An invasive clonal plant benefits from clonal integration more than a co-occurring native plant in nutrient-patchy and competitive environments

Abstract

Many notorious invasive plants are clonal, however, little is known about the different roles of clonal integration effects between invasive and native plants. Here, we hypothesize that clonal integration affect growth, photosynthetic performance, biomass allocation and thus competitive ability of invasive and native clonal plants, and invasive clonal plants benefit from clonal integration more than co-occurring native plants in heterogeneous habitats. To test these hypotheses, two stoloniferous clonal plants, Alternanthera philoxeroides (invasive), Jussiaea repens (native) were studied in China. The apical parts of both species were grown either with or without neighboring vegetation and the basal parts without competitors were in nutrient- rich or -poor habitats, with stolon connections were either severed or kept intact. Competition significantly reduced growth and photosynthetic performance of the apical ramets in both species, but not the biomass of neighboring vegetation. Without competition, clonal integration greatly improved the growth and photosynthetic performance of both species, especially when the basal parts were in nutrient-rich habitats. When grown with neighboring vegetation, growth of J. repens and photosynthetic performance of both species were significantly enhanced by clonal integration with the basal parts in both nutrient-rich and -poor habitats, while growth and relative neighbor effect (RNE) of A. philoxeroides were greatly improved by clonal integration only when the basal parts were in nutrient-rich habitats. Moreover, clonal integration increased A. philoxeroides's biomass allocation to roots without competition, but decreased it with competition, especially when the basal ramets were in nutrient-rich sections. Effects of clonal integration on biomass allocation of J. repens was similar to that of A. philoxeroides but with less significance. These results supported our hypothesis that invasive clonal plants A. philoxeroides benefits from clonal integration more than co-occurring native J. repens, suggesting that the invasiveness of A. philoxeroides may be closely related to clonal integration in heterogeneous environments.

Figure 1. Schematic representation of the experimental design.

Clonal fragments of the invasive plant A. philoxeroides or native plant J. repens, each consisting of three basal ramets (dark grey circles) and two apical ramets (light gray circles) with a stolon apex (horizontal arrow), were grown either with (competition) or without (control) competitive vegetation (J. repens or A. philoxeroides, spot-shadow) and with stolon connections between basal and apical ramets were either intact or severed (fork). Three integration treatments were used as follows: severed (stolon connections severed by the scissors), intact (stolon connections kept intact) and nutrient (stolon connections kept intact and with basal ramets in fertilized habitats).

Figure 2. Effects of integration treatments and competition on growth measures of the two clonal plants.

Total biomass, ramet number, stolon length, leaf number and total leaf area of the invasive plant A. philoxeroides (left: A, B, C, D, E) or native plant J. repens (right: F, G, H, I, J) in the apical sections, grown either with or without competitive vegetation (J. repens or A. philoxeroides) in three integration treatments. Data indicate the means ± SE. Bars sharing the same letter are not significantly different at P = 0.05.

Figure 3. Effects of integration treatments on the relative neighbour effect (RNE) of the two clonal plants.

The relative neighbour effect (RNE) of the invasive plant A. philoxeroides and native plant J. repens in the apical sections in three integration treatments. Data indicate the means ± SE. Bars sharing the same letter are not significantly different at P = 0.05.

Figure 4. Effects of integration treatments and competition on biomass allocation of the two clonal plants.

Biomass allocation (LMR, leaf mass ratio; SMR, stolon mass ratio; RMR, root mass ratio) of the invasive plant A. philoxeroides (left: A, B, C) or native plant J. repens (right: E, F, G) in the apical sections, grown either with or without competitive vegetation (J. repens or A. philoxeroides) in three integration treatments. Data indicate the means ± SE.

Figure 5. Effects of integration treatments and competition on photosynthetic performance of the two clonal plants.

The maximum quantum yield of photosystem II (F _v/F _m) and the effective quantum yield of PSII (Yield) of the invasive plant A. philoxeroides (left: A, B) or native plant J. repens (right: C, D) in the apical sections, grown either with or without competitive vegetation (J. repens or A. philoxeroides) in three integration treatments. Data indicate the means ± SE.