Book Review
Mycorrhizal Networks
In the summer of 1996 while preparing to re-enter grad school, I attended the first International Conference on Mycorrhizas (ICOM–1) at the University of California, Berkeley, hosted by Tom Bruns and a team of his students, including Tom Horton. Surprisingly, one of the plenary speakers was a grad student—Canadian Suzanne Simard, who was finishing her Ph.D. studies at Oregon State University. She presented the results of her dissertation project investigating whether carbon could move between different species of tree via fungal hyphae, a “mycorrhizal network,” or the “wood-wide web” as it was later captioned when her work was featured on the cover of the prestigious science journal, Nature. Although the existence of mycorrhizal networks had been documented by earlier workers, Simard’s presentation at ICOM and later Nature paper added pizzazz to the topic and spurred greater attention being paid to it, at least by those with an interest in mycorrhizas. It even tempted me to make networks the subject of my own dissertation work, but was advised against putting the fate of my degree in the hands of such a risky topic. Now, the concept has attracted a larger audience, as evidenced by the number of visitors to Simard’s videos on YouTube.
My advisers no doubt were wise to steer me in a different direction for, indeed, demonstrating that carbon, or other substances such as nitrogen, phosphorus, and plant defense chemicals, actually leave one plant, travel through the mycelium of a fungus, and then enter another plant is hugely difficult to do. Remember, all this is happening below ground and when we try to get close to see what’s going on, we inevitably disturb the system and so cannot be sure our observations reflect the natural processes. Thus, it is hardly surprising that there is still considerable controversy over the ecological importance of mycorrhizal networks, as is apparent in this new volume edited by molecular mycorrhizal ecologist Horton, now at Syracuse University.
The volume brings together nine contributions by a total of 26 authors from 12 countries, all of whom have been involved in research on various aspects of mycorrhizal networks. Following the Foreword by mycorrhiza and truffle guru, Jim Trappe, and a general introduction by Horton, the contents are presented in three sections—Network Structure, Nutrient Dynamics, and the Mutualism– Parasitism Continuum—although the sections are not noted as such in the table of contents or by use of section dividers and introductions as is typical for such a volume.
Network Structure includes two chapters—Mycorrhiza Specificity: Its Role in the Development and Function of Common Mycelial Networks, which adds to the classic 1992 treatment by Molina, Massicotte & Trappe, and Functional Significance of Anastomosis in Arbuscular Mycorrhizal Networks. Nutrient Dynamics includes three chapters—The Importance of Ectomycorrhizal Networks for Nutrient Retention and Carbon Sequestration in Forest Ecosystems, Nutrient Dynamics in Arbuscular Mycorrhizal Networks, and Resource Transfer between Plants through Ectomycorrhizal Fungal Networks. The Mutualism–Parasitism Continuum includes the final four chapters—The Role of Ectomycorrhizal Networks in Seedling Establishment and Primary Succession, Facilitation and Antagonism in Mycorrhizal Networks, Interspecific Mycorrhizal Networks and Non-networking Hosts: Exploring the Ecology of the Host Genus Alnus (for me, the best of the nine chapters), and Experimentally Testing Effects of Mycorrhizal Networks on Plant-Plant Interactions and Distinguishing among Mechanisms.
As would be expected given the large stable of authors, the chapters take varied approaches. Some are based primarily on the authors’ own research, whereas others provide a broader review including the work of others. The quality of the presentation and scholarship also varies—from good to excellent. A highlight is the treasure trove of reference citations, which are provided chapter by chapter, whereas the useful subject and taxonomic indexes are cumulative.
Inclusion of the mutualist–parasite continuum as a main organizing feature reflects the growing recognition of greater complexity in the concept of networks. It’s apparently not all rosy sharing among benevolent plants that were thought to be tooth-and-nail competitors. Things are more dynamic and more context-dependent than had been assumed, although the latter should not surprise anyone given that ecology can perhaps best be described as the “science of place.” The notion that, in a network involving multiple plants and multiple fungi, the participants receive benefit in direct proportion to what each contributes offers a contrast to the view of networks as involving altruistic support of the resource-poor by the resource-rich. However, if the former view is accurate, then there must be some means of “bookkeeping” among the participants if it is true that the two-way transfers of carbon and nutrients in mycorrhizal associations can be separated temporally, with, for instance, nutrients flowing to the plant early in the growing season and carbon deliveries to the fungus peaking later in the season. And how do benefits that don’t involve exchange of materials, such as protection from pathogens, figure in? Although the chapters make it clear that the arbuscular mycorrhizal and ectomycorrhizal fungi differ in many respects (this is probably at the heart of many disagreements over the ecological importance of mycorrhizal networks), we are still a very long way from understanding the implications of the differences when it comes to formation and functioning of networks involving them. In general, this compilation serves to demonstrate that much of what some think they know is not accepted by others. And, even if all that we think we know were to prove true, it represents only the thinnest skin of the onion. My principal complaint is that the book suffers from having too little to tie it all together. The introduction is the only place where this occurs other than occasional cross-references from one chapter to another. Having a wrap-up or capstone chapter that provided an overview of the state of knowledge of the subject, outlining what is generally agreed, pointing out the main areas of controversy, and suggesting some critical research needs, would have helped considerably. Some of this occurs here and there throughout, as well as in the final chapter, but not enough to be truly effective.
A lesser complaint relates to the production quality of the volume. For my review, I was given temporary access to an e-version of the book—I assume it is representative of the printed book. It contained more typos and misspellings than I’m used to seeing in professional books, especially ones offered at such a high price. Many of illustrations were very fuzzy and appear to have been produced by scanning previous publications rather than being re-drawn or utilizing the original art. In one figure, one acronym was used in the legend, another in the figure itself. Reference citations appeared out of alpha order. There was no consistency in the use of acronyms from chapter to chapter. Glitches in English usage were not cleaned up. It appears that the publisher skimped on the production editing.
The high price of the book is likely to prevent it from reaching more than a small audience of folks with access to university libraries. This is unfortunate, for the subject is one of great interest among many nature lovers and potentially could be of enormous ecological importance, for instance in better understanding carbon cycling and its relation to global climate change. Well worth reading if you can get your hands on a copy.
— Review by Steve Trudell
— Originally published in Fungi