ブータン西部のワン・チュ流域の海抜200 mから3800 mまでの植生の垂直分布を調査し，流域を8つの植生タイプ（植生帯）に区分した：①亜熱帯半常緑樹林（＜1100 m），②暖温帯照葉樹林下部（1100–2000 m），③暖温帯照葉樹林上部（2000–2800 m），④冷温帯針広混交林（2800–3200 m），⑤亜高山帯（亜寒帯）針葉樹林（3200–4000 m），⑥高山帯（寒帯）低木群落（4000 m＜），⑦乾燥マツ林（ブルーパイン型）（2000–2800 m），⑧乾燥マツ林（チャーパイン型）（1500 m＞）。①～⑥は湿性環境の植生，⑦と⑧は乾燥谷に成立する乾性植生である。この地域には，世界の植物地理区の全北界日華区系と旧熱帯界インド区系の境界が通っている。一般に，日華区系は暖温帯照葉樹林を含んだ範囲を指すと理解されているが，植物地理区の本義に従えば，第三紀周北極植物群を源とする落葉樹・温帯性針葉樹が日華区系の主要部分であり，熱帯・亜熱帯を源とする照葉樹林は旧熱帯界の側に位置づけるべきものである。東アジア全体の冷温帯夏緑樹林の成立過程を考えると，冬の気候が穏やかな中国南部が多様性のセンターであり，西へ行くほど乾燥の影響，北へ行くほど寒さの影響が現れて，多様性は減少する傾向にある。ブータンは西の端，日本は北の端に近いので，ブナ科の種数で見た多様性は，ブータンと日本であまり変わらない。しかし，高所まで照葉樹林の影響の及ぶブータンでは夏緑樹林が発達しないのに対し，日本では，第三紀周北極植物群の多くが生き残り，日本海側の多雪環境に適したブナが存在したため，日本特有の豊かな冷温帯夏緑樹林がつくり出された。

Vegetational surveys were undertaken along the Wan Chhu River and its tributaries in western Bhutan, between 200 m and 3800 m. The vegetation was classified into 8 types as follows: 1) subtropical semi-evergreen forest (＜1100 m); 2) warm-temperate laurel-leaved forest (lower part) (1100–2000 m); 3) warm-temperate laurel-leaved forest (upper part) (2000–2800 m); 4) cool-temperate mixed forest (2800–3200 m); 5) subalpine (subarctic) coniferous forest (3200–4000 m); 6) alpine (arctic) scrub (4000 m＜); 7) dry pine forest (blue-pine type); 8) dry pine forest (chir-pine type). A regional boundary runs along the southern slope of Bhutan Himalaya between the Sino–Japanese sub-region (Holarctic Region) and Indian sub-region (Palaeotropical Region). The laurel-leaved forest in East Asia is typically considered part of the Sino–Japanese sub-region, but should be considered part of Palaeotropical Region. The main components of the Holarctic Region, including the Sino–Japanese sub-region, are derived from Arcto-Tertiary flora, whereas the laurel leaved forest is closely associated with tropical and subtropical flora.<n>South China, with its moderate winter climate, has the highest species diversity as Arcto-Tertiary elements from the north and laurel-leaved forest elements from the south meet there to form a mixed forest. South China appears to be the main distributional center for cool-temperate forests in East Asia. The winter climate becomes increasing severe beyond this center, colder to the north and drier to the west. Bhutan and Japan are located at the western and northern periphery of this distribution, respectively. Because of the dominance of laurel-leaved forest components and the lack of Fagus and other deciduous trees belonging to the Arcto-Tertiary flora, cool-temperate forests are poorly developed in Bhutan. In contrast, Japan enjoys a vast species-rich cool-temperate deciduous forest because of the persistence of Arcto-Tertiary flora components and the presence of Fagus crenata, which survives in the snowy areas along the coastline of the Sea of Japan.