Phytogeographic Relationships of Pteridophytes Between Temperate North America and Japan

Abstract

The pteridophyte flora of North America comprises 325 species (Tryon, 1969) and the nonendemic species among them are referred to four geographic groups: circumpolar, amphioceanic, tropical, and Mexican. Japanese pteridophytes include about twice as many species as the North American, although the Japanese Archipelago is much smaller. The Japanese species are also more complex taxonomically and phytogeographically (Tagawa, 1959; Ohwi, 1965). The distribution pattern of the Japanese ferns is considered to be related to climatic zones by some authors including Nishida (Graham, 1972). They can be arranged roughly in the following phytogeographic groups: circumpolar, Eurasian, trans-Pacific (related to North America), Sino-Himalayan, and tropical Southeast Asian. The phytogeographic relationships between North America and Japan are found primarily in the temperate to boreal species. Since Asa Gray's time, phytogeographic relationships between eastern North America and eastern Asia have been discussed by many authors for pteridophytes as well as for flowering and non-vascular plants (Hulten, 1958, 1962; Li, 1952; Graham, 1972; Tryon, 1969; Tryon & Tryon, 1973). In spite of the accumulation of data, there still is no complete phytogeographic analysis of the biological relationship between eastern North America and the Far East. The complex geohistory often is taken into account and the floristic comparison often is made rather theoretically. Changes in land and climate in the Northern Hemisphere that have occurred since the Tertiary have given rise to replacement of boreal, temperate, or warm floras in the areas concerned, and have enabled plants to migrate between the North American and Asian continents via the Bering Strait (chiefly dry land in the Tertiary) and Aleutian chain. This migration probably occurred several times and in either eastward or westward directions. Similar events would have occurred in a south to north direction between temperate and tropical regions. Speciation and gene-flow caused by the geographic isolation and reunion might have taken place in both those areas. The fern floras of North America and eastern Asia include identical or related species. Some of the identical species might be younger and widespread in the Northern Hemisphere, whereas some related species might be older and become restricted to either North America or eastern Asia. Phytogeographic relationships of fern floras result from species migration that is initiated primarily by spore dispersal. Spore viability varies from about a week to more than several years; green spores are especially short-lived compared with non-green spores. It is reported that fern spores resist, to a considerable extent, physical and chemical environmental stress from low temperature, desiccation, UVand X-rays, and other factors that operate during the transport (Page, 1979). Tryon (1970) argued the effectiveness of long dispersal of fern spores in island fern floras and estimated that distances up to 300 miles are only a slight barrier to the migration of a fern flora and that a distance of 500 miles is not a significant barrier. In spite of this, most fern species are restricted in distribution and only a limited number have wide ranges. This means that the dispersal or migration of most ferns is regulated by some unknown factors. Page (1979) pointed out that many biotic and non-biotic ecological relations are involved in migration. In his study on genetic features of disjunct fern populations, Klekowski (1972) considered that the long-distance establishment and future evolutionary processes of fern populations are dependent on the mating system (intragametophytic vs. intergametophytic fertilization) and genetic variability as well as geographic reproductive isolation. Our knowledge of the mechanism of spore dispersal and species establishment at a new locality is still insufficient to fully understand the phytogeographic relationships of the fern flora in question, for which further experimental and field studies are desired.