As a result, the growth of forest decreased and the forest died, which not only caused great loss to the forest industry in China, but also caused serious damage to the ecological function of the forest. There are 5020 kinds of forest harmful insects, 2918 kinds of diseases and more than 60 kinds of rodents in China. Every year, the area of forest diseases and insect pests is more than 6 million hm2, the growth of trees is reduced by about 10 million m3, and the area of forest dying due to disasters is about 300000 hm2.
The main reason is that a large number of single plantation replaced the original forest with rich species diversity, which reduced the self-control ability of forest to harmful organisms.
Pine caterpillars are the main forest pests in China, which are distributed all over the country. The area of pine caterpillars is about 1.4 million Hm2 per year, and the pine production is reduced by 2 million m3. The main species of pine caterpillars in China are Dendrolimus punctatus, larch caterpillars, Chinese pine caterpillars, Yunnan pine caterpillars, red pine caterpillars, etc.
The distribution of forest diseases and insect pests is related to topography, altitude and temperature. Chinese pine caterpillars are distributed at the boundary of the second and third steps, but not in the high altitude area (except the north of Xinjiang) above the second step. The Dendrolimus punctatus is distributed in the middle temperate zone, the Dendrolimus punctatus is distributed in the warm temperate zone, and the Dendrolimus punctatus is distributed in the tropics and subtropics. Frequent occurrence area: in the area where the average temperature is equal to or higher than 25 ° C when the altitude is lower than 400m; occasional area: the altitude is 400-500m, and the temperature changes between 10-25 ° C, which is even disaster area; when the altitude is above 800m, the accumulated temperature is the minimum, which is a safe area.
Besides pine caterpillars, pine wood nematodes, poplar stem borers and Paulownia big bag moths are also serious pests. Among the forest diseases, there are poplar rot, pine blister rust, pine wilt, jujube witches' broom, leaf dissolving disease, Paulownia Witches' broom and so on. Each year, the loss of Paulownia Witches' broom is 23.41 million yuan, which is more serious in plain area than in mountainous area.
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