THE RELATION BETWEEN ABOVEGROUND AND BELOWGROUND BIOMASS ALLOCATION PATTERNS AND COMPETITIVE ABILITY

In a 2-year experiment, the evergreen shrubs Erica tetralix and Calluna vulgaris (dominant on nutrient-poor heathland soils) and the perennial deciduous grass Molinia caerulea (dominant on nutrient-rich healthland soils) were grown in replacement series in a factorial combination of four competition types (no competition, only aboveground competition, only belowground competition, full competition) and two levels of nutrient supply (no nutrients and 10 g N + 2 g P + 10 g K m-2 yr-1). Both in the unfertilized and in the fertilized treatments Molinia allocated about twice as much biomass to its root system than did Erica and Calluna. In all three species the relative amount of biomass allocated to the roots was lower at high than at low nutrient supply. The relative decrease was larger for Molinia than for Erica and Calluna. In the fertilized monocultures biomass of all three species exceeded that in the unfertilized series. Molinia showed the greatest biomass increase. In the unfertilized series no effects of interspecific competition on the biomass of each species were observed in either of the competition treatments. In the fertilized mixtures where only belowground competition was possible Molinia increased its biomass at the expense of both Erica and Calluna. When only aboveground competition was possible no effects of interspecific competition on the biomass of the competing species were observed. However, in contrast with the evergreens, Molinia responsed by positioning its leaf layers relatively higher in the canopy. The effects of full competition were similar to those of only belowground competition, so in the fertilized series belowground belowground competition determined the outcome of competition. The high competitive ability of Molinia at high nutrient supply can be attributed to the combination of (1) a high potential productivity, (2) a high percentage biomass allocation to the roots, (3) an extensive root system exploiting a large soil volume, and (4) plasticity in the spatial arrangement of leaf layers over its tall canopy. In the species under study the allocation patterns entailed no apparent trade-off between the abilities to compete for above- and belowground resources. This study suggests that this trade-off can be overcome by: (1) plasticity in the spatial arrangement of leaf layers and roots, and (2) compensatory phenotypic and species-specific differences in specific leaf area and specific root length.