Similar feedstock consisting of poplar stems and tops was fed to a commercial drum chipper, before and after a 12-month storage period. The chipper was fed alternately with poplar stems and tops, in order to determine the effect of piece size, tree part and storage period on machine performance. At the end of the storage period, both stems and tops were still almost as wet as at the beginning, and they showed visible signs of decay. Before storage, net chipping productivity was 72 m(3) h(-1) with stems and 30 m(3) h(-1) with tops. After storage, net chipping productivity dropped to 56 and 21 m(3) h(-1), respectively for stems and tops. Power and torque requirements were also reduced. Chip quality degraded with storage, as particle size distribution veered towards a larger incidence of the smallest fractions. In particular, fines (particles <3.15 mm) increased four-fold, reaching the proportion of 11% and 23% in weight for stems and tops, respectively. In general, storage effects were stronger for tops than for stems. Specific fuel consumption was not affected by tree part, storage or their combination. (C) 2015 Elsevier B.V. All rights reserved.