Deciduous forests have been remarkably resilient throughout their history, recovering from major shifts in climate and surviving periods of massive deforestation. But today the world’s great forests confront more ominous threats than ever before. Saving the World’s Deciduous Forests (Yale University Press, 2014) by Robert A. Askins examines forests to reveal their common origin back in time and the approaches to conservation that have been attempted on their behalf. The following excerpt is from chapter 1, “Parallel Worlds.”
The inspiration for this book was a walk along a forest stream in the mountains north of Kyoto on a clear morning in early spring. After working in the forests of eastern North America for many years, I found Japanese forests a mix of the familiar and the strange. My surroundings were mostly familiar. Leaves were just emerging from buds on the overhanging branches of maples and oaks. Splashes of color—clumps of violets, anemones, and trilliums—dotted the mottled brown leaf litter of the forest floor. Straight gray beech trunks, the shallow angle of spring sunlight, and the songs of returning migratory songbirds evoked the scenes and sounds of a New England forest in spring. The hemlocks, oaks, and Indian pipes were similar to those along a stream near my home in Connecticut.
On closer inspection, however, the details were distinctly different. The bird songs were new and strange. Although the general types of plants were familiar, the particular species were new to me. And there were many more species, making plant identification a greater challenge. Instead of a single species of beech there were two; instead of three types of maple trees there were more than a dozen; and the field guide showed a multitude of different species of similar-looking violets. In this respect, it was as if I were visiting a North American forest 8 million years ago, before the Pleistocene extinctions. The comparable North American forest can only be viewed dimly by examining fossil imprints of plants from that period.
Despite the differences in particular species of plants, however, East Asia and eastern North America have remarkably similar types of woodlands, with subtropical forests in the south, deciduous hardwood forests in the central region, and boreal coniferous forests in the north. The change in vegetation between Key West and Nova Scotia parallels the change between Okinawa and northern Hokkaido. A traveler on either journey begins on white sandy beach fringed by mangroves and ends on a rocky coast surrounded by spruce, fir, and birch. Both journeys traverse diverse deciduous forests for much of the way.
Of course, the geography and biology of Japan and North America differ in many ways. Japan lacks the extensive prairies, savannas, and deserts that dominate much of the North American landscape. Japan consists of a series of islands, so it has some of the biological characteristics of islands, such as lower species diversity than comparable areas on the continental mainland. And the history of land use and style of agriculture are distinctly different in most parts of Japan and North America. But the great expanses of deciduous forest dominated by oaks, maples, and other hardwoods in Japan and eastern North America result in parallel ecological worlds. If you want to see spectacular displays of autumn color, there are few places on earth that will rival the Appalachians of the United States or the mountains of Honshu. Outsiders may perceive both regions as densely populated and long settled, with little remaining natural habitat, but this is a misconception. Forest covers 60–70 percent of the land in both Japan and the northeastern United States. Both regions are dominated by second-growth woodland interrupted by roads and towns, but both have some surprisingly large expanses of continuous forest. And in both regions many forests are maturing and slowly acquiring the large trees, closed canopy, and dead wood of an ancient forest.
Fifteen million years ago, before the repeated advance and retreat of glaciers across northern Eurasia and North America, deciduous forest encircled the northern continents. These Miocene forests were remarkably diverse, with a great number of plants and animals that are now extinct. Some of these species succumbed to habitat change and hunting pressure as people spread throughout the Northern Hemisphere, but most disappeared long before the evolution of technologically skilled humans. They vanished because they did not survive a changing global climate, especially the severe disruptions of the Pleistocene, when kilometer-deep glaciers and cold, dry winds made much of the North Temperate Zone uninhabitable for all but the most flexible and hardy plants and animals.
Today you can find isolated regions of deciduous forest in the middle latitudes of eastern North America, Europe, and East Asia. The European forests have relatively few surviving tree species because Miocene cooling and Pleistocene glaciation were severe across much of western Europe, and the Mediterranean Sea blocked the spread of forests southward into warmer areas. Forests of eastern North America are more diverse because plants and animals could spread to relatively warm refuges on the Gulf Coast and what is now the continental shelf under the Gulf of Mexico. To experience the true diversity of the original deciduous forest, however, one must travel to eastern China, Korea, or Japan. These regions were not covered with continental glaciers during the Pleistocene, so the disruptions to forests caused by climate change were muted.
Superficially, the forests of Europe, East Asia, and eastern North America appear similar, but of course similar environments can lead to the evolution of similar organisms that are not necessarily related to one another. In the case of temperate deciduous forests, however, the three continents share the same taxonomic groups of plants. The dominant herbs, shrubs, and trees belong to the same plant families and often to the same genera (the plural of “genus”). A genus consists of a group of closely related species with a common ancestor, and the large number of shared genera among the three continents indicates that the forests of these regions were connected fairly recently in geological time. The list of shared genera is even greater if one compares fossils from the Paleogene, the relatively warm period between 65 and 23 million years ago, preceding the major climate changes that began during the Miocene. Numerous genera that are now found only in East Asia are known from the fossil record from this period for North America or Europe, or both. Similarly, genera now found in North America but not Europe are known from fossils in Europe. In fact, a majority of tree genera were once found in two or three of the current deciduous-forest regions. A large proportion of these plant groups disappeared from Europe and a smaller number disappeared from North America. Extinction rates of trees were especially high for western North America, where most broad-leaved forests were replaced by grassland, desert, or coniferous forest. In contrast, very few plant groups disappeared from East Asia.
