Effects of different pruning regimes on growth and sapwood area of Scots pine

Abstract

Scots pine trees (about 25 years old) were deprived of 50–75% of their needle biomass by three different pruning regimes, to study the allocation of growth, heartwood formation and homeostatic adjustment of the stem sapwood area to the reduced tree crown. After four growing seasons (including that of the pruning), differential effects in radial and basal area growth were observed at different stem heights. Unilateral stem pruning reduced growth all along the bole, whereas stem pruning from below caused greater reductions in the lower stem, and pruning from above actually increased radial growth above and decreased growth below the pruned crown fraction. Concurrent with the increase in radial growth, the branch basal area (representing the needle biomass) of the developing new whorls increased in the trees pruned from above, but not in other treatments. Height growth was only slightly affected by the treatments. The pruned trees had a total volume growth loss during the study period in the range 24–33%, compared with the controls. The pruning treatments resulted in a decrease in conducting sapwood area and an increase in non-conducting ‘heartwood’ (i.e. immobilized sapwood rather than true heartwood) area at breast height, but the homeostatic adjustment was still incomplete after four growing seasons, except for the upper whorls developed after the prunings. Thus, reliable needle biomass estimates cannot be derived from sapwood basal area data during the process of homeostatic adjustment after a sudden loss in needle biomass.