Global warming will cause massive declines in tree growth

By combining tree-ring data with U.S. Forest Service inventory data on Arizona’s ponderosa pines, a recent study led by the University of Arizona has investigated the possible future effects of climate change on tree growth. The results suggest that global warming may cause a 56 to 91 percent decline in individual tree growth.

Forests remove carbon dioxide from the atmosphere, thus offsetting greenhouse gas emissions and helping to mitigate climate change. “It’s a free service that forests provide, so forests have been touted as one of the many natural climate solutions that countries rely on to offset their emissions,” said study lead author Kelly Heilman, a postdoctoral researcher in the University of Arizona’s Laboratory of Tree-Ring Research. 

“But competition between trees, droughts and disturbances can reduce forest carbon uptake. Knowing how much carbon forests take up globally is essential to addressing the climate crisis and planning for a resilient future.”

The massive declines in tree growth that Dr. Heilman and her colleagues are forecasting will mean less uptake of atmospheric carbon dioxide in the future by Arizona’s forests. “While Arizona’s forests are relatively small in terms of their contribution to the total U.S. carbon sequestration, our approach can be used to make the same predictions for forests around the world,” she explained.

The causes of tree size decline are complex and varied, but the main one seems to be global warming. Since trees have to work against gravity to get water to their top, if temperatures rise, the water-transport system will be under increasing pressure and most likely become damaged. Warming will make trees more drought stressed, and reduce their growth. 

Although drought seems to make taller trees more vulnerable due to high temperatures, the scientists found that small trees will also suffer due to lack of water. Since their roots are smaller, they will need to struggle more to extract moisture from the soil.

Moreover, denser forests will likely fare worse during hotter and drier conditions, since there will be more competition between trees for increasingly rare resources such as water and nutrients. “Foresters can’t influence the climate, but they can change forest density to reduce competition for the remaining trees,” said study co-author Margaret Evans, an assistant professor of Dendrochronology in the tree-ring lab. “If you have both an overly dense forest and climate warming happening at the same time, that’s a double whammy. But if you thin the forests, you can remove one source of stress.”

In order to improve their tree growth predictions in future work, the scientists aim to include variables such as wildfires or insect disturbances into their analyses.

The study is published in the journal Global Change Biology. 

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By Andrei Ionescu, Earth.com Staff Writer