Trees soaking up less carbon than expected, study finds

Guelph, ON – Scientists and policy-makers hoping to use forests to naturally soak up increasing amounts of carbon dioxide may have overestimated the role of trees as carbon sinks, according to a new study by University of Guelph researchers.

Contrary to expectations, tree growth has declined over the past century despite rising amounts of CO2 in the atmosphere, said Madhur Anand, a professor in Guelph’s School of Environmental Sciences. Along with PhD students Lucas Silva and Mark Leithead, she co-authored a paper that appeared in the July issue of PLoS ONE, a peer-reviewed international journal published by the Public Library of Science. It was also featured by the journal as a “pick of the month.”

Their results challenge predictions that more atmospheric carbon will effectively “fertilize” forests. Under the Kyoto Protocol, countries are encouraged to use forests to sequester carbon and help meet targets for emissions of global-warming gases.

But the Guelph researchers say the predicted benefits of CO2 fertilization may be overestimated. They found that warming has caused a growth decline in temperate and boreal forests during the past century and especially since the 1950s.

Under warming-related stress, some trees use water more efficiently but grow more slowly. That means trees are storing less atmospheric CO2 than expected, said Dr Anand, who holds the Canada Research Chair in Global Ecological Change.

“We need to entirely rethink impacts of climate change. We need to consider multiple stressors and interactions with disturbances. More research is needed in these areas to better predict implications for forest productivity, management and even restoration.”

The team studied red oaks, red maples, red pine and black spruce at five sites from Long Point in southern Ontario to Moosonee near James Bay. They measured tree growth rings and studied carbon isotopes in those rings to gauge trees’ water-use efficiency and to distinguish climatic effects from other factors.

Some trees with growth declines not clearly connected to climate might have been affected by lack of nutrients, but more research is needed, said Dr Anand. They focused on Ontario forests after they discovered tree-growth declines in Brazil and then in other parts of the world.

The study was funded by the Inter-American Institute for Global Change Research (IAIGCR), the Natural Sciences and Engineering Research Council, the Canada Foundation for Innovation and U of G. Dr Anand leads the Canadian node of an international IAIGCR project studying changes in forests in North and South America caused by climate change and other disturbances.