EFFECTS OF SIMULATED DEFOLIATION AND DEBUDDING ON NEEDLE AND SHOOT GROWTH IN SCOTS PINE (PINUS-SYLVESTRIS) - IMPLICATIONS OF PLANT SOURCE-SINK RELATIONSHIPS FOR PLANT-HERBIVORE STUDIES

1. We studied how the outcome of simulated herbivory on the leading shoot of young Scots pine (Pinus sylvestris) depended on the extent and timing of the damage, and on whether the damaged part was physiologically a sink (bud) or a source (needle). In addition we studied how the position of the shoot in relation to the damaged shoot affected the outcome of treatments. 2. Damage to needles had either growth-stimulating or growth-suppressing effects, whereas damage to buds had a positive effect on growth. 3. Needles and shoots were able to compensate for a significant amount of defoliation but not for complete defoliation: 100% but not 50% defoliation of the leader shoot significantly decreased average needle mass and length, and to a lesser extent shoot size. 4. The impact of defoliation depended on the relative position of the shoot and on the timing of defoliation. The growth of new shoots, which were situated 'above' the defoliated shoot, was reduced. This was most apparent in the trees defoliated late in the season. The growth of shoots that were situated 'below' the defoliated shoot was increased. 5. The results show that the responses of Scots pine to simulated herbivory are dependent on the within-tree regulation of growth and that seemingly minor differences in the methods of simulated herbivory may explain the great variability in the results obtained in studies of herbivory-induced responses in trees.