Twig abscission (cladoptosis) and its ecophysiological significance for decline symptoms inQuercus robur L | Ökophysiologische Bedeutung von Zweigabsprüngen (Kladoptosis) unter besonderer Berücksichtigung der Symptomatologie vonQuercus robur L

The process known as cladoptosis has been investigated inQuercus robur L.. The results show an ontogenetic alteration of the hydraulic architecture which is additionally modified by the vigour of the tree.The preformation of the abscission zone is still present in all lateral buds during bud dormancy. Terminal shoots, however, do not form an abscission zone at their base at any time, which causes their inability to abscise. During juvenile ontogenesis the absciseability of the lateral buds is lost immediately after sprouting. In the further course of ontogenesis the formation of regular xylem (integration) within the stem junctions of the lateral shoots is retarded. The constricted conduits within the abscission zone limit hydraulic conductivity, equivalent to an increase in the hydraulic resistance. However, the absciseable twigs and branches are in an indifferent stage in which they are separated from the regular hydraulic system of the tree. In this stage hydraulically separated shoots are either connected by integration or disconnected by abscission. Negative environmental conditions increase the number of indifferent shoots. In this situation integration only 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 resistance of the shoot is increased by reduced integration dynamics. The increased number of abscission zones induced by unfavourable environmental conditions enables the tree to avoid runaway embolism during excessive droughts.The active control given by cladoptosis allows the tree to evaluate and control the xylem flow within its branching pattern. The integration and abscission of twigs varies according to environmental conditions. Years with water deficiencies reduce the photosynthesis of the indifferent (less vigorous) twigs and let them cavitate earlier, whereas photosynthesis of the youngest as well as the integrated twigs obviously remains unaffected.