Progress in Northern Hemisphere Phytogeography: An Introduction

Abstract

The problem of Northern Hemisphere plant geography was framed long ago by the observation of striking floristic similarities between eastern North America and eastern Asia. Of primary importance was the work of Asa Gray, who produced a series of detailed comparisons of these floras (see Li 1952; Boufford and Spongberg 1983; Wen 1999). During the past several decades, two major symposia, “Floristics and Paleofloristics of Asia and Eastern North America” (Graham 1972) and “Biogeographical Relationships between Temperate Eastern Asia and Temperate Eastern North America” (Davidse 1983), served to highlight both the nature of the problem and the variety of approaches used to examine intercontinental discontinuities in plants. Not long afterward, two landmark papers by Tiffney (1985a, 1985b) provided a synthesis of paleobotanical and neobotanical data in the context of a dynamic view of Earth history and emphasized floristic exchange via both the North Atlantic and the Bering Land Bridges. In this context, Tiffney (1985b, p. 73) noted that “three-area tests of the variety suggested by cladistic biogeographers need to be made and analyzed to determine whether the eastern Asia–eastern North American similarity is a unique pattern or simply a distinctive subset of a larger pattern.” Over the last two decades we have witnessed remarkable progress in elucidating phylogenetic relationships at all levels, and the number of phylogenies of Northern Hemisphere plant clades has grown to the point that meaningful comparisons are now feasible. Furthermore, there have been major developments in the theory of biogeography as well as in our knowledge of relevant fossils and Earth-history events. The symposium from which the articles in this volume emerged was part of the Botany 2000 meetings in Portland, Oregon. It was designed to create a forum for new analyses of the Northern Hemisphere problem, bringing together a set of paleobotanists and phylogeneticists who have concentrated on relevant areas and plant groups. The goal was to provide a focal point for investigators working from various perspectives on reconstructing relationships among the major areas of endemism and to identify key events that might be responsible for observed patterns of diversity and disjunction. The analyses presented here represent the initial steps toward assembling a modern synthesis based on phylogenetic knowledge, fossil distributions, estimates of sequence diver