Forests with a greater diversity of trees sequester more carbon, and in a way, this isn’t surprising. Different trees employ different survival strategies — a preference for shade or full sunlight, acidic or basic soils, partnerships with some bacteria and fungi over others, etc. The more strategies one crams into a given forest, the more efficiently that forest’s resources can be used.

This, so the theory goes, promotes more overall growth, resulting in the accumulation of more plant tissue and sequestered carbon, both above ground and below. More species of tree also means more strategies for collecting, recycling, and retaining nitrogen — the most important macronutrient for plant growth, and the primary limiting factor in the growth of many Canadian forests.

Anthony Taylor is an associate professor with the University of New Brunswick specializing in the relationship between forestry and climate. He demonstrated the link between tree diversity and carbon sequestration in a 2023 paper published in the journal Nature, comparing these two metrics on 406 sample plots across the country.

“If you take everything as a whole, a forest’s functional diversity increased the soil’s accrual of carbon and nitrogen by 30-50 per cent,” says Taylor.

Sequestering carbon is one thing, but keeping it sequestered is another. Taylor has been expanding his research into the domains of drought and wildfires, both of which will become more common in coming decades, and which are already being felt in the Maritimes. Here too, he’s found the diversity of trees in a given forest is a good indicator of how well they’ll weather a warming world, not only absorbing carbon, but holding onto it.

Maritime forests contain both coniferous and deciduous trees. Conifers (softwood trees with needles not leaves) [SK1] tend to grow more quickly — making them a favourite of loggers — but possess more flammable resins in their needles and bark than do deciduous trees, making them more vulnerable to wildfires.

“Leafy, green deciduous trees — hardwoods — also have higher percentages of moisture in and around their leaves,” he says. “…which reduces the risk of ignition.”

Maintaining a natural blend of coniferous and deciduous species in Maritime forests, therefore, would mean sequestering more carbon and suppressing more wildfires, but as Taylor outlined in a perspective paper published in 2024, regional forest management practices have been pushing in the opposite direction for decades.

“The ‘70s and ‘80s is when you begin to see a shift towards what we call sustained yield forestry, where the primary harvesting regime is clearcutting, and you plant back with commercially important conifers,” says Taylor. “Once you get those seedlings in the ground, you control competing vegetation (like deciduous trees) with early weeding and herbicides. You also manage the stand throughout its life cycle with pre-commercial thinnings, to steer its composition towards conifers.”

Today, many managed forests in the Maritimes suffer the unnatural dominance of White Spruce (planted and cultivated by loggers) and Balsam Fir, a conifer that, ironically, has benefited from a century of fire suppression. Normally they’d be the first to burn, making way for deciduous trees, but with a paucity of natural fires, they’ve spread to new extremes. Such White Spruce and Balsam Fir-dominated forests, lacking in native diversity, are sequestering less carbon than they should, and are especially vulnerable to a warming climate.

This was on display in 2018, says Taylor, when he and colleagues noticed the simultaneous and widespread death of Balsam Fir throughout western New Brunswick and eastern Maine. The cause, they determined, was draught, the species being poorly adapted to swings in available moisture — a troubling peek into the future.

One way or another, says Taylor, preparing our forests for a shifting climate will mean maintaining a more diverse landscape. Exactly how to achieve that is the subject of ongoing research: might the use of qualified, independent tree markers — whose job it is to help maintain the health and biodiversity of a forest, be part of the solution? Will controlled burns become necessary in the Maritimes, as a way of using up fuel on forest floors and promoting more deciduous tree growth? How rare must treatments of clearcutting and herbicides become? Taylor, colleagues, and graduate students are working on these and other questions, building a formula for climate-ready forestry in the Maritimes. The broad strokes, however, are already abundantly clear.

“If we, as a society, want to maximize the potential of our forests to sequester carbon and be resistant to fires, but also provide us with wood fiber and jobs, we should be trying to get better at various forms of partial cutting.”

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The Climate Story Network is an initiative of Climate Focus, a non-profit organization dedicated to covering stories about community-driven climate solutions.

Zack Metcalfe is a freelance journalist, columnist and author based in Salmon Arm, BC.

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Added to the Climate Story Network website: September 23, 2024

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