Abstract:Majority of the natural forests in UMR are mature and over-mature. Acturate estimation of the Biomass in the old-growth forests will help quatify actual carbon sequestration and carbon sink size. The allometric equations of the relationship between above-ground biomass (AGB) of various tree organs and diameter at breast height (DBH) and tree height, which are constructed on the 4 consecutive forest resource inventory data of 27 permanent plots, was used to explore the dynamics of the above-ground biomass density (AGBD) and its temporal and spatial variation of sub-alpine old-growth forest in UMR. The results showed: (1) the net increase in AGBD was (27.311±15.580) Mg·hm-2 while the mean annual growth rate and the mean annual mortality rate were (1.930±1.091) Mg·hm-2·a-1 and (2.271±1.424) Mg·hm-2·a-1 during 1988-2002, respectively. (2)the AGB depended largely on the growth and mortality rate of the remaining trees of different DBH classes and recruitment rate from one DBH class to another as well. The largest increment component of AGB came from DBH class at 20 cm to 40 cm, whereas the minimum increment component of AGB was above 80 cm. The net negative increment of AGB occurred at DBH ranging from 40-60cm to 60-80cm. (3)There were temporal and spatial variations of AGB in the alpine old-growth forests as AGB changed over time for the same sampling plot and AGB varied with location or sites for the same period. The variations not only reflected in numerical value but also in positive or negative biomass increment.