Interactions between arbuscular mycorrhizal fungi and intraspecific competition affect size, and size inequality, of Plantago lanceolata L.

Abstract

Journal of EcologyVolume 94, Issue 2 p. 285-294 Free Access Interactions between arbuscular mycorrhizal fungi and intraspecific competition affect size, and size inequality, of Plantago lanceolata L. RUTH L. AYRES, Corresponding Author RUTH L. AYRES *Present address and correspondence: Ruth L. Ayres, Centre for Academic Practice, University of Warwick, Coventry CV4 8UW, UK (tel. +44 2476 574121; fax +44 2476 572736; e-mail r.l.ayres@warwick.ac.uk).Search for more papers by this authorALAN C. GANGE, ALAN C. GANGE School of Health and Bioscience, University of East London, Romford Road, Stratford, London E15 4LZ, UK, and School of Biological Sciences, Royal Holloway, University of London, Egham Hill, Egham Surrey, TW20 0EX, UKSearch for more papers by this authorDAVID M. APLIN, DAVID M. APLIN School of Health and Bioscience, University of East London, Romford Road, Stratford, London E15 4LZ, UK, and School of Biological Sciences, Royal Holloway, University of London, Egham Hill, Egham Surrey, TW20 0EX, UK Present address: National Botanic Garden of Belgium, Domein van Bouchout, B-1860 Meise, Belgium.Search for more papers by this author RUTH L. AYRES, Corresponding Author RUTH L. AYRES *Present address and correspondence: Ruth L. Ayres, Centre for Academic Practice, University of Warwick, Coventry CV4 8UW, UK (tel. +44 2476 574121; fax +44 2476 572736; e-mail r.l.ayres@warwick.ac.uk).Search for more papers by this authorALAN C. GANGE, ALAN C. GANGE School of Health and Bioscience, University of East London, Romford Road, Stratford, London E15 4LZ, UK, and School of Biological Sciences, Royal Holloway, University of London, Egham Hill, Egham Surrey, TW20 0EX, UKSearch for more papers by this authorDAVID M. APLIN, DAVID M. APLIN School of Health and Bioscience, University of East London, Romford Road, Stratford, London E15 4LZ, UK, and School of Biological Sciences, Royal Holloway, University of London, Egham Hill, Egham Surrey, TW20 0EX, UK Present address: National Botanic Garden of Belgium, Domein van Bouchout, B-1860 Meise, Belgium.Search for more papers by this author First published: 14 February 2006 https://doi.org/10.1111/j.1365-2745.2006.01103.xCitations: 28 AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Summary 1 Intraspecific competition causes decreases in plant size and increases in size inequality. Arbuscular mycorrhizas usually increase the size and inequality of non-competing plants, but mycorrhizal effects often disappear when plants begin competing. We hypothesized that mycorrhizal effects on size inequality would be determined by the experimental conditions, and conducted simultaneous field and glasshouse experiments to investigate how AM fungi and intraspecific competition determine size inequality in Plantago lanceolata. 2 As predicted, plant size was reduced when plants were competing, in both field and controlled conditions. However, size inequality was unexpectedly reduced by competition. Plants may have competed in a symmetric fashion, probably for nutrients, rather than the more common situation, in which plant competition is strongly asymmetric. 3 Mycorrhizas had no effect on plant size or size inequality in competing plants in either field or controlled conditions, possibly because competition for nutrients was intense and negated any benefit the fungi could provide. 4 The effects of mycorrhizas on non-competing plants were also unexpected. In field-grown plants, AM fungi increased plant size, but decreased size inequality: mycorrhizal plants were more even in size, with few very small individuals. In glasshouse conditions, mycorrhizal colonization was extremely high, and was generally antagonistic, causing a reduction in plant size. Here, however, mycorrhizas caused an increase in size inequality, supporting our original hypothesis. This was because most plants were heavily colonized and small, but a few had low levels of colonization and grew relatively large. 5 This study has important implications for understanding the forces that structure plant communities. AM fungi can have a variety of effects on size inequality and thus potentially important influences on long-term plant population dynamics, by affecting the genetic contribution of individuals to the next generation. However, these effects differ, depending on whether plants are competing or not, the degree of mycorrhizal colonization and the responsiveness of the plant to different colonization densities. Introduction Arbuscular mycorrhizal fungi have a wide variety of beneficial effects on their host plants, including enhanced growth through nutrient acquisition (Smith & Read 1997), fecundity (Koide 2000), competitive ability (e.g. West 1996), improved drought tolerance (e.g. Ruiz-Lozano