Parallel Worlds

Spring Forests in New En gland and Kyoto

The inspiration for this book was a walk along a forest stream in themountains north of Kyoto on a clear morning in early spring. Afterworking in the forests of eastern North America for many years, Ifound Japanese forests a mix of the familiar and the strange. My surroundingswere mostly familiar. Leaves were just emerging from buds on theoverhanging branches of maples and oaks. Splashes of color—clumps ofviolets, anemones, and trilliums—dotted the mottled brown leaf litter of theforest floor. Straight gray beech trunks, the shallow angle of spring sunlight,and the songs of returning migratory songbirds evoked the scenes andsounds of a New En gland forest in spring. The hemlocks, oaks, and Indianpipes were similar to those along a stream near my home in Connecticut.

On closer inspection, however, the details were distinctly different. Thebird songs were new and strange. Although the general types of plantswere familiar, the particular species were new to me. And there were manymore species, making plant identification a greater challenge. Instead of asingle species of beech there were two; instead of three types of maple treesthere were more than a dozen; and the field guide showed a multitude ofdifferent species of similar-looking violets. In this respect, it was as if I werevisiting a North American forest 8 million years ago, before the Pleistoceneextinctions. The comparable North American forest can only be vieweddimly by examining fossil imprints of plants from that period.

Despite the differences in particular species of plants, however, EastAsia and eastern North America have remarkably similar types of woodlands,with subtropical forests in the south, deciduous hardwood forests inthe central region, and boreal coniferous forests in the north. The change invegetation between Key West and Nova Scotia parallels the change betweenOkinawa and northern Hokkaido. A traveler on either journey begins on whitesandy beach fringed by mangroves and ends on a rocky coast surrounded byspruce, fir, and birch. Both journeys traverse diverse deciduous forests formuch of the way.

Of course, the geography and biology of Japan and North America differin many ways. Japan lacks the extensive prairies, savannas, and desertsthat dominate much of the North American landscape. Japan consists of aseries of islands, so it has some of the biological characteristics of islands,such as lower species diversity than comparable areas on the continentalmainland. And the history of land use and style of agriculture are distinctlydifferent in most parts of Japan and North America. But the great expansesof deciduous forest dominated by oaks, maples, and other hardwoods inJapan and eastern North America result in parallel ecological worlds. If youwant to see spectacular displays of autumn color, there are few places onearth that will rival the Appalachians of the United States or the mountainsof Honshu. Outsiders may perceive both regions as densely populated andlong settled, with little remaining natural habitat, but this is a misconception.Forest covers 60–70 percent of the land in both Japan and the northeasternUnited States. Both regions are dominated by second-growth woodland interruptedby roads and towns, but both have some surprisingly large expansesof continuous forest. And in both regions many forests are maturing andslowly acquiring the large trees, closed canopy, and dead wood of an ancientforest.

Why Are North American and Japanese Forests Similar?

Fifteen million years ago, before the repeated advance and retreat of glaciersacross northern Eurasia and North America, deciduous forest encircledthe northern continents. These Miocene forests were remarkably diverse,with a great number of plants and animals that are now extinct. Some ofthese species succumbed to habitat change and hunting pressure as peoplespread throughout the Northern Hemi sphere, but most disappeared longbefore the evolution of technologically skilled humans. They vanished becausethey did not survive a changing global climate, especially the severedisruptions of the Pleistocene, when kilometer-deep glaciers and cold, drywinds made much of the North Temperate Zone uninhabitable for all butthe most flexible and hardy plants and animals.

Today you can find isolated regions of deciduous forest in the middlelatitudes of eastern North America, Europe, and East Asia. The Europe anforests have relatively few surviving tree species because Miocene coolingand Pleistocene glaciation were severe across much of western Europe, andthe Mediterranean Sea blocked the spread of forests southward into warmerareas. Forests of eastern North America are more diverse because plantsand animals could spread to relatively warm refuges on the Gulf Coast andwhat is now the continental shelf under the Gulf of Mexico. To experiencethe true diversity of the original deciduous forest, however, one must travelto eastern China, Korea, or Japan. These regions were not covered with continentalglaciers during the Pleistocene, so the disruptions to forests causedby climate change were muted.

Superficially, the forests of Europe, East Asia, and eastern North Americaappear similar, but of course similar environments can lead to the evolutionof similar organisms that are not necessarily related to one another. Inthe case of temperate deciduous forests, however, the three continents sharethe same taxonomic groups of plants. The dominant herbs, shrubs, and treesbelong 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 acommon ancestor, and the large number of shared genera among the threecontinents indicates that the forests of these regions were connected fairlyrecently in geological time. The list of shared genera is even greater if onecompares fossils from the Paleogene, the relatively warm period between65 and 23 million years ago, preceding the major climate changes that beganduring the Miocene. Numerous genera that are now found only in East Asiaare known from the fossil record from this period for North America orEurope, or both. Similarly, genera now found in North America but not Europeare known from fossils in Europe. In fact, a majority of tree genera wereonce found in two or three of the current deciduous-forest regions. A largeproportion of these plant groups disappeared from Europe and a smallernumber disappeared from North America. Extinction rates of trees wereespecially high for western North America, where most broad-leaved forestswere replaced by grassland, desert, or coniferous forest. In contrast, veryfew plant groups disappeared from East Asia.

