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 sh oot of young Scots pine (Pinus sylvestris) depended on the extent and timing of the damage, and on whether the damaged part was physiologica lly 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 ou tcome of treatments. 2. Damage to needles had either growth-stimulatin g 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: 1 00% but not 50% defoliation of the leader shoot significantly decrease d average needle mass and length, and to a lesser extent shoot size. 4 . The impact of defoliation depended on the relative position of the s hoot and on the timing of defoliation. The growth of new shoots, which were situated 'above' the defoliated shoot, was reduced. This was mos t apparent in the trees defoliated late in the season. The growth of s hoots that were situated 'below' the defoliated shoot was increased. 5 . The results show that the responses of Scots pine to simulated herbi vory are dependent on the within-tree regulation of growth and that se emingly minor differences in the methods of simulated herbivory may ex plain the great variability in the results obtained in studies of herb ivory-induced responses in trees.