Biomass from short rotation forestry can provide a valuable energy source but maintaining the quality of the material during handling and storage is challenging. The relatively young plant material is highly prone to microbial degradation and can consequently suffer changes in fuel quality. The aim of this work is to evaluate the role of particle size and pile height on the processes which occur during storage and how it affects fuel quality. Fresh willow shoots (Salix viminalis) were comminuted into chips and chunk wood (nominal size 22-45 mm), stored in 3 and 6 in high piles for 2 and 3 months respectively. Results showed that the four piles exhibited different behaviour. Temperature development was rapid and prominent in the chipped willow particularly in the 6 in high pile. In the chunk woodpiles. temperature rise was very slow and became notably higher than ambient temperature only after 2 months of storage in the 6 in high pile. The effect of storage on fuel quality was evaluated with respect to moisture content. heating value. particle size distribution, and ash content. In general, salix chips had a relatively lower moisture content and energy, value than chunk wood by the end of storage. Evaluation of fungal activity during storage showed that marked increase in spore counts was closely associated with heat development. (C) 2004 Elsevier Ltd. All rights reserved.