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 sylvestri s growing at the edge of a 'tree-patch' created in 1989, when surrounding t rees were removed, was compared with that of branches of trees growing in t he 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 paramete rs measured, the branch topology mapped, and the number of male and female cones counted. In 1993, the survival of growth increments and buds was reco rded and the branches were harvested to determine dry mass of growth increm ents and needles. 3 The size of growth increments produced after 1989, i.e. their length, tot al dry mass and needle dry mass, decreased in the order sun branches of edg e trees > branches of centre trees > shade branches of edge trees. Thus, th e growth increments produced on the shade branches of trees that also had b ranches 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 su n branches and lowest in shade branches of edge trees. Survival of growth i ncrements and buds was higher in edge than in centre trees; no difference w as found between sun and shade branches of edge trees. 5 In shade branches of edge trees, branching angles between first- and seco nd-order growth increments were highest and increased from older to younger growth increments. This was interpreted as 'bending' towards the edge of t he patch. 6 Production of female cones was almost totally restricted to edge trees an d higher in sun than in shade branches. 7 One prediction of an optimal foraging strategy, i.e. the production of mo re growth increments and buds in higher light, was supported by the data, w hereas the other prediction, i.e. decreased length of growth increments, co uld only be supported when it was expressed per unit dry mass. Thus, the hy pothesis 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 branc hes within trees are physiologically integrated and their 'foraging behavio ur' can only partly be understood in terms of their local environment. We i nterpreted the observed differences between shade branches of edge and cent re trees as correlative growth inhibition.