CANOPY STRUCTURE AND NITROGEN DISTRIBUTION IN DOMINANT AND SUBORDINATE PLANTS IN A DENSE STAND OF AMARANTHUS-DUBIUS L WITH A SIZE HIERARCHYOF INDIVIDUALS

The objective was to investigate how nitrogen allocation patterns in p lants are affected by their vertical position in the vegetation (i.e. being either dominant or subordinate). A garden experiment was carried out with Amaranthus dubius L., grown from seed, in dense stands in wh ich a size hierarchy of nearly equally aged individuals had developed. A small number of dominant plants had most of their leaf area in the highest layers of the canopy while a larger number of subordinate plan ts grew in the shade of their dominant neighbours. Canopy structure, v ertical patterns of leaf nitrogen distribution and leaf photosynthetic characteristics were determined in both dominant and subordinate plan ts. The light distribution in the stands was also measured. Average N contents per unit leaf area (total canopy nitrogen divided by the tota l leaf area) were higher in the dominant than in the subordinate plant s and this was explained by the higher average MPA (leaf dry mass per unit area) of the dominant plants. However, when expressed on a weight basis, average N contents (LNC(av); total canopy N divided by the tot al dry weight of leaves) were higher in the subordinate plants. It is possible that these higher LNC(av) values reflect an imbalance between carbon and nitrogen assimilation with N uptake exceeding its metaboli c requirement. Leaf N content per unit area decreased more strongly wi th decreasing relative photon flux density in the dominant than in the subordinate plants showing that this distribution pattern call be dif ferent for plants which occupy different positions in the light gradie nt in the canopy. The amount of N which is reallocated from the oldest to the younger, more illuminated leaves higher up in the vegetation m ay depend on the sink strength of the younger leaves for nitrogen. In the subordinate plants, constrained photosynthetic activity caused by shading might have reduced the sink intensity of these leaves.