The more his research progressed, the more he found that the current forest vegetation of Japan (24.1 million hectares, or 3.5 billion cubic meters of timber on more than 64% of the country) had moved away from potential natural vegetation, due to the introduction of alien species by man. He noted that conifers (still considered in the 1970s as indigenous by many Japanese, including botanists), which became dominant in many forests, are actually an introduced species, and were only naturally present at high altitudes and in extreme environments (such as mountain ridges and steep slopes). They have for centuries been planted there to produce timber faster, and they acclimated.