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Forests Use an Underground Supply Network

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It was a big surprise. Scientists at the University of Basel report an unexpected finding: trees in the woods — even unrelated species — trade large amounts of carbon with each other. How? They communicate through an even more unrelated organism: fungi.

Forest trees use carbon not only for themselves; they also trade large quantities of it with their neighbours. Botanists from the University of Basel report this in the journal Science. The extensive carbon trade among trees — even among different species — is conducted via symbiotic fungi in the soil. [Emphasis added.]

This is more than a free trade agreement. It’s a veritable economy, as the paper in Science describes:

Forest trees compete for light and soil resources, but photoassimilates, once produced in the foliage, are not considered to be exchanged between individuals. Applying stable carbon isotope labeling at the canopy scale, we show that carbon assimilated by 40-meter-tall spruce is traded over to neighboring beech, larch, and pine via overlapping root spheres. Isotope mixing signals indicate that the interspecific, bidirectional transfer, assisted by common ectomycorrhiza networks, accounted for 40% of the fine root carbon (about 280 kilograms per hectare per year tree-to-tree transfer). Although competition for resources is commonly considered as the dominant tree-to-tree interaction in forests, trees may interact in more complex ways, including substantial carbon exchange.

The carbon takes the form of “photoassimilates,” i.e., complex compounds produced by photosynthesis. 280 kilos is a lot. In English units, that’s over 600 pounds. In a five-year study, the team watched labeled carbon dioxide assimilated into the compounds traverse from the tree tops down through the root tips, and up into surrounding trees:

The only way the carbon could have been exchanged from spruce to beech, pine or larch tree — or vice versa — is by the network of tiny fungal filaments of the shared mycorrhizal fungi. Understory plants which partner up with other types of fungi remained entirely unmarked. The research group called the discovered exchange of large quantities of carbon among completely unrelated tree species in a natural forest “a big surprise”.

One of the scientists remarked, “Evidently the forest is more than the sum of its trees.” In a Perspective piece for Science, Marcel G. A. van der Heijden referred to this process as “underground networking” through “mycorrhizal pipelines.” Small seedlings had been known to share carbon this way, but not mature trees.

Does this improve forest fitness? Van der Heijden is not sure. Carbon does not seem to be a limiting resource. One could imagine that pathways for carbon could emerge haphazardly as symbiotic fungi spread their hyphae, and that resources would reach equilibrium by diffusion. There are hints more is going on, however. For one thing, the relationships are complex. For another, they function in symbiosis.

These underground networks can be highly complex because each individual tree and fungus has its own network and can associate with different partners.

The results reported by Klein et al. also have implications for key questions in mycorrhizal research: Why is this symbiosis so widespread and why has it evolved so successfully? The observation that 4% of net primary productivity is transferred to neighboring trees suggests that carbon is a nonlimiting resource, and not growth-limiting for these large trees. Thus, carbon allocation and loss to mycorrhizal fungi does not necessarily impair plant fitness. The exchange of “nonlimiting” carbon for nutrients may be one of the key factors responsible for the evolutionary stability of the mycorrhizal symbiosis.

If plants have an intranet (as we reported recently), why not an internet? One suspects that this system involves information transfer as well as carbon transfer. It’s already been determined that plants communicate through the air with volatile organic compounds. They can signal one another about threats, for instance. If they already communicate through one medium, why not another? It would be analogous to the Internet using both wired and wireless channels.

Other hints of regulated function include (a) hosts make specific connections, (b) the communication is bidirectional, and (c) the shared carbon products are diverse. Indeed, the authors know that theories of regulated sharing have been around for years.

It has been suggested that because of the unpredictability of disturbance events and the divergence of responses among plant communities, mycorrhizal fungi and their host plant species are under selective pressure to evolve generality. The groups of plants that are interlinked through a common mycorrhizal network are hence termed “guilds“. The identity and ensemble of fungal species may affect plant community structure and ecosystem productivity, with mycorrhiza improving plant fitness by increasing phosphorus and nitrogen uptake. As a result, mycorrhizal networks are considered an integral part of the autotrophic system and are essential components in ecosystem resilience to change. Yet, these benefits have traditionally been studied from a nutrient supply perspective, and the mycorrhiza “pipeline” was never shown to transfer considerable amounts (>1 g) of mobile carbon compounds among trees.

Contrary to evolutionary expectations, this network of supply lines is cooperative rather than competitive. It promotes ecosystem resilience to change. It looks designed for productivity of the community as a whole.

Determining the function of this carbon transfer will require additional research. Care for a prediction? The system likely includes bidirectional information transfer that leads to specific responses. It won’t reduce to random diffusion of compounds that happen to find pathways this way or that. The sharing of resources will be found to be regulated and purposeful. Perhaps it’s a form of cloud backup, where resources can be stashed for sharing in stressful times. Brian Owens at New Scientist suggests that this “wood wide web” will aid scientific “understanding of how forests can respond to the stresses of climate change, like drought or new insect pests.”

Intelligent design can prompt new research into this newly-recognized phenomenon, leading to understanding and appreciation for the overall beauty of a forest ecosystem. The science-stopper would be to shrug and say, “It evolved.”