This is of course not a natural colony, as Naoki intimates. In Japan gardens are often mass planted - a trend which probably is more a product of modern times. So it is with this colony. The trees are also planted - even aged Japanese cedars, Cryptomeria japonica.
In nature this species lives in far looser colonies, though no doubt many were quite large. Since Japan's temperate forests were dominated by deciduous trees of many types, it is likely that is where they were found historically. Currently the species is considered endangered throughout the country, but was historically widespread. Only in Nagasaki Prefecture is it considered probably extinct. That said, I can tell you it is not common these days due largely to loss of habitat. Fukuoka is said to have a large population, but its whereabouts is kept secret and it is watched carefully for poaching.
