Abstract: Pinus koraiensis have two distinguished features.One is the long lifespan and another is the large biomass in the natural mature forest.Just the two characteristics consist of their own specific pattern in when the light intensity in the community dominated by Pinus koraiensis becomes too poor for the pine seedlings to grow,the increasing rate of the pine tree population decreases.Then the fast-growing broad-leaved trees population increases with the decreasing pine individuals in the community.In the meantime the canopy of the broad-leavedtrees creates a suitable condition for the young pine trees to grow.Finally the pine trees become dominant species in the community again.

Key words: rotational tillage system, summer fallow period, winter wheat, soil water, yield, Southern Ningxia arid area