Biomass Allocation and Light Partitioning Among Dominant and Subordinate Individuals inXanthium canadenseStands

Patterns of above-ground biomass allocation and light captureby plants growing in dense stands or in isolation were studiedin relation to their height. A canopy model was developed tocalculate light absorption by individual plants. This modelwas combined with data on canopy structure and patterns of biomassallocation for solitary plants and for plants of different heightsin dense mono-specific stands of the dicotyledonous annualXanthiumcanadenseMill. There were four stands, and stand height increasedwith age and nutrient availability. The allometric relationshipbetween height and mass differed considerably between plantsin stands and those growing in isolation and also between plantsof different heights within stands. The proportion of shootmass in leaf laminae (LMR) decreased with increasing plant height,but solitary plants had a higher LMR than competing plants ofthe same height. Thus, in contrast to previous assumptions,LMR of competing plants is not strictly determined by biomechanicalconstraints but results from a plastic shift in biomass allocationin response to competition. Average leaf area per unit leafmass (SLA) decreased with increasing photosynthetic photon fluxdensity (PPFD) independent of nutrient availability. Consequently,taller, more dominant plants in stands had a lower leaf arearatio (LAR: LAR=LMRxSLA) than shorter, more subordinate plants.Dominant plants absorbed more light both per unit leaf area(