The Role of Ectomycorrhizal Networks in Seedling Establishment and Primary Succession

Abstract

In developed forests and secondary successional sites, host plants can readily access ectomycorrhizal (ECM) fungi because of the ubiquitous ECM mycelia and spores in soil, but this is not the case in some primary successional sites. In a volcanic desert on Mt Fuji, Japan, most of the area is non-mycorrhizal habitat and has poorly developed soil spore-banks. ECM habitat, i.e. pioneer willow shrubs and a small surrounding area containing ECM mycelia, are quite sparsely distributed, accounting for about 1 % of the ground surface in total. Such unique conditions provide us an interesting opportunity to explore the magnitude and role of direct mycelial connections between plants, i.e. ECM networks, in the field. It is difficult to observe individual ECM mycelial spread in soil, but the distribution of sporocarps and ECM roots having the same genotype indicate the spread of a mycelium in soil. We applied microsatellite markers to genotype sporocarps and ECM tips, and found that genets of two pioneer Laccaria species were small in size (mostly 10 m). These results indicate that ECM networks could vary considerably in size and longevity, even in the same site and associated with the same host species. Field transplanting experiment revealed that current-year willow seedlings rarely formed ECM associations in most habitats of the desert and showed poor growth. ECM infection from spores did not significantly improve seedling growth, indicating a small isolated mycelium on a tiny seeding may not be enough to acquire sufficient nutrients from extremely nutrient poor scoria. In contrast, seedlings transplanted near the pre-established willow shrubs, where ECM networks are available, readily formed ECM associations and grew well. Moreover, artificially reproduced ECM networks in previously non-mycorrhizal habitats significantly improved the growth of connected seedlings in 10 of 11 ECM fungal species in this desert. Therefore, ECM networks appear to be mostly positive and could be critical to seedling establishment, at least in this primary successional setting. Some previously proposed mechanisms may be less relevant to the observed positive effect of ECM networks on seedling establishment. For example, plant-to-plant carbon transfer through ECM networks might work for seedlings in dark forest floor, but not in primary successional settings characterized by strong sun light. More relevant mechanisms should include rapid ECM colonization with low costs, larger absorbing surface area than a solitary mycelium, and nutrient translocation within a network from nutrient rich soil patches to most demanding parts, often seedlings.