When it comes to climate change, not all forests are created equal

Reported and written by Grayson Badgley, Simone Barley-Greenfield, Renee Donovan, Patrick Freeman, Faine Greenwood, Alexandra Peers, Nicole Sarto and Shara Tonn.

Well-nourished forests, such as this one in Costa Rica, can capture and store vast amounts of carbon from the atmosphere. (Simone Barley-Greenfield / Peninsula Press)
Well-nourished forests, such as this one in Costa Rica, can capture and store vast amounts of carbon from the atmosphere. (Simone Barley-Greenfield / Peninsula Press)

Any gardener can tell you that plants need nutrients such as nitrogen and phosphorus to grow. The gardener who feeds homemade compost or fertilizer pellets to her plants can expect bountiful harvests and lush landscapes — while failing to do so could yield a much scrubbier planter box.

Forest trees are just as dependent on nutrients as a rosebush or a carrot, even though they obtain their fertilizer from minerals and organic material in soil, not from a garden store. But it turns out that nutrients don’t just keep the forest thriving: they may help the trees regulate Earth’s climate, too. New research published in Nature Climate Change concludes that nutrient-rich forests can more efficiently “sequester” (capture and store) carbon dioxide from the atmosphere than their nutrient-starved neighbors.

“It’s like eating junk food,” explains Marcos Fernández-Martínez, who led the international team of scientists that conducted the study. “A child who eats McDonald’s every day won’t grow as fast as a child who doesn’t.” In the report, the researchers show that only forests with healthy “diets” can grow well enough to combat climate change.

Each year, the world’s forests remove more carbon dioxide from the atmosphere than is produced by all of the cars and power plants in North America. This means that forests are an important force for mitigating climate change, but only if the forests can lock that carbon away securely, rather than letting it seep back into the atmosphere.

For decades, experts and climate models have assumed that this process of carbon sequestration is most closely tied to the total amount of carbon available in the atmosphere. But the new study adds to the growing body of evidence that suggests other factors are even more important.

Fernández-Martínez and his co-authors examined two decades of data from more than ninety forests around the world. They found that nutrients were more important in determining how efficiently forests were able to capture and store carbon than carbon dioxide levels in the atmosphere. Furthermore, nutrient-availability determined how securely the forests could sequester carbon once captured.

The key is that nutrient-poor forests can capture carbon, Fernández-Martínez says, but they aren’t very good at holding on to it. Unsatisfied by their junk-filled diet, trees in these forests spend their energy trying to get more nutrients instead of growing bigger and stronger.

“The result makes sense,” said Peter Vitousek, a professor of biology at Stanford University who has long studied the global nitrogen cycle. Nutrients help plants more efficiently capture light, making it easier for them to allocate those extra resources to producing wood, Vitousek explained.

Fernández-Martínez calls the shift in understanding “subtle.” But it still may have important implications for refining climate models and future scientific studies. Current models do not account for differences in soil quality when predicting how much carbon forests can be expected to take out of the atmosphere.

The result is unlikely to lead to new efforts to combat climate change directly, however. Large-scale fertilization could theoretically help increase the carbon stored in forests, but Fernández-Martínez is skeptical that it would make a difference at the global scale. Not only is fertilizer expensive, mass application presents numerous logistical and environmental issues, and it requires a tremendous amount of energy to produce. “And energy means carbon,” points out Fernández-Martínez. “Surely the carbon balance would be worse.”

Still, the diligent gardener values knowing what to expect come harvest time. Similarly, the more we know about how forests store carbon, the better we all can prepare other ways to face the challenges of climate change.

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