Effects of shoot pruning on stem growth, needle biomass, and dynamics of carbohydrates and nitrogen in Scots pine as related to season and tree age

Abstract

In a field experiment in central Sweden, current shoots representing one-fifth of the needle biomass were removed from Scots pine (Pinussylvestris L.) by hand pruning young and old trees and by inducing Tomicuspiniperda (L.) (Coleoptera: Scolytidae) attacks on caged young trees. Branches in the uppermost whorls were pruned in later summer, early summer, or both. Starch and nitrogen concentrations in needles were monitored for two growing seasons. After four seasons the trees were felled and effects on growth were recorded. Starch and nitrogen concentrations in needles of pruned branches decreased and increased, respectively, compared with the controls. In old trees, but not in young ones, the raised nitrogen level persisted for 3 years after pruning. Starch and nitrogen concentrations in unpruned branches of pruned whorls did not change compared with the control until these branches became pruned, i.e., each branch acted as a semiautonomous integrated physiological unit. At felling, pruned trees were short of a portion of needle biomass equal to that removed, while at the same time, the needle biomass grown out above the pruned whorls was larger than the corresponding part of the controls. Stem volume losses in old hand-pruned trees were larger than and lasted longer than in young ones and were not yet completed four growing seasons after treatment. In a stepwise linear regression analysis, final needle biomass explained most of the total variation in volume growth of young hand-pruned trees, whereas for old trees, intertree competition was more important. Responses of beetle-pruned trees were similar to those of young hand-pruned trees. Differences in response to pruning and defoliation and in recovery between young and old trees are discussed in terms of source and sink theory and of compensatory mechanisms and carbohydrate limitation, respectively.