et al. 1995) enhanced disease resistance (e.g. Borowicz 2001) and resistance to insect herbivores (Gehring & Whitham 2002). However, there are also many examples of AM colonization having a negative effect on plant growth and reproduction (e.g. Francis & Read 1995; Johnson et al. 1997), probably due to the degree of specificity of the symbiosis (Sanders 2002) or particular environmental conditions (such as high soil P) in which plants are grown (Gange & Ayres 1999). Such differences in responses to AM colonization have led to studies of the role of AM fungi in plant community structure. There are several experiments showing that they can increase the species richness of plant communities, either in microcosms or field situations (Grime et al. 1987; Gange et al. 1993; van der Heijden et al. 1998), although O’Connor et al. (2002) and Hartnett & Wilson (1999) found plant diversity or species richness increased following fungicide treatment to reduce mycorrhizal occurrence. Although no explicit test has been done, these community effects could well be due to a mycorrhizal effect on plant competition (van der Heijden 2002). Thus, if the competitive dominants in a community are strongly mycorrhizal, AM fungi will enhance their growth, leading to suppression of weaker competitors and thus reduced species richness. Meanwhile, if the competitive dominants are weakly mycorrhizal or non-mycorrhizal, AM fungi may enhance the growth of weaker, but mycorrhizal, competitors, promoting coexistence and an increased species richness. In reality, the situation is considerably more complicated, being affected by variations in mycorrhizal specificity and soil nutrient supply (Aerts 2002). It is implicit in the arguments regarding mycorrhizas and plant community structure that the fungi can affect the balance of plant competition. A number of studies have shown that AM fungi can affect the outcome of interspecific competition (e.g. West 1996; Marler et al. 1999), particularly when there is a difference in responsiveness of the two plant species to fungal colonization (Watkinson & Freckleton 1997). However, in many plant communities, individuals of a given plant species are most likely to be growing in close proximity to members of their own species (Harper 1977) and, thus, the role of AM fungi in affecting the outcome of intraspecific competition becomes critical. Several studies have shown that mycorrhizal presence increases the intensity of intraspecific competition in grasses (West 1996; but see also Watkinson & Freckleton 1997) and forbs (Shumway & Koide 1995; Moora & Zobel 1996; Facelli et al. 1999; Facelli & Facelli 2002), with possible consequences for the size inequality within populations. High-density plant populations are usually characterized by great inequality in size (Weiner & Thomas 1986), in which a few individuals usurp most of the available resources and the majority of individuals are small. These differences in size may be caused by any combination of environmental factors (such as nutrient availability or herbivores) and genetic differences between individuals, such as differential germination times or growth rates (Weiner 1990). Size inequality can have important consequences for the structure of plant populations, because an inequality in reproductive output will affect the genetic structure of subsequent generations (Shumway & Koide 1995), as well as affecting the structure of the current generation, when, if self-thinning occurs, there will be the deaths of smaller individuals (Weiner & Whigham 1988). In theory, mycorrhizas could reduce size inequality, by increasing the growth of weaker individuals, or increase it, by enhancing the growth of larger individuals at the expense of the weaker individuals. Here, we use even-aged populations of Plantago lanceolata L., a strongly mycorrhizal forb (Gange & West 1994), to address whether, and how, AM fungi affect size inequality in competing plant populations. Previous studies of the effects of AM fungi on size inequality have produced quite consistent results, in that mycorrhizas appear to increase size inequality when plants are grown at low density. At high densities, however, when resource competition is intense and nutrient depletion can occur, mycorrhizas have no effect on size inequality (Allsopp & Stock 1992; Facelli et al. 1999; Facelli & Facelli 2002). The one exception to this pattern is the work of Shumway & Koide (1995), in which AM fungi were found to increase the inequality in reproductive output of Abutilon theophrasti Medic. at both low and high density. It is interesting that the latter experiment was performed in the field, while other experiments have taken place in microcosms where nutrient limitation is likely to have occurred. Indeed, Facelli & Facelli (2002) suggest that at high-density plantings, AM fungi deplete the available soil resources, with the