The wood-wide web – mycorrhizal associations across individuals

It goes without saying that the world of mycorrhizal fungi is so vast and complex that we’re only just beginning to scratch away at the surface of understanding, though this doesn’t mean we haven’t made some very interesting developments over recent decades. For example, we know that trees may use mycorrhizal networks to ‘trade’ resources across a single species and across differing species, as do we know that they will ‘communicate’ to signal neighbours about upcoming defoliation events by insects. However, whilst we understand the concept, we aren’t necessarily in possession of an arsenal of data that can really begin to demonstrate the intricacies of mycorrhizal networks. I feel that this study is one that begins to establish knowledge of such intricacies, and therefore I hope that you find this as brilliant and mind-boggling as I did when I first read it a few months back (and also hopoe I am making sense with what I write!).

The focus of this study was a group of 67 Douglas fir (Pseudotsuga menziesii) of varying ages (courtesy of natural regeneration) in an area of 30m x 30m, and on the manner in which the ectomycorrhizal network of the species Rhizopogon vesiculosus and Rhizopogon vinicolor impacted upon the connectedness of Douglas fir individuals. By a similar token, it looked at the population structure of the two fungal species, across the study site.

In order to obtain data required to draw conclusions from the study aims, samples of soil were taken from four sides of each tree within the plot area (usually within the drip line, though if canopy cover was lacking then obviously not so). This enabled for the authors to theoretically obtain fibrous tree roots from each individual, and analyse the tree roots not only to identify the Douglas fir they were from, but to determine whether the two Rhizopogon species were present within both the root cambium and soil environment and, if so, of what genet they were from. The map below shows the plot area, and the sample locations. From each sample location, arrows are drawn to show what tree’s roots were found at each sample site, and from what Rhizopogon species (and genet) the roots were associated with. If we take, for instance, the upper-most blue-shaded area indicating a Rhizopogon vesiculosus genet, we can see how the genet is connected to many different trees across the site. These trees were thus deemed to be ‘connected’, as they shared an ectomycorrhizal network.

R. vesiculosus genets can be seen in the blue-shaded areas, and R. vinicolor genets in pink-shaded areas. The coloured lines around the shaded areas represent different genets of the two species, and the black dots within are the sample sites. The lines from the black dots show the links to the Douglas firs, which can be seen as the green star-like shapes (larger ones signify larger trees, and there are a total of four ‘cohorts’ marked by different-sized shapes). The arrow marks the Douglas fir most connected to other trees.

In light of the results obtained, which are shown above (visually), the authors identified a total of 56 Douglas firs that were connected with other trees in (largely) the plot area, via the ectomycorrhizal networks created by the two Rhizopogon species (and one fungal genet connected 19 trees!). 45 of the trees were inside the plot area, though a further 11 were outside (and this is why some are plotted outside the sample area). Within the site, 27 ectomycorrhizal genets were also found, of which 14 were from R. vesiculosus and 13 were from R. visicolor. 18 of the genets, 9 from each species, were found to connect at least two trees together. More associations were more frequently found amongst the larger and older individuals, most probably because they had been there longer and their larger rooting environments enabled them to assume more associations with ecotmycorrhizal genets. The table below provides a more detailed breakdown of the genets found and their associated with the different cohorts of Douglas fir.

A table showing the data obtained from the study.

In terms of the population structure of the mycorrhizae and its impact upon the Douglas firs, the authors found that two trees over 43m apart shared a connection via only two different ectomycorrhizal genets. Their connectedness had to span over more than one genet, as the maximum distance one genet (R. vesiculosus) spanned was around 20m. The ability of R. vesiculosus to span greater spatial distances may also be the reason behind why it was found to connect (10.2), on average, more trees per genet than R. vinicolor (4.4). The most connected tree (at 94 years of age), marked with the arrow in the first image, was considered to be ‘central’ to the overall ectomycorrhizal network, and had a relationship with 11 different ectomycorrhizal genets and 47 other Douglas fir.

A total of 62% of the Douglas fir from Cohort 1 and Cohort 2 were also found to be connected with trees from Cohort 3 and Cohort 4. This means that these younger specimens shared associations with the same fungal genets that older specimens were connected to, which the authors found interesting as it suggested that the fungal species surveyed had Douglas fir hosts that would ensure longevity of its existence within the landscape (as if the fungus has anticipated that, by only colonising older specimens, it could itself cease to exist when its old hosts all die out – succession-planning, if you will). Furthermore, it enables the younger specimens to share an already established inoculum base, from which carbon and water can be provided by the older specimens to aid with establishment. Beneath, a further image shows associations between individual Douglas fir studied during the research.

Showing how individual Douglas fir were linked to other individuals, the coloured circles vary in size depending upon tree DBH and colour (from yellow [young] up to dark green [old]) depending upon age. Thicker lines between individuals shows a greater degree ot connection, associated with how many different ectomycorrhizal genets linked them.

What we need to be aware of here is that this study was done over a tiny fragment of Douglas fir forest, and therefore if the associations were extrapolated out over an entire landscape, the connected nature of individuals to others would be absolutely incredible. Not only this, but because two trees were found to be connected at over 40m away, it highlights how the above-ground isolation of individuals in a stand masks the intricately-connected nature of the individuals beneath. Thus, we must really see a forest as a network, in place of individual trees.

The fact that older individuals were found to have many more connections, on average, than younger ones, also highlights the criticality of retaining older specimens in a stand – if only for the benefit of safeguarding ectomycorrhizal networks that aid with younger specimens obtaining required resources for their growth. However, we must also recognise that the mycelial networks of the two Rhizopogon species studied benefit hugely from the older trees, and retaining them is also of benefit to their existence. Targeted felling of large individuals, therefore, could wreak havoc (and rather quickly) upon the entire system, as the stand’s resilience is built upon these (and related) ectomycorrhizal networks that have established and persisted for a long time.

Even if, as the authors suggest, a connection does not provide the young tree with resources, it will still benefit from connecting with a well-established ecotmycorrhizal genet that is itself healthy and fully-functioning as a result of obtaining its carbon from upper-canopy, mature Douglas fir. It does not pay to be isolated from the crowd.

Source: Beiler, K., Durall, D., Simard, S., Maxwell, S., & Kretzer, A. (2010) Architecture of the wood‐wide web: Rhizopogon spp. genets link multiple Douglas‐fir cohorts. New Phytologist. 185 (2). p543-553.

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