Journal of Experimental Botany, Vol. 51, No. 347, pp. 1117-1125,
June 2000
© 2000 Oxford University Press
Exclusion of grass roots from soil organic layers by Calluna: the role of ericoid mycorrhizas
1 Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, UK
2 Institute of Terrestrial Ecology, Banchory Research Station, Banchory, Aberdeenshire AB31 4BY, UK
Received 21 December 1999; Accepted 9 February 2000
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Abstract |
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The role of ericoid mycorrhizal colonization in competition between the dwarf shrub Calluna vulgaris and coarse grass Nardus stricta was investigated. Nardus was grown alone, or in competition with Calluna, in a layered organic/sand substrate with and without inoculation with the ericoid mycorrhizal endophyte Hymenoscyphus ericae, and with and without the addition of nitrogen. Root length and allocation between different substrate layers was assessed along with plant biomass, nutrient uptake and mycorrhizal colonization. Calluna was the superior competitor for nutrients, probably because of its ability to concentrate root growth in the upper organic layer. In the presence of Calluna both the absolute amount and proportion of Nardus root length in the organic layer were reduced, and this reduction was greatest when Calluna was mycorrhizal. The presence of ericoid mycorrhizal colonization did not reduce Nardus shoot nutrient content or concentration, suggesting that ericoid mycorrhizal suppression of Nardus growth was not due to nutrient competition: alternative mechanisms of interference are discussed.
Key words: Below ground, Nardus, competition, layered substrate, Calluna heathland.
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Introduction |
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Mycorrhizal symbiosis may alter the competitive balance between species and hence influence community structure in nutrient-limited environments (Aerts, 1999
; Gange et al., 1993; van der Heijden et al., 1998a). For example, the outcome of competition between grass species can be altered by vesicular-arbuscular (VA) mycorrhizas when phosphorus is the limiting nutrient (Fitter, 1977; West, 1996). It has also been demonstrated that plant diversity increases with the number of VA mycorrhizal fungi present in an alpine meadow community (van der Heijden et al., 1998b). In acidic heathland ecosystems it is often nitrogen that limits growth, because mineralization rates are low and nutrients are locked up in complex organic compounds (Read, 1991). These heathland environments are dominated by ericaceous dwarf shrubs (Aerts and Heil, 1993) whose roots are colonized by ericoid mycorrhizal (ErM) fungi (Gimingham, 1972; Read and Stribley, 1973). ErM fungi are capable of utilizing nitrogen and phosphorus from a wide range of organic substrates (Haselwandter et al., 1990; Leake and Read, 1990; Myers and Leake, 1995), resulting in improved host nutrition (Kerley and Read, 1998; Read and Stribley, 1973).
Calluna vulgaris (L.) Hull is the ericaceous dwarf shrub that dominates many natural and semi-natural heathland communities in the oceanic north west of Europe (Gimingham, 1960; Rodwell, 1991). In many areas, including Scotland, this Calluna heath has been replaced in recent decades by grassland (Marrs, 1993; Sydes, 1988; Welch and Scott, 1995) of which the coarse grass Nardus stricta (L.) is a frequent and dominant component (Hill et al., 1992; Welch, 1986; Welch and Scott, 1995). The main cause of heathland decline in Scotland is over-grazing by sheep and deer, because Calluna is grazed in preference to relatively unpalatable grasses such as Nardus (Hartley, 1997; Hill et al., 1992). In other European heaths, particularly in the Netherlands, elevated nitrogen deposition (Aerts and Berendse, 1988), poor fire management and increased frequency of heather beetle (Lochmaea suturalis) attack (Aerts and Heil, 1993; Berdowski and Zeilinga, 1987) are important causes of Calluna decline. Because both defoliation (Hartley and Amos, 1999) and enhanced nitrogen fertilization (Stribley and Read, 1976; Yesmin et al., 1996) have been shown to decrease the proportion of host root colonized by ErM fungi, they may also influence the extent to which ErM colonization influences plant community structure.
