Tropical evergreen rain forest and semi-evergreen rain forest are distributed in the southern wing of the East Himalayan Mountains, with an altitude of 1, 000- 1, 100 meters, with tall trees and dense forests, usually as high as 30-40 meters. The main tree species are tropical, and the evergreen rainforest is composed of many trees such as Dipterocarpaceae, Moraceae and Siluriaceae. Semi-evergreen rain forest consists of lazy people in Li Qian and Ding Feng. The composition of these tropical forests is complex, and the levels in the forests are not obvious. The bark of many trees is smooth, grayish white or grayish brown, which is particularly eye-catching. Some trees have roots protruding from the ground, and some trees, such as verbena, have huge fruits, which grow directly on the trunk and have thick branches. This is the phenomenon of "old stems blooming". There are many large lianas and epiphytes in the forest, mainly composed of Araceae, Palmaceae, Orchidaceae and Piperaceae, as well as tropical ferns and mosses, including unicorn leaves with large leaves, Gesneriaceae with red corolla and orchids with pleasant fragrance. Evergreen broad-leaved forest is widely distributed in the south wing of the central and eastern Himalayas and Chayu area. It is composed of evergreen trees such as Castanopsis fargesii, Cyclobalanopsis glauca and Lithocarpus, Lauraceae and Magnoliaceae. It has a round and wavy crown, a large forest crown, a height of 20-30 meters, many layers, and many epiphytes and climbers. Some woodlands are very wet, and the stones or trunks on the ground are covered with moss, which has the characteristics of "fog forest" or "moss forest" Hard-leaved evergreen broad-leaved forest is a forest composed of Quercus species of FAGACEAE, such as Quercus montana, Quercus chuandianensis and Quercus flavescens. Different from the above evergreen broad-leaved forest tree species, Quercus variabilis has the characteristics of xerophytic ecological adaptation, with evergreen leaves, hard leathery, small and medium leaves, sharp spines or teeth at the edge of leaves, smooth leaves and short yellow or gray hairs on the back of leaves. The trunk is curved, the wood is hard, the bark is thick, and the branches are numerous and dense. The height of the tree is generally 15-25m, and the hierarchical structure is relatively simple. This kind of forest is yellow-green or yellow-brown, and yellowish pine cones are often hung on the branches of the crown. Deciduous broad-leaved forest is suitable for ecological conditions with cold climate, and it is mostly composed of positive tree species that shed leaves in winter, such as birch, alder and poplar. Betula scabra forest is more common in the upper part of the forest; Dry wax gourd in Alnus cremastogyne is a fast-growing tree species with wide adaptability, and often forms secondary forests at lower altitudes. The mixed forest of Populus davidiana and Betula platyphylla is a kind of natural regeneration forest after the destruction of coniferous forest, which is mainly distributed in the forest area in the southeast of the plateau. In addition, there is a small piece of Quercus liaotungensis forest on the shady slope in the eastern mountainous area of Qinghai.
These broad-leaved forest types have certain rules on the requirements of temperature and water conditions. With the change from warm humidity to cold humidity or cold drought, they generally change from tropical rain forest to evergreen broad-leaved forest to broad-leaved evergreen broad-leaved forest to deciduous broad-leaved forest. The forest is composed of many species, the forest structure is self-propagating and simple, the level is reduced and the height is shortened. Coniferous forest is the most widely distributed forest type in Qinghai-Tibet Plateau, among which evergreen coniferous forest includes pine forest, hemlock forest, spruce forest, fir forest and cypress forest. Deciduous coniferous forest has larch forest. The trees in coniferous forests are usually tall and straight, with high volume per unit area, which can provide economic wood and a large number of forest by-products, and have important economic value. Plateau coniferous forests are mostly distributed in the mountainous areas in the upper reaches of rivers, which have obvious ecological functions and benefits in soil and water conservation and environmental improvement.
Pine forest is mainly composed of Pinus yunnanensis, Pinus alpine, Qiao Song and Pinus longleaf. The first two species are distributed in Chayu and Hengduan mountain areas, and the last two species are distributed in the southern wing of Himalayas. They are drought-resistant and barren-tolerant positive tree species with strong adaptability. They usually grow rapidly, renew well and have strong fire resistance. Most of them form forests with few layers, good light transmittance and neat forest appearance on sunny and semi-sunny slopes. Taking Yunnan pine forest growing in Chayu area as an example, the average height of 130-year-old stands is close to 50 meters, and the volume per hectare reaches 1 1,000 square meters, which exceeds the Korean pine forest with superior growth conditions in Xiaoxing 'anling area of Northeast China.
Hemlock forest, which likes humidity, is mainly distributed on the hillside in the humid area on the southern edge of the plateau. Hemlock is vigorous and tall, up to 30-40 meters, and its drooping branches set it off more magnificently. Hemlock forest is humid, and there are many trees, shrubs and herbs in the lower layer. The trunk is covered with moss, which seems to be wrapped in a thick and soft green felt.
Forests composed of spruce and fir are generally called dark coniferous forests, which are widely distributed in mountainous areas in the southeast of the plateau. Abies likes wet and cold, the tree is 20-30 meters high, there are many trees and shrubs in the forest, the moss layer on the surface is developed, and the branches are often hung with long yellow-green pine branches, dotted with dark green forest crowns.
Spruce forests are usually distributed at a lower altitude than fir forests and can grow on sunny slopes in humid areas. Because of cold tolerance and drought tolerance, it often dominates on both sides of the watershed in Hengduan Mountain area, forming the upper limit of the forest and the northwest edge of the forest area. Common forests are Linzhi spruce forest and western Sichuan spruce forest. Picea crassifolia forest grows in the eastern Qilian Mountains on the northern edge of the Qinghai-Tibet Plateau. In the West Kunlun Mountain to the west of Yecheng, spruce forests in Ling Xue are scattered on the shady slopes of wet canyons.
The distribution of cypress forest is limited. Giant cypress is a tree species in Langxian County in the middle reaches of Yarlung Zangbo River, which is suitable for sand and drought resistance and has a unique style. In Jinchuan and Marcand in the upper reaches of Dadu River, cypress trees in Minjiang River are dominant, and most of them grow on the steep slopes of narrow valleys. Sabina vulgaris forest is mostly found in the sunny slope of the upper part of the forest belt in Hengduan Mountain area, with Sabina vulgaris as the main species. It is a typical drought-resistant, barren-resistant and cold-resistant forest type, mostly sparse forest with an altitude of about 10 m, and its structure is relatively simple. The eastern and northeastern parts of the plateau are dominated by Qilian around Berlin.
Larix gmelinii and Larix gmelinii are the main deciduous coniferous forests. The former is distributed in Hengduan mountain area, and the latter is mainly in the mountainous area on the southeast edge of Tibet. Cold-resistant and barren-tolerant, with low requirements for soil conditions and high distribution altitude. Some of them also form the upper limit of forests or stubbornly grow on moraines.