[Monitoring of seasonal variation of vegetation cover and evaluation of biological-control factor in orchards of China].
Biological-control factor (B) can quantitatively reflect the effect of vegetation on soil erosion. Vertical photography method was used to monitor the seasonal changes of orchard coverage in 18 small watersheds, covering all five water erosion zones in China. The canopy coverage and ground green coverage were then calculated, and the B values were estimated. The result indicated that the orchard canopy coverage varied with season and distributed with a bell shape. The trend of ground green coverage and total coverage changed with season and was affected by the land management. For the orchard where the land surface was not treated and the weeds could grow freely, the distribution of the green coverage season change for all five soil erosion zones had a bell shape. The total coverage in the Northwest Loess Plateau area changed with season with a bell shape as well. The total coverage in the other four areas did not vary with the season and almost kept constant. For the orchard where land surface was cleaned up from time to time, the green coverage and total coverage in all five water erosion areas irregularly varied with season. The average B value of orchards in Northeast black soil area was the minimum with a value of 0.0006, while that in the Northwest Loess Plateau area was the maximum (0.1212). The average orchard B values in the Northern rocky earthy area, the Southern red soil area and the Southwest rocky earthy area were not significantly different and were 0.0548, 0.0627 and 0.0639, respectively.