Woody trunks and branches of trees in the wet tropical rainforests of Queensland are losing their ability to absorb excess carbon dioxide.
That’s according to an analysis of 49 years’ worth of data, published in Nature today, which shows this “woody biomass” has switched from being a net carbon absorber to an emitter.
And this shift occurred about 25 years ago.
Ecophysiologist and study lead author Hannah Carle, from Western Sydney University, said the historical assumption has been wet tropical rainforests around the world and Australia were “carbon sinks”.
“Which means they’re taking in more carbon per annum than they release,” she said.
Woody biomass is part of this storage system, and sequesters carbon within the structure that forms the woody parts of trees.
But the new study showed woody biomass was now, on average across 20 study sites, a carbon source because carbon lost to trees dying and decaying outstripped the carbon gained by trees growing to replace them.
“Trees are dying more than they were decades ago and we attribute that change to climate change,” Dr Carle said.
This may be a sign that these Australian wet rainforests as a whole ecosystem were in decline, and could switch from being net carbon sinks to carbon sources in the future, according to the study.
“We have in this study evidence that Australia’s moist tropical forests are the first of their kind globally to to exhibit this [woody biomass] change,” Dr Carle said.
“And that that’s really significant. It could be a sort of canary in the coal mine.”
Wet tropical rainforests in Queensland have closed canopies which means little light gets through spaces between the treetops to the forest floor. (Supplied: Alexander Shenkin)
How do wet tropical rainforests store carbon?
Queensland has two main types of rainforest: wet and dry.
Wet rainforests are carbon dense with hundreds of species and layers of trees and plants crammed together with canopies that block out a lot of sunlight.
They cover about 1 million hectares of mountainous and coastal land from Cape York down to the Mackay region.
Intact rainforests are an important part of the world’s carbon cycle.
Trees take carbon dioxide or CO2 from the air and turn it into sugars to help them grow, with the oxygen we breathe created as a byproduct of this process called photosynthesis.
Carbon is also stored in leaves, trunks, branches and roots of trees, and roots transport carbon into soil too.
“Woody biomass is a large component of forest carbon, alongside pools of carbon in tree canopies and in soils and in other vegetation in the forest,” Dr Carle said.
When a tree is alive, carbon can be stored in its woody biomass for decades.
But when it dies, that carbon can be released into the atmosphere as it rots.
There aren’t enough new trees living long enough to replace the loss of woody biomass, resulting in a carbon dioxide storage shift. (Supplied: Andrew Ford)
Flicking a switch from storage to emitter
A previous study suggested wet rainforest trees in Queensland were dying at a higher rate because of climate-change-driven heat and drier air.
That study used the same monitoring sites as the new research, which was a collaboration between Australian and international universities and the CSIRO.
Each site is half a hectare in size in untouched patches of forest ranging in elevation from 15 metres to 1,200m above sea level.
Only trees more than 10 centimetres in diameter were monitored at the sites, which were established between 1971 and 1980, resulting in a dataset of 11,000 monitored trees representing 474 species.
The new study found woody biomass in the plots absorbed an average of 620 kilograms of carbon per hectare annually from 1971 to 2000.
But from 2010 to 2019, they emitted 930kg per hectare annually.
Such a switch hasn’t been seen in places like the Amazon rainforest, which is known as the “lungs of the Earth” and covers 600 million hectares of South America.
Although the Amazon as a whole is believed to have shifted from being a carbon sink to a carbon source, it’s had more tree growth due to rising levels of atmospheric CO2.
“Having elevated CO2 should enhance the capacity of a forest to absorb carbon and help mitigate climate change,” Dr Carle said.
“Perhaps it’s the case that the [Queensland] forests are not able to take advantage of that elevated atmospheric CO2 because they’re limited by what they can get from the soil.”
Dr Carle said it could be that other climate extremes found in an Australian setting could be limiting growth.
Sample sites ranged from low lying to mountainous areas. (Supplied: Alexander Shenkin)
Heather Keith, a Griffith University forest ecologist who was not involved in the study, said the fact the CO2 fertilisation effect was not observed in Queensland forests was a warning it could not be relied upon to maintain forests as a carbon sink.
“Other factors, such as moisture and nutrient limitation, either are currently or will increasingly in the future counteract the CO2 fertilisation effect,” she said.
Cyclones were also found in the study to suppress how much carbon could be absorbed by the forests, with less carbon stored by rainforests in the six years following a cyclone.
What does this mean for Queensland’s rainforests?
We still don’t know enough about what this all means for the future of Queensland’s wet tropical rainforests.
Dr Carle said it was extremely unlikely we end up with them suddenly becoming grasslands in the immediate future.
There could, however, be changes to forest structure, function and floral species diversity, which could also impact the animals that lived there.
While there is uncertainty, the new study suggested a decline of carbon storage in woody biomass in Australia could be a sign of things to come for rainforests elsewhere in the world.
But Dr Keith said responses by forests from climate change would differ regionally, “due to both the differing characteristics of the ecosystems and differing climate conditions”.
“In northern Queensland, forests are more likely to suffer from moisture limitation and cyclones than in some other regions,” she said.
“The relative impact of different factors may vary but all are important in understanding the changes in carbon dynamics of tropical forests.”
The sites used in the new study were established in the 1970s. (Supplied: Andrew Ford)
But if these changes do occur around the world, it could impact calculations for how much greenhouse gases we can still put in the air before exceeding targets to limit global warming.
“Maintaining the global carbon budget depends on retaining stable carbon stocks in ecosystems,” Dr Keith said.
“As well as reducing emissions from fossil fuels.”
Dr Carle said, ultimately, we can’t just assume ecosystems like forests would do the work for us of mitigating climate change.
“They’re actually in a delicate balance with the atmosphere,” she said.
“Forests need the right conditions in order to act as carbon sinks.”