The aim of this study was to determine the ability of Nardus seedlings to grow in the presence of established mycorrhizal and non-mycorrhizal Calluna plants. Although ErM colonization has been shown to improve host plant nutrition, the effect on competition is difficult to predict. Read (Read, 1996) and Aerts (Aerts, 1999) suggest that, in low nutrient environments, the accessibility of organic nitrogen conferred by ErM colonization may be a mechanism for niche differentiation between ericaceous plants and species such as Nardus which may only be able to access inorganic nitrogen sources. If this hypothesis is correct, Nardus seedling growth should be affected less by competition with ErM Calluna than by non-mycorrhizal Calluna. This alleviation of competition could be a mechanism that contributes to the maintenance of species diversity in heathland plant communities. However, Calluna is able to form low-diversity swards under certain conditions. It may be that ErM colonization facilitates this dominance by enabling Calluna to compete more effectively for other resources such as light and water. If this second hypothesis is correct Nardus should grow less when establishing in the presence of mycorrhizal Calluna. In either case, increased inputs of inorganic nitrogen to heathlands may make the role of ErM colonization redundant (Aerts, 1999). Therefore, in this study it was important to establish how ErM colonization influences plant competition with and without the addition of inorganic nitrogen.
Calluna and Nardus have contrasting root systems. The fine hair-roots of Calluna (
0.1 mm diameter) are produced mainly in the upper organic soil horizons (Aerts, 1993; Gimingham, 1960) where the greatest ErM colonization also occurs (Caporn et al., 1995). Nardus produces a combination of thick unbranched roots (1–2 mm diameter) that penetrate down to depths of 50 cm or more, and shallower fine lateral roots (<0.5 mm diameter) (Chadwick, 1960). These differences in rooting strategy may result in different competitive outcomes depending on substrate heterogeneity (Caldwell and Pearcy, 1994). Fitter demonstrated greater coexistence of grass species competing in a layered substrate than in a uniform substrate and suggested that this might be because different species differentially exploit distinct soil horizons (Fitter, 1982). In this study, spatial heterogeneity was created between organic and mineral substrates by over-laying sand with an organic layer as frequently observed in natural soil profiles.
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Materials and methods |
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Preparation of materials
Even-sized Calluna cuttings (6 cm long) were collected in August 1998 from a large number of plants at Grandhome Moss near Aberdeen, NE Scotland (grid reference NJ907123). After removing the leaves from the bottom 2 cm of each cutting they were planted in an unheated propagator in a mixture (1 : 1, v : v) of autoclaved, washed sand and TerragreenTM in a greenhouse until roots were produced. Nardus seeds were collected in late August 1997 from Glen Derry, Aberdeenshire (grid reference NO040958). After 5 min surface-sterilization with 1.0% sodium hyperchlorite the seeds were germinated on autoclaved sand.
Mor humus collected from below Calluna-dominated heath in Glen Clunie, Aberdeenshire (grid reference NO825139) was dried and sieved (2 mm) prior to sterilization by gamma-irradiation (8.19 kGy) to kill sources of ErM inoculum. After sterilization the humus was rehydrated and labile carbon and nitrogen was removed by repeated washing in a 0.1 M chloride salt solution containing cation ratios comparable to those found in local rainwater (Ca : Mg : Na : K=2 : 1 : 10 : 2, by vol.) (R Smart personal communication). The salt solution was not sterile, but distilled water was used here and throughout the remainder of the experiment to minimize the risk of ErM fungal contamination. The humus was then washed five times in distilled water and the pH adjusted to 3.8. (with HCl).
The ErM endophyte Hymenoscyphus ericae (Read) Korf & Kernam (Strain He 101, obtained from Professor DJ Read) was grown on malt extract agar. Fresh mycelium from the margins of actively growing colonies was grown in shaking modified Melin-Norkans liquid medium (pH 5.8) (Brundrett et al., 1996) at room temperature for 14 d. The mycelium was filtered and washed five times with distilled water before being mixed with distilled water and macerated in a blender to produce a slurry containing 7.2 mg ml-1 of dry mass mycelium.
