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Driven by climate change, forests in Australia’s tropical north have become the first in the world to transform from carbon sink to a source of emission, a study has found.
The study was published by scientists from the Australian National University in the journal Nature. They found the change in the forests located in northeastern Australia, including the state of Queensland, began about 25 years ago in the year 2000. The findings have direct implications on vegetation estimates and determining the carbon budget to ensure that global temperatures remain under 2 degrees Celsius.
But data suggests the transition may have begun around 1998.
The scientists used long-term forest inventory data from 1971 to 2019 to assess carbon balance on woody aboveground standing biomass (AGB), the demographic process accounting to it and the climatic drivers (including cyclones) influencing them.
It found that from 1971 to 2000, these forests functioned as a carbon sink, accumulating carbon at a rate of 0.62 ha (megagrams) of carbon accumulate over an area of one hectare for one year (Mg Cha).
But the shift to carbon source occurred when the same was recorded to be -0.93 Mg Cha annually, with sink capacity declining at a rate of 0.041 Mg C ha for the same time frame.
Lead author of the study Hannah Carle, from Western Sydney University, said in a press statement that the capacity of woody biomass to continue working as a carbon sink is at risk.
“The change our study describes is largely due to increased tree mortality driven by climate change, including increasingly extreme temperatures, atmospheric dryness and drought. Regrettably, the associated increase in carbon losses to the atmosphere has not been offset by increased tree growth. This is surprising because higher carbon dioxide levels should make it easier for plants to scavenge carbon dioxide from the air, leading to more tree growth and greater carbon sink capacity,” she noted.
The study found that the rising temperatures and associated anomalies in VPD (vapour pressure deficit) and MCWD (maximum climatological water deficit) are implicated as likely drivers of increasing AGB loss, often acting as indirect effects of temperature. These climate extremes have increased in magnitude and frequency over time, driven by anthropogenic climate change.
The escalating losses were linked to increasing tree mortality and the carbon gains from biomass growth did not offset them.
Forest dynamics were further influenced by cyclones, which negatively affect the carbon balance, shrinking the sink capacity in the six years following a cyclone. The study noted that on average, cyclones increased the mortality rate above background levels by 19 per cent.
Carle said, “This is cause for concern, with cyclones projected to become increasingly severe under climate change, and to impact areas further south, affecting additional stretches of forest to a potentially greater extent.”
“Although projections remain highly uncertain, cyclone regimes are expected to become more severe and possibly more frequent in response to future climate change forcing. Although cyclones increased per-hectare mortality by a similar magnitude to 39 years of increasing VPD (1980-2019), shifting climate may alter forest carbon fluxes more strongly than cyclones when considered at the regional scale,” the study said.
However, cyclones were not the sole drivers of the shift as long-term trend of escalating AGB loss was evident during the analysis.