Scientists have found a way to use satellites to track photosynthesis in evergreens—a discovery that could improve the ability to assess the health of northern forests amid climate change.

Photosynthesis is easy to track in deciduous trees—as leaves bud or turn yellow and fall off. But until recently, it had been impossible to detect in evergreen conifers on a large scale.

Researchers are reviewing years of satellite data on northern forests to determine whether photosynthetic cycles are happening earlier because of climate change. Image credit: Pixabay. An international team of researchers led by John Gamon, a University of Alberta biologist, has now used satellite sensor data to identify slight color shifts in evergreen trees that show seasonal cycles of photosynthesis—the process by which plants use sunlight to convert carbon dioxide and water into glucose. They combined two different satellite bands—one of which was used to study oceans and only recently made public by NASA—to track seasonal changes in green (pigment created by chlorophyll) and yellow (created by carotenoid) needle color.

The index they developed provides a new tool to monitor changes in northern forests, which cover 14% of all the land on Earth.

“Photosynthesis is arguably the most important process on the planet, without which life as we know it would not exist,” Gamon says. “As the climate changes, plants respond—their photosynthesis changes, their growing season changes. And if photosynthesis changes, that in turn further affects the atmosphere and climate.”

Through their CO2-consuming ways, plants have been slowing climate change far more than scientists previously realized. The big question is whether this will continue as the planet continues to warm due to human activity, Gamon says.

Scientists have two hypotheses—the first of which is that climate change and longer growing seasons will result in plants sucking up even more CO2, helping to slow climate change. The other predicts a drop in photosynthetic activity due to drought conditions that stress plants, causing them to release CO2 into the atmosphere through a process called respiration—thereby accelerating climate change.

Gamon's lab is reviewing 15 years’ worth of satellite data on forests in Canada and Alaska to determine whether photosynthetic cycles are happening earlier because of climate change and whether forests are becoming more or less productive at absorbing CO2. The first results should be available in the next several months.