On Japan's Biological Richness: Why and How Plants Walk and Swim

by Conrad Totman One fascinating truism of the global biosystem is that islands, no matter how remote from other parcels of land, manage somehow to acquire vegetational coverage of one sort or another. And with vegetation comes a select population of microbes and animals, creating complex systems of biological interaction and interdependence. How, one wonders, did the islands of the Japanese archipelago acquire their rich array of plants?

Japan and the East Asian biome
Among temperate-zone regions of the world, East Asia (the China-Korea-Japan region) is known for its unusually rich assemblage of plant life, with native species of trees, forbs, and grasses outnumbering those of either Europe or North America. And Japan, despite its island character, has shared fully in that vegetational richness.

To explain how Japan has been able to share in that richness, three variables seem deserving of particular note. Most noteworthy is the region's experience during Pleistocene glacial ages. But one must note, also, the roles of nearby ocean currents and of the archipelago's proximity to the continent. Finally, we should note, in passing, the recent contribution of humans.



The Role of Pleistocene Ice Ages
The glacial cold phases of the Pleistocene Epoch (Earth's most recent 2 million years) ravaged the biosystems of northwestern Europe and large parts of North America by burying them under immense layers of ice for millennia at a time. East Asia was largely spared the icing, but the region did grow very cold, and its entire biosystem experienced a massive southward adjustment.

The ice for these glaciers came, of course, from the world's oceans, which meant that as ice piled up on the land, sea level dropped and shorelines advanced seaward. In the vicinity of Japan, sea level dropped an estimated 400-450 feet below present-day levels during the three or four coldest phases of the past million years.

That extent of sea-level drop was of major consequence for Japan because it transformed the place from a set of islands into a part of the continent. The Sea of Japan, which currently separates Japan from the mainland, was transformed into a huge freshwater lake, roughly the size of today's Black Sea. North of the lake, the continent was joined to Sakhalin Island. East and south of the lake lay most of present-day Japan, and to its southwest extended an immense coastal plain that encompassed today's Yellow and East China seas.



That transformation of Japan from island chain to continental edge was crucial to its vegetational richness, and two aspects of the situation merit note --the rate of climate change and the region's topography.

(1) Rate of Climate Change. Birds, of course, migrate by flying, land animals by walking, and fish and cetaceans by swimming. These options are not available to plants, which mostly travel by inter-generational relocation.

To explain, during a cooling phase, plants situated on the northern edge (or higher elevation) of a species' growing range fare poorly and produce fewer seed that survive, enabling other, more hardy plants gradually to displace them. Meanwhile, those plants situated on the southern edge (or lower elevation) flourish. Their seeds are scattered around by Wind, water, and animal digestion, enabling some to sprout, displace. other less hardy vegetation, and repeat the cycle. Slowly, generation by generation, a few score yards at a time, this process shifts the species' growing range southward. And during warming periods, the same process works in the reverse direction.

Plant migration is thus a slow process and requires sufficiently slow climate change. It appears that, due mainly to fossil-fuel consumption, climate change warmed the globe by about 1.3 degrees Fahrenheit during the twentieth century.¹ Although our knowledge of rates of temperature-change during the Pleistocene epoch is imprecise, the present-day rate may be about seven-to-ten times as fast as that of glacial-age norms.² Suffice to note that during Pleistocene phases of cooling and warming, plant life in East Asia was able to make the necessary relocation southward and later northward.3

(2) Topography of the Region. For vegetation in Japan, the gradual creation of coastal lowlands in the southwest was crucial to that process of southward relocation. In addition, it was important that most of the high mountain chains of Japan trend roughly north to south. That orientation enabled plants in most places to relocate along valley floors and lower hillsides without being thwarted by Japan's high ridgelines. Moreover, the fact that much of eastern China is lowland or low hill country and that the Himalaya Mountains bend southward in their eastern extremity meant that lowlands in east and southeast Asia were contiguous. That openness optimized the capacity of vegetation throughout the region to adapt to climate change by relocating as needed, thereby preserving its diversity.

