm in the dryest month of the year. However, the rainfall data alone can be misleading, especially at higher elevations. Turrialba on the Caribbean slope has an average annual rainfall of around 2400 mm, not much greater than some areas on the Pacific slope, such as Puriscal. The dry season on the Pacific slope, however, is much more severe and lacks the frequent cloudiness and misting of Turrialba and the Caribbean slope. These seasonal differences in cloud-cover and in rainfall affect the vegetation, and one can