From above the Amazon rainforest may look like an endless, uniform sea of greenery, but it turns out there are sharp lines through it separating very different ecosystems with distinct inhabitants. And these lines are drawn by the region's geology.
An innovative study published in Journal of Biogeography and led by Mark Higgins of Duke University is the first to combine large-scale data from satellites with painstaking work on the ground, sampling the plant types found in particular areas.
It shows an abrupt boundary between two distinct kinds of forest, running some 300km through northern Peru. The method also reveals a similarly sharp disjunction in western Brazil, running from north to south for more than 1500km. The researchers suggest this effectively marks the boundary between western and central Amazonia.
The earth is very different on either side of these boundaries. On average the soil on one side contains 15 times as many cations - tiny particles that plants need for nourishment - as that a short distance away on the other side. The plant communities living in these different kinds of soil are almost completely distinct.
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