A new study suggests that young forests that are thinned early can grow bigger, healthier trees that are more adapted to a shifting climate. Moreover, although a thinned forest has far fewer trees than an un-thinned one, in time the total amount of carbon sequestrated by the two should be nearly identical.

thin forest

Credit: Pexels.

Don’t put all your hopes on trees, though

By the middle of this century, average CO2 levels in the atmosphere might be higher than they ever were in the last 50 million years. As things stand today, humanity is unwillingly embarking on a huge science experiment which all signs point to catastrophic climate change. Mitigating greenhouse gases will undoubtedly be one of the biggest challenges we will face in the coming decades as people transition from “this might be a problem” to “oh, crap!”.

The problem with CO2 is that it’s a very resilient molecule that can remain suspended in the atmosphere for up to two centuries. This means that even if we somehow miraculously halt all the CO2-producing industry in the world, the temperature won’t roll back to pre-Industrial Age levels. It won’t dip at all, for that matter, since the damage we’ve caused so far has already locked us in for 1.5 degrees C of warming by some estimates.

A lot of people would feel the solution to our problem is trivial: just add more trees. Things aren’t that simple though.

Typically, a tree absorbs as much as 48 pounds (21 kg) of carbon dioxide per year and can sequester 1 ton of carbon dioxide by the time it reaches 40 years old. The average North American generates about 20 tons of CO2-eq each year, which means every year you’d need to plant about 500 trees to offset your carbon footprint, that’s not taking into account the time it takes for a tree to mature and reach the optimal carbon-sinking age. If you’re a New Yorker and need to fly to Berlin, your seat is responsible for generating 10,285 pounds (4,675 kg) of CO2. Essentially, your 8.5-hour-long flight just offset roughly 223 trees. Kudos! If you think this isn’t fair, that’s just life for ‘ya because neither is digging up and burning billions of dead trees accumulated over millions of years which until not too long ago safely stayed miles beneath the ground. It’s no surprise that U.S. forests only capture 10 to 20 percent of the nation’s greenhouse emissions each year.

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I somewhat digress because trees are definitely a go-to solution for tackling climate change, which is why scientists are trying to find out not only which are the best species that can handle rising temperatures and dwindling water, but what an idle forest might look like.

Common sense says you shouldn’t cut any more trees from a forest, no matter how bad the market might want that wood resource. That’s not necessarily the case, according to Andrew Larson, a forest ecologist at the University of Montana in Missoula and the author of a new study which looked at data from a long-running experiment in northwestern Montana.

The experiment

In the experiment which first began in 1961, the U.S. Forest Service researchers were interested to see whether forest thinning helps timber grow faster and more abundantly. The study involved growing a young forest of western larch, a common conifer found in the American Northwest, which was divided into plots. In some of these plots, the 8-year-old trees were thinned from tens of thousands per hectare down to only 494 trees per hectare.

Like any plant, a tree needs sunlight to grow and in a crowded forest, many trees compete for sunshine. In some very forests, like the tropical variety in the Amazonian rainforest, the canopies block a huge portion of the sunlight from reaching the bottom of the floor.

On the plots where the USFS let the forest alone, the trees grew in larger numbers but were taller and skinnier. On the thinned plots, the trees grew much thicker trunks and broader canopies. But which plot sequestered the most carbon?

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Larson and colleagues calculated the amount of carbon that was stored in the mass of the tree by measuring their height, diameter, or branch widths, but also the dead wood and forest floor debris.

According to this analysis, the total carbon trapped by the trees was even. The plot that was left alone had more trees, but the thinned plots compensated by growing bigger, tougher trees, the scientists reported in the journal Forest Ecology and Management.

“When it comes to carbon sequestration and climate change adaptation, we can have our cake and eat it too,” Larson said. “It’s a win-win.”

Larson says that thinning doesn’t necessarily work every time. They key is to do it early, in a young forest before the population has time to fight over water and sunshine. If you thin early, the remaining trees can grow rapidly. In those cases where thinning treatments were done on mature trees, results were poor because the leftover trees had already been weakened by the competition.

Bigger and tougher trees are more resilient in the face of drought and they also resist fires better, both occurrences destined to increase in frequency due to climate change. As such, the findings might help forest management to prepare new populations of trees that are cleverly thinned to make them better adapted to climate change.

“We infer that early PCT (precommercial thinning) can be used to simultaneously achieve climate change mitigation and adaptation objectives, provided treatments are implemented early in stand development before canopy closure and the onset of intense intertree competition,” the authors of the new paper concluded.

At the same time, much more work is needed to unravel all the subtleties. Not only do the researchers have to prove this works for other species of trees, they also need to show what happens to a habitat when you thin a forest. Unscrupulous developers might also see the findings as an opportunity to justify clearing down more trees for wood resources.

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