The new findings announced this week that bristlecone pines in the Great Basin are experiencing a growth spurt not seen in 3,700 years has thrust the issue of forests and climate change into the news.
Purely by coincidence, I invited an expert on carbon cycling between forests and the atmosphere to speak with my graduate level Science Writing course this past Monday, and he shared news with us that I thought was just as important — and more surprising.
It turns out that as the growing season in the Colorado Rockies has grown longer as a result of climate change, growth of trees in the forests has actually slowed down. And this has caused uptake of atmospheric carbon dioxide by subalpine forests to decline, contributing to climate change, according to Russel Monson, a scientist at the University of Colorado and the Cooperative Institute for Research in Environmental Sciences.
This goes against the conventional scientific wisdom, Monson said. With a longer growing season, one would naturally expect, well, more growth. And that would mean more carbon dioxide drawn out of the atmosphere (since photosynthesis takes the carbon and uses it to produce the carbohydrates that support growth).
Monson’s results, published online in the journal Global Change Biology, suggest a more complex picture.
If you think about it, the length of the growing season should not be the only limiting factor on tree growth. Water could be another important factor as well. So Monson and his colleagues did a nifty bit of scientific detective work to see what effect changes in water availability might be having on the trees.
The researchers started with a 9-year record of CO2 exchange between the subalpine forest at a site on Niwot Ridge west of Boulder, Colorado. This record showed that the trees took up less CO2 when the growing season was longest.
“This was sort of our wow moment,” Monson said.
To find out what was going on, he and his colleagues analyzed the isotopic fingerprint of water sampled from the xylem of the three dominant tree species, lodgepole pine, Engelmann spruce, and subalpine fir. And this revealed another surprising result (at least it was surprising to me): “What we found is that all way into September, 70 percent of the water that the trees were using came from snowpack,” Monson said. Not rain. Water from snow.
And this was true even though snow pretty much disappears in summer. The trees “went through all that summer rain, and still, 70 percent of the water they used came from snowpack.”
Monson explains that the trees have deep roots, which tap into ground water that originally fell as snow months earlier. In fact, it was spring snows that turned out to be most important, because they carry the most water. ”That’s what’s giving the forest its water,” Monson said.
What does this have to do with CO2 and climate change? The earlier onset of spring documented in the Rockies is eating into spring snowpack — and thus the water the trees need to grow.
“If we get an earlier spring — if spring eats into that snow — these trees have less access to water,” he said. And the result is that “the trees are suffering.” They are growing less, and therefore taking up less CO2.
Why is this significant? From a scientific perspective, if you want a full understanding of the climate system, and how we are altering it, you have to follow the carbon — the natural fluxes of carbon between the atmosphere and the Earth system, as well as the 9.1 billion metric tons that we’re now spewing into the atmosphere every year. And it turns out that not all of that carbon stays there.
About 5.1 billion tons is coming back into the Earth system, while 4 billion tons stays behind in the atmosphere. “Where in the Earth system is that CO2 going back in?,” Monson asks. Some is going into the oceans. That number is fairly well constrained. And some is also being taken up by terrestrial ecosystems — by plants. “We’re trying to figure out more accurately what this number is.”
But there’s another significance to Monson’s findings. Some climate change skeptics have argued that by burning coal and other fossil fuels, the carbon dioxide we’re adding to the atmosphere will fertilize plant growth, ushering in a great greening of the Earth. And there’s no question that more CO2 can mean more robust growth, and in turn greater uptake of CO2.
But Monson’s study shows that it’s really not that simple at all. In some ecosystems, such as the high mountain subalpine forests of the Rockies, you not only have to follow the carbon. But you also have to follow the water.
And when you do, you can also see yet another impact of climate change.
“Here is a case where increased CO2 causes climate change which causes less growth of forests,” Monson said. And this is contributing to climate change — a positive feedback that makes climate change even worse, not the negative feedback that many scientists expected.”