It was a warm summer day when my colleagues and I hiked down to an abandoned estate. The land was soon to be “developed” to accommodate one really tall skyscraper. And in doing so, a tree survey was needed to assess the feasibility of existing tree retention.
What awaited us was a sanctuary for life. Woods and shrubs ran wild regenerating themselves when left untouched by busy human hands. Pretty much like woodland trees growing tall in search of sunlight, the trees here leaned away from the city’s shadow in the same manner. Despite the isolation, many of them remained in good health. Surprisingly, we found an old veteran Banyan tree, a mother tree, one of many that we set out to save against all odds (and bulldozers). To protect them from site construction, some started to talk about tree relocation, root ball wrapping, and a standard 2x2x2 sq.m. concrete tree pits.
Instead, our senior arborist looked at us straight in the eyes and gently shook his head “We don't contain life in a two metre cage.” He insisted, pointing down at the complex supporting the root structure that was visible above the ground:
“The key is to let them spread out, then we might succeed at making them a little happier.”
Several years later, I have started to understand the meaning behind his words.
RIGHT BENEATH OUR FEET
Contrary to popular belief, tree roots predominantly develop horizontally closer to the soil surface rather than elongating downward. The secret behind tree life support begins where the root system ends. Colonised inside and surrounding the roots is a “Mycorrhizal Network” or a system of fungi shaped of tiny white filaments called “Mycelium.” Scientists believe that more than 90% of land plants on planet Earth have a mutually beneficial association with mycorrhiza fungi. The fungi hold up their end of the bargain by mining soil, seeking and supplying plants with resources available further away, while receiving sugar in return.
THE WOOD WIDE WEB
Dr. S. Simard, a forest ecologist, discovered that the mycorrhizal network spans across a forest connecting trees and plants enabling them to communicate with their kin and form alliances cross-species as long as they share the same fungi type. Through mycorrhiza communication, trees could:
exchange resources like carbon, nitrogen, nutrients, water, and minerals;
an unhealthy or dying tree could dump its resources back into the network;
transmit distress signals triggering neighbouring trees’ to gear up defensive enzymes if danger approaches. For example, predator, invasive species, or forest disturbance like drought, disease, or deforestation;
mother trees nurture young seedlings for their better chance of survival;
however, as mentioned by BBC CrowdScience, some plants could hack the system and steal nutrients or spread poison into the network to sabotage their rivals.
The network has been nicknamed a “wood wide web.” According to Prof. J. Leake and Prof. D. Read, the overall distance of mycelium globally was estimated to extend over 450 quadrillion km. - about half the width of our galaxy!
THE NEXT LEVEL CARBON SINK
Forests absorb and store carbon dioxide in tree trunks and the fungi network:making plants and mycelium a dream team for sinking carbon dioxide. Through the fungal activity, carbons are fixed within the soil stabilising the ecosystem which accounts for 75% of terrestrial carbon. R.L. Gadgil and R.D. Gadgil further demonstrated mycorrhizal capability for suppressing decomposition rate in mature forest, resulting in greater carbon sink.
It is clear that land use altering processes in favour for urbanisation, agriculture, and forestry has polluted, severed, and overwritten the fungi germination - soil become susceptible leading to a release of sequestered carbon into the atmosphere. But climate change reversal schemes sometimes disregard the underground world and solely prioritise above ground restoration where in fact, it could actually go hand in hand.
For instance, mainstream tree planting actions typically focus on the number of trees grown rather than diversity which can be seen in mono type reforestation. While some efforts even have negative outcomes turning carbon sink into carbon source such as harvesting of short-live-same-age trees in commercial plantations.
But there is still hope. Some scientists are on the quest to identify a mycorrhizal hotspot that could store more carbon in over 10,000 locations cross-continent. Currently underway included a reintroduction of native mycorrhizal fungi by biotechnology.
HOW ABOUT AN URBAN ECOSYSTEM?
Urban trees, on the other hand, are trickier because tree-mycorrhizal associations occur much less in comparison to wild habitats. When we plant trees along the street, the trees are mostly confiscated in a concrete box deprived of connection with their peers. Even in a sizable urban landscape, mycorrhizal activities are even more rare due to lesser diversity and depleted nutrient soil. As it happens, a soil treatment may still not give the best result. Firstly from a mismatch of host and fungi, not to mention a case where existing fungi outcompete the garden plants.
In this sense, trees could survive such conditions but their thriving mechanism is compromised. If there is a lack of understanding of ecosystem composition and how trees and plants thrive as a community, an increasing tree numbers is no guarantee to enhance the services they provide (e.g. capturing carbon, biosecurity, keeping the soil nutrients enriched, therefore, reducing eutrophication, benefiting the city’s dweller health, and so on.)
I don’t know whether the Banyan tree and her children survived the construction. But it surely was a hard lesson learned for nature conservation and town planning.
BBC CROWDSCIENCE, 2018. How trees secretly talk to each other [video]. Available from: https://www.youtube.com/watch?v=yWOqeyPIVRo
DOBSON, M., 1995. Tree Root Systems. Arboricultural Advisory and Information Services [online]. Available from: https://www.trees.org.uk/Trees.org.uk/files/61/6181f2b7-e35d-4075-832f-5e230d16aa9e.pdf
GADGIL, R., GADGIL, P., 1971. Mycorrhiza and Litter Decomposition. Nature [online]. Available from: https://doi.org/10.1038/233133a0
HARVEY, F., 2021. World’s vast networks of underground fungi to be mapped for first time [online]. The guardian. Available from: https://interestingengineering.com/trillions-of-miles-of-underground-fungus-networks
KOZIOL, L., et al., 2018. The Plant Microbiome and Native Plant Restoration: The Example of Native Mycorrhizal Fungi, BioScience [online]. Available from: https://doi.org/10.1093/biosci/biy125
LAGOMARSINO, V., 2019. Exploring The Underground Network of Trees – The Nervous System of the Forest [online]. Harvard University. Available from: https://sitn.hms.harvard.edu/flash/2019/exploring-the-underground-network-of-trees-the-nervous-system-of-the-forest/
TREE, I., 2019. Wilding. Picador.