subsequent limitation of plant growth negating the benefit derived from the symbiosis. Such a situation is much more likely to occur in controlled experiments and so we hypothesized that an effect of AM fungi on plant size inequality in crowded populations (i.e with plants experiencing intraspecific competition) is unlikely to be seen in microcosms, although it may be apparent in the field. A second common feature of previous studies is that the analysis of size inequality has been rather limited, although Shumway & Koide (1995) used both Lorenz curves and the Gini coefficient. The Lorenz curve allows for graphical examination of the relative contribution of large or small individuals to a plant population, while the total amount of inequality is summarized by the Gini coefficient (see Shumway & Koide 1995). Facelli & Facelli (2002) calculated just the Gini coefficient in their analysis of how mycorrhizas, intraspecific competition and nutrients affect size inequality in Trifolium subterraneum L. However, as different Lorenz curves can possess identical Gini coefficients, the calculation of this statistic alone can produce misleading results if we are trying to understand how AM fungi affect the contribution of large or small plants to the total biomass of a population. Damgaard & Weiner (2000) therefore proposed an alternative statistic, the Lorenz Asymmetry Coefficient, and their re-analysis of the data of Shumway & Koide (1995) showed that the increase in reproductive inequality of Abutilon theophrasti when mycorrhizas were present was caused by the contribution of a small number of very large individuals. To date, no study has applied the methodology of Damgaard & Weiner (2000) to the analysis of mycorrhizal effects in competing plant populations. Here, we take this approach, enabling a more detailed analysis of how mycorrhizas affect plant size inequality. Materials and methods study system This investigation was carried out on Plantago lanceolata L. (Plantaginaceae), a common perennial forb that forms an arbuscular mycorrhiza and which shows a significant growth reduction when the mycorrhiza is reduced (Gange & West 1994). The investigation had two simultaneously conducted components: a field trial, in which plants were grown in natural soil, and a controlled experiment, where plants were grown in pots of the same natural soil in a glasshouse. Seeds of P. lanceolata were sown in sterile potting compost (John Innes number 1, Roffey Ltd, Bournemouth, UK) and maintained at a temperature of 20 °C. After 14 days, emerged seedlings were at the three-leaf stage (two cotyledons plus one true leaf) and individuals selected for uniformity of size, based on the length of the true leaf, were planted into the field and glasshouse trials. field trial A 500 m2 area of land at Silwood Park, Ascot, Berkshire, UK, was treated with the herbicide ‘Round Up’ (Monsanto plc, Leicester, UK) containing 360 g l−1 glyphosate in autumn, shallow ploughed in winter and hand raked in early spring, to remove any vegetation. A randomized block design was set out, consisting of four treatments, with 36 replicates of each. Two experimental conditions were created, consisting of presence or absence of intraspecific competition, with or without natural mycorrhizal colonization in 0.5 × 0.5 m plots. The experiment was therefore a 2 × 2 factorial with four treatments in total. No competition plots consisted of one centrally planted individual, giving a density of 4 m−2, while competing plants consisted of groups of 16 in a 4 × 4 grid (i.e. 12.5 cm apart, 64 m−2). These plant densities were chosen to represent the typical range of this species in early successional communities on this site (V.K. Brown, personal communication). Each plot was separated from its neighbour by 2 m and all other plants that appeared in the experimental plots through natural germination were hand-weeded out. Reduced mycorrhizal colonization was achieved by application of the fungicide ‘Rovral’ (Bayer Crop Science, Hauxton, UK) (containing 40% w/w iprodione) to the soil. This was applied at a rate of 2 g m−2 of formulated product at 2-week intervals from March to August. The soil was a sandy loam, with a pH of 5.4 and a bicarbonate extractable P content of 3.9 µg P g−1 and nitrogen content of 2.1 µg g−1. Plants were watered immediately after transplanting, but once established no supplementary water was given. A total of four plants did not survive transplanting and these were replaced within the first week of the trial. Thereafter, no plants died during the course of the experiment. The site was fenced to exclude rabbits and although molluscs were rare on the acidic sandy soil, a few pellets of the molluscicide MifaSlug (containing metaldehyde) (Farmers Crop Chemicals Ltd, Worcester, UK) were placed around the perimeter of each plot once a month. Plants were maintained for 20 