The root cause of tropical forest diversity

Oreomunnea mexicana Photo: James Dalling

Tropical forests are renowned for their high diversity, yet in many sites a single tree species accounts for the majority of the individuals in a stand. An explanation for these monodominant forests remains elusive, but may be linked to mycorrhizal symbioses.

The types of beneficial fungi that associate with tree roots can alter the fate of a patch of tropical forest, boosting plant diversity or, conversely, giving one tree species a distinct advantage over many others. A new study sought to explain this phenomenon that occurs in some tropical forests: Small patches of "monodominant forest," where one species makes up more than 60 percent of the trees, form islands of low diversity in the otherwise highly diverse tropical forest growing all around them. These patches dominated by a single species are rare but understanding how monodominant forests arise and persist could help explain how tropical forests otherwise maintain their remarkable diversity.

The new study focused on mountain forests in Panama that harbor hundreds of tree species, but which include small patches dominated by the tree species Oreomunnea mexicana. A team of US researchers focused on two types of fungi that form symbiotic relationships with trees: arbuscular mycorrhizas and ectomycorrhizas. Arbuscular mycorrhizas grow inside the roots of many different tree species, supplying phosphorus to their tree hosts. Ectomycorrhizas grow on the surface of tree roots and draw nitrogen from the soil, some of which they exchange for sugars from the trees. Ectomycorrhizas cooperate with only a few tree species, 6 % or less, of those that grow in tropical forests.

The researchers tested three hypotheses to explain the high abundance of Oreomunnea. First, they tested the idea that Oreomunnea trees are better able to resist species-specific pathogens than trees growing in more diverse forest areas. They actually found the opposite: The trees suffered more from pathogen infection when grown in soil from the same species than in soil from other species.

The researchers next tested whether mature Oreomunnea trees supported nearby Oreomunnea seedlings by sending sugars to them via a shared network of ectomycorrhizal fungi. But they found no evidence of cooperation between the trees.

In a third set of experiments, the team looked at the availability of nitrogen inside and outside the Oreomunnea patches. They found three-fold higher soil nitrate and ammonium concentrations outside than inside Oreomunnea-dominated forest and a correlation between soil nitrate and Oreomunnea abundance in plots.

Ectomycorrhizal fungi make some of their nitrogen available to the trees while starving other plants and soil microbes of this essential nutrient. The lack of adequate nitrogen means bacteria and fungi are unable to break down organic matter in the soil, causing most other trees to suffer because they depend on the nitrogen supplied by the microbial decomposers.