Satellites reveal surprising growth patterns follow fires in far-northern forests

By Nicholas Gerbis
Published: Thursday, October 26, 2023 - 3:54pm

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Scott Goetz
The team computed the fraction of deciduous trees for the years 1990, 2000, 2005, 2010 and 2015, along with uncertainty estimates. The boxout (left side) shows Fort Greely, Alaska in 2010.

Boreal forests comprise one-quarter of the world’s woodlands and exert a substantial influence on Earth’s climate.

Now, new research in the journal Nature Climate Change is altering our picture of how these massive carbon storehouses react to forest fires.

“Those are quite flammable systems, especially with all the increase in storm activity and lightning that we see in these high latitudes now,” said senior author Scott Goetz of NAU. “So they burn very rapidly and readily.”

When fires sweep through the black spruce and jack pine forests of Canada and Alaska, it’s thought the confers are replaced by leafy deciduous trees.

“The more severe the burning, the more it comes back as a deciduous forest and persists for some period of time,” said Goetz.

That differs from Arizona’s ponderosa pines and pinyon-juniper woodlands, which revert back to conifers fairly soon after burning.

Deciduous trees take up more carbon, reflect more light, cool the climate and lower fire risk.

Based on data from previous ground-based sampling, Goetz and his team expected to find plenty of leafy trees when they used satellite imagery to study areas across the high latitudes burned over the past several decades.

“But in fact, what we found is, after a few decades, the forests — they initially are deciduous, but then they gradually transition back to being evergreen conifers,” he said.

Ken Tape
When fires sweep through the black spruce and jack pine forests of Canada and Alaska, it’s thought the confers are replaced by leafy deciduous trees.

So, viewed from a broad perspective, the coniferous-deciduous shift was minor and temporary during the period studied — especially since confers continued to spread to unburned areas in the interim.

“When we integrated across both space and time, the changes — thus far, at least — are fairly small, on a net basis,” said Goetz.

That could change, though, if more intense and frequent forest fires due to climate change burn away the soils conifers depend on. Goetz says fires can reach deep into soils — half a meter or more — and that soil combustion puts “a whole lot of carbon” in the air.

“When that layer is removed, then deciduous vegetation is able to establish and outcompete conifers for some period,” he said.

Goetz said this is an evolving story, and that his team is already starting to extend the time period and data they are studying. He cited the potentially large implications of their findings on other concerning processes underway in the Artic.

That includes permafrost thaw, which experts worry will release huge amounts of carbon, destabilize soils, coasts and infrastructure, and harm ecosystems and indigenous communities.

“If forests could actually mitigate all the warming that we're seeing happening, that would be a very positive outcome,” he said.

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