weeks, after which time each was carefully dug up and the roots washed free of soil. The extreme sandy nature of the soil meant that we were able to recover virtually all of each root system intact. Total vegetative biomass (separately for roots and shoots) was recorded as dry weight and the number of inflorescences counted on every plant. To minimize edge effects, we conducted our analyses on competing plants (below) using the means of the four plants in the middle of the plot, in a similar manner to the designs of Shumway & Koide (1995) and Facelli & Facelli (2002). Before drying, a 2-g portion of fresh root was removed from each plant, washed and examined at ×200 using a Zeiss Axiophott epifluorescence microscope equipped with a UV lamp and filters giving a transmission of 450–490 nm blue light. Under these conditions, the arbuscules fluoresce (Ames et al. 1982) and arbuscular colonization was recorded as percentage root length colonized (%RLC) by the cross hair eye piece method of McGonigle et al. (1990). Values for dry root biomass were corrected for the loss of the 2-g sample in each case. This method was chosen because it produces more consistent and reliable results in P. lanceolata than any of the conventional stains (Gange et al. 1999). However, as non-mycorrhizal fungal material will not be seen, we also subjected roots to a conventional staining procedure (Vierheilig et al. 1998), to check for such infection. glasshouse experiment The experiment was conducted under controlled conditions in a glasshouse at the University of East London, Stratford, UK. Seedlings at the three-leaf stage (see above) were transplanted into 250-mm diameter pots containing 24 L of soil taken from an area adjacent to that of the field study area at Silwood Park. The soil was placed into the pots and allowed to equilibrate for a 2-month period prior to transplanting. After this time, N and P contents were measured and found to be 2.9 µg g−1 and 4.4 µg P g−1, respectively. Neither of these two values was significantly different from those obtained in the field site (P > 0.05). The no competition treatment consisted of one plant in the middle of each pot (equivalent to 20 m−2), while the competition treatment consisted of three plants, 12.5 cm apart from each other (61 m−2), to give plant densities as similar as possible to those in the field trial. Within blocks, competition pots were arranged adjacent to each other on the glasshouse bench, with extra ‘dummy’ pots around the edge. Only pots inside this arrangement (i.e. not edge pots) were sampled, to minimize edge effects and to be as close a mimic as possible of the field plot design and those of Shumway & Koide (1995) and Facelli & Facelli (2002). Mycorrhizas were reduced by addition of iprodione at the same application rate as in the field (i.e. 2 g m−2, 0.1 g per pot) applied at 2-week intervals. There were 25 replicate pots of each of the four treatments and these were arranged in a randomized block design on the glasshouse bench. Plants were maintained for 20 weeks, during which time no supplementary fertiliser was given, but each pot received variable amounts of water per week, to maintain a soil moisture level equal to that occurring in the field. At the end of the growth period, all plants were carefully removed from the pots and their roots washed free of soil. Foliar and root biomass was obtained for all individual plants, but for those in the competition treatment, roots could not be separated and so mean biomass per pot was calculated by dividing the total by three. Dry biomass was recorded, together with the total number of inflorescences produced per plant. Mycorrhizal colonization of each plant was obtained in an identical manner to that in the field trial. statistical analysis Plant growth data (foliar and root biomass and flower number) were tested for normality and homogeneity of variances prior to analysis, and underwent log transformation, where appropriate. Mycorrhizal percentage colonization data were subjected to the angular transformation prior to analysis (Zar 1996). For non-competing plants, we examined the relation between mycorrhizal colonization and the degree of ‘benefit’ received by the plant (defined as the percentage change in a parameter of a mycorrhizal plant relative to a mean value for plants without AM colonization (Gange & Ayres 1999)). Foliar biomass was used as the response variable in this analysis. Data were analysed by randomized block analysis of variance, including mycorrhizas and competition as main effects, using the UNISTAT® statistical package. To examine size inequality, we calculated the Gini coefficient and constructed Lorenz curves for each treatment, as described by Shumway & Koide (1995), to examine the relative contribution of large or small individuals to the inequality of the populations. If all individuals in a population are the same