A New Look At Old Forests

All forests get old; they die—due to fire, insects, hurricanes, et cetera—and regenerate. Most US forests are maturing from regeneration that began about 100 years ago when extensive clear-cutting occurred.

Jianwu (Jim) Tang, assistant scientist at the Marine Biological Laboratory’s Ecosystems Center, is lead author of a recent paper that demonstrates that when a forest gets old, its ability to grow decreases because it takes up less carbon dioxide and sunlight and respires less.

The traditional theory had maintained that when a forest gets old, it would respire more and use more energy. But Dr. Tang and his research group found both energy production (photosynthesis) and energy consumption (respiration) decrease with age, resulting in an overall decrease of growth rates. Their finding suggests that the growth of forests in the US will decline. This finding will help in developing models to predict future changes in forests.

Forests are also big carbon sinks that offset, in part, human-induced carbon emissions. The scientists’ finding also suggests the forest carbon sink may decrease in the US because of the slowdown in forest growth. In some other countries, young forests could grow faster and take up more carbon dioxide.

To mitigate human-induced climate change and global warming, either more forests need to be planted from non-forested lands, or the use of fossil fuels needs to decrease. Publication: Tang, J., S. Luyssaert, A. D. Richardson, W. Kutsch,  I.A. Janssens. “Steeper declines in forest photosynthesis than respiration explain age-driven decreases in forest growth.” Proceedings of the National Academies of Science.

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