Researchers at UBC Forestry have identified a previously underestimated carbon flux between trees and soils that has implications for forest operations and long-term ecosystem health.
“There is a continuous replenishment of carbon stocks that retains soil carbon equilibrium,” says UBC Forestry Prof. Cindy Prescott, who is at the forefront of this potentially revolutionary area of soil carbon research.
Active carbon comes from photosynthate, which is the sugar and other substances generated by plants when exposed to sunlight through photosynthesis.
Dead leaves and branches — including slash from forest operations — were once thought to contribute the vast majority of carbon found in soils, which is the repository of more carbon than that found in the atmosphere and all vegetation on Earth combined. Scientists also believed that most of the carbon from plant matter ended up being released into the atmosphere as carbon dioxide gas.
Cindy’s research forms part of a more nuanced understanding of the carbon cycle, and one that involves a complex carbon feedback loop extending from treetop to root tip and into the soil.
In their 2023 paper on continuous root forestry, Cindy and UBC Forestry Prof. Sue Grayston reviewed evidence of a carbon exchange between photosynthate exuded from tree roots and from the branching hyphae of mycorrhizal fungi, as well as soil biodiversity, organic matter and carbon stocks.
Their paper explained how some of the carbon photosynthetically fixed by a tree is released through its roots or transferred to mycorrhizal fungi. This carbon is then released into the surrounding soil, feeding belowground biodiversity, which accounts for around one quarter of all biodiversity on the planet.
However, the carbon flux dissipates quickly the further afield it extends from a tree trunk. By the 10-metre mark, it becomes insignificant, Cindy notes.
critters digest cellulose and lignin, eating 10% to 20% of annual forest stand waste, and cycling nutrients back to into the earth through their feces — shown on the far, upper left — and other remains.
Retaining more trees during harvest can protect the integrity of the carbon flux, ensuring soil carbon renewal, and the long-term health of the many organisms it supports, is preserved.
For example, partial, continuous-cover or selective retention harvesting that involves the removal of only a portion of trees from a stand can prevent a cumulative shortfall in photosynthate-derived carbon, Cindy explains.
Larger trees fix more carbon than smaller ones, and broadleaved trees, such as red alder, contribute more carbon and nitrogen than coniferous trees. Other strategies to consider in support of a robust carbon flux that replenishes soil organic matter and carbon stocks include leaving in place larger trees and repopulating harvest zones with more broadleaved trees rather than only conifers, says Cindy.
Likewise, when trees are retained close enough to each other, the carbon flux can extend throughout belowground ecosystems, feeding a multitude of plants, bugs and microorganisms.
“Based on our research, we propose an inter-tree distance of 15 metres be practiced to keep the soil alive and functioning, and to support soil carbon replenishment,” adds Cindy.
Cindy’s research is one example of how innovative work at UBC Forestry is pushing boundaries to enhance the forestry profession.
“The findings from this research have changed my views on clear-cutting and sustainable forest management. While clear-cut harvest produces a lot of dead organic matter, we now know that living trees are a critical source of active carbon that generates
persistent soil carbon,” says Cindy.
Watch Cindy’s “Carbon Flux of Life Webinar”
This article was originally published in the Spring 2024 issue of Branchlines Magazine. View the full issue here.