Floristic plant geography: the classification of floristic areas and floristic elements

Abstract

Floristic plant geography is concerned with narrow timescales, narrow to broad spatial scales and entire floras. A major objective is the identification and classification of floristic areas and floristic elements. The spatial scale of plant geography can be divided into three segments: microregional, mesoregional and macroregional. Microregional areas are those included within the boundaries of a single traditionally defined floristic province; mesoregional areas include entire floristic provinces or span the boundaries of two adjacent floristic provinces; and macroregional areas include several floristic provinces. At the microregional scale, species distributions appear as a continuum. The pattern of floristic areas and floristic elements is an emergent one found at macroregional scales. Methods for developing floristic classifications include qualitative modifica tions of previous classifications, visually inspecting and sorting range maps of taxa, and the use of complex numerical techniques requiring computer analyses. These numerical analyses require a database consisting of thorough, accurate lists of all (or selected) species present within various OGUs - subdivisions of or samples (local floras) from the region of interest. A matrix of association among OGUs - similarity, dissimilarity or distance - can be analysed by one or more techniques for clustering and ordination. Historical biogeographies, including vicariance biogeography and panbiogeography, are based on the assumption that most species are relatively old. It is likely, however, that major extinctions and speciation events have reshaped the world's flora every 20 000 to 100 000 years in response to cyclic variation in the earth's orbit. Floristic plant geography, with its detailed evaluation of modem patterns and whole floras, has thus much to contribute to a general understanding of biogeographic processes.