Taking mycocentrism seriously: mycorrhizal fungal and plant responses to elevated CO2
Author for correspondence: Odair Alberton Tel: +31 317 482371 Fax: +31 317 483766 Email: firstname.lastname@example.org
- • The aim here was to separately assess mycorrhizal fungal and plant responses under elevated atmospheric CO2, and to test a mycocentric model that assumes that increased carbon availability to the fungus will not automatically feed back to enhanced plant growth performance.
- • Meta-analyses were applied across independent studies. Responses were compared in ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungi, and ECM and AM plants.
- • Responses of both mycorrhizal fungi and mycorrhizal plants to elevated CO2 were significantly positive. The response ratio for ECM fungi was 1.34 (an increase of 34%) and for AM fungi 1.21 (21%), indicating a significantly different response. The response ratio for ECM plants was 1.26, similar to that of AM plants (1.25). Fractional colonization proved to be an unsuitable fungal parameter. Evidence was found for the mycocentric view in ECM, but not in AM systems.
- • Fungal identity and plant identity were important parameters that affected response ratios. The need for better descriptors of fungal and plant responses is emphasized.
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The potential consequences of rising atmospheric CO2 levels for plant growth, crop production, and ecosystem functioning have resulted in a large number of studies to assess plant performance under elevated CO2, both in the greenhouse and under field conditions. To better understand and predict responses, we need to study below-ground dynamics, including both roots and associated microorganisms. One major association is the mycorrhizal symbiosis, a mutualistic symbiosis with certain soil-inhabiting fungi. It has been estimated that up to 30% of total photoassimilate products can be used in growth and maintenance of the fungus (Ek, 1997; Nehls & Hampp, 2000; Nehls et al., 2001). Bhupinderpal-Singh et al. (2003) reported that respiration by mycorrhizal roots and the mycelia of mycorrhizal fungi can account for more than 50% of total soil respiration.
Research on mycorrhizas has often been plant-centred (Fitter et al., 2000), and many phytocentric models treat mycorrhizal fungi primarily as extensions of the plant's root system. A mycocentric view on carbon (C) use by the fungus for its own growth and maintenance was proposed by Fitter et al. (1998). This mycocentric view assumes that carbon is allocated between intraradical and extraradical mycelia according to the fungal carbon demands, not to the demands of their autotrophic hosts (Robinson & Fitter, 1999).
From a mycocentric perspective it is essential to include measurement of the extraradical mycelium because this parameter is less dependent on root growth responses than the intraradical mycelium. It is known that elevated CO2 tends to increase mycorrhizal parameters, such as amount of extraradical hyphae (Tingey et al., 2000; Treseder & Allen, 2000). Ceulemans et al. (1999) suggested that this parameter is important for soil exploration and the potential for subsequent enhanced nutrient translocation to the host. However, Fransson et al. (2005) suggested that a possible increase in fungal biomass production may not necessarily result in increased transfer of N to the host plant, since the fungus can also become a larger sink for nutrients. Evidence for the importance of fungal nutrient immobilization, leading to the lack of plant responses (or even a negative plant response) have been described by Colpaert et al. (1992), Colpaert & Verstuyft (1999), and Treseder and Allen (2002).
Since global perturbations of the environment are increasing, it is very important to improve the predictive power of models by taking into account the functioning of soil microorganisms (Lindahl et al., 2002). Moreover, it is imperative to know how the rising atmospheric CO2 concentration influences the mycorrhizal symbiosis. An understanding of mycorrhizal fungal and plant controls over additional carbon may help to improve the predictive power of models of the effects of elevated CO2. As a first step in improving our knowledge of the effects of elevated CO2 on mycorrhizal fungal and plant responses, a meta-analysis may provide a quantitative statistical means of integrating independent results and of identifying aspects of experimental design that might contribute to variation among studies (Gurevitch & Hedges, 1999, 2001).
