WHY DOES A BRANCH SUFFER MORE AFTER BRANCH-WIDE THAN AFTER TREE-WIDE DEFOLIATION

We investigated the way in which the pattern and extent of defoliation (a single branch or all the branches of the tree) and fertilization affected the growth and foliage carbon/nitrogen ratios of Scots pine (Pinus sylvestris) branches of different ages and thus different relative growth rates. Our objective was to contrast two hypotheses, operating at different levels, explaining consequences of defoliation: the carbon/nutrient (C/N) balance hypothesis, which builds on defence-related changes in foliage carbon vs nutrients, and the source/sink hypothesis which emphasizes defoliation-caused disturbances in the source/sink; balance, and the consequent quantitative changes in growth. Regardless of the age of the branch, the consequences of defoliation for branch growth depended less on the extent of the damage than on the pattern of the damage. When a single branch was defoliated, its growth retarded more than when other branches in the tree were also defoliated. However, older and more slowly growing branches in totally defoliated trees, but not in trees in which a single branch was defoliated, compensated better for the loss of foliage than younger and more rapidly growing branches. The predictions of the CIN hypothesis were fulfilled after tree-wide but not after branch-wide herbivory: defoliation significantly decreased the foliage carbon/nitrogen ratio in two- and five-year-old branches in whole-tree but not in single-branch defoliation trees. We discuss why the mechanisms behind these responses probably were not those assumed by the C/N hypothesis. A key problem is that the CIN hypothesis has to take into account the changes occurring in the allocation of resources within a tree after defoliation. Our results demonstrate that the sink/source hypothesis, based on relative resource competition among branches, satisfactorily explained changes in branch growth activity after defoliation. It also offered a natural explanation for the stronger quantitative and weaker qualitative responses to branch-wide than to tree-wide herbivory.