The long-term effect of partial defoliation on the yield of short-rotation coppice willow

Five varieties of willow were planted in a randomised block design at the Northern Ireland Horticulture and Plant Breeding Station, Loughgall, County Armagh in 1997. Three treatments were applied: control (no treatment), manual defoliation and routine control (spraying) of willow beetles. The defoliation was carried out at 75% for the first 2 years and thereafter 25% up to the first harvest in January 2001. During the second production cycle, no manual defoliation was applied in order to observe the ability of the different varieties to recover from earlier damage. The variety 78112 exhibited the highest yield loss in the first cycle as a result of defoliation (∼70%), while the vigorous variety Tora suffered least yield loss (35-43%). In the recovery phase (second production cycle 2001-2003), all the varieties showed little improvement in yields in plots that had been defoliated previously, indicating a long-term legacy of yield loss as a result of defoliation. The possible reasons for this inability to recover are discussed.