DECOMPOSITION OF DEADWOOD DEBRIS LEFT OVER FROM TROPICAL RAINFOREST LOGGING OPERATIONS

点击翻译按钮获取中文摘要

The widespread and rapid logging of tropical forests may mean their global carbon emissions have been underestimated because they do not fully account for the dead wood left behind. Living trees take in carbon dioxide whereas dead and decaying ones release it. It is the balance between these two processes that determines whether a large area of forest is a source of carbon dioxide, or a 'sink' that helps to absorb carbon dioxide from the atmosphere. Partially-logged tropical rainforests are probably emitting more carbon than assumed because they contain a high proportion of dead wood. Loggers will typically leave around one quarter of all the trees they cut, or that get damaged during logging operations, to rot in the forest. This dead wood can make up almost two third of the biomass, the biological material found above ground, in logged forests. Compare that proportion to untouched forests, where dead wood is created through natural processes, and where it makes up around 20 per cent of the total aboveground biomass. Around one third of all tropical forests have been logged and that proportion is increasing rapidly, meaning tropical forests contain much larger amounts of dead wood than previously thought. Learning how rapidly that dead wood will decompose is a key ingredient for calculating the carbon balance of tropical forests. And the carbon balance of forest could, in turn, hold the key to combating climate change.We don't know if dead wood in a logged forest will take longer to decompose than dead wood in an untouched forest, and getting that answer is a complicated business. Not only do we need to know how fast the dead wood left behind by the loggers will decompose, we also need to know how fast new pieces of dead wood are created. And logged forest creates a lot because the trees left behind in logged forests live and die faster than the trees the loggers take away, and when they die they turn into dead wood. One of the reasons these trees live and die so fast is that they have low wood density - their wood is relatively soft meaning it is fast to grow - and soft wood decomposes faster than hard wood so at first glance we would expect the dead wood in logged forests to decompose rapidly. But it's not so clear cut. One of the most important animal groups in tropical rainforests are termites, many of which are specialist dead wood eaters, and the amount of wood these millions of small animals munch their way through is one of the key reasons why decomposition happens so fast. Termites have very soft bodies and don't respond well to logging. Logging removes a lot of the forest canopy, opening it up to the sun and making the forest floor a hotter and drier place. Because of that, logged forests only have about one third the number of termites that you find in untouched forest and that, in turn, means dead wood might decompose much slower.This new research sets out to discover whether the high speed life and death of trees in logged forests counterbalances the loss of termites, or if one of these forces is stronger and outweighs the other. The answer is important. Protecting tropical forests is one of the most cost-effective ways for taking carbon dioxide out of the atmosphere. But by leaving behind large piles of dead wood, and potentially slowing the rate at which that dead wood decomposes, logging of those forests has the potential to render this key mechanism for combating climate change ineffective.