Decaying forest wood transmits alarming rates of carbon
If you've wandered through a forest, you've probably dodged dead, rotting branches or stumps scattered on the ground. This is "deadwood", and it plays several vital roles in forest ecosystems. It provides habitat for small mammals, birds, amphibians and insects.
But there's another important role we have little understanding of on a global scale: the carbon deadwood releases as it decomposes, with part of it going into the soil and part into the atmosphere. Insects, such as termites and wood borers, can accelerate this process. The world's deadwood currently stores 73 billion tonnes of carbon. The research calculated that 10.9 billion tonnes of this (around 15%) is released into the atmosphere and soil each year — a little more than the world's emissions from burning fossil fuels.
But this amount can change depending on insect activity, and will likely increase under climate change. It's vital deadwood is considered explicitly in all
A global effort
Forests are crucial carbon sinks, where living trees capture and store carbon dioxide from the atmosphere, helping to regulate climate. Deadwood — including fallen or still-standing trees, branches and stumps — makes up 8% of this carbon stock in the world's forests. Our aim was to measure the influence of climate and insects on the rate of decomposition — but it wasn't easy. Our research paper is the result of an extraordinary effort to co-ordinate a largescale cross-continent field experiment. More than 30 research groups worldwide took part.
Wood from more than 140 tree species was laid out for up to three years at 55 forest sites on six continents, from the Amazon rainforest to Brisbane, Australia. Half of these wood samples were in closed mesh cages to exclude insects from the decomposition process to test their effect, too. Some sites had to be protected from elephants, another was lost to fire and another had to be rebuilt after a flood.
What was found?
The rate of deadwood decay and how insects contribute to it depend very strongly on climate. It increased primarily with rising temperature, and was disproportionately greater in the tropics compared to all other cooler climatic regions. The interactions between insects and climate on deadwood carbon release makes future climate projections a bit tricky.