Trees release carbon during heatwaves

(ORDO NEWS) — The tall eucalyptus forests of South Tasmania are exceptionally good at taking carbon dioxide out of the atmosphere and turning it into wood.

For many years, we thought that these forests had an adequate margin of safety against climate change due to the cool, humid environment.

Unfortunately, the results of my research published today show that these forests are closer to the edge than we had hoped. I have found that these forests go from taking in carbon to releasing it during a heat wave.

This is not good news, as it is expected that as the planet warms, the number of heat waves will only increase. As we work to reduce emissions, we need to explore ways to make these vital forests more resilient.

From the intake of carbon dioxide to its removal

Through forest sampling, it has been determined that a humid, cool environment, such as in southern Tasmania, provides ideal conditions for the growth of tall eucalyptus forests.

We thought that these types of forests would buffer against the worst effects of coming climate change and perhaps even benefit from limited warming.

But this is no longer the case.

I watched what happened to a forest of prickly eucalyptus (Eucalyptus obliqua) during a three-week heat wave in November 2017. Under these conditions, the forest has become a net source of carbon dioxide, each hectare emitted about 10 tons of greenhouse gas during this period.

A year earlier, under more normal conditions, the forest was a net sink of carbon dioxide, absorbing about 3.5 tons per hectare.

How do we know this? The forest I studied is in the Warra Supersite in the upper Huon Valley, one of 16 intensive ecosystem monitoring field stations in the Australian Terrestrial Ecosystem Research Network.

Instruments mounted on an 80-meter-high tower in Warr give us the opportunity to understand how the forest behaves. We can measure how much and how quickly carbon dioxide, water and energy move between the forest and the atmosphere.

What actually happened in the forest during the heat? Two important things.

First, the forest exhaled more carbon dioxide. This is to be expected, since the living cells of all air-breathing life forms (yes, this includes trees) respire more actively when it gets warmer.

But the second turned out to be very unexpected. The ability of the forest to photosynthesize has decreased, meaning less solar energy has been converted into sugars. This happened at a time when the trees were rapidly transpiring (giving off water vapor).

So far, we’ve seen photosynthetic productivity drop during heatwaves because trees try to limit water loss. To do this, they close the pores on the leaves (stomata). When a tree closes its stomata, it makes it difficult for carbon dioxide from the air to reach the leaves and stimulates the process of photosynthesis.

On the contrary, during this heat wave, the trees released water and at the same time produced less food.

So what’s going on? In short, the temperature was simply too high for the forests of southern Tasmania. Every forest has an ideal temperature for the best photosynthesis results. We now know that in Australia this temperature is related to the historical climate of the local region.

This means that the trees in Warra require cooler temperatures for optimal nutrition compared to most other Australian forests.

During the heat wave of 2017, the temperature went far beyond the comfort zone of the forest. During the hottest part of the day, the forest could no longer produce enough food to feed itself.

Out of the comfort zone of the forest

So far, the forest in Varra remains untouched. After the heat wave, the dwarf bark forest quickly regained its ability to feed and again became a carbon sink.

But as the climate warms, these forests will move more and more outside their comfort zone. They can only withstand so many of these heatwaves. If they continue, there will come a turning point, after which the forest will no longer be able to recover.

What then? We can see a disturbing glimpse when we look at Tasmania’s oceans, which are a hotspot for marine heatwaves. About 95% of Tasmania’s giant kelp have already disappeared, dying from temperatures they can’t handle.

It is no exaggeration to say that the temperature rise is the most serious threat to the health of high eucalyptus forests that I have encountered in 40 years of studying the health and threats of forests in Tasmania.

Unlike kelp, our tall eucalyptus forests have not yet reached breaking point. We still have time to reduce the risk that global warming brings.

Work is already underway to test new promising methods to make future forests more adaptable to the new climate in which they find themselves.

These methods include climate-adjusted provence, where arborists plant seeds of native species collected in hotter climates. Another method they are trying to apply to the giant kelp is to find individual plants with better heat tolerance and select them.

Our eucalyptus forests will need our help more and more. The more we are engaged and informed about the risks to forests that we have long considered very resilient, the more likely we will be able to conserve them.

One way to do this is to make real-time monitoring data publicly available so we can understand the stress our forests are under as the climate warms.


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