In the second paper in this series factorial combinations of two short rotation coppice (SRC) willow varieties (S. viminalis cv. Jorunn and S. x dasyclados) and five planting densities (10,000-111,000 plants ha(-1)) were compared at a site in Cambridgshire, UK. Frequent measurements of above-ground and below-ground biomass accumulation (AGBA and BGBA), canopy radiation interception (I), green leaf area index (GLAI), radiation use efficiency (RUE), and attenuation coefficient (k) were made in 1997, 1998 and 1999 in order to explain density, variety and harvest interval related yield effects. At full canopy, the highest GLAIs attained, usually by the highest plant density, were in the range 5-6.5. GLAI declined rapidly once this peak had been attained. GLAI decreased following canopy closure (radiation interception > 90%), due to loss of leaves lower down the canopy profile. Despite a more rapid loss of leaf area by the highest planting density, GLAD values were consistently greater at the highest planting density (P > 0.05), and GLAD for S. dasyclados was significantly greater than S. viminalis in each year (P < 0.05). GLAD was related to final dry matter production. Higher planting densities also had earlier dates of canopy closure, which were correlated with increased final yield. k values were in the range -0.2-0.5 1, indicating differences in morphology between the varieties: although no consistent trends could be determined. Patterns of AGBA mirrored canopy development. Peak AGBA in the first season was 19 and 14 t ha(-1) for S. viminalis and S. x dasyclados, respectively. End of season yield was significantly lower (11.5 and 12.9 t ha(-1) for S. viminalis and S. x dasyclados, respectively) due to leaf fall and carbohydrate movement. End of season yields followed a clear, significant trend of increased yield with increasing plant density in the case of S. viminalis. This trend was not apparent with S. x dasyclados after the first season. RUE values were significantly increased (P < 0.05) by planting density and variety, ranging from 1.55 to 2.55 g MJ(-1) for 10,000 and 111,000 plant ha(-1), respectively, in S. viminalis, which had significantly higher RUE values than S. x dasyclados. Plant mortality, after three seasons, was approximately 15% at the highest density but less than 5% at the three lowest densities. The two varieties behaved similarly. Thus, very high plant populations were sustained at the high planting densities. An inverse relationship between individual plant weight and planting density was evident, but there was no linear relationship. Higher plant densities supported larger plants than would have been expected from a linear relationship. As a consequence, yields at high densities were higher than yields at low densities.