Silent Eminence in Forests: Re-Discovering Wood-Decay Fungi

The Special Issue is devoted to the diversity, abundance, and distribution of wood-decay fungi, which historically have sparked little attention in both scholars and amateurs, despite the refinement of cultivation techniques and valorization of medicinal species over the course of several centuries, especially in Eastern Asia. Apart from a few edible species with special nutritional and/or medicinal properties, wood-decay fungi have mainly been studied in the field of forest pathology. Contradictory paradigms have been proposed regarding this research topic, with speculation about whether the occurrence and development of wood-decay fungi species are a consequence of causal factors or the decline in woody plants [1,2].

We often find ourselves lost in a foggy landscape as we are seldom aware of the extremely variable forms that plant–fungi relationships can take, especially regarding their diversity in both over-sampled areas (such as Europe) and neglected areas (such as the tropics). We sometimes realize that the same genus is unexpectedly present in different macroclimates, leading us to explore new biogeographic hypotheses, such as in Entonaema Möller [3] ascomycetes, which have been increasingly reported in temperate holarctic regions. The same study by Pošta et al. [3] also highlights another important point: when we study wood-decay fungi, we inevitably must address their plant hosts. In this sense, mycology meets botany as substrate preference both reflects species specificity and the local occurrence of certain hosts. The distribution of hosts changes over time, and they function as carriers and bridges for fungal distribution. Thus, Fraxinus L. species are suggested to be key factors of Entonaema distribution, whereas the study by Bolaños-Rojas et al. [4] shows Phellinotus teixeirae Salvador-Mont., Elias & Drechsler-Santos as an example of a species that can adapt to locally available, native resources, such as Pithecellobium dulce (Roxb.) Benth.

Taxonomical diversity in wood-decay fungi is a true challenge to old phenetic methods; if we consider the most widely represented class, i.e., Agaricomycetes, we find the highest variety in morphology regardless of the phylogenetics at higher ranks and even at the family rank. This does not mean you cannot trust the knowledge of experts in morphology, you are simply encouraged to merge morphological, molecular, biochemical, mating, and ecological data.

Pošta et al. [3] provide accurate morphological descriptions of sporomata spores and germination and multi-locus molecular phylogeny, evaluating the pros and cons of Bayesian inference and maximum likelihood methods. Microscopy techniques are also described in detail, providing the reader with useful and clear protocols. Similarly, Bolaños-Rojas et al. [4] also adopt a maximum likelihood approach and rely on the Shimodaira–Haegawa approximate likelihood-ratio test to optimize tree selection for molecular phylogeny.

Although these studies analyzed material from sporomata, strains isolated from pure cultures represent a remarkable source of reproducible, axenic material and fresh DNA. Culture collections are becoming increasingly significant for both pure and applied research, across a range of disciplines from systematics to biotechnology. This is the basis of the collaboration between Pavia and Salamanca University, in which the resources of the most neglected category in wood-decay fungi, corticioids, are being researched. Few specialists have truly devoted their studies to corticioid diversity, which involves keen observations, both with the naked eye and the microscope [5].

Moving from the taxon to the community, we still have to calibrate our tools appropriately for our analysis, and this leads us to another major question: do we want to survey the visible level (either macro or micro), usually based on the sexual reproduction (sporomata or similar structures), of the species that occur in a certain area? Do we want to also survey invisible species, that is, all those members of the community that occur in woody substrates and presumably contribute to degradation (although we cannot see their sporomata nor other structures, and perhaps we never will)? Is one approach more correct than another, or is it simply a matter of aims and scope instead?

Prof. Garbelotto’s team has several years of specialist experience in wood metabarcoding and offers us an excellent example of how this methodology can shed some light on the complex dynamics of wood degradation [6]. Firstly, this study reminds us that wood is much more than a substrate: there are likely to be as many different ecological niches available in one plant as there are fungal species, and different plants (individuals, species) contain distinct niches. Secondly, it reminds us that wood-decay fungi penetrate the host through wounds, and opportunist species can take advantage of the degradation caused by pioneer species. Thus, the wood community is shaped by substrate features and the distance from penetration points, and suitable methods and tools (such as PCR primers) are required depending on the sub-area that is being surveyed. Finally, such unexpectedly high fungal diversity may be discovered, suggesting that part of these species has not been classified yet, even in genetically “poor” hosts such as Populus tremuloides Michx. in North America.

On the other hand, we are aware that metabarcoding is not the only solution: this method can only be applied on limited sample sets due to its high cost and workload; therefore, if you wish to survey a large number of wood samples (which are discrete and extremely variable), possibly in different seasons, you should shift to more traditional techniques. It may also be interesting to observe which species are truly able to reach the optimal conditions that allow them to reproduce in a certain environment. This is the basis for most ecological studies and for assessing the conservation status of species and habitats (and data that are comparable with those of other authors are required). The study by Ponce et al. [7] provides us with a prime example from a naturalist perspective, supported by strong evidence, and both perspectives (metabarcoding and sporome survey) agree on a fundamental point: wood is a highly variable resource, and each degradation stage has its own peculiarities and hosts its own communities, even if the same plant species are being analyzed (the European oak Quercus cerris L. in Ponce et al. [7]). Is this a simple process? One must try to examine and classify all the wood debris in his/her plots, identifying all the collected sporomata, and then repeat this as many times as is necessary to reach a diversity plateau. This is a typical aspect of a naturalist’s research: providing the scientific community with fundamental knowledge. Briefly, Ponce et al. [7] remind us that the ecological role of wood-decay fungi at the ecosystem level is fundamental to all stages of degradation.

In conclusion, we encourage you to treat the content of this Special Issue as a brief introduction to the main research topics regarding wood-decay fungi diversity and ecology. Forest and bush ecosystems would simply not exist without these fungi, and the function and community complexity of fungi should be considered beyond their pathogenic role. We thank Forests for proposing this Special Issue, allowing us to reach a readership of forestry scientists and remind them that the few specialists in wood-decay fungi are found in their research field.