https://www.cbc.ca/news/technology/co2-trees-1.5000709
[Oops, another climate change denial myth goes down. Also, old studies
where plants were grown in greenhouses with higher CO2 levels showed
that vegetables and fruits (tomatoes were of particular interest) tended
to become woodier with lower sugar levels.
links in online article]
Rising CO2 won't make trees grow more, study suggests
Research into ancient cedars in Quebec shows we may need to change
predictions of vegetation, water cycle
Emily Chung · CBC News · Posted: Feb 04, 2019 4:00 AM ET
Everyone knows plants need CO2 to grow. So it seems logical that the
extra carbon we're spewing into the atmosphere will make plants grow
more, capturing more carbon and mitigating climate change, right? That's
something argued by climate change skeptics and assumed by some
scientific models used to predict future changes in the Earth's vegetation.
But growing evidence suggests that extra CO2 in the atmosphere isn't
making trees grow more. And, in fact, climate change is generating
warmer, drier conditions that could make them grow less in many places.
A recent study of extraordinary Quebec cedars that are between 600 and
1,000 years old adds to that evidence.
It found rising CO2 levels in the atmosphere since pre-industrial times
made trees more efficient at using water, but didn't increase the growth
of their trunks — and therefore the amount of carbon they stored, the
researchers reported in the Proceedings of the National Academy of
Sciences last week.
"What we bring as a hypothesis is if you don't have the water and
nutrients to consume this supplementary CO2, well, you cannot grow
faster," said Claudie Giguère-Croteau, who conducted the research while
she was a master's student at the University of Quebec in Montreal.
That's consistent with what's been found in other studies of tree growth
with climate change, both in the tropics and in northern ecosystems.
Oldest boreal trees
What's special about the trees in the recent Canadian study is they're
North America's oldest boreal trees, allowing researchers to track
changes much farther back in time, said Étienne Boucher, a geography
professor at the University of Quebec in Montreal who co-authored the study.
The researchers drilled into the trees and took core samples about the
width of a pencil so they could examine the growth rings. Then they
ground samples from each ring for chemical analyses.
The rare trees grow on small, very rocky islands in Lac Duparquet in
western Quebec. Despite their impressive age, their trunks max out at
about the width of a water cooler. That means the rings are extremely
small — fractions of millimetres wide — and need to be measured with a
microscope.
The trees' growth patterns showed no obvious changes before and after
the industrial revolution, Giguère-Croteau said.
But the chemical analysis showed something interesting.
Plants typically lose water during photosynthesis through the leaf pores
that they open to let the CO2 in. The researchers noticed that after the
start of the industrial revolution, as CO2 levels started climbing, the
ancient cedars became dramatically more efficient at using water during
photosynthesis.
That's something that's been previously shown to happen in other trees
when they're exposed to increased CO2 levels, allowing them to
photosynthesize more efficiently.
But the increased efficiency didn't lead to any increased growth in the
ancient cedars.
And the efficiency gains started levelling off after 1965, when the
region began seeing more extreme heat events in summer that cause trees
to shut their leaf pores to prevent water loss, slowing down photosynthesis.
Boucher said that's what we can expect to happen in a drier climate.
Trees will rely more on their own reserves of carbon instead of sucking
it from the atmosphere.
The researchers said the findings about trees' water use mean we may
need to revise our understanding of how the water cycle will be affected
by rising CO2 and climate change, as a lot of the Earth's water moves
through trees and plants.
The results also imply that scientific models used to predict future
vegetation growth may need to be adjusted. Many assume trees will grow
more quickly and capture more carbon with higher CO2 levels in the
atmosphere.
"Actually, in nature, it's more complicated than that," Boucher said.
"Maybe trees will not grow as much as expected."
That also suggests a need to plant new trees if we want to capture more
carbon from the atmosphere, Giguère-Croteau said. "We cannot count so
much on the already existing trees."
The study was funded by the Natural Sciences and Engineering Research
Council and the Fonds de Recherche Québécois Nature et Technologies, the
Quebec government's non-profit agency that provides grants for
scientific research.
