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C is for Canopy

One of the first things I learned about woodland ecosystems is that all trees are in competition with each other for that most precious of substances, light.

Light shapes trees. An oak grown in an open field will become wider than it is tall, a form that resists the wind. Inside woodland oak grows tall as its stretches skyward, however its spread will be limited by neighbouring trees.

This paints an image of trees battling it out for survival, of trees that out shade and outgrow their neighbours. Although some woodland trees, such as beech and sycamore, do cast deep gloom it is more common for native broadleaf trees to form an open canopy defined by lighter foliage.

Birch woodland has a light and airy canopy

Walk though a coppiced oak-birch woodland, a habitat that once covered much of the Pennines. Oak and birch co-exist with ash and an understory of both hazel and rowan. Beneath this grows bluebells, wood anemones, wood sorrel, yellow archangel, violets, wild garlic and more – all of which have enough light to survive.

For too long we have viewed woodlands through the lens of our own behaviour. We have imagined that the woodland ecosystem was one of out-and-out competition. Yet in the oak-birch woodland there exists a delicate balance of interests that requires co-operation between plant species to ensure that every organism gets its share of the bounty. Considering our lack of understanding it is a happy accident that this natural system was aided by centuries of regular coppicing that allowed even more light to reach the woodland floor.

Although trees need to assert their own space in the world, put down good roots and produce many leaves to photosynthesise their food, it turns out that woodland trees and plants were supporting each other all along. Only we were so busy looking up into the canopy that we missed what was happening beneath the soil’s surface.

Ash leaves emerge late and form an open canopy

Tree roots are connected via fungal mycelium – thread-like filaments – to a vast network including other woodland trees and plants. When these mycorrhizal fungi were first discovered it was assumed that they were parasites, draining the trees’ jealously guarded resources. The news that these fungi might be actually helping the trees was a revelation.

Although these fungi take nutrients for themselves they also pass on minerals and sugars, along with messages, from tree to plant to tree. Some of these communications are warning signals, such as telling neighbours that a leaf-eating insect has just landed and that they need to ramp up the chemicals that make leaves unpalatable. Saving trees from possible destruction ensures a stable supply of nutrients for the system.

Plants use this mutually beneficial network to access more food and information than their form allows. Some propose that the woodland ecosystem more resembles a multi-faceted super-organism connected by one nervous system than it does a highly networked collective of individuals.

Rowan in the dapped shade of the understory

So closely attuned are each to the other that a heavily shaded seedling may receive a nutrient boost from its taller neighbours. And, in perhaps the ultimate act of altruism, as death approaches trees may choose to support life by transmitting their resources to surrounding organisms.

Seen from this perspective, the woodland canopy is less of a ceiling marking the competition for light and more of an all encompassing shelter feeding the life below. Yet this more nuanced understanding of how woodland organisms co-operate came as a surprise. Is it because we have stepped so far out of our own natural networks that competition is the only way we know? Trees have much to teach us all.