Severe conditions caused rainforest to suck in lower levels of carbon dioxide, raising concern over future climate impacts
By Gerard Wynn
Severe drought five years ago caused an observed doubling in the rate of tree mortality in the Amazon rainforest, according to a study published in the journal Nature on Wednesday.
In addition, the drought caused the forest to take up about 1.4 billion tonnes less carbon dioxide from the atmosphere.
The Amazon forest acts as a carbon sink, because trees suck the greenhouse gas CO2 out of the atmosphere as they grow, during photosynthesis, converting this to plant matter including bark, wood and roots.
The Nature article showed how droughts may disrupt this carbon sink, both as a result of reduced photosynthesis and tree die-back. The study may be a concern, if climate change in future caused more frequent, severe droughts.
The main finding of the study was that trees still grew even during a severe drought, but that was at the expense of using energy for tissue maintenance, defence and putting aside sugars against future stresses.
Weakened maintenance may have contributed to greater death tree mortality observed in the years following the drought, the authors said.
They found that rates of tree mortality at least doubled in the drought-affected areas, to 4-7%, from a long-term trend of 1.6%.
“Our data indicate that mortality rates peaked one to two years after the drought, consistent with the hypothesis that trees were weakened during the drought from decreased maintenance but only succumbed later.”
Trees had to ration their energy use in this way because of lower rates of photosynthesis through the drought.
“Both leaf-level and plot-level measurements indicate that severe drought suppresses photosynthesis,” said the authors, who were based in research institutes in Europe, Australia and South America.
“Scaling (our) measurements to the entire Amazon basin with rainfall data, we estimate that drought suppressed Amazon-wide photosynthesis in 2010 by 0.38 petagrams of carbon (equivalent to 380 million tonnes carbon, or 1.4 billion tonnes of CO2).”