To date, meta-analyses of plant responses to elevated CO2 have been published by Curtis & Wang (1998), Poorter & Pérez-Soba (2001), and Jablonski et al. (2002). Treseder (2004) was the first to perform a meta-analysis of mycorrhizal responses to elevated CO2. She noted an increase in colonization by 36%. However, Treseder's meta-analysis was performed with few data points (14), as she only included field studies. Her work did also not allow to test for differences in measurement types (fungal responses assessed by mycelial measurements compared with changes in colonization). When more than one index of mycorrhizal performance was available, fractional colonization was used in order to facilitate comparisons between studies. In the paper she also did not compare the response of mycorrhizal fungus and mycorrhizal plant over the same range of studies.
This meta-analysis aims to integrate research results from all published studies on effects of elevated CO2, focusing on the contrast of mycocentric and phytocentric views. It contrasts mycorrhizal fungal and plant responses for both ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) systems and compares differences between ECM and AM systems, both for fungal and plant responses. More specifically we attempted to answer the following questions: (1) Does elevated CO2 differentially affect mycorrhizal fungal responses for ECM and AM systems? (2) Does elevated CO2 differentially affect mycorrhizal plant responses for ECM and AM systems? (3) Are there significant differences between fungal and plant responses in both ECM and AM systems? (4) Is the magnitude of the effect of elevated CO2 comparable between experiments with individual mycorrhizal fungal species and experiments with mixed fungal communities? (5) Is the magnitude of the effect of elevated CO2 comparable between short-term (< 1 yr) and long-term (> 1 yr) studies? (6) Is there a significant difference between laboratory and field experiments?
Materials and Methods
One major assumption of meta-analysis is that studies and data points are independent of one another (Gurevitch & Hedges, 1999, 2001). If particular publications reported data from more than one study system (different mycorrhizal fungal species, plant species, nitrogen and phosphorus levels, temperature, soil moisture, light availability and/or ozone), those systems were considered independent data points. Our choice to consider these different data points as providing independent data points is based on the conditionality or context-dependency of mycorrhizal functioning (van der Heijden & Kuyper, 2001). Levels of ambient CO2 ranged from 340 to 380 ppm (in one study, 400 ppm), those of elevated CO2 from 540 to 750 ppm (in one study, 10 000 ppm; note that this study did not yield an outlier). In most cases the ratio between elevated and ambient CO2 was close to 2. Four studies contained three CO2 levels, (350, 525, and 700 ppm); in those cases we selected the higher and lower level only. Papers that only reported mycorrhizal fungal or mycorrhizal plant responses were not considered. If parameters had been measured several times in the same study, only the last sampling date was used for meta-analysis. Only one parameter for fungal and plant performance was used for every data point. When different measurements were taken, we determined an order of priority (see later).
The data collection consisted of obtaining the means of the two groups: experimental (elevated CO2) and control (ambient CO2), with standard deviation (SD) and replicate number (n). Papers where SDs or replicate numbers were missing were excluded. If data were presented graphically, values were estimated from figures manually digitized. The units with which measurements were reported were not considered since the calculated response ratio is dimensionless. In case only standard errors (SEs) were given, these were transformed to SD according to the equation: SD = SE × √n. Unidentified error bars were assumed to represent SE. If several values of n were given, the lowest value was taken.
Literature search was performed through ‘Web of Science’ with keywords mycorrhiza(l) and carbon dioxide. In total, 28 papers on ECM systems and 24 papers on AM systems were analysed. As several papers included either more fungal or plant species, or different experimental conditions, we analysed 65 data points for ECM systems and 77 data points for AM systems. We further subdivided the data in various subgroups: (1) data on individual fungal species and on mixed communities; (2) data obtained from conifer and broad-leaved ECM systems; (3) data from woody and herbaceous AM systems; (4) data on short-term (< 1 yr) and long-term (> 1 yr) experiments; (5) data on laboratory experiments and on field experiments. The literature search was ended on March 13, 2005.
Order of importance for parameter choice
Data from the literature contained in most cases more than one measured parameter in each study. However, meta-analysis requires that only one parameter be used, as the use of multiple parameters violates the assumption of independence of the data. It was therefore essential to a priori rank the parameters used to assess plant and fungal responses to arrive at a less biased estimate. Parameter choice was based on the following rationale. Measurements that included the symbiotic interface (root biomass, fractional colonization) were considered to provide a less accurate picture of separate fungal and plant responses than measurements that pertain more or less exclusively to fungus or plant only. Above-ground plant measurements (shoots) were preferred over root measurements, and measurements of the extraradical mycelium were preferred over measurements of colonization. This choice should allow a maximum separation between phytocentric and mycocentric perspectives. As meta-analysis allows testing of the extent to which the final outcome depends on parameter choice, we also executed separate meta-analyses for every parameter.