size, then the Lorenz curve is a straight diagonal line, called the line of equality, but otherwise, it is a curve below the line and the area between it and the line is measured by the Gini coefficient or Gini ratio, with the latter defined as the ratio of the area bounded by the line and the curve to the total area beneath the line (Damgaard & Weiner 2000). In competition treatments, the coefficient was calculated using the four middle plants (field plots) or all three plants (glasshouse pots), with each plot or pot as a replicate. As it is possible for different Lorenz curves to have the same Gini coefficient, the Lorenz Asymmetry Coefficient (S) was calculated in each case, following Damgaard & Weiner (2000). This is done by measuring the asymmetry of the Lorenz curve around the axis of symmetry (the other diagonal). Specifically, the Asymmetry Coefficient is the point at which the slope of the Lorenz curve is equal to 1 (i.e. equal to that of the line of equality) and can be used to examine whether the total biomass of a population is being made up by a few very large individuals (curve ‘a’ in Damgaard & Weiner 2000) or many small individuals (curve ‘b’ in the same paper). When the Lorenz curve is parallel with the line of equality at the axis of symmetry, S will equal 1, as all individuals are the same size. If this point occurs below the axis of symmetry, S < 1, indicative of a population with many small individuals that contribute little to the population's total biomass. If, however, this point occurs above the axis of symmetry, S > 1, indicative of a population with a few very large individuals, which contribute the majority of the population's biomass. Confidence intervals for S were obtained with a bootstrap procedure (Dixon et al. 1987). Results mycorrhizal colonization In both field- and glasshouse-grown plants, application of fungicide was successful in reducing the abundance of AM fungi (Fig. 1). Infection by non-mycorrhizal fungi was extremely low and the highest level recorded in any sample was that for glasshouse-grown plants in the non-fungicide treatment at 3.1% RLC (Root Length Colonized). It is therefore most unlikely that any confounding effects of non-mycorrhizal fungi existed. In contrast, levels of arbuscular colonization were exceptionally high in glasshouse plants, with a mean of 50% in non-competing, untreated plants (Fig. 1b). Some individual plants in this treatment had levels of arbuscular colonization alone over 70%. Figure 1Open in figure viewerPowerPoint Mycorrhizal colonization of Plantago lanceolata, measured by percentage root length colonized (%RLC, arbuscules only) and grown with or without competition (see text for explanation). Open bars, natural mycorrhizal levels; shaded bars, application of fungicide to reduce colonization. Intraspecific competition significantly reduced AM colonization in both field (F1,140 = 38.2, P < 0.001) and glasshouse plants (F1,96 = 12.5, P < 0.001). In field plants, there was a significant interaction term between mycorrhizas and competition (F1,140 = 6.9, P < 0.01), because the fungicide effect was only clearly seen when plants were not competing (Fig. 1a). plant growth Not surprisingly, plants undergoing competition produced significantly smaller amounts of both foliar and root biomass than those not competing, in both experiments. Of more interest was the fact that AM fungi also affected biomass, but this was not consistent between the experiments. In field-grown plants, mycorrhizas resulted in plants with greater foliar biomass. However, because this effect was only seen in non-competing plants, there was a significant interaction term between mycorrhizas and competition. No interaction was seen with root biomass, as mycorrhizas increased the amount of root, irrespective of the density at which plants were grown (Table 1). In glasshouse plants, however, mycorrhizas decreased both foliar and root biomass significantly. In both parameters, there was a significant interaction between the treatments, as the mycorrhizal-induced reduction in growth was only seen in non-competing plants, where the response was quite dramatic, with mycorrhizas causing a reduction of over 25% in each case. Table 1. Means (with one SE in parentheses) and summary of statistical analysis of growth parameters of Plantago lanceolata, grown in conditions of low or high density, with mycorrhizas (+AM) or with reduced mycorrhizas (–AM). Statistical values tabulated are F ratios from anova, testing for the main effect of mycorrhizas (M), intraspecific competition (C) or the interaction between them (M × C). Degrees of freedom for field plants = 1,140, and for glasshouse plants = 1,96. *P < 0.05; **P < 0.01; ***P < 0.001 –Competition +Competition anova summary +AM –AM +AM –AM M C M × C Field-grown plants Foliar biomass (g) 27.8 (2.2) 18.5 (1.9) 5.9 (0.3) 5.7 (0.3) 11.3*** 231.3*** 