One of the big uncertainties about climate change is how much carbon dioxide will be taken up by terrestrial ecosystems. There is hope that tropical forests will absorb some of the excess carbon in the atmosphere, incorporating it into their tissues as they grow and storing it for hundreds of years. However, plants need more than just carbon dioxide to grow and flourish; they also need nutrients and other essential building blocks. The degree to which nutrients limit the tropical carbon sink remains unclear, especially in forests recovering from agricultural land use.
Ecologists have long thought that phosphorus is the main nutrient that restricts tree growth in tropical forests, but evidence to support this idea has been limited. In fact, our lab is finding that nutrient limitation changes throughout tropical forest succession. We made this discovery using a landscape-scale nutrient addition experiment, where we have been tracking tree dynamics in 76 0.16 ha plots treated with nitrogen, phosphorus, nitrogen plus phosphorus, or a control for up to 26 years.
The experiment revealed that in young tropical forests recovering from disturbance — when carbon accumulation is highest — nitrogen is actually the main factor limiting forest carbon accumulation. As tropical forests age, evidence of nitrogen limitation on forest growth disappears. We find no evidence of phosphorus limitation on carbon accumulation in forests of any age at our study sites.
This work has clear implications for tropical forest management as a natural climate solution, suggesting that if nitrogen limitation were alleviated, tropical reforestation could sequester up to an additional 1.1 gigatons of carbon dioxide per year. For example, forest managers could consider reforesting in areas with high nitrogen deposition, allowing the trees to clean up nitrogen pollution as they grow.
Next, we hope to use our landscape-scale fertilization experiment to better understand how nutrients limit carbon sequestration and leaf, fine root and wood productivity over tropical forest secondary succession, and how trees use nutrient strategies to overcome nutrient limitation.
In addition to informing best practices for policymakers and practitioners seeking to use tropical reforestation as a natural climate solution, this work promises to improve climate change models by providing a more realistic picture of how much carbon tropical forests can be expected to absorb.
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