After having defined the rank order of parameters, the parameter with the lowest rank was chosen first. If parameter 1 was absent in the study considered, parameter number 2 was used; if parameter 2 was also absent, the next parameter number 3 was used, and so on. In all cases only one parameter was used to assess the response of the mycorrhizal fungus, and one parameter to assess the response of the mycorrhizal plant.
The following rank order for mycorrhizal fungal responses was determined.
- 1Dry weight of extraradical mycelium
- 2Area or development of extraradical mycelium
- 3Hyphal length in root-free compartment
- 4Hyphal length in root compartment
- 5Specific phospholipid fatty acid (PLFA) content
- 6Ergosterol content
- 7Glycogen content of hyphae
- 814C in substrate
- 9Below-ground carbon use efficiency
- 1014C respired from the soil
- 1114C distributed in ECM tips
- 12ECM biomass
- 13Fractional colonization (%)
- 14Total number of root tips
- 15Nitrogen (N) and phosphorus (P) concentration in ectomycorrhizas.
Note that parameters 11–15 could provide a more biased assessment of the mycocentric view.
The following rank order for mycorrhizal plant responses was determined.
1 Leaf or needle biomass or area
- 2Stem or stump biomass
- 3Shoot biomass
- 414C in shoots
- 5Total plant biomass
- 6Total root length
- 7Total root biomass
- 814C in roots
- 9Root density
- 10Woody or coarse root biomass
- 11Fine root biomass
- 12Fine root production
- 13Carbon transferred to fine roots
- 14Root: shoot ratio
- 15Standing root biomass
- 16Net assimilation rate
- 17Measurements of individual sugars
- 18Measurements of individual enzymes
- 19Nitrogen and P concentration or content in the plant.
Note that parameters 5–15 could provide a more biased estimate of the phytocentric view.
The meta-analysis was performed separately for mycocentric and phytocentric views. A random effect model was used if the value of pooled within-class variance (σ2 pooled) was higher than zero, and a fixed effect model was used if that quantity was equal to or lower than zero (Hedges et al., 1999; Rosenberg et al., 2000).
A quantitative index of the effect size in each experiment was calculated by the natural log of the response ratio. The response ratio was calculated by the mean of the experimental group (elevated CO2) divided by the mean of the control group (ambient CO2) (Hedges et al., 1999; Rosenberg et al., 2000; Gurevitch & Hedges, 2001). In the Results section we report the weighted mean response ratio (R), the 95% confidence interval (CI) for R and the number of observations (n). The mean of a response variable was considered significantly positive if the lower limit of the 95% CI was larger than 1. The means of two different response variables were tested for significant differences based on the model heterogeneity test (Q-test), which is tested against a χ2 distribution with 1 d.f., as implemented in metawin (P = 0.05) (Rosenberg et al., 2000).
Calculations were performed using metawin 2.0 and in Microsoft Excel worksheets.
Data on the responses by fungi and plants in both mycorrhizal symbioses are given in Table 1. Responses of both mycorrhizal fungi and mycorrhizal plants were significantly positive under elevated CO2. The mean response ratio was 1.34 (CI = 1.25–1.43) for ECM fungi, an increase of 34%, and 1.21 (CI = 1.12–1.32) for AM fungi, an increase of 21%. The Q-test indicated a significantly different response of ECM and AM fungi to elevated CO2 (P = 0.02). The response ratio was 1.26 for ECM plants and 1.25 for AM plants (P = 0.84).
