Trees are recognized as one of our best tools for fighting climate change, but researchers conducting experiments east of Fredericton say there’s no guarantee future forests will absorb more gases. greenhouse effect than they emit.
And, some argue that we should adjust forest management practices now to improve the odds.
“When you hear on the news how we’re trying to limit global warming to 1.5 degrees Celsius, that projection of warming is based on things like this measurement of forest carbon that we take,” said Loïc D’Orangeville, the one of many scientists collecting data in the Acadia Experimental Forest near Highway 10, north of Burpee Creek.
Wood is a “nice, long-term” way to sequester carbon, said D’Orangeville, an associate professor in the University of New Brunswick’s faculty of forestry and environmental management.
Trees use energy from the sun and convert carbon dioxide and water into sugars, wood and oxygen.
All climate models assumed that with longer growing seasons, trees would grow more and, overall, forests would store more carbon, D’Orangeville said.
But his research suggests that may not be the case.
“It’s a lot more complicated than that,” he said.
In an article published in the journal Nature in August, D’Orangeville and some American colleagues report finding that trees in research forests at Harvard University in Massachusetts and the Smithsonian Institution in Maryland produced no more than wood with longer growing seasons. The growth period has just changed.
“If there was a warmer spring, they would start growing early in the season, but they would also stop growing earlier in the season.”
“It was a big shift in our understanding, in my opinion,” D’Orangeville said.
“A certain level of warming can be good up to a point, and then that beneficial effect becomes detrimental.”
“This study told us that we may actually have passed that tipping point in these forests.”
According to D’Orangeville, the lesson is that we cannot assume that the forest will adapt to climate change and “do the work for us”.
“We actually have to make sure that we reduce carbon emissions into the atmosphere,” he said.
But what happens in the experimental forests of Harvard and the Smithsonian is not necessarily what happens here.
Most of New Brunswick and all of Nova Scotia and Prince Edward Island have a type of forest called “Acadian”. He has some of the same trees that grow in the region of New England where he did his study — known as the “temperate” forest — and some of the trees that grow in the “boreal” forest that covers most of from Canada. The Acadian forest also has a species of its own: the red spruce.
In contrast, boreal forest tree species are expected to grow more with warming, D’Orangeville said, especially in eastern Canada, where water is abundant.
He expects the future growth of the Acadian forest to vary by tree species, and he has experiments underway to get a better idea of how they will behave.
Balsam fir, which now dominates local forests, may grow a little better with warming, D’Orangeville said, but only up to a point.
“It will probably slow down and be replaced by other species.”
While D’Orangeville focuses on how individual trees grow and breathe, Manuel Helbig, his colleague from the Department of Physics and Atmospheric Sciences at Dalhousie University, is one of the people looking at the big picture. – how the forest as a whole absorbs carbon and gives it away.
A quarter of the carbon dioxide created by humans goes into the land, including the forest, Helbig said.
“We want to understand exactly how much of this anthropogenic CO2 that we emit will end up in the forest, which somehow slows down climate change.”
“It can go both ways,” D’Orangeville said, because when leaves and wood decompose, for example, it generates carbon dioxide.
Scientists call the flow of carbon dioxide into and out of the forest a “flux”. They measure it in a handful of “flow towers” across the country.
The Acadia Experimental Forest has the only flux tower in Atlantic Canada.
Instruments on the flux tower count CO2 molecules in the air above the treetops.
UNB researchers began collecting this type of data about a decade ago.
Helbig upgraded the equipment this fall to keep up with his work.
Sensors placed at regular intervals in the tower also measure temperature and humidity.
Helbig expects the forest around the Acadia Stream Tower to still absorb CO2, but to a lesser extent during hot, dry summers.
It can even give off CO2 sometimes, he said, “if it’s really dry.”
Canadian forests are thought to have been net emitters of greenhouse gases for the past 15 years or so due to disturbance, D’Orangeville said, but we still have a “limited understanding” of how the flow changes in response to heat, fires, droughts and insect infestations.
D’Orangeville would like to see many more stream towers to take measurements in different types of forest stands – some that have more hardwood trees, and some that have younger and older trees.
He also thinks more studies of tree growth are worthwhile, like the one described in his Nature article.
He said the information was gathered using metal bands that cost around $15 each.
“It’s not a massive investment of money…but it ends up giving us critical information about the entire forest response.”
It could mean the difference, he said, between the forest actively storing carbon or slowly dying.
In the meantime, he said, we should use the tools we have to help him.
“We plant millions of trees every year. The human power that exists in the woods… every day to manage the forest, we could use it to really help the forest cope with climate change.”
“Things like increasing species diversity is like increasing your investment portfolio.”
It would help the industry, he said, and also help the environment.
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