Pot preparation and growth conditions
Plant pots (80 mm diameter, 60 mm depth) were prepared containing 40 mm of sterile, acid-washed medium sand (mean particle size=0.1 mm, Hepworth Minerals and Chemicals Ltd, Cheshire, UK (pH adjusted to 3.8 with HCl)) overlain with a 20 mm deep organic layer of mixed irradiated humus and sand (1 : 1, v : v) (Fig. 1a). Three rooted Calluna cuttings were planted into 28 ‘mixed species’ pots and allowed to establish for 2 weeks and then three Nardus seedlings (1 cm high) were added in an evenly spaced arrangement (Fig. 1b). Sixteen ‘Nardus-only’ pots were established at the same time by planting six Nardus seedlings in the same arrangement. All of the pots therefore had a constant plant density. After a further week half of each pot type received a 2 ml inoculation of live H. ericaeinoculum into the organic layer, the other half received an equal volume of autoclaved inoculum. In a fully crossed factorial design, half the pots received no additional nitrogen and the others received 10 ml of 50 µg N ml-1 nitrogen (as ammonium sulphate, pH adjusted to 3.8 with HCl) per day for the remainder of the experiment. All of the pots received 60 ml of half-strength nitrogen-free Rorisons solution (Hewitt, 1966) at the start of the experiment, and again 2 months later. The substrate was kept moist with distilled water (pH adjusted to 3.8 with HCl). The pots were placed in a randomized array (re-randomized every 2 weeks) in a controlled environment room (18 h day, 20 °C day, 15 °C night, irradiance of 600 µmol m-2 s-1) employing a combination of 55 W fluorescent tubes and 60 W tungsten bulbs to provide a R : FR ratio of 1.8.
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Harvesting
After the first 2 months, monthly non-destructive counts of Nardus leaf blades per plant in the low nitrogen, mixed-species pots were made. Destructive harvesting started 4 months after the Nardus was planted. The number of leaf blades per plant and maximum leaf blade height was recorded for each Nardus plant in the mixed species pots and three randomly selected plants from the Nardus-only pots. After oven-drying at 80 °C for 48 h, Nardus shoots, Calluna leaves and Calluna stems were weighed. Nardus shoots and Calluna leaves were analysed for total nitrogen and phosphorus content after sulphuric acid/hydrogen peroxide digestion (Allen, 1989). Nitrogen was measured as ammonium using the continuous flow, indophenol-blue method (Rowland, 1983), and phosphorus was measured as phosphate using the molybdenum blue method (Allen, 1989).
The pH of the organic layer was measured in distilled water. After separating the organic layer from the sand layer, all roots were carefully extracted by washing with a water jet over a 100 µm sieve, and manually separated by species with fine tweezers. A subsample of Calluna root was removed for ErM determination (see below). Roots that had grown out of the bottom of the pots were harvested separately. Each root sample was scanned on a flat-bed scanner and image-processed using WinRhizoTM v3.10b (Regent Instruments Inc., Canada) software to measure total root length and diameter class distribution. Where the sample was too large to be scanned (total root length >35 m) a subsample was used. After being dried in an oven at 80 °C for 48 h the root samples were weighed.
ErM colonization of the Calluna roots was determined using high pressure liquid chromatography to measure the concentration of ergosterol, a fungal-specific metabolite (Weete, 1980), in the root tissue (Caporn et al., 1995; Martin et al., 1990). A representative subsample (~80 mg) of fresh Calluna hair-root was taken from each organic layer sample and extracted in methanol using a pestle and mortar. Polyvinylpyrrolidone (PVP) was added (10%, w/v) to the methanol to precipitate phenolic compounds. The extract was centrifuged and the supernatant made up to 1.5 ml before being filtered through a Whatman® 0.45 µm syringe tip filter. 50 µl of sample was injected into a mobile phase (1.0 ml min-1) of 100% methanol running through a 12 cm C-18 HighChrome column at 27 °C using an HPLC (Thermo Separation Products® UK). External standards from 0.05–5 µg ml-1 were used to determine that ergosterol had a retention time of ~6.0 min (detecting at 280 nm) and to calculate ergosterol concentrations from the peak integration. Root ergosterol concentrations were calculated as µg g-1 (fr. wt.). The presence and absence of ErM infection in the roots was confirmed visually by clearing in 10% KOH (20 min, 80 °C), acidifying in 1% HCl (1 h) and staining (20 min, 80 °C) with 0.05% trypan blue in acid glycerol.
