Litter Accumulation and Its Impact Factor of Three Main Forest Communities in Xinglong Mountain, Gansu Province The accumulation, decomposition and ecological effects of above-ground litter in terrestrial ecosystem STUDY ON THE LITTER ACCUMULATION IN A LEYMUS CHINENSIS GRASSLAND: THE LITTER ACCUMULATION UNDER NATURAL CONDITIONS AND THE EFFECTS OF GRAZING AND CUTTING ON LITTER ACCUMULATION Seasonal dynamics of litter accumulation in major forest communities on the northern slope of Changbai Mountain, Northeast China