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Southern Tasmania’s tall eucalyptus forests are exceptionally good at taking carbon dioxide from the atmosphere and converting it into wood.
These forests have been believed to provide a reasonable buffer against climate change for many years. This is due to their cool, moist environments.
Unfortunately, my Today’s researchThese forests are closer than we thought to the edge. During heatwaves, I discovered that these forests switch from taking carbon in to pumping it out.
That’s not good news, given heatwaves are only expected to increase as the world heats up. We must work to reduce carbon emissions, but we also need to find ways to make these vital forests more resilient.
From carbon dioxide in, to carbon out
It’s well established from forest samplingThe ideal conditions for tall eucalypt forests are found in moist, cool areas like southern Tasmania.
We had hoped that these types forests would become a reality. Have a bufferYou can help mitigate the worst effects of climate changes and even gain from some limited warming.
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This is not the case anymore.
I observed what happened to stringybark, the messmate.Eucalyptus obliquaDuring a three-week heatwave that occurred in November 2017, the forest was covered in carbon dioxide. These conditions led to the forest becoming a net source for carbon dioxide. Each hectare released close to 10 tonnes of the greenhouse gases over the three-week period.
One year earlier, under more normal conditions, the forest was a net source of carbon dioxide, absorbing around 3.5 tonnes per ha.
How do we know this? The forest I studied is at the Warra Supersite in the upper reaches of the Huon Valley, one of 16 intensive ecosystem monitoring field stations making up Australia’s Terrestrial Ecosystem Research Network.
An 80-metre tall tower at Warra houses instruments that give us great insight into the forest’s behavior. We can measure the amount and speed of the carbon dioxide, water, and energy shuttle between the forest & the atmosphere.
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As climate change worsens worldwide and more severe fires are common, Australian forests will be less carbon-rich.
What happened in the forest during that hot spell? These are two crucial facts.
The first was that the forest exhaled more carbon dioxide. This was to be expected as all living cells in all forms of air-breathing lifeforms (yes this includes trees) are capable of expending more carbon dioxide.
As temperatures rise, breathe in more.
The second surprise was quite unexpected. The forest’s ability to photosynthesise fell, meaning less solar energy was converted to sugars. This occurred while the trees were rapidly transpiring (releasing water vapour).
Until now, we’ve seen falls in photosynthesis output in heatwaves because the trees are trying to limit their water loss. This is done by closing the pores of their leaves (stomata). It makes it more difficult for carbon dioxide in the air to enter the leaves and fuel photosynthesis by closing its stomata.
This heatwave, however, saw trees release water and produce less food.
So what’s going on? The temperatures in southern Tasmania were simply too high. Every forest has the ideal temperature for photosynthesis. This temperature is what Australia has. This is in relation to the historic climateYou can find out more about the area.
Warra’s trees require lower temperatures than most Australian forests to thrive.
During the 2017 heatwave, the temperatures soared well outside the forest’s comfort zone. The forest couldn’t produce enough food during the hottest hours of the day.
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Outside the forest’s comfort zone
The Warra forest is still intact for now. After the heatwave the messmate stringybark forests quickly recovered their ability to feed themselves and became a carbon sink.
As the world heats up, these forests will be forced to move further from their comfort zones. They can only withstand so many heatwaves. If they keep coming, they will become more severe. Tipping pointBeyond this point, the forest is not able to recover.
What then? We can see a disturbing glimpse when we look at Tasmania’s oceans, which are a marine heatwave hotspot. Fully 95% of Tasmania’s giant kelp forests are now gone, Get killedThey are unable to tolerate temperatures above their limits.
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It is no exaggeration to say that the rapid increase in temperatures are the most serious threat to the health of tall eucalypt forests I’ve encountered during 40 years of studying forest health and threats in Tasmania.
Our tall eucalyptus forest has not yet reached its tipping point, unlike the kelp and kelp forests. We still have plenty of time to reduce the dangers that global warming poses.
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Can selective breeding of super kelp’ save our cold water corals from the heat?
It is already possible to test promising new methods to make future forests more able to adapt to the changing climate.
These techniques include climate adjusted provenancingThis is where forest managers sow seeds from local species that have been collected from the hottest end of their range. Another possibility for giant kelp was another. Individual plantsThey are more resistant to heat and can be bred.
Our support will be more important for the survival of eucalyptus trees. The more we are informed about the risks forests face, the more likely we will be able preserve them.
We could do this by making our monitoring data public in real-time, so that we can see the strain on our forests as the world heats.