Table 1. Meta-analysis of the effects of elevated CO2 on mycorrhizal systems
|ECM fungi||1.34||1.25–1.43||65|| 0.02|
|AM fungi||1.21||1.12–1.32||77|| |
|ECM plants||1.26||1.19–1.34||65|| 0.84|
|AM plants||1.25||1.19–1.31||74|| |
|ECM fungi||1.34||1.25–1.45||65|| 0.17|
|ECM plants||1.26||1.19–1.34||65|| |
|AM fungi||1.21||1.12–1.32||77|| 0.32|
|AM plants||1.25||1.19–1.31||74|| |
|ECM extraradical||1.45||1.30–1.65||38||< 0.01|
|ECM colonization percentage||1.19||1.09–1.28||31|| |
|AM extraradical||1.23||1.07–1.40||46|| 0.65|
|AM colonization percentage||1.17||1.06–1.30||30|| |
|ECM extraradical||1.45||1.30–1.65||38|| 0.04|
|AM extraradical||1.23||1.07–1.40||46|| |
|ECM extraradical||1.45||1.30–1.65||38|| 0.03|
|ECM plants||1.26||1.19–1.34||65|| |
|AM plants||1.25||1.19–1.31||74|| 0.55|
|AM extraradical||1.23||1.07–1.40||46|| |
Although the response of ECM fungi was larger than that of ECM plants, the difference was not significant (P = 0.17). The responses of AM fungi and of AM plants were similar (P = 0.32). These data appear to indicate that the phytocentric and mycocentric view do not make a difference. However, the full data set included papers where fungal performance measurements were based on fractional colonization data. In order to obtain a less biased view of mycocentric responses, we assessed response ratios separately for the extraradical mycelium and for fractional colonization. For ECM fungi the extraradical mycelium (R = 1.45) responded to a larger extent to elevated CO2 than fractional colonization (R = 1.19), and the difference between both was significant (P < 0.01). For AM fungi both responses were similar (extraradical mycelium, R = 1.23; fractional colonization, R = 1.17) and not significantly different (P = 0.65). The responses of the extraradical mycelium of ECM and AM fungi were also significantly different (P = 0.04). A comparison between ECM plant response and fungal response based on the performance of the extraradical mycelium indicated that the phytocentric and mycocentric views were quantitatively different (P = 0.03), while no difference in either view was evident for AM systems (P = 0.55).
Could fungal identity matter from the phytocentric perspective (Table 2)? A subdivision of experiments with fungal communities and experiments with individual fungal species showed large and significant differences between ECM plants in both kinds of experiment (P = 0.02). Plants colonized by an ECM fungal community showed a much larger response (R = 1.39) than plants colonized by single fungal species (R = 1.16). However, fungal responses in both kinds of experiment were very similar (P = 0.91). The AM plants also showed differences between experiments with fungal communities and experiments with individual species (P = 0.08). Again, plants colonized by AM fungal communities responded (R = 1.31) more strongly to elevated CO2 than plants colonized by individual species (R = 1.20). From a mycocentric perspective there were no significant differences (P = 0.19).
Table 2. Meta-analysis of the effects of elevated CO2 on mycorrhizal systems
|Plants associated with ECM fungi species specific||1.16||1.06–1.27||37|| 0.02|
|Plants associated with ECM fungal community||1.39||1.23–1.57||28|| |
|ECM fungi species specific||1.32||1.22–1.43||36|| 0.91|
|ECM fungal community||1.31||1.20–1.45||29|| |
|Plants associated with AM fungi species specific||1.20||1.12–1.28||32|| 0.08|
|Plants associated with AM fungal community||1.31||1.21–1.40||42|| |
