Carbon gain in the competition for light between genotypes of the clonal herb Potentilla reptans

Different views exist as to what traits will lead to dominance when plants compete for light. One view is that taller plants with better relative positions in the canopy will exclude shorter plants because they intercept almost all light and thus can achieve a higher carbon gain. Alternatively, resource competition models predict that plants that are capable of positive net photosynthesis at the lowest light level will win. In a 5-year-old dense competition experiment with 10 genotypes of the clonal plant Potentilla reptans, both these views were tested to see if either of them could explain the dominance of one of the genotypes, or the possible coexistence of several others. 2. Using a combination of measured morphological and physiological traits, a canopy model was constructed to calculate whole-shoot daily photosynthetic rates of the genotypes in the different layers of the canopy in relation to the invested mass. 3. Results show that the dominant genotype exhibited characteristics of relative shade tolerance: low rates of light-saturated photosynthesis and respiration. This resulted in a calculated daily carbon gain at the bottom of the canopy, where other genotypes could not achieve that. However, the dominant genotype did not have the highest photosynthetic rates throughout the whole canopy. Some genotypes that persisted in the stand in coexistence with the dominant one achieved greater daily carbon gain at the top of the canopy. 4. Synthesis. The dominant genotype had characteristics similar to those predicted by resource competition models such as the ability to have positive growth at lower light levels. The persistence of several other genotypes, in contrast, may be explained by traits that allowed them to achieve higher carbon gains at the top of the canopy. This suggests that the light gradient formed by the plants themselves creates enough heterogeneity for strategies for dealing with different light requirements to coexist, even within a single species.