8.7** Root biomass (g) 23.5 (2.1) 18.1 (1.8) 13.3 (0.6) 11.7 (0.4) 7.2** 30.1*** 0.9 Root:shoot ratio 0.65 (0.06) 0.87 (0.09) 1.57 (0.06) 1.45 (0.07) 0.5 104.3*** 5.1* Inflorescence number 39.6 (2.5) 31.6 (2.5) 13.9 (0.9) 13.3 (0.4) 6.1* 183.3*** 4.8* Glasshouse plants Foliar biomass (g) 8.8 (0.9) 11.9 (0.9) 4.2 (0.3) 4.1 (0.2) 4.6* 136.7*** 6.2* Root biomass (g) 15.6 (1.5) 20.9 (1.6) 8.3 (0.4) 8.1 (0.5) 4.1* 82.9*** 5.6* Root:shoot ratio 1.6 (0.1) 1.6 (0.1) 2.0 (0.2) 2.0 (0.1) 0.04 4.4* 0.0 Inflorescence number 30.8 (2.5) 30.4 (2.8) 10.1 (0.8) 10.4 (0.6) 0.01 70.4*** 0.7 Mycorrhizas had no effect on the root:shoot ratio in either experiment, but this parameter was consistently increased by competition. In the field trial, non-competing plants produced more shoot than root biomass, giving a ratio less than unity, whilst the reverse was true for competing plants where ratios were greater than one (Table 1). This resulted in a significant interaction term for root:shoot ratio in field grown plants. In glasshouse plants, however, all treatments produced ratios over one (indicating a greater amount of root), but the effect of competition was still significant, albeit weak. The number of flowering stems was greatly reduced by competition in both experiments, a likely result of the overall effects on plant size. The mycorrhizal effect was not consistent because inflorescence number was significantly increased by mycorrhizas in non-competing, field-grown plants, but unaffected by AM fungi when plants experienced competition. This resulted in a significant interaction term for field-grown plants (Table 1). In contrast, the number of flowering stems produced by glasshouse plants was unaffected by mycorrhizas, even though overall foliar biomass was altered (Table 1). For plants grown in the field, the range in colonization across fungicide-treated and untreated plants was 2–35%. A significant positive relationship was found between AM colonization and plant ‘benefit’ (as defined in Gange & Ayres 1999), indicating that the association was generally beneficial to the plants, and was best fitted by a second order polynomial (F2,70 = 155.5, P < 0.001, R2 = 81.6%) (Fig. 2a). Meanwhile, for glasshouse plants, the range in colonization was 9–71% but the significant negative relationship, also fitted by a second order polynomial (F2,48 = 37.4, R2 = 60.9%) (Fig. 2b), indicated that the association was mostly antagonistic to the plants. In the glasshouse, plants with very high levels of colonization were smaller than mycorrhizal free plants grown in the same conditions. Figure 2Open in figure viewerPowerPoint Relationships between mycorrhizal colonization and the degree of ‘benefit’ (sensuGange & Ayres 1999) derived by the plant. Data portrayed is that for all low-density plants, combined across fungicide treatments. The equation of the fitted line for field-grown plants is y = 6.6x – 0.1x2 while that for glasshouse plants is y = 9.1x − 0.1x2. size inequality It should be noted that comparisons of Gini coefficients are only unambiguous if populations share the same type of Lorenz curve. As this was not so in this study, we report only qualitative differences between the coefficients. In field-grown plants, size inequality was reduced by competition, as indicated by the reductions in Gini coefficients (Fig. 3a). Mycorrhizas also had an effect on size inequality, which varied according to the level of competition. In non-competing plants, AM fungi reduced inequality by about 25%. However, in the competition treatments, no effect of mycorrhizas was found (Fig. 3a). These results form an interesting comparison with those of total foliar biomass (Table 1), because when mycorrhizas increased plant size, inequality was reduced. Figure 3Open in figure viewerPowerPoint Graphical analysis of inequality in field-grown plants. Total inequality is measured by the Gini coefficient in non-competing and competing plants. Open bars, natural mycorrhizal levels; shaded bars, application of fungicide to reduce colonization. Lower graph shows the Lorenz curve for each treatment. +C and –C, with and without competition, respectively; +AM and –AM indicate natural mycorrhizal levels or reduced levels. The diagonal solid line is the line of equality. The reduction in total inequality in competition treatments can be seen clearly in the two Lorenz curves being closer to the line of equality than either of the two non-competition curves (Fig. 3b). When plants were grown singly, the Asymmetry Coefficient, S, was 0.872 for mycorrhizal plants and 0.713 for plants where mycorrhizas were reduced. The interpretation of this is that, as the coefficient is closer to one for the mycorrhizal plants, this population contained fewer very small individuals and plants were more even in size. However, when plants experienced competition, S for mycorrhizal plants was 1.105, while that for red