|AM fungi species specific||1.31||1.12–1.52||32|| 0.19|
|AM fungal community||1.15||1.03–1.30||45|| |
|ECM fungi conifer tree||1.35||1.25–1.46||50|| 0.73|
|ECM fungi broad-leaved tree||1.31||1.13–1.52||15|| |
|ECM fungi conifer tree extraradical response||1.52||1.35–1.70||27|| 0.05|
|ECM fungi broad-leaved tree extraradical response||1.23||1.00–1.52||10|| |
|AM fungi associated with woody species||1.38||1.09–1.73||14|| 0.18|
|AM fungi associated with herbaceous species||1.17||1.07–1.30||63|| |
|ECM conifer tree||1.30||1.21–1.40||51|| 0.06|
|ECM broad-leaved tree||1.11||0.94–1.30||14|| |
|AM woody species||1.32||1.17–1.51||14|| 0.26|
|AM herbaceous species||1.23||1.16–1.30||60|| |
|ECM fungi short-term (< 1 yr)||1.27||1.17–1.38||34|| 0.20|
|ECM fungi long-term (> 1 yr)||1.36||1.25–1.49||31|| |
|AM fungi short-term (< 1 yr)||1.22||1.11–1.35||60|| 0.62|
|AM fungi long-term (> 1 yr)||1.16||0.94–1.42||17|| |
|ECM plants short-term (< 1 yr)||1.13||1.02–1.25||32|| 0.01|
|ECM plants long-term (> 1 yr)||1.39||1.25–1.54||33|| |
|AM plants short-term (< 1 yr)||1.22||1.15–1.28||59|| 0.08|
|AM plants long-term (> 1 yr)||1.38||1.21–1.57||15|| |
|ECM plants in laboratory experiments||1.16||1.07–1.27||41||< 0.01|
|ECM plants in field experiments||1.45||1.28–1.62||24|| |
|ECM fungi in laboratory experiments||1.35||1.25–1.46||43|| 0.41|
|ECM fungi in field experiments||1.28||1.15–1.43||22|| |
|AM plants in laboratory experiments||1.28||1.21–1.38||47|| 0.08|
|AM plants in field experiments||1.16||1.05–1.28||27|| |
|AM fungi in laboratory experiments||1.22||1.08–1.39||48|| 0.67|
|AM fungi in field experiments||1.17||1.04–1.32||29|| |
Could plant identity matter from the mycocentric perspective (Table 2)? Responses of ECM fungi on conifers and broad-leaved trees were similar (R = 1.35 and 1.31, respectively; P = 0.73). However, the response ratios for the extraradical mycelium showed larger differences (ECM fungi with conifers, R = 1.52; ECM fungi with broad-leaved trees, R = 1.23; P = 0.05). Responses of AM fungi when associated with woody species were larger than responses of AM fungi when associated with herbaceous plants (R = 1.38 and 1.17), but the difference was not significant (P = 0.18). From a phytocentric perspective, ECM conifer tree species responded more strongly than broad-leaved tree species (R = 1.30 and 1.11; P = 0.06), but AM trees and AM herbs responded similarly (R = 1.32 and 1.23, respectively; P = 0.26).
Fungal responses in both short-term (< 1 yr) and long-term (> 1 yr) experiments (Table 2) were not significantly different in both ECM and AM systems. However, both ECM and AM plants showed a larger response to elevated CO2 in long-term experiments than in short-term experiments (P = 0.01 and 0.08, respectively). The ECM plants showed a much larger response ratio in field experiments (R = 1.45) than in laboratory experiments (R = 1.16) and the difference between both kinds of experiment was significant (P < 0.01). Conversely, the fungal response was not different between both classes of experiments (P = 0.41). There were no significant differences in response between field and laboratory experiments for AM fungi (P = 0.67). For AM plants the difference was marginally significant (P = 0.08; Table 2).
Table 3 lists the results of the meta-analysis for the various ECM fungal and AM fungal parameters. Since for many parameters sample size was small, CIs tended to be (very) large and there were few consistent patterns.
