Habitat fragmentation disrupts forest carbon cycles

We all know that fragmenting forests is bad for biodiversity.  But it's also bad for the planet -- because it screws up carbon cycles, makes forests more likely to burn, and promotes global warming. 

  Fires... not natural in rainforests  (photo by Mark Cochrane)

Fires... not natural in rainforests (photo by Mark Cochrane)

Prior studies, including those led by ALERT director Bill Laurance and ALERT member Tom Lovejoy, have shown that fragmented forests in the Amazon lose a lot of their carbon.  This is evidently because the hot, dry conditions near forest edges kill many trees.  Additional trees are snapped or toppled by wind gusts near edges.

This is bad news because the carbon stored in the trees eventually decomposes and ends up as carbon dioxide and other greenhouse gas emissions. 

Laurance and colleagues estimated that the fragmentation of tropical forests creates up to 150 million tons of atmospheric carbon emissions per year -- equivalent to the entire annual emissions of Great Britain.  And this 'fragmentation effect' is on top of the massive carbon emissions that result from forest destruction.

And now a new study shows that habitat fragmentation also slows down the rate of organic decomposition.  This occurs because the warm, dry conditions near forest edges inhibit wood-eating fungi, which are important decomposers.  In the new study, the rate of decomposition near forest edges was about half that in forest interiors.

This means that dead trees, limbs, leaves, and other woody material will accumulate near edges.  Why is this important?  What happens if you take a lot of fine woody material and dry it out? 

It becomes very, very easy to burn.

In the Amazon, fragmented forests are hugely vulnerable to fires.  Not only does forest fragmentation create a lot of dry, flammable material near forest edges, but cattle ranchers like to burn the pastures surrounding the fragments -- to control weeds and produce a flush of new grass for their cattle.  Many of these fires burn the forests as well.

The results can be devastating.  Satellite images show that fragmented forests in drier parts of the Amazon virtually 'implode' in the first few years after fragmentation -- the result of a withering recurrence of destructive, edge-related fires. 

The bottom line: One of the best things we can do for forests is not to fragment them. 

This isn't just about conserving a few pretty birds or butterflies.  By reducing forest fires and rampant carbon emissions, it's about keeping our planet more livable for all of us.