Statistical analysis
Due to the death of plants during the initial stages of the experiment, the design was unbalanced, with four pot replications for each Nardus- only treatment and six and seven replications, respectively, for the high and low nitrogen, mixed-species pots. The further death of a Calluna and a Nardus plant later in the experiment resulted in the removal from the analysis of one pot from the high nitrogen, uninoculated and one pot from the low nitrogen, inoculated, mixed-species treatments.
All variables are analysed on a per plant basis. Where root length between layers is compared, values are expressed as length cm-3 of substrate. Total root mass and length equalled the sum of root growth in the organic and sand layers and out of the bottom of the pot.
The results were analysed using GLM based ANOVA in Minitab. Box-Cox transformations were used for variables that had non-parametric residuals.
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Results |
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Above-ground measurements
Calluna:
Addition of nitrogen resulted in over a 3-fold increase in shoot biomass (F1,20=758.9, P<0.001). Leaf mass was increased to a greater extent than stem mass, resulting in a higher leaf : stem ratio in the pots that received high nitrogen addition (F1,20=18.03, P<0.001) (Fig. 2
). Leaf nitrogen concentration was increased from 0.56% (SE=0.02%) to 0.65% (SE=0.02%) by nitrogen addition, but phosphorus concentration was reduced by 28% despite a 150% increase in total foliar phosphorus content (Table 1). The proportion of pots containing flowering plants increased from 0.50–0.92 with nitrogen addition.
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There was no significant effect of H. ericae inoculation on total shoot mass, but stem mass was significantly higher in the inoculated pots, resulting in a 24% reduction in leaf : stem ratio (F1,20=15.67, P=0.001). H. ericaeinoculation did not affect nitrogen or phosphorus concentration or content of the leaves (Table 1
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Nardus:
Competition with Calluna reduced Nardus shoot biomass by 79% (F1,32=282.3, P<0.001), resulting in a consequent reduction in leaf blade number (F1,32=45.95, P<0.001) and height (F1,32=18.21, P<0.001) compared with plants from Nardus- only pots. The effect of competition on shoot mass and leaf blade number was greatest in pots that received no additional nitrogen as shown by a significant competitionxnitrogen addition interaction (F1,32=5.00, P<0.05; F1,32=5.77, P<0.05) (Fig. 3). The increase in shoot nitrogen concentration in plants grown in competition with Calluna was almost significant (F1,32=3.73, P=0.063), however, total foliar nitrogen content was reduced by 76% (Table 1). Shoot phosphorus concentration was increased by 34% in plants grown in competition with Calluna although there was no difference when the plants received high nitrogen addition (Table 1).
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Nitrogen addition resulted in an almost 4-fold increase in shoot biomass (F1,32=278.9, P<0.001), resulting in greater leaf blade number (F1,32=63.4, P<0.001) and height (F1,32=70.57, P<0.001). Although the increase in shoot nitrogen concentration was not quite significant (F1,32=3.31, P=0.078), the total nitrogen content was over five times higher in the high nitrogen pots (Table 1
). Total shoot phosphorus content was also increased by nitrogen addition but concentration was reduced by 22% (Table 1).
H. ericae inoculation did not have a significant effect on final shoot mass or leaf blade number at the final harvest, but the mean values were, respectively, 15% and 19% lower when in competition with mycorrhizal Calluna. Measurements of leaf blade number in the mixed species pots that received low nitrogen addition one month (F1,12=13.21, P<0.01) and two months (F1,12=10.44, P<0.01) prior to the final harvest do, however, show a significant 39% reduction in leaf blade number in the presence of H. ericae (Fig. 4). Shoot nitrogen content and concentration was not influenced by H. ericae inoculation (Table 1). Nardus plants competing with Calluna in inoculated pots had a 27% higher phosphorus concentration than the other treatments (Table 1; Fig. 5).