Table 3. Meta-analysis for the rank order of parameters to ectomycorrhizal (ECM) fungi and arbuscular mycorrhizal (AM) fungi under elevated CO2
|Parameter for ECM fungi|
|1. Dry weight of extraradical mycelium||1.45||1.16–1.79|| 9|
|2. Area or development of extraradical mycelium||1.39||0.86–2.25|| 5|
|3. Hyphal length in root-free compartment||1.57||0.73–3.42|| 3|
|4. Hyphal length in root compartment||1.12||0.73–1.72|| 5|
|5. Specific phospholipid fatty acid (PLFA) content||1.13||0.88–1.43|| 4|
|6. Ergosterol content||1.60||1.34–1.90|| 8|
|7. Glycogen content of hyphae||3.49||0.00–247.71|| 2|
|8. 14C in substrate||1.92||0.10–36.23|| 2|
|9. Below-ground carbon use efficiency||0.82||0.00–184.93|| 2|
|10. 14C respired from the soil||1.86||0.11–30.57|| 2|
|11. 14C distributed in ECM tips||1.00||0.79–1.27|| 1|
|12. ECM biomass||1.48||1.06–2.08|| 8|
|13. Fractional colonization (%)||1.19||1.09–1.28||31|
|14. Total number of root tips||1.21||1.04–1.40||26|
|15. Nitrogen concentration or content||0.82||0.44–1.54|| 4|
|15. Phosphorus concentration or content||0.70||0.47–1.04|| 3|
|Parameter for AM fungi|
|1. Hyphal length||1.23||1.05–1.43||46|
|2. Hyphal length with roots||1.49||1.09–2.05|| 1|
|3. Fractional colonization (%)||1.17||1.06–1.30||30|
Meta-analysis is a combination of data from independent studies to estimate the magnitude of the effect across such studies and to check potentially causative differences in the effect among them (Gurevitch & Hedges, 2001). Because many factors, in addition to elevated CO2, could have caused the different responses (choice of mycorrhizal fungal and plant species, duration of the experiment, field and laboratory conditions) we also subdivided experiments in a number of separate classes to test for these subsidiary factors. Two major complicating issues of meta-analysis, as mentioned by Gurevitch & Hedges (2001) are publication bias and research bias. Publication bias (under-reporting of experiments without significant results) will likely lead to an overestimation of the number of significant results and an overestimation of effect size. Research bias is more problematic (Gurevitch & Hedges, 1999). Research bias can be manifested through the tendency to preferentially choose certain organisms or experimental conditions under the expectation of obtaining significant results (again resulting in overestimation of effect size) or through the choice of model species and experimental conditions that are easy to handle. This may result in choosing fast-growing mycorrhizal species with a higher nutrient demand or experimental conditions that are too nutrient-rich from the point of view of mycorrhizal functioning (Read, 2002). Nutrient-rich conditions may explain experimental results where ECM trees performed less than the nonmycorrhizal controls (Gebauer et al., 1996; Rouhier & Read, 1998). Research bias may also result in investigating parameters that are easier to assess (fractional colonization), even if these are not the most suitable parameters. Finally, Klironomos et al. (2005) showed that pulse experiments with instantaneous doubling of CO2 levels, while more easy and rapid to execute, are likely to overestimate effects compared with conditions where CO2 levels are increased gradually.
Our meta-analysis showed that both mycorrhizal fungi and mycorrhizal plants respond positively to elevated CO2. The magnitude of the plant effect (25% for AM plants, 26% for ECM plants) is similar to the effects reported by Curtis & Wang (1998), who noted an overall response of 29% and Jablonski et al. (2002) who noted an overall effect of 31%. Our effect size is smaller than that reported by Poorter & Pérez-Soba (2001) who observed an effect of 47% under optimal conditions, but noted that under nutrient-poor conditions, where mycorrhizal symbioses are likely of largest benefit, effects can be much smaller. The average effect sizes of 21% for AM fungi and 34% for ECM fungi are lower than the results of the analysis by Treseder (2004) who noted an average enhanced effect of 47%. She noted that measurements of fractional colonization (36%) gave a lower response. While she deliberately selected fractional colonization in order to facilitate comparisons between studies, we tried to make a clear separation between the mycocentric and phytocentric view.
There are limits to this conceptual separation of mycocentric and phytocentric perspectives. Under conditions of normal soil fertility almost all mycorrhizal fungi and mycorrhizal plants can only complete their life cycle in the symbiotic condition. Responses to elevated CO2 must therefore be coordinated to some extent, and in a strict sense there may be no exclusive fungal and plant performance parameters. However, coordinated responses have often been assumed rather than demonstrated. Therefore, testing whether the magnitude of effects of elevated CO2 on fungus and plant are significantly different remains important. For proper testing the issue of parameter choice is relevant. For our analysis we selected parameters that appear indicative of a fungal or plant response over parameters that are more directly affected by the coordinated response of the symbiotic interface (the mycorrhizal root: measurements of root biomass and fractional colonization).
