Small changes in carbon mean big changes in forests

Some effects of climate change are dramatic and obvious: more severe storms, wildfires and sea-level rise. But a new study funded by Science Foundation Ireland has found a deep and quiet change in forests that could dramatically affect how water moves around the planet.

The study, led by Professor Jennifer McElwain at Trinity College Dublin, shows that forests around the world are changing how they handle their water in response increases in atmospheric carbon dioxide, and that evergreen plants are showing more rapid adaptation than their deciduous or leaf-shedding counterparts.

“We started the study wanting to know more about how forests have been responding to recent changes in the Earth’s atmosphere, over the last three decades or so,” explains Professor McElwain, Chair of Botany at TCD’s School of Natural Sciences. As a palaeobotanist, she has made important discoveries from the fossil record about how plants adapted to changes in atmospheric oxygen and carbon dioxide millions of years ago, but this project needed a different approach. Instead of digging into the fossil record, the researchers looked at historical written accounts and samples from various forests taken by scientist Jack Wolfe at the Smithsonian Institute in Chicago.

“He kept meticulous records from the work he did in the late 1980s and early 1990s, when the levels of carbon dioxide in the Earth’s atmosphere were about 50 parts per million lower than they are today. We wanted to see had there been changes in those forests during that increase in climate change.”

So the team returned to 21 forest sites around the world and compared what they saw and found there with Wolfe’s older records from the same locations - they explored trees and shrubs from the warm tropical and desert landscape of Fiji, Puerto Rico and Arizona to the cold temperate and boreal forests of Alaska. “We created this beautiful experiment, comparing representative species from forests around the world 30 years ago and today,” she says.

What they found was surprising: even in that relatively short period with a jump in atmospheric carbon dioxide, the forests had changed at a deep physiological level, notes Dr Wuu Kuang Soh, a research associate at Trinity’s Botany Department and lead author on the Science Advances paper that details the study.

“We looked at how the woody plants handle water, which they release through the leaves as water vapour, and we could see plants had become more efficient at holding on to water,” says Dr Soh, who will be taking up a position as botanist in the National Botanic Gardens in the new year. “They were losing less water for every molecule of carbon they were taking up from the atmosphere.”

Taken at scale, this change could have a profound effect the capacity for forests to hold on to water, he adds: “If across the board plants are losing less water for every molecule of carbon they take up, that means globally forests are recycling less water and this could increase the risk of floods.”

The team of scientists from Ireland (Trinity and Maynooth University), Sweden, Australia and the UK also found that evergreen trees and shrubs are more efficient in using water than deciduous plants in cooler climate locations but there was no evidence for such pattern in parts of the world with warmer climates.

“It is a big result, really simple and unexpected, and it shows that even at an increase of 50 parts per million of carbon dioxide in the atmosphere we can see a strong differential response between plant types,“ says Professor McElwain.

“We think that if we continue changing the climate and atmospheric composition, with an expected doubling of carbon dioxide in the atmosphere by the end of this century, we could see a big change in ecology in forests globally. Our results indicate that future increases in atmospheric carbon dioxide may confer a competitive advantage to woody evergreen trees in cooler parts of the world, and this insight will improve our ability to build models on vegetation response to future climate change.”