Using periodic remeasurements of tagged trees in nine 0.4-ha sample plots in aPiceasitchensis(Bong.) Carr. –Tsugaheterophylla(Raf.) Sarg. forest at Cascade Hand Experimental Forest, Oregon, we calculated that biomass of bolewood increased from 570 Mg•ha−1at age 85 years to 760 Mg•ha−1at age 138 years. Net primary production of bolewood declined from 11 to about 6 Mg•ha−1•year−1, and mortality loss increased from 2 to about 6 Mg•ha−1•year−1. Values for 37-year-old plots in the same area were 210–360 Mg•ha−1•year−1bole biomass, 7–20 Mg•ha−1•year−1bolewood production, and 0–2 Mg•ha−1•year−1mortality loss. Indications are that bolewood production and biomass were lower in the older plots when they were 37 years old. In the older plots, biomass did not increase between ages 120 and 138. Of the photosynthate potentially available for bolewood production, some replaces biomass lost via mortality and some is allocated to maintenance (respiration plus allocation to fine roots). We estimate that one-quarter to one-half of the production is lost by mortality, and that mortality loss may thus be an important factor limiting forest biomass accumulation.