Five willow clones and one hybrid poplar clone were planted during 1987 at 0.3 x 0.3 m spacing and harvested annually for 10 years. Half of the trees were fertilized annually with N, P and K and all trees were irrigated beginning in the third growing season. Annual biomass production fit the logistic growth curve well for four of the clones with r(2) values ranging from 0.91 to 0.54, suggesting that well-adapted willow clones can be consistently productive for at least 10 years with annual harvesting. Fertilizer did not increase the maximum biomass production level attained, but it reduced the time required to reach maximum production by 1 year. The correlation between annual biomass production and the number of growing degree days during years 4-10 was high, ranging from 0.95 to 0.66. (C) 2001 Elsevier Science Ltd. All rights reserved.