Low extinction rates for plants in East Asia and eastern North America and high extinction rates in the other parts of Eurasia and western North America resulted in an odd pattern of plant distributions. Particular groups of plants are found only in the deciduous forests of East Asia and the eastern deciduous forest of North America; they are missing (except in the fossil record) from other parts of the temperate zone. The remarkable similarity of the flora of East Asia and eastern North America was thoroughly documented in the 1950s by Hui-Lin Li, whose monograph includes dozens of maps showing the geo graphical distribution of genera restricted to these two widely separated regions. Some examples include such familiar North American plants as witch hazel (Hamamelis), Virginia creeper (Parthenocissus), catalpa (Catalpa), trailing arbutus (Epigaea), and partridgeberry (Mitchella). About 65 genera of plants show this pattern. Comparisons of genetic sequences in different species in many of these genera confirm that the East Asian and North American representatives of each genus evolved from a common ancestor millions of years ago. Hence their similarities do not result merely from convergent evolution in similar habitats.
During the period when the important families of deciduous forest plants were evolving, North America, northeastern Asia, and Europe were intermittently connected by land via Greenland and the Bering Strait, so it isn’t surprising that they share or shared many types of temperate-zone plants. These connections ceased about 15 million years ago, after which plant groups that became extinct on one continent were not replaced by immigration of related species from other continents, and some continents lost a high proportion of their original diversity. The current differences between Europe an deciduous forests and the forests of North America and East Asia are mainly due to extinctions resulting from a progressively colder climate and the severe effects of glacial periods during the past few million years. This resulted in the exceptionally low diversity of plant species in European forests.
The differences in diversity between East Asia and eastern North America appear to be more complex. East Asia has higher plant diversity because it has retained more species in ancient groups such as magnolias and ginkgos that became extinct in North America. Other groups are more diverse in East Asia because the rate of evolution of new species apparently was greater in East Asia than in North America. This is partly a product of the mountain or ocean barriers that separate Japan and Korea from mainland China and each other. As described in Chapter 2, this set the stage for isolated populations of a single species diverging into separate species. Also, new deciduous-forest species may have frequently evolved in East Asia due to the close connection between subtropical and tropical forests and temperate deciduous forests in China, permitting many species to move northward and adapt to a more strongly seasonal climate. This may have been facilitated by the mountains of southern China, which support tropical and temperate vegetation in close proximity at different elevations. In contrast, the deciduous forests of eastern North America are connected only tenuously to lowland tropical forests in the West Indies and Mexico. They have been separated by ocean barriers and—for millions of years—by deserts and semi-deserts of the southwestern United States and northern Mexico. The result is that forests in North America are similar to those in East Asia, but are less diverse due to both more frequent extinction and less frequent addition of new species. Although Japan is now separated from China by ocean barriers, it was connected to mainland Asia recently (in geological terms) when sea levels were lower during glacial periods, so it shares a relatively high diversity of plants with the mainland.
The similarities among the deciduous forests of East Asia, Europe, and North America create the conditions for a “natural experiment.” Natural experiments are not designed by a researcher, and therefore are not well controlled or replicated. Natural experiments are inferior to designed experiments in every way except one: they test processes on time scales (thousands or millions of years) and geographical scales (entire islands or continents) that are not practical for experimental manipulations by researchers.
During my first visit to Japan I was particularly interested in how the parallel evolution of bird migration had played out on deciduous forests on different sides of the world. The summer forests of both Japan and New England are inhabited by songbirds that migrate hundreds or thousands of miles south each autumn to spend the winter in the tropics. Species on both sides of the world have similar behavior. Males defend small territories that support the breeding pair and their young. They announce their territorial boundaries by singing. Most species feed on insects that they find in the complex, multilayered foliage of the forest. Many species specialize on particular layers of the forest, such as the shrub layer or the tree canopy. Some species glean insects from the undersurface of leaves while others specialize in probing under bark or pursuing flying insects. The overall organization of these bird communities is similar in many ways, but virtually all of the species are different. In many cases, they are not even closely related to one another—they don’t have recent common ancestors. Some of the most important families of birds in North America, such as the vireos, New World warblers, and tyrant flycatchers, are missing from Japan. Other bird families, such as the bulbuls and Old World flycatchers, are found in Japan but not in North America. Hence we have the makings of a particularly interesting natural experiment: a similar ecological stage (to paraphrase G. Evelyn Hutchinson), but an almost completely different set of actors. Did the play proceed in the same way? Did similar ecological patterns emerge? Are the conservation problems and solutions similar? If so, then Japanese, American, and Canadian ecologists and conservationists can learn much from one another, and the emerging principles may also apply to deciduous forests in Europe, Korea, and China.
Once I began thinking about parallel evolution in Japanese and North American forests, all sorts of questions occurred to me. How has the relatively recent extinction of wolves in both Japan and most of eastern North America affected natural forest environments? Have recent population explosions of deer in Japan and North America resulted in similar ecological changes? How have species adapted to young forest and forest openings fared as forest has matured in the two regions? Does timber harvesting have similar effects on biological diversity in Japanese and North American forests? If there are similar patterns for forests of Japan and eastern North America, can these be generalized to other temperate deciduous forests in Europe, Korea, and China?
These comparisons particularly intrigue me because some of the basic approaches to conservation—and even the way people perceive natural habitats and natural beauty—differ in Japan and North America. Is one approach to conservation more effective, or do both approaches have strengths and weaknesses? Could we combine these different approaches to better protect temperate deciduous forests, with their many special qualities—colorful fall foliage, spring woodland flowers, four visually distinct seasons—and their high diversity of species (among the highest north of the moist tropics)? And how does this comparison relate to conservation in China and in England and other parts of Europe, which have different traditions of land use and wildlife conservation from either North America or Japan?
This excerpt has been reprinted with permission from Saving the World’s Deciduous Forests by Robert A. Askins and published by Yale University Press, 2014.