Forces that structure plant communities: quantifying the importance of the mycorrhizal symbiosis
Letter to the Editor
School of Natural Sciences
Understanding forces that structure plant communities is a key issue in plant community ecology. Traditionally, ecologists have focused on the absolute and the relative roles of competition and herbivory (Sih et al., 1985) and on the relative role of dispersal limitation vs biotic interactions (Huston, 1999), as well as on neutral models (Hubbell, 2001) and on their niche-based counterparts (Cadotte et al., 2008). Increasingly, mycorrhizal ecologists have argued that mycorrhizal fungi should also be considered as fundamentally important determinants of plant community composition. This conviction is motivated by a growing body of research which demonstrates that mycorrhizal fungi can have large effects on plant growth and may modify the strength of other interactions (van der Heijden, 2002), which is taken as proof that mycorrhizas should have a strong effect on plant community structure. We agree that the potential influence of mycorrhizal fungi on structuring plant communities has been conclusively shown. However, the realization of such potential under variable environmental conditions and complex biotic interactions, and the relative influence of the mycorrhizal symbiosis compared with other biotic and abiotic drivers, have only been addressed in a few studies (see Table 1) and remain uncertain for most systems. Interestingly, we find no studies where the relative influence of ectomycorrhizal or ericoid mycorrhizal fungi on plant community structure has been quantified. This may reflect the difficulty of measuring plant community outcomes (e.g. plant community diversity or evenness) in these symbioses, which tend to be formed by long-lived plants. Nonetheless, the widespread failure of ectomycorrhizal plants to establish in the absence of ectomycorrhizas suggests that the effect size of ectomycorrhizal fungi on plant community structure is likely to be large. Quantification, in this case, may only be possible through the creative utilization of individual-based models of forest community dynamics (e.g. SORTIE; Pacala et al., 1996). Similar debates about the relative importance of predation and competition in structuring plant communities have been addressed through syntheses and meta-analyses of published experimental studies (Sih et al., 1985; Gurevitch et al., 1992). These approaches have shown that for terrestrial plants, herbivores have more frequent and stronger effects on community structure than interspecific competition. Currently there is an insufficient number of published studies on the relative contribution of the mycorrhizal symbiosis (compared with other factors) to attempt a meta-analytical approach.