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Below-ground measurements
Root length and diameter distribution:
Fine hair-roots (<0.3 mm) accounted for the greatest root length produced by Calluna plants, resulting in a strong positively skewed root length diameter class distribution (Fig. 6). The Nardus root systems had a more normally distributed root length diameter class distribution with a modal diameter range between 0.3 mm and 0.6 mm (Fig. 6). Calluna and Nardus had mean root lengths per plant of 154 m (SE=14 m) and 32 m (SE=2 m), respectively.
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Calluna:
Nitrogen addition more than doubled total root mass, but the increase was greatest in plants not inoculated with H. ericae (F1,20=5.21, P<0.05) (Fig. 7). Total root length was also doubled by nitrogen addition (F1,20=49.9, P<0.001). Overall, there was almost twice the root length density in the organic layer than in the sand layer (F1,40=27.61, P<0.001). Live H. ericae inoculation resulted in substantially higher concentrations of ergosterol in the hair-roots, irrespective of nitrogen addition (37–104 µg g-1 (fr. wt.) (F1,20=23.97, P<0.001)) and ErM colonization was confirmed visually. H. ericae inoculation did not influence total root length or distribution between the organic and sand layers.
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Nardus:
Nardus root mass was decreased by 59% when in competition with Calluna (F1,32=290, P<0.001) and increased almost 3-fold by nitrogen addition (F1,32=132, P<0.001). Root : shoot ratio was increased 10% by competition with Calluna (F1,32=6.67, P<0.05) and decreased by half in pots that received nitrogen addition (F1,32=276, P<0.001). Total root length was increased 40% by nitrogen addition (F1,32=14.83, P=0.001), but this did not affect relative root length production between the organic and sand layers. When Nardus was grown alone, root length density was three times greater in the organic layer than in the sand layer (Fig. 8). Competition with Calluna halved the root length density in the organic layer (F1,64=, P<0.001), but did not influence root length density in the sand layer (Fig. 8). H. ericae inoculation resulted in a further 33% reduction in root length density in the organic layer when Nardus was in competition with Calluna (F1,64=3.99, P=0.05) (Fig. 8). H. ericaeinoculation had no effect on root mass or length in the absence of Calluna.
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Peat layer pH
Nitrogen addition reduced the pH of the organic layer from 4.67 to 3.80 (F1,32=148.4, P<0.001). Although not quite significant (F1,32=3.86, P=0.058) the presence of Calluna slightly increased the organic layer pH from 4.16 to 4.31. H. ericae inoculation did not affect pH.
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Discussion |
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Calluna was the superior competitor under all combinations of nitrogen and mycorrhizal treatments. Nardus plants grown with Calluna had smaller root and shoot masses and fewer leaf blades than when grown with other Nardus plants. The use of a layered organic and sand substrate had important effects on root growth and therefore competition between the two species. Both Nardus grown alone and Calluna produced greater root length density in the organic layer. Quartz sand has a very low cation exchange capacity (Rowell, 1994
) so it is likely that nutrients mineralized from the organic matter remained concentrated in the organic layer. These species demonstrate a typical root growth response to nutrient availability as demonstrated by Drew et al. (Drew et al., 1973). Competition with Calluna reduced the root length density of Nardus, but only in the organic layer, demonstrating that below-ground effects were most pronounced in the soil layer of greatest nutrient availability.
Both species took up more nitrogen and grew bigger in response to nitrogen addition, which confirmed that growth of plants in the low nitrogen treatments was nitrogen limited. Nitrogen addition increased the nitrogen : phosphorus ratio of both Calluna and Nardus leaves indicating that phosphorus availability might limit growth in the high nitrogen pots. Nardus plants grown in competition with Calluna without added nitrogen had the highest shoot phosphorus concentrations, suggesting that Calluna was out-competing Nardus for nitrogen rather than phosphorus in these pots. Ericaceous shrubs have been shown to have a competitive advantage over grasses in low nutrient conditions (Berendse and Aerts, 1984; Hartley and Amos, 1999). In this study Calluna was also a better competitor when nitrogen was added, irrespective of the presence of ErM colonization. In contrast, Hartley and Amos (Hartley and Amos, 1999) found that Calluna did not respond to fertilization when in competition with Nardus, and similar results have been obtained in experiments with Erica tetralix and Molinia caerulea (Berendse and Aerts, 1984). This difference may arise because these previous experiments were carried out in homogenous substrates where the ericoid species did not gain an advantage by being able to out-compete Nardus roots in the upper organic layer of substrate.
