Leaf investment and light partitioning among leaves of different genotypes of the clonal plant Potentilla reptans in a dense stand after 5 years of competition

Abstract

Background and aims: While within-species competition for light is generally found to be asymmetric - larger plants absorbing more than proportional amounts of light - between-species competition tends to be more symmetric. Here, the light capture was analysed in a 5-year-old competition experiment that started with ten genotypes of the clonal plant Potentilla reptans. The following hypotheses were tested: (a) if different genotypes would do better in different layers of the canopy, thereby promoting coexistence, and (b) if leaves and genotypes with higher total mass captured more than proportional amounts of light, possibly explaining the observed dominance of the abundant genotypes.

Methods: In eight plots, 100 leaves were harvested at various depths in the canopy and their genotype determined to test for differences in leaf biomass allocation, leaf characteristics and the resulting light capture, calculated through a canopy model using the actual vertical light and leaf area profiles. Light capture was related to biomass to determine whether light competition between genotypes was asymmetric.

Key results: All genotypes could reach the top of the canopy. The genotypes differed in morphology, but did not differ significantly in light capture per unit mass (Phi(mass)) for leaves with the laminae placed at the same light levels. Light capture did increase disproportionately with leaf mass for all genotypes. However, the more abundant genotypes did not capture disproportionately more light relative to their mass than less-abundant genotypes.

Conclusions: Vertical niche differentiation in light acquisition does not appear to be a factor that could promote coexistence between these genotypes. Contrary to what is generally assumed, light competition among genetic individuals of the same species was size-symmetric, even if taller individual leaves did capture disproportionately more light. The observed shifts in genotype frequency cannot therefore be explained by asymmetric competition for light.

Fig. 1.

Average frequency per plot (% of total leaves + 1 s.e.) of all genotypes in the three layers harvested: bottom, middle and top layer. Data are from J. F. Stuefer et al. (unpubl. res.).

Fig. 2.

Allometric relationships between leaf characters of eight genotypes of Potentilla reptans and relative PPFD at the position of each leaf lamina: (A) total leaf weight (g); (B) specific lamina area (m² g⁻¹); (C) lamina mass ratio (g g⁻¹); (D) lamina area ratio (m² g⁻¹). Lines represent linear regression lines of the genotypes as indicated, based on log-transformed data of all plots pooled together. Covariance analysis is given in Table 1.

Fig. 3.

Relationship between light capture efficiency (Φ_mass, mol g⁻¹ d⁻¹) and relative PPFD. Lines represent linear regression; for explanation of genotypes, see key in Fig. 2A. Note log scales. For statistics see Table 1.

Fig. 4.

Power relationship between light capture (mol d⁻¹) and mass (g). Data are represented on a normal scale for clarity. (A) Power relationship between light capture of individual leaves (mol d⁻¹) and total leaf weight (TLW, g). β is the regression coefficient in the linear expression: log Φ_d = log α + β logTLW, with β > 1 indicating a disproportional increase of Φ with total leaf weight. Different lines indicate the regressions for different plots. Average β = 1·56, which was significantly larger than 1 (t = 5·42, P = 0·001). (B) Power relationship between average total light capture of the genotypes (mol d⁻¹) and total weight of the genotypes (g). The line represents the regression of all measurement of all eight plots. Average β for all eight plots = 1·03, which was not significantly different from 1 (t = 0·034, P = 0·974).

Fig. 5.

Plot Φ_mass (mol g⁻¹ d⁻¹) for the eight genotypes (+ 1 s.e.) as calculated from all leaves within a plot. Results of ANOVA analysis: genotype, F = 3·49, P = 0·005; plot, F = 3·09, P = 0·01.