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The secret language of trees – Camille Defrenne and Suzanne Simard

Most of the forest lives in the
shadow of the giants that make up the highest canopy. These are the oldest trees, with hundreds of children and
thousands of grandchildren. They check in with their neighbors,
sharing food, supplies, and wisdom gained over their long lives. They do all this rooted in place, unable
to speak, reach out, or move around. The secret to their success lies
under the forest floor, where vast root systems support the
towering trunks above. Partnering with these roots are symbiotic
fungi called mycorrhizae. These fungi have countless branching,
thread-like hyphae that together make up the mycelium. The mycelium spreads across a much larger
area than the tree root system and connect the roots of different
trees together. These connections form
mycorrhizal networks. Through mycorrhizal networks, fungi can pass resources and signaling
molecules between trees. We know the oldest trees have the
largest mycorrhizal networks with the most connections to other trees, but these connections are incredibly
complicated to trace. That’s because there are about a hundred
species of mycorrhizal fungi– and an individual tree might be colonized
by dozens of different fungal organisms, each of which connects to a
unique set of other trees, which in turn each have their own unique
set of fungal associations. To get a sense of how substances flow
through this network, let’s zoom in on sugars, as they travel from a mature tree to a
neighboring seedling. Sugar’s journey starts high above
the ground, in the leaves of the tallest trees
above the canopy. The leaves use the ample sunlight up there
to create sugars through photosynthesis. This essential fuel then travels
through the tree to the base of the trunk in the thick sap. From there, sugar flows down to the roots. Mycorrhizal fungi encounter the
tips of the roots and either surround or penetrate
the outer root cells, depending on the type of fungi. Fungi cannot produce sugars, though they
need them for fuel just like trees do. They can, however, collect nutrients from the soil much
more efficiently than tree roots— and pass these nutrients into
the tree roots. In general, substances flow from where they are more
abundant to where they are less abundant, or from source to sink. That means that the sugars flow from the
tree roots into the fungal hyphae. Once the sugars enter the fungus, they travel along the hyphae through
pores between cells or through special hollow
transporter hyphae. The fungus absorbs some of the sugars, but some travels on and enters
the roots of a neighboring tree, a seedling that grows in the shade and
has less opportunity to photosynthesize sugars. But why does fungus transport resources
from tree to tree? This is one of the mysteries of the
mycorrhizal networks. It makes sense for fungus to exchange soil
nutrients and sugar with a tree— both parties benefit. The fungus likely benefits in less obvious
ways from being part of a network between trees, but the exact ways
aren’t totally clear. Maybe the fungus benefits from
having connections with as many different trees as possible, and maximizes its connections by shuttling
molecules between trees. Or maybe plants reduce their
contributions to fungi if the fungi don’t facilitate exchanges
between trees. Whatever the reasons, these fungi pass an incredible amount
of information between trees. Through the mycorrhizae, trees can tell
when nutrients or signaling molecules are coming from a member of their
own species or not. They can even tell when information is
coming from a close relative like a sibling or parent. Trees can also share information
about events like drought or insect attacks through their
fungal networks, causing their neighbors to increase
production of protective enzymes in anticipation of threats. The forest’s health relies on these
intricate communications and exchanges. With everything so deeply interconnected, what impacts one species is bound
to impact others.

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