Biomass allocation and light partitioning among dominant and subordinate individuals in Xanthium canadense stands

Patterns of above-ground biomass allocation and light capture by plants growing in dense Stands or in isolation were studied in relation to their height. A canopy model was developed to calculate light absorption by individual plants. This model was combined with data on canopy structure and patterns of biomass allocation for solitary plants and for plants of different heights in dense mono-specific stands of the dicotyledonous annual Xanthium canadense Mill. There were four stands, and stand height increased with age and nutrient availability. The allometric relationship between height and mass differed considerably between plants in stands and those growing in isolation and also between plants of different heights within stands. The proportion of shoot mass in leaf laminae (LMR) decreased with increasing plant height, but solitary plants had a higher LMR than competing plants of the same height. Thus, in contrast to previous assumptions, LMR of competing plants is not strictly determined by biomechanical constraints but results from a plastic shift in biomass allocation in response to competition. Average leaf area per unit leaf mass (SLA) decreased with increasing photosynthetic photon flux density (PPFD) independent of nutrient availability. Consequently, taller, more dominant plants in stands had a lower leaf area ratio (LAR: LAR = LMR x SLA) than shorter, more subordinate plants. Dominant plants absorbed more light both per unit leaf area (Φ(area)) and per unit mass (Φ(mass)) than subordinate plants. Their greater Φ(area) more than compensated for a lower LAR (Φ(mass) = Φ(area) x LAR). We conclude that the greater Φ(mass) of dominant plants is quantitative evidence that success in competing for light is disproportionately related to the size of the shoot (i.e. asymmetric competition). We know of no other study which has demonstrated this with quantitative estimates of light acquisition in relation to plant size. This result is contrasted with previous studies on multi-specific stands in which Φ(mass) of dominant and subordinate species is similar, and we discuss why light competition in mono-specific stands is asymmetric whereas it can be size-symmetric in multi-specific stands, with similar or greater LAI.