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Although it is generally accepted that the rate of accumulation of biomass declines as forests age, little is known about the relative contributions to this decline of changes in net primary production (NPP) and tree mortality. We used 10–15 years of observations of permanent plots in three small watersheds in and near the H.J. Andrews Experimental Forest, Oregon, to examine these issues. The three watersheds are of similar elevation and potential productivity and support young (29 years at last measurement), mature (∼100 years) and old (∼400 years) forest dominated by  Pseudotsuga menziesii  (Mirb.) Franco and  Tsuga heterophylla  (Raf.) Sarg. Accumulation of tree bole biomass was greatest in the young stand, reaching ∼7 Mg ha −1  year −1  in the last measurement interval. Bole biomass accumulation was relatively constant (∼4–5 Mg ha −1  year −1 ) in the mature stand, and there was no net accumulation of bole biomass in the old-forest stand. The NPP of boles increased with time in the young stand, from ∼3 to ∼7 Mg ha −1  year −1 , but was nearly constant in the mature and old-forest stands, at ∼6 and 3–4 Mg ha −1  year −1 , respectively. Mortality increased slowly in the young stand (from &lt; 0.1 to 0.3 Mg ha −1  year −1 ), but fluctuated between 1–2 and 2–6 Mg ha −1  year −1  in the mature and old-forest stands, respectively. Thus, declining biomass accumulation with stand age reflects, in approximately equal amounts, both decreasing NPP and increasing mortality.

Trends in bole biomass accumulation, net primary production and tree mortality in Pseudotsuga menziesii forests of contrasting age