Abstract:

There were great differences in the growth laws of stands and productivity of forest between afforestation by water-harvesting and conventional afforestation on the Loess Plateau. The basic reason was that afforestation by water-harvesting improved the condition of soil moisture in forest owing to increase of runoff volume collected in the planting strip. 15 years old four black locust forests with different densities such as 840 stands·hm^-2, 1110 stands·hm^-2,2220 stands·hm^-2 and 3330 stands·hm^-2 built by natural slope water-harvesting, and 9 years old two forests built respectively by stamped slop water-harvesting and conventional measure were studied in this paper. The results showed that the forest density was thinner (the size of water micro catchment area was larger), the runoff ratio of water micro-catchment area was higher, and the runoff volume collected in the planting strip was more. If it was 100 percent in forest with density of 840 stands·hm^-2, the runoff volume in other forests from low to high density made respectively up 93.60, 81.77 and 84.43 percent. The annual soil moisture in the lowest density forest increased 20～25 percent more than that in one of the two highest density forests. The runoff volume collected in the planting strip in the afforestation by water-harvesting was 30 percent more than that in the conventional afforetation, and the difference of soil moisture between the two forests was about 10 percent. Because of the different water condition and dominant effect of forest density, the growth of height, DBH (diameter at breast height) and volume between different treatment forests were greatly different. Especially, the difference of total increment was the most remarkable. The growth status would be worse when the forest density was more than 1000 stands·hm^-2. The current annual increment of atands put up an annual variation law with precipitation. The maximum of the current annual increment arose in the year with highest precipitation. The maximum of the current annual increment in the highest density forest had arisen at the earlier age.

Key words: Afforestation by water-harvesting, Runoff volume collected in the planting strip, Growth of forest tree, The Loess Plateau