Getting warmer: Slower forest growth means less carbon storage

Forests and land play an important role in absorbing carbon dioxide emissions, but current models and forecasts don’t incorporate a new and surprising ecological discovery: Despite more available carbon, climate change and warmer temperatures are slowing forest growth.

A new study, published June 18 in Geological Research Letters, considers for the first time the impact of the discovery on climate models, finding that one of the most-used land models for determining the impacts of climate change may overestimate forests’ future potential for carbon storage by as much as 30%.

“Knowing how well the land will be able to keep taking up carbon in the future is really important for knowing how much CO2 you’ll have in the atmosphere, and how much warming you’ll have,” said first author and postdoctoral researcher Brendan Clark. “But the land models are probably underestimating the effects of hotter, drier air on actual growth.”

The land currently absorbs approximately 27% of the carbon dioxide produced from the burning of fossil fuels, with the ocean taking up another 25% and the rest staying in the atmosphere, leading to warming. Slower forest growth could reduce the land’s capacity for carbon storage, accelerating warming and its impacts in a way that’s not currently captured in climate models.

“The more we look, the clearer it becomes that with further warming it will become harder for nature to keep up,” said senior author Daniele Visioni, assistant professor of earth and atmospheric sciences in the College of Agriculture and Life Sciences. “Brendan’s research is an important step in improving our ability to quantify just how harmful future warming will be.”

Clark and his team used recent research from forests in Switzerland that measured the growth rate of both broadleaf and coniferous tree species over eight years, finding that drier, hotter weather led to reduced growth. Ecologists have found that the slower growth, which has now been documented across North America, may be due to lower turgor pressure, or the amount of water in the tree’s cells. The lower pressure impedes cell division and carbon storage, even while photosynthesis occurs, a process not currently represented in land models.

“So the tree may be photosynthesizing, but it’s not growing,” Clark said. “Land models operate on the assumption that photosynthesis and growth are the same thing, but new evidence shows they often are not.”

With the data from Switzerland, Clark built a statistical model that predicts tree growth and carbon storage through 2069 and compared it to simulations from an open-source, widely used land surface model. Clark found that the land model simulations may overestimate tree growth by a factor of 2 for broadleaf trees and a factor of 3 for coniferous trees. Smaller trees would result in a reduction in carbon storage, especially in areas forecasted to become hotter and drier.

The discrepancy between the two models signals the importance of incorporating the processes that slow growth in modeling more broadly – and could partly account for land models’ inaccuracy to date.

“Models have been shown to overestimate the land’s ability to store carbon, contrary to what the observations show, and this could be one reason why,” Clark said. “It’s really important that we know the biases in these models and what’s driving them, and that they match observations, because uncertainty makes planning for the future really hard.”

Clark first learned about the new ecological findings from co-author and ecologist Shan Kothari, assistant professor at the University of Alberta, and immediately wondered how they might impact climate models. He began attending conferences on forest ecology to understand the latest research.

“There can be a disconnect between ecologists and modelers,” Clark said. “It’s important to bring them together, to bring this new idea in the ecology world to the land-modeling community, because I think this is going to be something we need to think about.”

Clark hopes to now develop code to account for the slower forest growth – that researchers can incorporate directly into their models.

“Actually implementing this in the code and showing other land modelers how we did it and why it’s important – that’s really the next step,” he said.

Manuel Lerdau from the University of Virginia is a co-author.

Funding for the study came from the Quadrature Climate Foundation and was made possible thanks to the resources of the National Science Foundation’s National Center for Atmospheric Research.