The addition of live H. ericae resulted in ErM colonization of Calluna hair roots, with ergosterol concentrations comparable to those found in field-collected roots (Caporn et al., 1995). The low levels of ergosterol found in the non-mycorrhizal treatment may have been due to small amounts of colonization in some parts of the root system or contamination by other soil fungi. In some studies (Hartley and Amos, 1999; Stribley and Read, 1976; Yesmin et al., 1996), nitrogen addition has reduced ErM colonization, but this is not always the case (Caporn et al., 1995). In this study nitrogen addition resulted in foliar nitrogen concentrations around 0.8%, which may not be high enough to reduce levels of colonization.
ErM colonization had little effect on the overall growth and nutrition of Calluna in this study. Strandberg and Johansson did not detect an increase in mass or nitrogen uptake by Calluna in response to ErM colonization (Strandberg and Johansson, 1999), but Read and Stribley and Stribley et al. detected an increase in growth and nitrogen uptake in Calluna and Vaccinium (Read and Stribley, 1973; Stribley et al., 1975). High nutrient availability was not a factor in the lack of response of Calluna to mycorrhizas in this study, because there was no response in the low nitrogen addition pots where plant growth was clearly nitrogen limited. Although there was not an effect of ErM colonization on Calluna root length or distribution, root mass of mycorrhizal plants did not show as great a response to nitrogen addition as non-mycorrhizal plants. Stribley and Read also reported a reduction in root : shoot ratio of mycorrhizal V. macrocarpon seedlings grown in sand culture (Stribley and Read, 1976). Further investigations are required to determine the reasons for variable responses to ErM colonization.
Nardus was not affected in any way by inoculation with live H. ericae in the absence of Calluna. Effects of inoculation on Nardus growth in competition with Calluna must, therefore, result from ErM formation rather than direct effects of H. ericae on Nardus. This study demonstrates that Nardus roots were less able to exploit the organic layer when Calluna was mycorrhizal. Although there was no significant effect of this reduced root length density on the final shoot harvest, measurements of leaf blade number in low nitrogen pots prior to harvest did demonstrate a significant reduction in blade number in those pots containing mycorrhizal Calluna. Nardus plants growing with mycorrhizal Calluna were able to acquire the same amounts of shoot nitrogen and phosphorus as those growing with non-mycorrhizal Calluna, indeed these smaller plants had the highest shoot phosphorus concentrations. Because the effects of mycorrhizal colonization of Calluna on Nardus were present in pots that received both high and low nitrogen levels and did not alter the nutrient content of the plants, increased competition for nutrients by Calluna cannot be the mechanism for these mycorrhizal effects. Similarly, mycorrhizal effects on Nardus root distribution were not the result of enhanced Calluna growth. It is unlikely that light or water limitation were the mechanisms of competition, because of the high PAR with overhead and side illumination, and constantly maintained soil moisture levels. ErM colonization did not affect substrate pH, and acidification caused by ammonium uptake in the high nitrogen pots did not alter the mycorrhizal effects, thus pH alteration can be ruled out as a mechanism of inhibition. In conclusion, the observed effects of ErM colonization on Nardus/Calluna competition seem unlikely to be resource-based and other interference mechanisms such as allelopathy must be considered.
Allelopathic interference has been implicated in many negative interactions between species (Rice, 1984). There are, however, a number of problems in identifying allelopathic effects because of the difficulties in eliminating resource competition (Harper, 1975). Nilsson attempted to separate resource competition from allelopathic interference of Empetrum hermaphroditum extracts on Pinus sylvestris growth (Nilsson, 1994). However, in Nilssons' study the allelochemical (identified by Odén et al., 1992) reduced both nutrient uptake and mycorrhizal colonization of P. sylvestris (Nilsson et al., 1993), so it was not possible to separate resource competition from allelopathic interference. Subsequently, Michelsen et al. demonstrated that the E. hermaphroditum extract only appeared to be allelopathic because it stimulated microbial activity, and resulted in enhanced plant/microbial nutrient competition (Michelsen et al., 1995). In this experiment the effect of ErM colonization on the interaction between Calluna and Nardus did not result in reduced nutrient uptake by Nardus, so if allelopathy is a factor it is not acting through inhibition of nutrient uptake.
