By Kevin Short/Special to The Japan NewsPeople have been living in the Kanto region for close to 40,000 years. During most of this time, however, they made their living by hunting and gathering. They certainly had some impact on the regional ecosystems, but the disturbances they caused were small in scale and usually only temporary in nature.
All this changed a little over 2,000 years ago, when the technology for growing rice in irrigated paddies arrived from western Japan. The first rice farmers began draining and diking the marshlands that thrived on the floors of narrow valleys. These spongy marshes were originally home to typical wetland habitats such as reed beds and alder and willow forests, but were soon converted to rice paddies.
Rice paddy cultivation in the valleys has continued unbroken here for two millennia — a tribute to the sustainable lifestyles and nature-friendly folk spirituality of the local farmers. Over the past half-century, however, many paddies have been abandoned as young people leave the land for more stable jobs in towns and cities. These abandoned paddies are now in the process of reverting to their original pre-farming ecosystems.
There is one narrow valley near my home where paddies were abandoned in the early 1970s, when construction began on the huge Chiba New Town residential area. The entire valley floor, from side to side, is now covered by a dense forest of hannoki, or Japanese alders. Alders (classified in the genus Alnus) along with willows (classified in the genus Salix) are major components of wetland forests across the northern hemisphere. These trees are among the few species able to flourish in wet, marshy soils and can even tolerate being immersed in shallow standing water.
The Japanese alder (A. japonica) is very common in the lowlands of the Kanto region. Right now, these trees are in full bloom, their branches densely decorated with long, yellow-brown male catkins.
Alders, like most members of the Birch family (kabanoki ka), are monoecious (shiyudoshu), meaning the male and female flowers bloom separately, but that each individual tree has both types. The male flowers form into long catkins that hang down in bunches from the tips of the branches. The tiny female flowers crowd together in small reddish cones, on short stalks that are placed well back from the branch tip. These are wind-pollinated flowers (fubaika). The male catkins ooze prodigious amounts of pollen, which is carried on the wind to the female flowers of other trees. Once fertilized, the female flowers grow slowly into a woody cone-like structure called a strobile. Inside each strobile are dozens of tiny flat seeds.
As the cones open and disintegrate, the seeds are whisked away on the wind. These seeds are also impervious to water and can survive a long soaking. Alder seeds that fall into the water are thus often dispersed downstream. Sometimes a clear size gradation can be seen in a valley, with the first tall older pioneer trees upstream and smaller younger generation trees further down.
Natural habitats can be very resilient. Even after being suppressed for more than 2,000 years, the alder forests have been able to reclaim lost territory in just a few decades. All they need is a few survivors to provide the first stock of seeds.
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