2.3 Biomass determination
For each treatment, three 20 × 20 m plots were set aside. The diameter at breast height (DBH), height of the tree, and basal diameter were recorded for all the trees in each plot. Also, a total of 8 trees ofPopulus × beijingensis , 11 of P. sylvestris var.mongolica , and 13 of U. pumila were harvested from the respective plots, including roots to a depth of 100 cm. The trees were washed and cut into 2 m sections, and each section was separated into stems, branches, and leaves. Portions of stems, branches, leaves, and roots were sampled from each tree. All samples were weighed in the field, packed into polythene bags, transported to the laboratory, and dried to constant weight at 80 °C for estimating their C content. Biomass of individual trees was estimated using allometric regressions (Table 1).
Biomass of the herbs component of the vegetation was measured by destructive sampling in five randomly chosen 1 × 1 m subplots within each main plot. The above-ground plant parts and roots were harvested in each subplot, and their fresh weights recorded in the field. Subsamples of above-ground plant parts and roots were packed into polythene bags, transported to the laboratory, and dried to constant weight at 80 °C for estimating their C content. The biomass of the herbs component was determined from the ratio of the dry weight to the fresh weight.
Biomass of the litter was estimated the same way as that for the herbs component.