Branch morphology and abscission in kauri, Agathis australis (Araucariaceae)

Abstract

Agathis australis is unusual in that self-pruning of branches occurs. This capability persists throughout the life of the tree and all branches have this potential. Four morphologically different branch types were identified. The first-formed (early) branches on kauri saplings are longer, more slender, and longer lived than those formed later. They also have a juvenile leaf form whereas the later-formed branches on the saplings have a transitional leaf form. In the mature tree, the adult support branches bear foliage branches, which have the shortest life span of all the branch types. Analysis of shed branches showed that both early- and late-formed sapling branches have a greatly enlarged branch diameter at the base, a smooth separation face, and a reduction in cross-sectional area of vascular tissue at the point of separation when the branch abscises. In contrast, adult foliage branches show little indication of swelling of the branch base, no reduction of vascular tissue, a rough separation face, and broken vascular tissue after abscission, apparently from mechanical force breaking the wood. It appears that mechanical force is necessary for abscission to occur in adult foliage branches. When early-formed sapling branches were cut back to 50 mm stumps, the stumps abscised within six weeks in almost all cases. This occurred irrespective of the time of year that the treatment was carried out. Intact control branches did not abscise within the same time period.