An international research team has discovered that forest fragmentation poses an even greater threat to Amazonian biodiversity than previously thought. Their findings, to be published next week (27 November 鈥� 1 December) in the journal Proceedings of the National Academy of Sciences USA, summarizes key findings from the world鈥檚 largest and longest-running experimental study of habitat fragmentation.

The Amazon contains the planet鈥檚 most biologically diverse tree communities, with up to three hundred species occurring in an area the size of just two football fields. These forests are being rapidly felled and fragmented for timber operations, cattle ranches and industrial soy farms.

The team, led by William Laurance of the Smithsonian Tropical Research Institute in Panama, has been studying the fates of nearly 32,000 Amazonian trees since 1980.

The most striking finding, say the authors, is the remarkable speed at which tree communities are changing in forest fragments.

鈥淩ainforest trees can live for centuries, even millennia,鈥� said Laurance, 鈥渟o none of us expected things to change too fast. But in just two decades鈥攁 wink of time for a thousand year-old tree鈥攖he ecosystem has been seriously degraded.鈥�

The main driver of these changes, say the authors, is ecological changes near the margins of forest fragments. 鈥淲hen you fragment the rainforest, hot winds from the surrounding pastures blow into the forest and kill many trees, which just can鈥檛 handle the stress,鈥� said Henrique Nascimento, a team member from Brazil鈥檚 National Institute for Amazonian Research in Manaus. 鈥淎lso, winds build up around the fragment and knock down a lot of trees.鈥�

The trees that regenerate in their place are very different from the trees that died. 鈥淲hen you fragment a forest, the winners are common pioneer and generalist species that like forest disturbance,鈥� said Laurance. 鈥淭he losers are rare, slow-growing tree species that provide fruit, nectar, and homes for a diversity of rainforest animals.鈥�

To understand how fragmentation is affecting the trees, the team studied 22 different characteristics of the increasing and declining species. 鈥淥ur results show that tree communities in fragments are being completely restructured,鈥� said Nascimento. 鈥淢ost vulnerable are trees specialized for living in the dark forest understory that need animals such as birds or bats to disperse their seeds and pollen.鈥�

Fragmentation is also changing the dynamics and structure of the forest. Tree communities in fragments are highly unstable, losing and gaining species at a high rate. Fragments also tend to lose many of their large trees and become dominated by small, fast-growing species.

Forest fragmentation may even increase global warming. The authors demonstrate that the small, fast-growing trees that proliferate in fragments contain less biomass, and hence store less carbon, than do the original rainforest trees they replaced. The carbon from the dead rainforest trees is broken down by microbes and fungi to become carbon dioxide, the most important greenhouse gas.

鈥淔ragmentation is affecting the forest in a lot of ways,鈥� said Laurance. 鈥淭hese changes occur remarkably fast, and when you completely alter something as basic as the trees, the other species that live in the rainforest will surely be affected too.鈥�

Source: Smithsonian Tropical Research Institute