The Eastern Asian-Eastern North American Floristic Disjunction

Botanists recognized the great similarity between the forests and woodlands of eastern Asia and those of eastern North America as early as the first decades of the 19th century. Asa Gray, a renowned botanist of that century, was the first scientist to quantify the similarity. He listed 538 plant species found in both regions. Later scientists realized these species were not the same, though they were similar and closely related. Based on paleontological evidence, scientists determined most of these similar species diverged during the late Miocene, following the uplifting of the Rocky Mountains. Throughout most of the Miocene, a warm temperate forest zone existed from eastern North America across the Bering land bridge and extending into Asia and Western Europe. The uplift of the Rocky Mountains and the Himalayas disrupted the widespread equable climate that supported this warm temperate zone of forest. Species that preferred temperate forests became restricted to areas of eastern Asia and eastern North America. After becoming isolated from common parent species that ranged across this Miocene forest, American species diverged from Asian species. A study of plant DNA from 22 similar species found in both regions supports the paleontological evidence. Most closely related species diverged between 10 million years ago to 5 million years ago. The oldest divergence took place 12 million years ago, and the youngest took place 3 million years ago.

Forests and woodlands in this part of Asia are very similar to those of eastern North America. Image from Harvard University.

North American pachysandra next to a patch of Asian pachysandra. Photo by Peter Del Tredici.

550 year old Japanese Oak located in Korea. Eastern Asia is dominated by forests of oak like much of eastern North America.

North American trumpet honeysuckle.

Japanese honeysuckle is probably more common now in America than trumpet honeysuckle. The latter is prettier.

Eastern North America has more plant species related to those of Eastern Asia than to those of Western North America, and Eastern Asia has more species related to those of Eastern North America than to those of Western Europe, despite the wider geographical separation. Both regions are richer in species than Western Europe and Western North America. Ice Age glaciations drove more species into extinction in those 2 regions. Eastern Asia has 33% more plant species than eastern North America. This suggests more abundant refugia from Ice Age glaciations, and it also points to Asia as the region where most genera and families originated.

Closely related species on both continents include many species of oaks, walnut, chestnut, buckeye, arrowwood viburnum, elder, magnolia, clematis, catalpa, honeysuckle, white pine, and cedar. Scientists have also found the same pattern of similarity with fungi, spiders, millipedes, and fish.

Reference:

Tiffney, B.

“Perspectives on the Origin of the Floristic Similarity between Eastern Asia and Eastern North America”

Journal of the Arnold Arboretum 66 1985

Xiang, Q., D. Soltis, P. Soltis, D. Crawford

“Timing the Eastern Asian-Eastern North American Floristic Disjunction: Molecular Clock Corroborates Paleontological Estimates”

Molecular Phylogenetic Evolution 325 (2) 2000