Carbon dioxide makes trees thriftier with water
Forests are doing more with less as carbon dioxide levels rise, suggests new research in the journal Nature. Specifically, trees seem to be using water more efficiently, making it easier for them to grow and lock up atmospheric carbon in the process.
Scientific experiments have long suggested plants could become more thrifty with water under high levels of carbon dioxide. But when scientists tested the theory out, they noticed trees were using water even more efficiently than expected. So are the lab experiments underestimating the phenomenon, or could something else be making trees in the real world look much better at super-scrimping than they really are?
Less is more
The leaves on plants and trees contain small holes called stomata which can be opened and closed to let gases in and out. The authors behind the research suggest some trees are responding to rising levels of carbon dioxide by partially closing these stomata.
They suggest that as a result, trees in warm and cool climes have been able to retain water which would otherwise have escaped. Since water and carbon dioxide are the two basic things plants need to grow, losing less water makes it easier for plants to grow. This in theory makes forests better carbon sinks, removing more greenhouse gases from the atmosphere.
Image - Temperate -forests _1753 (note) Image - Boreal -forest _76fd (note)
The research studied temperate and boreal forests, whose locations are shown above. Source: GRIDA
To test the theory, the researchers mounted instruments above the canopy of forests in 21 locations in the US, and measured carbon uptake and water use over a 20-year period. From this data they calculated that compared to the amount of carbon the plants were locking up, the amount of water they were losing was shrinking.The trees were using water more efficiently, and in many cases that meant they were gaining mass.
The authors concluded that rising carbon dioxide levels seemed the most plausible reason for the trees’ water efficiency.
Six times more efficient
Across all the sites examined, forests had become on average three per cent more efficient at using water over the 20-year period. This may not sound much, but it’s actually a much greater rate than controlled experiments in a lab suggest.
The lab experiments suggest that increase in efficiency should be proportional to the increase in carbon dioxide levels in the atmosphere. But the measurements taken in the US forests suggest the increase in efficiency is six times greater than the rise in carbon dioxide levels.
Other factors – such as changes in the the availability of minerals in the soil or fluctuations in rainfall – could have affected the results. The tools used to measure carbon uptake and water use can also become unreliable over long time periods. The authors systematically ruled these factors out, though. None could explain why water use was so much more efficient than theory said it should be.
Image - Measuring Carbon Fluxes (note)
Equipment mounted above the tree canopy measures changes in carbon and water vapour. Source: Chris Vogel
Cause for concern?
The authors say their study suggests rising levels of greenhouse gases are having an “unexpectedly strong” effect on the carbon cycle in many temperate forests, such as those found across much of Europe, and boreal forests found further north. And this could have implications for climate projections.
As a discussion article in the same journal notes, there’s a significant increase in how efficiently forests use water that can’t be explained. Either plants are more responsive to rising carbon dioxide levels than lab experiments suggest, or there are other factors contributing to the trend the new study found in US forests.
If trees are more responsive to carbon dioxide than previously thought, models of the carbon cycle might need to be “reassessed”, say the authors. That could even alter climate model projections of how plants and trees will respond to climate change, since these models assume efficiency will rise at a proportional rate to carbon dioxide levels.
For now, though, more research is needed to back up or disprove these findings. The results should be interpreted carefully as there are many other aspects of climate change, besides rising emissions, which could affect how much carbon forests lock up. Changes in precipitation, temperatures, drought and wildfire could all have an impact on how well trees convert carbon dioxide into plant growth.
On its own, this study isn’t proof climate change will be good news for trees and forests.