How many cookies fit on a baking sheet? How many oranges fit in a bag? In order to calculate how much carbon a tropical forest can store, scientists working with data from the Smithsonian’s ForestGEO network came up with biologically sound explanations behind the simple mathematical rules of thumb that can be used to determine how many trees fit in a tropical forest and how big they get, essential information for calculating the ability of forests to store carbon.
Their results will be published online by the journal Science Jan. 8.
“The models commonly used to estimate how much carbon tropical forests pull out of the atmosphere and store are based on ‘ideal’ trees—rough estimates of how many trees are out there and how big they are,” said Stephen Hubbell, staff scientist at the Smithsonian Tropical Research Institute (STRI) and distinguished professor of life sciences at UCLA. “Based on 30 years of data from Barro Colorado Island in Panama, we discovered that trees seek light and grow to fill gaps caused by windstorms and falling trees in predictable ways, which explains why tropical trees have roughly the same size distributions in forests around the world.”
“After a storm or other disturbance opens up a hole or gap in the forest, many small trees establish in the abundance of light but there is a race to stay in the light as the gap closes,” said Stephanie Bohlman, STRI research associate and assistant professor at the University of Florida. “As the diameter of the tree trunks increase, there are fewer and fewer individuals that succeed in staying in the sunlit layer of the forest canopy. That number can be approximated by mathematical relationships. The rest of the trees are the ‘losers’ and are doomed to grow very slowly in the forest understory.”
When one quantity in an equation (such as the size of trees in the forest canopy) changes in a way that is proportional to the change in another quantity (like the number of canopy trees in an area), mathematicians say the relationship between the two obeys a ‘power law.’ Many small cookies fit on a baking sheet or fewer, bigger cookies fit.
This team, led by Caroline Farrior, a post-doctoral fellow at the U.S. National Institute for Mathematical and Biological Synthesis—soon to be assistant professor at the University of Texas at Austin—and including Princeton University’s Steven Pacala, was able to describe a simple mechanism of competition for light following small-canopy openings. This explains why tree-size distributions consistently seem to follow a power law based on the winners and losers of the race to stay in the sunlight after a gap is formed.
Understanding such an important, simple property of forests will enable estimations of the amount of carbon tropical forests contain and will lead to more confident predictions of how that amount could change in the future. The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is part of the Smithsonian Institution. The Institute furthers the understanding of tropical nature and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems. Website: www.stri.si.edu.
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Farrior, C.E., Bohlman, S.A., Hubbell, S., Pacala S.W. 2015. Dominance of the suppressed: Power-law size structure in tropical forests. Science.
SI-2-2016
Beth King
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