For more than 200 years, scientists have known that tree diversity — the number of different types of trees found near each other — is highest near the equator and diminishes moving into the middle and higher latitudes. The conventional explanation for this decline in local species richness has been temperature and precipitation.
A mammoth project, organized by Purdue University and involving scientists from 50 nations, says multiple factors, such as landforms, soil and human impact, control local species richness — especially in the tropics. Purdue associate professor Jingjing Liang said the combination of factors was “one of the surprises we found, especially in the tropical region.”
The findings, published in the journal Nature Ecology and Evolution, were drawn from 1.3 million sample plots and 55 million trees archived in the Global Forestry Biodiversity Initiative. There are an average of 98 tree species per hectare (2.47 acres) in the tropics and four per hectare 50 degrees north or south of the equator. The most diverse tropical forests, in Amazonia, have more than 200 tree species per hectare.