Our results provide evidence for problems associated with the use of fractional colonization for assessing mycorrhizal responses to elevated CO2. Extraradical mycelial performance parameters showed a significantly higher response ratio than fractional fungal colonization in ECM fungi. In AM fungi the effect was not significant. While interpretation of these results is inevitably constrained by the fact that there are very few studies where fungal growth in or around roots and in the soil have been measured in directly comparable units, we would argue that measurement of fractional colonization is an unsuitable index to determine fungal responses to elevated CO2. Fractional colonization is the result of plant root growth and fungal growth on or around roots. A differential effect of elevated CO2 on both parameters can be positive or negative (Sanders et al., 1998; Rillig & Allen, 1999; Allen, 2001; Rillig et al., 2002). Furthermore, levels of mycorrhizal colonization change over time and depend on the root : shoot ratio. Changes in plant allometry as a result of responses to elevated CO2 could therefore result in changes in fractional colonization (Staddon, 1998; Staddon & Fitter, 1998).
Differences in response ratio between the extraradical mycelium and fractional colonization, and differences in response ratio between ECM fungi and plants support the need for conceptually separating mycocentric and phytocentric views, as advocated by Fitter (2001). Such a separation also contributes to understanding changes in C partition under elevated CO2, which can result from downregulation of plant C-transporters and upregulation of fungal C-transporters (Nehls & Hampp, 2000; Nehls et al., 2001). Old models of mycorrhizal feedback to elevated CO2 have not made this conceptual separation and have treated the fungi primarily as an extension of the root system. Under such a model elevated CO2 will increase C availability to the fungus, resulting in increased uptake of limiting nutrients such as N and P, finally resulting in enhanced plant performance as a result of this positive feedback. However, under a mycocentric view, elevated CO2 will increase carbon availability for the fungus, by which the fungus increases its own biomass and fitness, irrespective whether this increased fungal fitness will feed back into enhanced nutrient uptake and increased plant performance.
Increased fungal biomass could increase competition for nutrients. Several parameters in the data set of Table 3, while still based on a very low number of observations and hence with large confidence intervals, are consistent with this suggestion. Larger mycelial dry weight (R = 1.45), mycelial area (R = 1.39), hyphal length in root-free compartment (R = 1.57) and 14C incorporation in biomass (R = 1.92) can be juxtaposed next to decreased N and P concentrations of the ectomycorrhizas (R = 0.82 and R = 0.70, respectively). As a consequence of increased nutrient competition the efficiency with which fungi use the additional C can decrease resulting in a larger C respiration by the fungus. This pattern of decreased C-use efficiency was observed by Gorissen & Kuyper (2000) for the nitrotolerant Laccaria bicolor but not for the nitrophobic Suillus bovinus, suggesting that species-specific differences in fungal nutrient demand are a major factor in determining the potential of feedback. While there is large interest in ECM fungal behaviour to elevated N availability, another major driver of global change (Lilleskov et al., 2002; Rillig et al., 2002; Taylor et al., 2003; Treseder, 2004), the way fungal N use feeds back to plant performance requires more attention.
While for ECM symbioses the need to separate mycocentric and phytocentric perspectives is evident, the pattern for AM symbioses is less clear. Neither differences in response ratios between AM fungi and AM plants nor differences between the extraradical fungal response parameters and AM fractional colonization were significant. It may be possible that this difference between ECM and AM symbioses reflects a genuine difference in mycorrhizal functioning. The amount of extraradical mycelium of ECM fungi is an order of magnitude larger than that of AM fungi. This could result in a greater potential for nutrient immobilization even if the fungi respond similarly to additional C. Considering these differences in the amount of extraradical mycelium an increase in mycelial size has a lower chance of increasing nutrient uptake for ECM fungi than for AM fungi, even in the case of uptake of the same immobile elements. This possibility had also been raised by O’Neill (1994), who, in her review, suggested that undisturbed ecosystems may be ‘saturated’ with regard to mycorrhizas. This issue of mycelial abundance being greater than optimal from the plant's perspective for uptake rates should be further explored in models of nutrient uptake by mycorrhizal fungal mycelia (Yanai et al., 1995).
