Competition between grassland plants of different initial sizes

Abstract

Four species of grassland plant, Plantago lanceolata, Holcus lanatus, Lolium perenne and Rumex acetosa, were grown as monocultures and mixtures in pots of nutrient poor soil in a glasshouse for 8 months. There were four plants per pot and these were arranged in two competition modes: either root and shoot interactions were permitted, or only roots allowed to interact by using above-ground partitions. Time of introduction of seedlings was varied to give a range of plant size ratios at the start of the experiment. The factorial design catered for all combinations of species, competition modes and planting times, replicated in four blocks. The shoots were clipped at a fixed height at each of five harvests. Rumex grew badly and was mostly omitted from analysis of the data.By (i) following the change in the relationship of clip dry weights against planting time with successive harvests, (ii) plotting the change in the logarithm of the ratio of cumulative clip dry weights with time and (iii) the use of de Wit logarithmic ratio plots it was demonstrated that each monoculture and mixture combination's ratios of plant weights converged towards stable equilibrium values. Three hypotheses are put forward to explain why in monocultures a smaller plant was at a competitive advantage relative to a larger neighbour and was not suppressed in its growth by the latter. In mixtures this plant size effect was superimposed to different extents on the relative aggressiveness of the species considered. It was concluded that in a nutrient poor soil, when competition for light was low, root interactions can promote the co-existence of neighbouring plant species.

Root foraging for patchy resources in eight herbaceous plant species

“…However, many of the experiments showing size symmetry of root competition have used homogenized soil (Newbery and Newman 1978;Weiner et al 1997;Wilson 1988). Belowground size asymmetry requires small-scale heterogeneity of resources.…”

Mechanisms determining the degree of size asymmetry in competition among plants

“…The published experiments of which we are aware indicate that when competition is primarily for soil resources it is size symmetric (Casper and Jackson 1997). However, most of the evidence has been from greenhouse populations where root competition took place in the con®ned spaces of soil containers (Newbery and Newman 1978;Weiner 1986;Biere 1987;Wilson 1988;Weiner et al 1997). In the one ®eld study (Gerry and Wilson 1995), the eect of initial transplant size on subsequent growth was examined for six grassland species grown under dierent competitive regimes.…”

“…Competition does not seem to be size asymmetric when plants grow (a) from seed for a very short period of time (Rabinowitz 1979;Turner and Rabinowitz 1983), (b) on very poor soils (Newbery and Newman 1978), (c) at low density (Stoll et al 1994), (d) competing only for below-ground resources (Weiner 1986;Wilson 1988;Gerry and Wilson 1995;Weiner et al 1997).…”

Soil nutrient heterogeneity and competitive ability of three grass species (Festuca ovina, Arrhenatherum elatius and Calamagrostis epigejos) in experimental conditions

“…Most of the competition experiments have been carried out under homogeneous conditions and some studies have shown that smaller plants (individuals) have a competitive advantage when compared with their larger neighbour (e.g., Newberry & Newman 1978;Weiner et al 1997). The results published by indicate that heterogeneity alone, without a change in the overall nutrient availability of the soil, can change the relative competitive ability of two perennial plant species.…”