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News Article

Diverse tropical forest growth not limited by phosphorus

Study finds that phosphorus limits growth of individual species but not entire forest communities

According to widely accepted ecological theory, nutrient poor soils limit the productivity of forests in tropical regions but applying nutrients as a fertilizer seldom increases tree growth, suggesting that productivity is not limited by the level of nutrients.  In a new study, published in Nature earlier this month, researchers from the Smithsonian Tropical Research Institute have sought to resolve this contradiction, showing that phosphorus can limit the growth of individual tree species, but not entire forest communities, which, it is anticipated, will have implications for the way we understand forest growth and change.

Vast areas throughout the tropics occur on ancient landscapes where nutrients derived from rocks have been leached away, from years of heavy rainfall.  Phosphorus is particularly scarce, because the iron oxides in the soil bind to the phosphorus, making it unavailable to plants.  However, the application of fertilizer to diverse forests in Africa, Southeast Asia and the Americas has not increased tree growth.  Hawaii was the only place where fertilization resulted in tree growth, where the forest is dominated by a single tree species.

Another method of analyzing nutrient limitation is by comparing the growth rates of trees in forests that naturally differ in soil nutrient availability.  The researchers found that the small but biodiverse country of Panama provided a useful setting for their study.  Central Panama has complex geology which means that natural levels of plant-available phosphorus in the soil differ more than 300-fold, which is similar to the range of phosphate availability in tropical soils found in other parts of the world.  The soils in Panama also vary in moisture and other nutrients such as calcium, nitrogen and potassium and researchers are able to study the effects of these variables on growth at the same time.

The team measured 19,000 trees from 541 different tree species in a series of long-term forest monitoring plots that are part of the Forest Global Earth Observatory network, in order to examine the effect of phosphorus on tree growth. They found that, on average, growth rates of the individual tree species increased in soils that had higher levels of plant-available phosphorus.  This is consistent with the ecological theory.  However, they were surprised to find that tree species that were present on low phosphorus soils actually grew faster on average, than species growing on soils with a high phosphorus content.  Also, variation in the tree species present across the plots meant that community-wide growth rates did not differ according to the level of soil phosphorus.

"Finding that species adapted to low phosphorus soils are growing so fast was a real surprise," said study lead and author Ben Turner. "We still don't understand why this occurs, nor why high phosphorus species are not growing faster than they are. Perhaps trees invest extra phosphorus in reproduction rather than growth, for example, because seeds, fruits and pollen are rich in phosphorus. For now, these results help us to understand how soil fertility influences tree growth in tropical forests, and demonstrate once again the power of tropical diversity to surprise us."

Co-author Jim Dalling added, "This study highlights our limited understanding of how plants cope with phosphorus-poor soils, a significant challenge to farmers through much of the tropics.  Comparing how plants adapted to high versus low phosphorus availability acquire and use this critical nutrient could suggest new approaches for increasing food production without relying on costly fertilizers." 

Article details

  • Author(s)
  • Stephanie Cole
  • Date
  • 28 March 2018
  • Source
  • Smithsonian Tropical Research Institute
  • Subject(s)
  • Environment