What will tropical forests do with more CO2?

Carbon dioxide is the fuel of photosynthesis, the sunlight-driven chemical reaction that kick-starts plant growth. As humans pump more of this greenhouse gas into the atmosphere, many scientists wonder if future tropical forests will grow faster than they do now. There are two ways to determine if this will be the case, says STRI plant physiologist Klaus Winter. The first involves monitoring forests until 2100 and beyond, when atmospheric CO2 could be 800 parts per million, twice today’s concentration. The other is to artificially supercharge forest parcels with carbon right now.

The latter is no easy task but a group of Smithsonian scientists proposes to do just that. Predicting how tropical forests, which have the largest stores of carbon in the terrestrial biosphere, will respond to a doubling in CO2 levels "represents a major source of uncertainty that limits our capacity to understand tropical ecosystem processes, assess their vulnerabilities to climate change and improve their representation in Earth system models," wrote Winter and other authors in the journal Functional Plant Biology. The review paper resulted from a symposium held at the Smithsonian Tropical Research Institute in 2011, bringing together experts from around the globe to assess the state of carbon science. Most importantly, they sought a path forward. They identified key carbon questions and decided how to best address them experimentally.

Based on decades of experimental work on tropical seedlings and saplings, Winter has high confidence in two observed trends. When he adds CO2 to young trees in growth chambers they tend to grow faster and require less water in the process. But this doesn't necessarily mean that tropical forests in an environment with more carbon will be larger or more tolerant to drought: soil nutrients and water availability are important factors that could limit the growth-boosting power of CO2.

"The challenge is now to scale up from seedling and saplings to the full-grown forest," says Winter, who has published more than 20 peer-reviewed articles on how tropical seedlings and saplings respond to atmospheric and climate change. "Manipulative experimentations at a large scale are what we need, and we need them now."

Winter asks if adult trees will grow faster in a CO2 enriched atmosphere. If so, will they die faster? Will forests hold more or less carbon? Will species composition change?

Winter expects forests to be different in a carbon-enriched world but the quickest way to be certain is to expose forests to high levels of CO2. Experiments include free-air CO2 enrichment experiments, which blast forest trees with CO2 but have yet to be conducted in the tropics. Winter also proposes to construct massive, naturally lit domes like those used in England’s Eden Project. Domes would allow scientists to study tall tropical vegetation under scenarios where future atmospheric and climate conditions - including temperature and moisture - can be strictly controlled. Instead of having to wait a century, scientists could conceivably reduce future climate uncertainty in a few years.

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