Australian tropical rainforest #trees switch in world first from #carbon sink to #emissions source
Researchers say carbon emissions change in #Queensland tropical rainforests may have global climate implications
Australian tropical rainforest trees have become the first in the world to switch from being a #CarbonSink to an #emissions source due to increasingly extreme #temperatures and drier conditions.
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The change, which applies to the trees’ trunks and branches but not the roots system, began about 25 years ago, according to new research published in Nature.
Trees store #carbon as they grow and release it when they decay and die. Overall, #TropicalForests are thought to be #CarbonSinks – absorbing more #CO2 than they release – and uptake is assumed to increase amid rising atmospheric concentration.
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Tropical forests act as important global carbon sinks1, and Earth System Models predict increasing near-term carbon sink capacity for these forests, with elevated atmospheric carbon dioxide concentration thought to stimulate tree growth2,3. However, current forest inventory data analyses suggest that the carbon sink capacity of intact tropical forests may be in decline, portending a possible future switch from carbon sinks to carbon sources3–7. Here we use long-term forest inventory data (1971–2019) from Australian moist tropical forests and a causal inference framework8–10 to assess the carbon balance of woody aboveground standing biomass over time, the demographic processes accounting for it, and its climatic drivers, including cyclones. We find that a transition from sink (0.62 ± 0.04 Mg C ha−1 yr−1: 1971–2000) to source (−0.93 ± 0.11 Mg C ha−1 yr−1: 2010–2019) has occurred for the aboveground woody biomass of these forests, with sink capacity declining at a rate of 0.041 ± 0.032 Mg C ha−1 yr−1. The transition was driven by increasingly extreme temperature and other climate anomalies, which have increased tree mortality and associated biomass losses4, with no evidence of the carbon fertilization (stimulation) of woody tree growth. Forest dynamics underlying carbon sink capacity were also punctuated by cyclones, with impacts of a similar magnitude to long-term climate-induced changes. Our findings suggest the potential for a similar response to climate change by woody aboveground biomass in moist tropical forests globally, which could culminate in a long-term switch from carbon sinks to carbon sources. A transition from carbon sink to source for the aboveground woody biomass of moist tropical Australian forests has occurred, driven by increasingly extreme climate anomalies.
But nearly 50 years of data collected from tropical forests across #Queensland has revealed this crucial #CarbonSink could be under threat.
About 25 years ago, tree trunks and branches in those #forests became a net emitter, with more #trees dying and insufficient new growth, according to the research.
“It’s the first #TropicalForest of its kind to show this symptom of change,” said lead author #DrHannahCarle from Western Sydney University.
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“We know that the moist tropics in #Australia occupy a bit of a warmer,drier #climate space than tropical forests on other continents, and therefore it might serve as a future analog for what tropical #forests will experience in other parts of the world.”
#ProfAdrienneNicotra from the Australian National University,a co-author of the study, said it remained to be seen whether Australia’s tropical forests were a harbinger for other tropical forests globally and further research was needed.
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But if so, the findings could have significant implications for global #ClimateModels, carbon budgets and climate policies.
“This paper is the first time that this #TippingPoint of a switch from a #CarbonSink to a carbon source in tropical rainforests has been identified clearly – not just for one year but for 20 years,” said #ProfDavidKaroly , an emeritus professor at the #UniversityOfMelbourne and an expert in #ClimateChange science.
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Even though the balance between gains and losses had shifted, these #forests were still playing an important role in soaking up #CO2, #Karoly said. But their reduced capacity to absorb extra #carbon would make #emissions cuts “a lot harder” and require an even more rapid transition away from #FossilFuels.
The analysis drew on a unique set of #ForestData dating back to 1971, including records tracking roughly 11,000 #trees across 20 forest sites in #Queensland.
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It considered the #carbon stored above ground in tree trunks and branches but not the gains and losses below ground from #soil and #roots.
#DrRaphaelTrouve, who studies forest dynamics at the #UniversityOfMelbourne and was not involved in the study, said the research highlighted the value of collecting and maintaining long term data.
Trouve’s own research published earlier this year, which found #MountainAsh forests were thinning rapidly under drier and hotter conditions,
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