Plant foraging and dynamic competition between branches of Pinus sylvestris in contrasting light environments

1 The morphological plasticity of sun and shade branches of Pinus sylvestris growing at the edge of a 'tree-patch' created in 1989, when surrounding trees were removed, was compared with that of branches of trees growing in the centre of the patch. 2 In 1992, we selected 10-year-old branches. Yearly growth increments (long-shoots) and emerging buds were individually marked, architectural parameters measured, the branch topology mapped, and the number of male and female cones counted. In 1993, the survival of growth increments and buds was recorded and the branches were harvested to determine dry mass of growth increments and needles. 3 The size of growth increments produced after 1989, i.e. their length, total dry mass and needle dry mass, decreased in the order sun branches of edge trees < branches of centre trees < shade branches of edge trees. Thus, the growth increments produced on the shade branches of trees that also had branches in the sun were consistently smaller than the growth increments on the branches of centre trees growing completely in the shade. 4 The number of new growth increments produced after 1989 was highest in sun branches and lowest in shade branches of edge trees. Survival of growth increments and buds was higher in edge than in centre trees; no difference was found between sun and shade branches of edge trees. 5 In shade branches of edge trees, branching angles between first- and second-order growth increments were highest and increased from older to younger growth increments. This was interpreted as 'bending' towards the edge of the patch. 6 Production of female cones was almost totally restricted to edge trees and higher in sun than in shade branches. 7 One prediction of an optimal foraging strategy, i.e. the production of more growth increments and buds in higher light, was supported by the data, whereas the other prediction, i.e. decreased length of growth increments, could only be supported when it was expressed per unit dry mass. Thus, the hypothesis of an adaptive foraging strategy in plants was rejected in favour of a `passive' growth null hypothesis. 8 The results suggest that both growth increments within branches and branches within trees are physiologically integrated and their `foraging behaviour' can only partly be understood in terms of their local environment. We interpreted the observed differences between shade branches of edge and centre trees as correlative growth inhibition.