This article was originally published in The Conversation

and in Ars Technica

Forests using less water because of climate change – a good thing?

Global warming is primarily driven by increasing emissions of greenhouse gases from human activities. Chief among these gases is carbon dioxide (CO2), which warms the planet by trapping heat that would otherwise radiate into space.

But carbon dioxide has effects on things other than the climate. In the journal Nature, Trevor Keenan of Macquarie University and his colleagues report that trees in some forests are using less water to maintain growth than they did only 20 years ago. This puzzling finding has been attributed to the increased levels of CO2, which trees utilise as their carbon source.

Instruments placed in various US forests have been measuring CO2 and water concentrations in the air for many years. These measurements have been correlated with the amount of CO2 locked up by trees over the same period to show that forests have become more efficient at storing carbon. What is more important is that the measurements suggest that the increase in efficiency of storing carbon is six times greater than would be expected if it was just proportional to the increase in atmospheric CO2 concentration during the same period.

Increased CO2 availability means that trees have to restrict the opening of their breathing pores (stomata) so that CO2 levels inside their leaves remain constant. But this has additional consequences for the trees. Smaller pores means less water evaporates from their leaf surfaces through these stomatal openings. This effect has been called the “CO2 fertilisation effect”, which means plants can utilise more CO2 to make more carbohydrates, like cellulose and sugars, while using smaller volumes of water overall than previously required.

The fine balance between CO2 uptake and water loss is critical for plant survival. Early predictions by climate scientists were that increasing temperatures would devastate forests. That is because elevated temperatures increase the rate of evaporation and transpiration at leaf surfaces, potentially causing trees to suffer from “water-stress”. Instead, this paper suggests that increased efficiency of water-use by forests might mean that water does not become a limiting factor in productivity as temperatures rise.

This new finding seems like unadulterated good news, therefore, until you factor in the effect that water usage by forests has on components of the ecosystem. Trees move an incredible volume of water from the ground into the atmosphere. That water then forms rain, which helps the connected ecosystem thrive. Large forested areas play a very important role in the water balance and ecology of most agricultural land on Earth.

Keenan and his colleagues examined 21 forest sites going back as much as 20 years, with their data limited to the temperate and boreal forests of the Northern Hemisphere. Although this is still a relatively small sample size, this work will probably result in a flurry of research activity to establish what will happen to plant primary productivity in other areas in response to elevated CO2. Keenan said: “We’ve examined the trend upside down and inside out as much as we can, and it is wholly robust.”

Climate scientists use data from studies such as this one to build long-term computer simulations that help them examine potential effects of alterations in variables like temperature, ocean currents and rainfall. Decreased volumes of water being moved by transpiration into the atmosphere will now be added as an input in these simulations in an attempt to predict what the medium and long-term effects of the new observations might be.

On the one hand, more consumption of CO2 by forests will help stem global warming. But on the other hand, less water circulated through more efficient use by trees will mean that non-forest ecosystems may get into trouble.