No one expected this. In the longest-running experiment of its kind, the plants that were supposed to thrive in higher carbon dioxide levels have instead done less well.
The finding suggests plants will not take up as much CO2 in the future as climate models assume – potentially leading to greater warming in the long term.
“To get this result is startling,” says of the University of Minnesota in St Paul. His team has run a 20-year experiment looking at how higher CO2 levels affect grasslands.
Advertisement
The findings show we need to be cautious about predicting how complex systems behave, he says. “There could be surprises in store.”
Plant food
Plants make themselves out of the carbon they obtain from carbon dioxide. For most of Earth’s history, CO2 levels in the atmosphere have been much higher than they are now, and the planet has been much hotter. But 30 million years ago, CO2 levels plummeted and the ice ages began.
Low CO2 is a huge problem for plants, because the enzyme that captures CO2 often grabs hold of oxygen instead – a mistake that wastes lots of energy. The lower CO2 levels fall, the more often this wasteful mistake occurs.
But some plants, known as C4 plants, have come up with a solution. They concentrate CO2 in their cells, effectively recreating the ancient atmosphere.
This trait evolved independently in several groups over the past 30 million years. C4 plants have been outcompeting normal plants – known as C3 plants – ever since. Just 3 per cent of land plant species – mostly grasses – are C4, but they make up a fifth of plant biomass.
This basic science suggests that C4 plants will not benefit much from rising CO2, whereas C3 plants will. This idea was well on its way to becoming a “scientific fact”, because it is exactly what has happened in many short-term experiments in which plants have been exposed to higher levels of CO2.
Shock finding
It’s also what Reich saw during the first 12 years of his experiment, which involves more than 300 grassland plots planted with 16 species of grasses and herbs. But afterwards, C3 plants grown in higher CO2 started producing less biomass than the same plants grown in ambient CO2. Meanwhile, the C4 plants grown in higher CO2 started outperforming the C4 plants grown in normal levels.
“It’s a result that contradicts well-established theory,” says at Western Sydney University in Australia, who studies how plants respond to higher CO2. “It is indeed surprising.”
Further studies by Reich’s team suggest an explanation. In high-CO2 air, less nitrogen is available to the C3 plants while more is available to the C4. The team thinks this is because the higher CO2 is affecting either the symbiotic fungi associated with the roots of plants, or the soil microbes that break down organic matter and release the nitrogen. “This is not easy to study,” Reich says.
If he is correct, land areas are likely soak up less carbon in the future, given that most plants are C3 plants. However, the findings shouldn’t apply to crops fertilised with nitrogen.
Greener world?
More work needs to be done to confirm both the basic finding and the reason for it, say other researchers.
“I really think the study should be replicated elsewhere, though I’m not aware of such a long-running experiment anywhere else in the world,” says , also at Western Sydney University. His work has suggested that a lack of phosphorus will prevent forests benefitting from higher CO2.
The trouble is, no one wants to provide such long-term funding, says Reich. He has only kept the experiment running for so long by constantly applying for two-year grants.
Globally, satellite studies suggest the planet is already getting greener as a result of climate change, but the reasons are complex. For instance, trees are sprouting in northern regions where it used to be too cold for them to grow. And while in Reich’s experiment only CO2 levels changed, in the real world there will also be big changes in temperatures and rainfall as the planet warms.
Science
