Plastic responses in the competition for light among genotypes of a stoloniferous species

The ability to adjust the phenotype (e.g. plasticity) is thought to be beneficial for the performance of a plant, because it prevents overinvestment in support structures when plant density is low, and allows a plant to position its leaves high in the canopy when density is high. However, as the plastic responses differ within species, exclusion of less responsive genotypes from dense vegetation is likely to occur. A 5 year old competition experiment between 10 genotypes of the stoloniferous plant Potentilla reptans was used to study the consequences of the genotypic differences in plasticity, and the interplay between these genotypes, in order to gain more insights in the mechanisms behind the competition for light Within the competition experiment, leaves that were positioned higher in the canopy had higher light interception and higher photosynthetic rates per unit invested biomass than lower places leaves. Placing laminas at the top of the canopy is thus of great importance to the performance of a genotype. Using artificially created light gradients, it was also shown that the speed of height increase determines which genotypes can place their leaves at the top. Height increase of the canopy differed between mono-genotypic stands, and thus depends on the genotypes that form the canopy. A canopy model of the competition experiment showed that the dominant genotype had rather shade tolerant photosynthetic characteristics, which allowed it to have positive carbon gain at the bottom of the vegetation. Therefore, low leaf turnover may have contributed to the abundance of this genotype. Coexistence could than occur because other genotypes have a higher carbon gain in the top layers of the canopy. A game theoretical analysis of the canopy model showed that for all genotypes the total lamina area that would maximize the photosynthetic rate of the genotype itself was higher than total the lamina area that would maximize the photosynthetic rate of the vegetation as whole. Selection would thus favor the former strategy, leading to lower biomass production of the whole canopy. Finally, the analyses indicate that a decrease of the total lamina area of the dominant genotypes would increase its own photosynthetic rate, but that it would increase the performance of other genotypes more. Yet although a mutation in the lamina area of the dominant genotype would not increase its performance, the vegetation is not evolutionarily stable, as a mutant in the lamina area of the other genotypes would lead to better performance of these mutants. Overall the results suggest that the performance of an individual is not only determined by its own characteristics, but also by that of its competitors. In addition to the ability to reach the top of canopy, relatively shade tolerant characteristics may be selected for if plant density is high. The findings also demonstrate that is important to know at what level of organization selection operates. The understanding of competitive exclusion would greatly benefit if plastic response were studied in relation to photosynthetic traits, and to temporal changes that occur during the growing season.