Twig abscission (cladoptosis) and its ecophysiological significance for decline symptoms in Quercus robur L.

The process known as cladoptosis has been investigated in Quercus robur L.. The results show an ontogenetic alteration of the hydraulic architecture w hich is additionally modified by the vigour of the tree. The preformation of the abscission zone is still present in all lateral bud s during bud dormancy. Terminal shoots, however, do not form an abscission zone at their base at any time, which causes their inability to abscise. Du ring juvenile ontogenesis the absciseability of the lateral buds is lost im mediately after sprouting. In the further course of ontogenesis the formati on of regular xylem (integration) within the stem junctions of the lateral shoots is retarded. The constricted conduits within the abscission zone lim it hydraulic conductivity, equivalent to an increase in the hydraulic resis tance. However, the absciseable twigs and branches are in an indifferent st age in which they are separated from the regular hydraulic system of the tr ee. In this stage hydraulically separated shoots are either connected by in tegration or disconnected by abscission. Negative environmental conditions increase the number of indifferent shoots. In this situation integration on ly occurs in the junctions of the most vigorous twigs. This special reaction optimizes the hydraulic conductivity by distributing a given water flow selectively within the branching. The hydraulic resistan ce of the shoot is increased by reduced integration dynamics. The increased number of abscission zones induced by unfavourable environmental condition s enables the tree to avoid runaway embolism during excessive droughts. The active control given by cladoptosis allows the tree to evaluate and con trol the xylem flow within its branching pattern. The integration and absci ssion of twigs varies according to environmental conditions. Years with wat er deficiencies reduce the photosynthesis of the indifferent (less vigorous ) twigs and let them cavitate earlier, whereas photosynthesis of the younge st as well as the integrated twigs obviously remains unaffected.