DECOMPOSITION OF DEADWOOD DEBRIS LEFT OVER FROM TROPICAL RAINFOREST LOGGING OPERATIONS

热带森林的广泛和快速砍伐可能意味着它们的全球碳排放量被低估了，因为它们没有充分考虑到留下的枯木。活的树木吸收二氧化碳，而死的和腐烂的树木释放二氧化碳，这两个过程之间的平衡决定了大片森林是二氧化碳的来源，还是帮助吸收大气中二氧化碳的“汇”。部分砍伐的热带雨林可能比假设的排放更多的碳，因为它们含有很高比例的枯木。伐木工人通常会留下大约四分之一的树木，他们削减，或在伐木作业中受损，在森林中腐烂。这些枯木几乎可以构成三分之二的生物量，即在被砍伐的森林中发现的地面上的生物材料。将这一比例与未受影响的森林相比，在未受影响的森林中，枯木是通过自然过程产生的，占地上生物量总量的20%左右。大约三分之一的热带森林已经被砍伐，而且这一比例正在迅速增加，这意味着热带森林中的枯木比以前想象的要多得多。了解枯木分解的速度是计算热带森林碳平衡的关键因素。而森林的碳平衡反过来又是对抗气候变化的关键。我们不知道被砍伐的森林中的枯木是否比未被砍伐的森林中的枯木需要更长的时间来分解，得到这个答案是一件复杂的事情。我们不仅需要知道伐木工人留下的枯木分解的速度，我们还需要知道新的枯木产生的速度。砍伐的森林创造了很多东西，因为被砍伐的森林里留下的树木比砍伐者带走的树木活得更快，死了之后就变成了枯木。这些树木存活和死亡如此之快的原因之一是它们的木材密度低-它们的木材相对较软，这意味着它生长得很快-软木比硬木分解得更快，所以乍一看，我们会认为砍伐森林中的枯木分解得很快。但这不是那么明确。热带雨林中最重要的动物群体之一是白蚁，其中许多是专门的枯木食者，这些数百万的小动物咀嚼的木材数量是分解如此之快的关键原因之一。白蚁的身体非常柔软，对伐木反应不佳。伐木活动使森林的树冠层大量消失，暴露在阳光下，使森林地面变得更加炎热干燥。正因为如此，砍伐森林中的白蚁数量只有原始森林中的三分之一，这意味着枯木分解的速度可能会慢得多。这项新的研究旨在发现砍伐森林中树木的高速生死是否抵消了白蚁的损失，或者是否其中一种力量更强大，超过了另一种力量。答案很重要。保护热带森林是将二氧化碳从大气中清除的最具成本效益的方法之一。但是，由于留下了大量的枯木，并可能减缓枯木分解的速度，这些森林的砍伐有可能使这一应对气候变化的关键机制失效。

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.