Equations were developed to estimate components of above-ground woody biomass, as a function of diameter, height, spacing and age for two hybrid poplar clones in western Washington. Independent and harmonized fitting techniques are compared. With the small sample sizes that are unavoidable in such experiments, harmonized equations provided more useful and consistent estimates of biomass increment than did independent equations by age and spacing. They were also better suited to interpolation and extrapolation of long-term trends of biomass increment than those based on the independent fits. Copyright (C) 1996 Elsevier Science Ltd.