Himalayan subalpine forests with a wide altitudinal range of distribution are an ideal target for quantifying altitudinal change in storage and turnover of stand biomass. Over the entire range from 3100 to 3900 m in a subalpine forest in Langtang, altitudinal change in aboveground biomass and its turnover rate was examined by tree stem allometry and stem wood increment. With altitude, tree height at any given stem diameter decreased. Growth rate in stem diameter was similar across altitude. Large aboveground biomass of about 500 t/ha was maintained by slow demographic biomass turnover rate of 1% per year compared to usual forest ecosystems (1.5 to 2% per year). Decrease in canopy height, aboveground biomass and biomass turnover rate with altitude suggested architectural adaptation to severe conditions for primary production in higher altitude.