Warmer temperatures and high soil nitrogen levels are causing Earth’s largest land biome to advance northward, according to a new study.
The boreal forest (or “taiga”) is the world’s largest land biome and accounts for close to 25 percent of the planet’s forested areas. The boreal ecozone principally spans 8 countries: Canada, China, Finland, Japan, Norway, Russia, Sweden, and the United States.
Most boreal landscapes are characterized by a low diversity of tree species, of which gymnosperms such as Abies, Larix, Pinus, and Picea species usually dominate, with varying proportions of angiosperm Populus, Betula, and Alnus species.
However, a recent analysis of satellite images, published in Global Change Biology, showed that anthropogenic climate change has reached the biome—and it’s a scene worthy of Tolkien: The entire boreal forest is on the move.
In the study, ecologist Logan Berner and remote sensing expert Scott Goetz, both from Northern Arizona University, used Landsat satellite images from 1985 to 2019 to examine changes in land cover.
According to CTV News Canada, the researchers found that along the southern edge of boreal forests, conditions there are now too hot and dry for the trees to survive. And to the north, the scientists found that the temperatures gave become more hospitable for trees, like conifers.
“It clearly shows that one of the world’s largest biomes is in the midst of a massive transition,” said Terry Chapin, an ecologist from the University of Alaska Fairbanks who was not involved in the study. “This puts another nail in the coffin in terms of showing what climate change is doing to our planet.”
The more than 15 million square kilometers (about 5.8 million square miles) of boreal forest features deciduous trees and conifers that can survive cold temperatures year-round.
Prior to Berner and Goetz’s work, these forests had been projected to move northward because of climate change, but whether the biome was already shifting remained uncertain.
Berner and Goetz’s study is the first to reveal the global picture using Landsat’s high-resolution images. To isolate the effects of climate change, the researchers excluded forest areas that were greening because they were recovering from disturbances like wildfires and logging.
The researchers based their conclusions on two factors – one is direct and the other is indirect. In northern latitudes, warmer temperatures lead to longer growing seasons, which speed up forest development. Meanwhile, temperatures along the southern edge are becoming too hot to sustain forest growth, leading to browning.
Also, comparing greening and browning rates from 1985 to 2019 with a database of soil nitrogen levels uncovered a previously overlooked indirect effect: Areas experiencing greening tended to have high soil nitrogen levels.
However, the rate of greening over the past 2 decades revealed that the shift is slowing – a trend Berner thinks will continue.
“With continued climate change, we’re likely to see actually a contraction of the boreal forest overall, because the rate at which trees die is much faster than the rate at which they’re able to expand,” he said.
According to Goetz, including data on the forest shift in models “will be our best way for projecting what’s likely to happen over the next 2–3 decades.”
Needless to say, the shift will have far-reaching consequences, some of which could further exacerbate climate change, according to the authors.
Tree mortality along the southern border will increase carbon dioxide in the atmosphere if the forests aren’t replaced with plants that are equally good at fixing carbon.
Increasing tree cover in the north will cause this region to absorb more sunlight, further warming the region, while vegetation also traps snow that insulates permafrost, potentially causing it to melt and lead to the release of methane and carbon dioxide.
All this will have a trickle-down effect that will include changes in wildlife, temperature levels, droughts, and forest fires. These devastating events could have a monumental impact on the boreal forest biome in the next few decades, say the authors.
