EFFECTS OF BRANCH LENGTH ON CARBON-ISOTOPE DISCRIMINATION IN PINUS-RADIATA

Gas exchange was measured on a pruned Pinus radiata D. Don hedge and o n a long-branch P. radiata tree near Hamilton, New Zealand, in spring 1993 when soil water content was close to field capacity. Foliage at t he end of long branches (9.0 m) showed a marked drop in net photosynth etic rate and stomatal conductance as the saturation deficit increased , whereas foliage on short branches (0.5 m) showed little change. Mean foliage delta(13)C was -30.1 parts per thousand for short branches an d -26.3 parts per thousand for long branches. Foliage delta(13)C was c orrelated with branch length in two genetically improved P. radiata se edlots at four stocking densities. The multinodal seedlot had shorter branches and more C-13-depleted foliage compared with branches and fol iage from the long internode seedlot. There was a strong effect of sto cking density on carbon isotope composition in both seedlots. We concl ude that branch morphology affects foliage gas exchange properties and foliage carbon isotope composition.