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In recent years, the Great Barrier Reef off Australia’s northeast coast has seen multiple events of mass coral bleaching as human-caused global warming has driven sustained high temperatures in the ocean.
The Coral Sea is not the only natural wonder: the rainforests listed in the World Heritage list are another. Wet tropics Queensland.
It turns out that the same climate change factors that contribute to coral bleaching have also affected the trees that live in these magnificent tropical rainforests.
Our co-authors and we discovered in new research that These trees have twice the mortality rateThe reason for this is likely warmer air and greater drying power. These trees, like coral reefs provide energy, nutrients, and structure to their diverse ecosystems.
A 50-year-old record
Our study was based upon 20 plots of trees in rainforests of northeast Queensland. These were created and monitored as part of a project that began in 1971 by Geoff Stocker, a forest scientist.
These plots were later incorporated into Wet Tropics of Queensland World Heritage Area. Monitoring has been done by CSIRO scientists based out of Atherton in Queensland.
The plots are typically half a hectare (5,000m²) in size. The species and diameter of any trees with a diameter greater than 10 cm at breast height were recorded in each plot.
Alexander Schenkin, Author provided
The plots were checked at intervals of two to five years. The tree diameters were again recorded, as well as any new trees that had grown to the 10+ cm size class and any trees that had already died.
A few more plots were created over the years and contributed to our analyses. These 20 plots were the most extensive and formed the basis of the dataset.
The tree’s life expectancy
With many plots visited multiple times, and many tree species on each plot, we were able to estimate the average percentage of trees in each species that died in a given year (the “annual mortality rate”). We also looked at how the rate has changed over time.
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The average annual mortality rate was 1% until the mid 1980s. This means that every tree had an approximately one in 100 chance for dying in any given year.
This is equivalent to a tree’s average lifespan of approximately 100 years.
However, the annual mortality rate began rising in the mid-1980s. The average annual mortality rate had increased to 2% by the end of the 2019 dataset.
These results match Similar pattern for tree deathsThe increase in tree mortality in the Amazon rainforest may also be occurring simultaneously, which could indicate that the problem is widespread.
A doubled annual mortality rate means trees are not living as long as they used to. This means that they are only storing carbon half as long.
If the trend we observed is indicative for tropical forests in general, it could have huge implications for the ability of tropical forests to absorb carbon dioxide emissions from humans.
Thirsty Air
What is causing the rising mortality rates of tropical trees?
Temperature stress is a possible first thought: The average air temperature in the plots has risen in recent decades.
We did not find that temperature was directly responsible for the rising mortality rates. Instead the mortality rates correlated better with the drying power or “thirstiness” of the air, which scientists call the “air vapour pressure deficit”.
You’re probably familiar with the idea of relative humidity. It tells you how much water vapour is in the air as a percentage of what the air can hold.
Alexander Schenkin, Author provided
When temperatures rise, the air’s capacity to hold water vapour increases exponentially. Each degree of heating allows the air to hold approximately 7% more water vapour.
If the air temperature rises and the relative humidity remains the same, the air will have more capacity to absorb water vapour.
This is the result of global warming. The air temperature has increased, the relative humidity has remained roughly constant, and it has become more thirsty.
This means the drying power of the atmosphere (or “evaporative demand”) has increased. This is what we believe best explains the rising mortality rates in Australian tropical trees.
What’s next
If greenhouse gas emissions are not curtailed, both the temperature and the deficit in air vapour pressure will continue to rise. Our research suggests that this will likely cause an increase in the mortality rates of tropical rainforest tree species.
As coral reefs, the tropical rainforests could experience rapid changes in their species composition, biodiversity, three-dimensional structure, and other factors that could threaten these precious ecosystems. This threat can be mitigated by reducing urgently the emissions of carbon dioxide, and other greenhouse gases, to slow global warming and stabilize the global climate system.
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