Effects of soil nutrient heterogeneity on intraspecific competition in the invasive, clonal plant Alternanthera philoxeroides

Abstract

Background and aims: Fine-scale, spatial heterogeneity in soil nutrient availability can increase the growth of individual plants, the productivity of plant communities and interspecific competition. If this is due to the ability of plants to concentrate their roots where nutrient levels are high, then nutrient heterogeneity should have little effect on intraspecific competition, especially when there are no genotypic differences between individuals in root plasticity. We tested this hypothesis in a widespread, clonal species in which individual plants are known to respond to nutrient heterogeneity.

Methods: Plants derived from a single clone of Alternanthera philoxeroides were grown in the greenhouse at low or high density (four or 16 plants per 27·5 × 27·5-cm container) with homogeneous or heterogeneous availability of soil nutrients, keeping total nutrient availability per container constant. After 9 weeks, measurements of size, dry mass and morphology were taken.

Key results: Plants grew more in the heterogeneous than in the homogeneous treatment, showing that heterogeneity promoted performance; they grew less in the high- than in the low-density treatment, showing that plants competed. There was no interactive effect of nutrient heterogeneity and plant density, supporting the hypothesis that heterogeneity does not affect intraspecific competition in the absence of genotypic differences in plasticity. Treatments did not affect morphological characteristics such as specific leaf area or root/shoot ratio.

Conclusions: Results indicate that fine-scale, spatial heterogeneity in the availability of soil nutrients does not increase competition when plants are genetically identical, consistent with the suggestion that effects of heterogeneity on competition depend upon differences in plasticity between individuals. Heterogeneity is only likely to increase the spread of monoclonal, invasive populations such as that of A. philoxeroides in China.

Fig. 1.

Experimental design. Darkly shaded and unshaded squares represent high and low soil nutrient patches, respectively; lightly shaded squares received the mean of the high and low levels. Black circles mark positions where plants of Alternanthera philoxeroides were planted.

Fig. 2.

Effects of heterogeneity of soil nutrients and density of plants on (A) total dry biomass, (B) mass of stolons, (C) mass of leaves, (D) mass of roots, (E) total leaf area, (F) total stolon length, (G) number of new ramets and (H) number of new stolons. See Table 1 for ANOVA results.

Fig. 3.

Effects of heterogeneity of soil nutrients and density of plants on (A) internode length, (B) specific internode length, (C) specific leaf area and (D) root to shoot ratio of plants of Alternanthera philoxeroides. Values are means + s.e. See Table 1 for ANOVA results.