In consequence of this emergence of a broad coastal lowland, the temperate-zone vegetation that inhabits most of Japan during warm periods such as today was able to move southwestward during glacial phases. In its place, boreal-forest species (notably spruce and fir trees), which during warm periods flourish only at higher elevations of northern Japan and north of there in Sakhalin and Siberia, were enabled to advance southward and dominate the Japan region until warming re-occurred. When that warming did occur, vegetation was able to relocate northward, with temperate-zone flora regaining predominance in the archipelago.

However, as sea level rose and coastal lowland gradually re-flooded, the trek to Japan became more difficult, and doubtless some species failed to complete the journey before Japan was again transformed into an archipelago. For those species, successful return required mechanisms other than "walking."

Proximity to the Continent; Ocean Currents
For some species travel was still possible because of Japan's proximity to the continent. Insofar as migratory or meandering birds might consume fruit or seed on the continent and not defecate until over or near Japan,
such air travel could re-introduce plant species to the islands. Or high winds might blow minute organisms or viable seed eastward to the islands.

More important, however, may have been the role of ocean currents, in particular the Japan Current. It is the western segment of the great clockwise-circling, northern hemispheric Pacific Current. It turns northward east of the Philippines, flows past up Taiwan, along the Ryukyu Island chain, and then splits south of Kyushu. Part of it flows into the Sea of Japan and up its eastern edge while the main part flows eastward along the southern edge of Japan, past the Kanto Plain, and on toward North America.

As the Japan Current moves northward from the Philippines, it transports such floating materials as it acquires from nearby land, especially when heavy monsoon rains or typhoons produce substantial flooding and erosion that wash debris into the ocean. Such debris may include viable seeds, seedlings, plant parts, and even tree trunks, which can carry diverse living passengers. Some of this material is devoured by sea life, and some rots or sinks, but some eventually washes ashore, of that which washes ashore, a portion survives to contribute to the local biosystem if the ambient environment is favorable to it.

Due to this maritime "conveyor belt," vegetation from more southerly areas is continually introduced to Japan, and over time it has helped make the archipelago a fully participating part of the rich East Asian biome. In a sense then Japan has a rich biosystem because plants do indeed both "swim" and "walk" --and even "fly" --when conditions permit it.

The Human Contribution
The mechanisms described above have sustained Japan's rich floral heritage for thousands of millennia. In recent centuries, moreover, humans -themselves an "exotic" (i.e., non-native) species of relatively recent presence in Japan --have introduced to the archipelago a wide array of other exotic species of plants, animals, and smaller organisms.

The single most consequent of these exotics has been domesticated rice. But diverse other non-native grains, garden vegetables, fruit and other trees, as well as myriad ornamental plants, have also reached the archipelago through human effort, along with horses, cattle, other domesticated animals, and miscellaneous unintended imports, such as the smallpox, syphilis, and tuberculosis organisms and diverse unwanted plants and animals ("weeds" and "pests"). By way of a crude summation, one could argue that these human-introduced exotics now dominate the 20% of Japan that is lowland while native species dominate most of the mountainous 80%.

ENDNOTES
¹ Nature Conservancy, 57-2 (Summer 2007), p. 14, discusses 20th-century temperature change. The rate of change accelerated as the century advanced, and it still appears to be accelerating.

² This estimate of "seven-to-ten times" reflects information on long-term temperature change in the Japan vicinity during ca. 20,000-to-6,OOO years ago, a topic treated by Junko Habu, Ancient Jomon of Japan (Cambridge UP, 2004), and by several earlier authors, whom she cites. The rate of change surely varied over time, as, for example, when a meteor assault (and/or the collapse of a vast North American lake?) around 13,000 years ago may have led to a brief but abrupt period of cooling. New York Times (Jan. 2, 2009), p. A13.

³ Pleistocene plant movement also depended on available nearby terrain. Where broad areas are given over to agricultural and urban uses, as in today's industrialized societies, plant mobility becomes nearly impossible, and even animal migration is severely impaired.