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‘A real surprise’: Study reveals low phosphorus doesn’t hinder rainforest growth

  • Phosphorus is an important nutrient for plants and is required for protein synthesis and cell division, among other critical processes. But phosphorus is typically scarce in the soils underlying tropical forests.
  • A group of researchers investigated this by looking at trees in the forests of Panama, where soil phosphorus levels vary considerably.
  • The researchers were surprised to find that tree species in low-phosphorus soils grew faster on average than species in high-phosphorus soils. Their results also indicate the growth rates of tree communities comprised of a variety of species doesn’t change doesn’t change in relation to soil phosphorus amount.
  • The authors and other scientists say the study’s findings further our understanding of the dynamics between tropical plants and phosphorus and could help farmers grow crops more effectively without having to use environmentally harmful fertilizers.

Scientists have long believed the lack of nutrients in tropical forests soils meant that they weren’t very productive when it came to plant growth. But at the same time, trees didn’t seem to grow any taller when fertilizer was added to increase soil nutrient levels. A group of researchers sought to resolve this apparent paradox by looking at one specific, important nutrient – phosphorus – and how differing amounts in the soil affect tree growth.

The study’s results reveal that tropical forest trees have an unexpected ability to persist in low-phosphorus soils. The researchers say their study, published recently in Nature, could have big implications for human understanding and management of forests and crops.

Phosphorus is an important nutrient for plants and is required for protein synthesis and cell division, among other critical processes. But phosphorus is typically scarce in the soils underlying tropical forests, having been washed away by frequent heavy rains and consumed by iron oxide compounds, which readily bind up phosphorus into a form that can’t be used by plants.

While rainforest trees can apparently handle low-phosphorus soils, crops often become stunted without nutrient inputs. This is often remedied through the application of fertilizer.

But still, despite their poor soils, rainforests are host to a huge variety of plant life – including very tall trees. And when scientists have added phosphorus fertilizer to the soil of tropical forests around the world, it didn’t seem to have much of an effect on tree growth.

So just how important is phosphorus to rainforest trees? A group of researchers from the Smithsonian Tropical Research Institute (STRI) investigated this by looking at trees in the forests of Panama, where soil phosphorus levels vary considerably. They examined 19,000 individual trees from 541 different species, measuring their growth and the amounts of phosphorus and other nutrients like calcium and nitrogen present in the soil.

Their results revealed that, overall and as expected, the average growth rates of tree species tended to go up with phosphorus amount. But when the researchers zoomed in on the areas with the highest and lowest growth rates, they were surprised to find that this correlation was switched – tree species in low-phosphorus soils grew faster on average than species in high-phosphorus soils.

The study also finds the growth rates of tree communities comprised of a variety of species doesn’t change doesn’t change in relation to soil phosphorus amount.

“Finding that species adapted to low phosphorus soils are growing so fast was a real surprise,” Ben Turner, an STRI staff scientist who led the study, said in a statement. “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.”

The authors and other scientists say the study’s findings further our understanding of the dynamics between tropical plants and phosphorus and could help farmers grow crops more effectively without having to use environmentally harmful fertilizers.

“This study highlights our limited understanding of how plants cope with phosphorus-poor soils, a significant challenge to farmers through much of the tropics,” said Jim Dalling, STRI research associate and professor and head of the Department of Plant Biology at the University of Illinois Urbana-Champaign. “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.”

The authors also write that their study may help us understand how tropical forests react to environmental changes like global warming.

“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,” Turner said.

 

Citation:

Turner, B. L., Brenes-Arguedas, T., & Condit, R. (2018). Pervasive phosphorus limitation of tree species but not communities in tropical forests. Nature.

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