Like other flora in Japan, the diversity of trees here, especially in the southern forests, is inspiring. This is due to a variety of reasons. First of all, rainfall is plentiful and the soils are rich such that most areas have been historically forested. The varying elevations as well add to diversity, particularly in the south where forest type varies from nearly subtropical to cool temperate, since many microclimates exist because of them. Third, due to Japan's extensive north to south latitudinal reach the species found here range from truly boreal to truly tropical and everything in-between.
Human influence on forests has been extensive. Lowland forest in particular is nearly gone, therefore virtually all of the forested area is in the mountains. This fact is born out in statistics: approximately 73% of the nation is mountainous and about 67% is forested. The similarity of these numbers isn't a coincidence. Generally speaking mountain areas are not built on since they tend to be unstable, so the mountains here are more or less usually forested.
A country that is 2/3 forested sounds quite impressive, especially considering that this small nation also has nearly 130 million permanent human residents. The truth of the matter is that much of this forest is not natural, but rather cut over and planted in specific lumber trees. In the south sugi (Cryptomeria japonica) and hinoki (Chamaecyaris obtusa) are the main species used for this purpose and their plantations often make up the bulk of the forest on any given mountain. These plantations are composed of same age trees that are usually not allowed to grow very large before being harvested. Additionally, such forests lack biological diversity being unsuitable for most plant species and therefore are not good habitat for many animals either. The sad truth is that only about 1% of Japan's forests can be considered primeval, old growth.
The forests of Kyushu, Shikoku, and half way up the coasts of Honshu are home to the northernmost extension of southeast Asia's broadleaf evergreen forests. Here the forests are dominated by species from the beech family (Fagaceae), the laurels (Lauraceae), and the Asterids (Theaceae). Numerous other trees are represented here as well, including conifers and a wide range of deciduous trees, but often they are in the minority. The feel of the forest is very nearly subtropical, but this far north these forests are in warm temperate climes. Typical genera found here include Quercus (subgenus Cyclobalanopsis), Lithocarpus, Castanopsis, Cinnamomum, Machilus, and Camellia. Since this is the forest region I live in and have most examined, many of the trees treated here will be from this forest type. Fortunately, I happen to live near lovely examples of old growth forest, albeit small remnants. One can only wonder at the forests of old - they must have been breath-taking.