Inferior plant competitor allocates more biomass to belowground as a result of greater competition for resources in heterogeneous habitats

Abstract

Nutrient heterogeneity in soil widely exists in nature and can have significant impacts on plant growth, biomass allocation, and competitive interactions. However, limited research has been done to investigate the interspecific competitive intensity between two clonal species in a heterogeneous habitat. Therefore, this greenhouse experiment was conducted with two clonal species, Phragmites australis and Scirpus planiculumis, exposed to heterogeneous and homogeneous patches of soil nutrients at five different planting ratios (0:4, 1:3, 2:2, 3:1 and 4:0), to assess the effects of both soil heterogeneity and interspecific competition on plant growth. It was found that soil nutrient heterogeneity significantly enhanced P. australis' interspecific competitive capacity and biomass by promoting a 20% increase in belowground allocation. Interestingly, the planting ratio did not affect the magnitude of this net outcome. In contrast, the superior competitor S. planiculumis did not exhibit significant change of growth indicators to the heterogeneous soil patches. These findings imply that the uncertainties associated with human-induced redistribution of plant species may lead to a shift in dominance from other species to those like P. australis, which have strong nutrient foraging abilities in response to heterogeneity in emergent wetland plant communities.

Keywords: competitive hierarchy; interspecific interaction; resource heterogeneity; root-to-shoot ratio; wetland plant.

Figure 1

Schematic representation of the experimental design. Dark grey and white squares represent high-nutrient commercial potting soil (A) and low-nutrient washed sand (B) patches, respectively; light grey squares received the mean nutrient level between the high and low levels. The total amount of soil nutrients in a container was the same in all treatments. Open circles and filled triangles mark the positions where plants of Scirpus planiculumis and Phragmites australis were planted, respectively. There are six replicate containers (28 cm long × 28 cm wide × 20 cm deep) for each of the ten treatments and thus 60 plastic containers in total.

Figure 2

Effects of soil nutrient heterogeneity (heterogeneous and homogeneous soil) and planting density ratio (the proportions of Phragmites australis and Scirpus planiculumis were 1:3, 2:2 and 3:1) on the mean (± SE) log response ratio (LnRR) of (A) Phragmites australis based on the total biomass, (B) Scirpus planiculumis based on the total biomass, (C) Phragmites australis based on the aboveground biomass, (D) Scirpus planiculumis based on the aboveground biomass, (E) Phragmites australis based on the belowground biomass, and (F) Scirpus planiculumis based on the belowground biomass. A more negative value of the LnRR indicates greater competition intensity between two species, and a more positive value indicates a more facilitative interaction. Symbols (*) at the ends of the bars indicate that biomass significantly differed between the two soil nutrient heterogeneity treatments (paired t-test, P < 0.05); “ns”: P > 0.05. Means + SE are given. See Table 1 for ANOVA results.

Figure 3

Effects of soil nutrient heterogeneity (heterogeneous and homogeneous soil) and planting density ratio (the proportions of Phragmites australis and Scirpus planiculumis were 0:4, 1:3, 2:2, 3:1 and 4:0) on the mean (± SE) above- and belowground biomasses of (A) Phragmites australis and (B) Scirpus planiculumis. The dashed line represents a separation between treatments with competition and without competition. Symbols (*) at the ends of the bars indicate that biomass significantly differed between the two soil nutrient heterogeneity treatments (paired t-test, P < 0.05); “ns”: P > 0.05. See Table 2 for ANOVA results.

Figure 4

Effects of soil nutrient heterogeneity (heterogeneous and homogeneous soil) and planting density ratio (proportions of Phragmites australis and Scirpus planiculumis were 1:3, 2:2, 3:1 and 4:0) on the mean (± SE) (A) rhizome length, (B) number of ramets, (C) root to shoot ratio and (D) relative growth rate (RGR) of Phragmites australis. The dashed line represents a separation between treatments with competition and without competition. Symbols (*) at the ends of the bars indicate that the parameter significantly differed between the two soil nutrient heterogeneity treatments (paired t-test, P < 0.05); “ns”: P > 0.05. Means + SE are given. See Table 2 for ANOVA results.

Figure 5

Effects of soil nutrient heterogeneity (heterogeneous and homogeneous soil) and planting density ratio (proportions of Phragmites australis and Scirpus planiculumis were 0:4, 1:3, 2:2 and 3:1) on the mean (± SE) (A) rhizome length, (B) number of ramets, (C) root to shoot ratio and (D) relative growth rate (RGR) of Scirpus planiculumis. The dashed line represents a separation between treatments with competition and without competition. Symbols (*) at the ends of the bars indicate that the parameter significantly differed between the two soil nutrient heterogeneity treatments (paired t-test, P < 0.05); “ns”: P > 0.05. Means + SE are given. See Table 2 for ANOVA results.