O’Neill (1994) also mentioned the possibility that increasing CO2 levels could result in differences in ECM fungal species composition rather than changes in mycorrhizal performance. A change towards ECM fungi that produce a larger amount of extraradical mycelium under elevated CO2 was reported by Godbold & Berntson (1997), although Fransson et al. (2001) found no evidence for such a change. The interesting question then is whether such changes in the mycorrhizal fungal community structure may decrease or enhance plant responses compared with situations where just one fungal species is present. Such knowledge is highly pertinent for upscaling mycorrhizal plant responses to elevated CO2 (Staddon et al., 2002). A comparison of experiments with single mycorrhizal species and fungal communities did not show significant differences from a mycocentric perspective but, interestingly, a large effect from a phytocentric perspective. In both ECM and AM plants, community-wide responses were much larger than responses in experiments with single fungal species. This result contradicts the insurance hypothesis of theories on species diversity, which predicts an evening-out of species effects in multispecies communities (Loreau, 2000). We offer two suggestions to explain this discrepancy. It could be the case that elevated CO2 results in fungal community shifts towards species that are apparently less nutrient-limited (i.e. that make more fungal biomass at the same levels of carbon and nutrient availability). Such species shifts then increase the potential for positive feedback. The results by Klironomos et al. (2005), where a pulse of CO2 led to a loss of the most C-demanding AM fungi (Gigaspora, Scutellospora), and hence release from a C-drain, are consistent with this hypothesis. An alternative possibility is research bias: selection of species that grow well under experimental conditions but that are not fully representative for field conditions. Such research bias may result in over-representation of r-selected, nitrotolerant species, resulting in an underestimation of effects in the real world. Further experiments are needed to test both hypotheses.
Ectomycorrhizal fungi responded more strongly than AM fungi under elevated CO2. O’Neill (1994) had previously suggested that ECM fungi might respond more strongly than AM fungi. However, this outcome contrasts with the results of Treseder (2004). She noted, based on data of fractional colonization, a much larger response for AM fungi (R = 1.84) than for ECM fungi (R = 1.19), even though the difference was not significant (P = 0.11). Her data set contained only one data point for extraradical hyphal responses of both ECM and AM fungi, and a direct comparison between her results and ours is therefore impossible.
The ECM fungal responses to short-term and long-term experiments did not differ significantly. Ectomycorrhizal plants showed a larger response to elevated CO2 in long-term experiments than in short-term experiments. The same pattern was noted for AM fungal and plant responses. These observations contradict the generally held view of downregulation of photosynthesis under elevated CO2, if enhanced photosynthesis does not lead to a similarly enhanced nutrient uptake. We cannot offer an explanation for this difference, but note that it may well be artefactual (research bias) as most short-term experiments were done in pots with single species, and most long-term experiments in the field with fungal communities.
In conclusion, while both mycorrhizal fungi and mycorrhizal plants are stimulated under elevated CO2, the magnitude of response of both organisms in the symbiosis may differ to some extent, both for ECM and AM symbioses. A conceptual separation of mycocentric and phytocentric views is therefore necessary to understand such differential responses and the consequences of these differences for feedbacks. Ectomycorrhizal systems respond more strongly than AM systems to elevated CO2, although it is not clear whether this difference results from research bias or reflects some underlying fundamental differences in either the biology of the fungi or the plants. Because elevated CO2 may affect species composition of mycorrhizal fungi, and species identity is a large determinant of outcome in individual studies, our analysis suggests the importance of studies of species interactions and of argued choices of species selection in experiments.
This study derives an important inspiration from work by Alastair H. Fitter, who has emphasized in various publications the need to conceptually separate mycocentric and phytocentric views. We thank Jessica Gurevitch, Glaciela Kaschuk, Lijbert Brussaard and three anonymous referees for useful comments on the manuscript and all authors whose studies were included in the analyses. This study was funded by the C.T. de Wit Graduate School for Production Ecology and Resource Conservation (PE & RC) of Wageningen University and Research Centre, the Netherlands.
References on ECM for meta-analysis
References on AM for meta-analysis