Simulated beetle defoliation on willow genotypes in mixture and monotype plantations

The effect of simulated beetle damage (0%, 25%, 50% and 75% mechanical defoliation) on 12 willow genotypes, grown in short-rotation coppice, was studied in a modified criss-cross experimental design. The design enabled the above-ground effects Of monoculture and mixed planting to be assessed. Repeated measurements were modelled to produce derived variables in terms of time or, more appropriately, in terms of accumulated day length (i.e. 'developmental time') units. These derived variables were then analysed using the residual Maximum Likelihood (REML) method implemented in GenStat (TM) (2001). No significant. competition effect between the genotypes due to planting regime was detected. Genotypes Salix viminalis x Salix schwerinii 'Beagle' and S. viminalis x S. schwerinii 'Torhild' were found to have the greatest rate of increase in leaves regardless of defoliation and also the greatest height prior to defoliation. Genotype Salix dasyclados 'Loden' showed the highest rate of growth under the stress of defoliation. When assessing height at the end of the growing season, S. viminalis x S. schwerinii 'Olof' was the highest genotype for 25% and 75% levels of defoliation, but genotypes Salix aurita x Salix cinerea 'Delamere', Loden and S. viminalis x Salix burjatica 'Ashton Parfitt' appeared to be most tolerant by having consecutively lower base day lengths (i.e. increasing the accumulation of developmental units and the length of the growing season) for increasing defoliation. Shorter genotypes tended to be more tolerant, but of the higher genotypes reaching a control height of greater than 3 in by the end of the growing season, S. viminalis x S. schwerinii 'Tora' and Beagle performed best to 50% defoliation.