Short-chain fatty acids, thought to be products of Calluna litter decomposition, have been identified in heathland soils at concentrations potentially inhibitive to root growth (Jalal and Read, 1983a). Utilization of lipids by H. ericae may result in production of these toxic fatty acid residues, providing a potential mechanism by which ErM colonization indirectly inhibits growth of non-ericaceous competitors (Jalal and Read, 1983b). This process may have been responsible for inhibition of Nardus root growth in this study, particularly since the organic matter used in this study originated from Calluna heath. As a note of caution, gamma-irradiation, like all sterilization procedures for organic matter, has unwanted side-effects. For example, if irradiation altered the physico-chemical composition of labile organic material, its subsequent transformation by H. ericae (as suggested above) might not be typical of non-irradiated organic material. Furthermore, the post-irradiation microbial community within which the Calluna/H. ericae/Nardus interaction took place is unlikely to be the same as that in the field. Extrapolation of these findings to Calluna/Nardus interaction in the field should bear this in mind.
AM mycorrhizas may have antagonistic effects on both non-host and host species (Francis and Read, 1994, 1995; Grime et al., 1987). In some cases this has been attributed to hyphal penetration of non-host roots resulting in deformation or mortality of the infected root (Allen et al., 1989; Francis and Read, 1995). In this investigation no penetration of Nardus roots by H. ericae was found. Inhibition of non-host root growth has also been demonstrated in the absence of hyphal penetration (Francis and Read, 1994). In this case, water extracts from the substrate containing mycorrhizas caused inhibition of root growth, which resulted in the conclusion that the suppression may be allelopathic.
It has been demonstrated that the ascomycete fungi involved in ErM associations have a high genetic diversity both within and between individual host plants (Hutton et al., 1994; Perotto et al., 1996), which may be reflected in functional diversity (Mitchell and Read, 1985). In this experiment Hymenoscyphus ericae isolated from V. macrocarpon was used as the sole inoculum (Bajwa and Read, 1985; Stribley and Read, 1976). In Calluna heath, H. ericae is the dominant endophyte (Smith and Read, 1997), however, this experiment should be repeated with a wider range of ErM fungal isolates. Nardus is normally colonized by AM endophytes in the field (Ali, 1969; Heijne, et al., 1994). The plants in this study were not inoculated with AM endophytes and did not become colonized over the period of study. There is evidence from field surveys that although colonization is still present, levels are significantly lower in plants establishing among Calluna swards (DR Genney, unpublished data). The lack of AM colonization in this study may have influenced the outcome, but reflects to some extent the situation when Nardus establishes into Calluna swards in semi-natural heathland communities.
The results presented here demonstrate that ErM colonization of Calluna is able to reduce the competitive vigour of Nardus in a layered organic/inorganic system. This may be one mechanism by which Calluna dominated swards are maintained in heathland ecosystems. However, there is no evidence that the competitive balance between Calluna and Nardus is in the conditions of this experiment altered by mycorrhizal enhancement or amelioration of nutrient competition, for example, by selective uptake of organic nitrogen. Other mechanisms such as allelopathy must be considered. In pot experiments, grazing and elevated nutrient levels are known to reduce ErM colonization in ericoid hair-roots. If these processes can be demonstrated in the field, the loss of ErM colonization may be one factor facilitating the ability of grasses such as Nardus to invade and replace Calluna heath.
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Acknowledgments |
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We thank Professor David Read for providing the H. ericae culture and Richard Smart for local rain-water chemistry data. We are also grateful to the Invercauld Estate for allowing access to collect materials and to Janet Woo for help maintaining the plants. This work was funded by an NERC studentship to DRG.
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Notes |
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3 To whom correspondence should be addressed. Fax: +44 122 427 2703. E-mail: d.r.genney{at}abdn.ac.uk
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References |
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