Low extinction rates for plants in East Asia and eastern North Americaand high extinction rates in the other parts of Eurasia and western NorthAmerica resulted in an odd pattern of plant distributions. Particular groupsof plants are found only in the deciduous forests of East Asia and the easterndeciduous forest of North America; they are missing (except in thefossil record) from other parts of the temperate zone. The remarkable similarityof the flora of East Asia and eastern North America was thoroughlydocumented in the 1950s by Hui-Lin Li, whose monograph includes dozensof maps showing the geographical distribution of genera restricted to thesetwo widely separated regions. Some examples include such familiar NorthAmerican 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 geneticsequences in different species in many of these genera confirm that the EastAsian and North American representatives of each genus evolved from acommon ancestor millions of years ago. Hence their similarities do notresult merely from convergent evolution in similar habitats.

During the period when the important families of deciduous forestplants were evolving, North America, northeastern Asia, and Europe wereintermittently connected by land via Greenland and the Bering Strait, so itisn't surprising that they share or shared many types of temperate-zoneplants. These connections ceased about 15 million years ago, after whichplant groups that became extinct on one continent were not replaced byimmigration of related species from other continents, and some continentslost a high proportion of their original diversity. The current differencesbetween Europe an deciduous forests and the forests of North America andEast Asia are mainly due to extinctions resulting from a progressively colderclimate and the severe effects of glacial periods during the past few millionyears. This resulted in the exceptionally low diversity of plant species inEurope an forests.

The differences in diversity between East Asia and eastern North Americaappear to be more complex. East Asia has higher plant diversity becauseit has retained more species in ancient groups such as magnolias and ginkgosthat became extinct in North America. Other groups are more diversein East Asia because the rate of evolution of new species apparently wasgreater in East Asia than in North America. This is partly a product of themountain or ocean barriers that separate Japan and Korea from mainlandChina and each other. As described in Chapter 2, this set the stage for isolatedpopulations of a single species diverging into separate species. Also, newdeciduous-forest species may have frequently evolved in East Asia due to theclose connection between subtropical and tropical forests and temperatedeciduous forests in China, permitting many species to move northward andadapt to a more strongly seasonal climate. This may have been facilitated bythe mountains of southern China, which support tropical and temperatevegetation in close proximity at different elevations. In contrast, the deciduousforests of eastern North America are connected only tenuously to lowlandtropical forests in the West Indies and Mexico. They have been separated byocean barriers and—for millions of years—by deserts and semi-deserts ofthe southwestern United States and northern Mexico. The result is that forestsin North America are similar to those in East Asia, but are less diversedue to both more frequent extinction and less frequent addition of new species.Although Japan is now separated from China by ocean barriers, it wasconnected to mainland Asia recently (in geological terms) when sea levelswere lower during glacial periods, so it shares a relatively high diversity ofplants with the mainland.

Searching for General Patterns in Forest Ecology

The similarities among the deciduous forests of East Asia, Europe, andNorth America create the conditions for a "natural experiment." Naturalexperiments are not designed by a researcher, and therefore are not wellcontrolled or replicated. Natural experiments are inferior to designedexperiments in every way except one: they test processes on time scales(thousands or millions of years) and geographical scales (entire islandsor continents) that are not practical for experimental manipulations byresearchers.

During my first visit to Japan I was particularly interested in how theparallel evolution of bird migration had played out on deciduous forests ondifferent sides of the world. The summer forests of both Japan and New Englandare inhabited by songbirds that migrate hundreds or thousands ofmiles south each autumn to spend the winter in the tropics. Species on bothsides of the world have similar behavior. Males defend small territories thatsupport the breeding pair and their young. They announce their territorialboundaries by singing. Most species feed on insects that they find in thecomplex, multilayered foliage of the forest. Many species specialize on particularlayers of the forest, such as the shrub layer or the tree canopy. Somespecies glean insects from the undersurface of leaves while others specializein probing under bark or pursuing flying insects. The overall organizationof these bird communities is similar in many ways, but virtually all of thespecies are different. In many cases, they are not even closely related to oneanother—they don't have recent common ancestors. Some of the mostimportant families of birds in North America, such as the vireos, New Worldwarblers, and tyrant flycatchers, are missing from Japan. Other bird families,such as the bulbuls and Old World flycatchers, are found in Japan but not inNorth America. Hence we have the makings of a particularly interestingnatural experiment: a similar ecological stage (to paraphrase G. Evelyn Hutchinson),but an almost completely different set of actors. Did the playproceed in the same way? Did similar ecological patterns emerge? Are theconservation problems and solutions similar? If so, then Japanese, American,and Canadian ecologists and conservationists can learn much from oneanother, and the emerging principles may also apply to deciduous forests inEurope, Korea, and China.

Once I began thinking about parallel evolution in Japanese and NorthAmerican forests, all sorts of questions occurred to me. How has the relativelyrecent extinction of wolves in both Japan and most of eastern North Americaaffected natural forest environments? Have recent population explosionsof deer in Japan and North America resulted in similar ecological changes?How have species adapted to young forest and forest openings fared as foresthas matured in the two regions? Does timber harvesting have similar effectson biological diversity in Japanese and North American forests? If there aresimilar patterns for forests of Japan and eastern North America, can thesebe generalized to other temperate deciduous forests in Europe, Korea, andChina?

These comparisons particularly intrigue me because some of the basicapproaches to conservation—and even the way people perceive natural habitatsand natural beauty—differ in Japan and North America. Is one approachto conservation more effective, or do both approaches have strengths andweaknesses? Could we combine these different approaches to better protecttemperate deciduous forests, with their many special qualities—colorful fallfoliage, spring woodland flowers, four visually distinct seasons—and theirhigh diversity of species (among the highest north of the moist tropics)?And how does this comparison relate to conservation in China and in En glandand other parts of Europe, which have different traditions of land use andwildlife conservation from either North America or Japan?