Over a decade ago Jim Trappe and Dan Luoma entitled an article "The Ties That Bind: Fungi in Ecosystems." Reading that article was an important influence in my decision to return to school and study the ecology of forest fungi. The authors summarized the importance of fungi succinctly: "The roles of fungi are much more pervasive and varied than commonly realized. Fungi interact with all other organisms in an ecosystem, usually directly but often indirectly. They are involved in all ecosystem processes, directly or indirectly. Success of the other organisms in the system and even survival of the system itself depend on fungal activity."
At the time those words were written, mycorrhizal ecology was a little-studied discipline. However, in the intervening years, greatly increased attention has been paid to it, although mycorrhizal ecologists still constitute a small minority of mycorrhiza scientists and an even tinier proportion of ecologists. Nonetheless, this recent compilation provides ample proof that Jim's and Dan's words were right on the money. Marcel van der Heijden and Ian Sanders have assembled a collection of varied and interesting contributions from 36 (mostly) mycorrhizal ecologists. Although by no means an exhaustive review of the subject, the book does provide a good picture of the state of the field.
The 17 chapters are organized into six sections:
Section A, "Introduction," consists of a single chapter by David Read, "Towards ecological relevance: Progress and pitfalls in the path towards an understanding of mycorrhizal functions in nature." In it Read, who is perhaps the person most responsible for the recognition of the importance of mycorrhizas to ecosystems, discusses how the narrowly focused ("reductionist") approach that is characteristic of today's science has critical limitations when it comes to understanding ecological systems. Although closely controlled experiments are necessary for elucidating many details of how things work, laboratory set-ups provide only a faint representation of the real-world environment. Yet the complexity of natural systems makes it difficult fox field studies to yield unequivocal results. Thus, Read argues, a creative synthesis of field and laboratory approaches will be required if we are to advance our knowledge much further.
Section B, "Ecophysiology, Ecosystems Effects, and Global Change," includes five chapters. In "Carbon and nutrient fluxes within and between mycorrhizal plants," Suzanne Simard et al. focus on material flows within ectomycorrhizal systems. The possibility that fungal hyphae link multiple plants together into so-called common mycorrhizal networks is an exciting one with profound implications for how ecologists view ecosystems. Competition might not be so dominant a force as is usually believed. Iver Jakobsen et al. cover similar ground with respect to arbuscular mycorrhizas in "Function and diversity of arbuscular mycorrhizae in carbon and mineral nutrition." Pal Axel Olsson et al. contribute "Foraging and resource allocation strategies of mycorrhizal fungi in a patchy environment." Given the heterogeneity of the soil, its not surprising that fungi play such a key role in nutrient acquisition and cycling. Rien Aerts discusses "The role of various types of mycorrhizal fungi in nutrient cycling and plant competition." Mycorrhizas are diverse, and their occurrence seems to be closely tied to environmental conditions. The section closes with a broad view of things, "Global change and mycorrhizal fungi" by Matthias Rillig et al. These authors point out that different plants and fungi respond differently to changed conditions such as increased temperature and carbon dioxide levels. Thus, partners in mycorrhizal associations might not be able to react in synch to the environmental changes we are causing. Given the reliance that most plants have on mycorrhizal fungi, this should concern us.
The five chapters in Section C, "Biodiversity, Plant and Fungal Communities," show how molecular techniques and other approaches are allowing great strides to be made in characterizing the diversity of mycorrhizal fungal communities. However, our understanding of why patterns of diversity are the way they are is still in its infancy. Nonetheless, indications are that fungal diversity has an important influence on plant diversity. The chapters include Suzanne Erland and Andy Taylor, "Diversity of ecto-mycorrhizal fungal communities in relation to the abiotic environment"; Justin Clapp et al., "Genetic studies of the structure and diversity of arbuscular mycorrhizal fungal communities"; Miranda Hart and John Klironomos, "Diversity of arbuscular mycorrhizal fungi and ecosystem functioning"; Marcel van der Heijden, "Arbuscular mycorrhizal fungi as a determinant of plant diversity: In search for underlying mechanisms and general principles"; and Jim Bever et al., "Dynamics within the plant-arbuscular mycorrhizal fungal mutualism: Testing the nature of community feedback."
The three chapters in Section D, "Multitrophic Interactions," look at relations between mycorrhizal fungi and organisms that occupy different trophic /"feeding") levels in food chains. Catherine Gehring and Tom Whitham disclose some surprising links between soil fungi and aboveground plant-eaters in "Mycorrhizae-herbivore interactions: Population and community consequences." Alan Gange and Valerie Brown do the same for tiny soil animals in "Actions and interactions of soil invertebrates and arbuscular mycorrhizal fungi in affecting the structure of plant communities." The third chapter, "Interactions between ecto-mycorrhizal fungi and saprotrophic fungi," by Jonathan Leake et al., adds to the complexity of the below-ground picture by showing how mycorrhizal and decomposer fungi interact in their respective quests for nutrients and energy. Such observations have led some to suggest that nutrient cycling in northern forests is governed largely by mycorrhizal-saprotrophic fungal encounters.
It has long been said that mycorrhizal associations lack specificity. That is, within a given mycorrhiza type, just about any fungus can team up with just about any plant. Section E, "Host-specificity and Coevolution," sheds new light on this belief. In both "Mycorrhizal specificity and function in myco-heterotrophic plants" by Lee Taylor et al. and "Specificity in the arbuscular mycorrhizal symbiosis" by Ian Sanders we learn that there is much less promiscuity in these symbioses than we thought. Some of the associations are, in fact, highly specific. For instance, Allotropa virgata, the candy stick plant, appears to associate exclusively with Tricholoma magnivelare, the American matsutake. Here again, molecular methods of identification are playing a big role in advancing our knowledge.
Section F, "Conclusions," wraps things up with a summary and look toward the future by the editors, "Mycorrhizal ecology: synthesis and perspectives." They close with a list of conclusions that highlight the fact that terrestrial systems are highly dependent on the activities of mycorrhizal fungi. Unfortunately, this fundamental fact still is largely lost on the general ecological community. In "The ties that bind ..." Trappe and Luoma state, "An understanding of ... ecosystems cannot approach reality if major participants are neglected. Such neglect has been the norm rather than the exception in regard to fungi in most ecosystem research." Things have improved a bit, but there's still a long way to go before mycorrhizal fungi are routinely included in ecological investigations. Perhaps this book can contribute to making that happen.
Is this book for everyone? No. Mycorrhizas are not all that photogenic, and, as far as I know, no one eats them. However, for those of you who know a bit of the basics about mycorrhizas, this should be of interest. An understanding of college-level biology and chemistry will help you get the most out of it. For those with a keen interest in mycorrhizas or plant or fungal ecology, this is must reading.
— Review by Steve Trudell, Seattle, WA
— Originally published in The Mycophile 44:5, 2003