What happens to a poor tree when you withhold rain for a whole month, then hit it with four days in a row of 43C temperatures? It was so hot, some of the leaves on these trees got close to 49-50 °C.
In at least one gum species in Australia, the answer is “not much”. They suck up lots of water from their deep roots and sweat it out til the heatwave passes. The trees become evaporative coolers “siphoning up” water. They cope so well, that not only did the trees not die, but their trunk and height growth were unaffected. Indeed, only about 1% of the leaf area even exhibited browning.
Whole tree chambers in Richmond, New South Wales, Australia. Twelve 9-m-tall chambers in a field setting (a) enclose the canopies of individual Eucalyptus parramattensis trees rooted in soil (b). Two heatwave chambers can be seen on the left of the infrared image, along with several control chambers (c; temperature in °C).
But with global warming running at a heady 0.13C per decade, you might wonder how many years will it take for the trees to adapt?
From the paper — “one day”:
The gums rapidly increased their tolerance for extreme heat, the researchers found. Within a day the threshold temperature for leaf damage had increased by 2C.
Righto. At the current rate of warming, the world might get two degrees hotter in 150 years. So these trees can adapt 55,000 times faster.
The researchers say the trees were not just likely, but remarkably good with heatwaves:
“We conclude that this tree species was remarkably capable of tolerating an extreme heatwave via mechanisms that have implications for future heatwave intensity and forest resilience in a warmer world.”
This research (yet again) fits the hypothesis that life on Earth is well adapted to a wildly variable climate, probably because it happened all the time. The researchers even looked to see if exposing trees to hot weather first would help adapt them to extreme heat, but found it didn’t matter. The trees ability to adapt was innate. They just coped.
The models didn’t predict this
As the trees transpired more, they also stopped photosynthesising — they shut down in a survival mode. This breaks a pretty long standing biology rule, and thus breaks most plant growth models (and some climate ones too). It’s pretty central to plant biology, leaves give up water to bring in CO2. As plants transpire more, they absorb more CO2 and turn it into carbohydrate (i.e. more plant) which is photosynthesis. We now know that rule breaks under extreme heat when trees take a sauna-break, stop working, and just … sweat. I’d probably do the same if my leaves were 50C.
As usual in the news, no one mentions that the models were totally wrong on this, they just say, they found “the opposite” and it needs revising.
Scientists have long known about this evaporative cooling mechanism, known as transpiration. But current climate models suggest transpiration is closely related to trees’ photosynthesis rates, and that it declines during heatwaves.
The researchers found the opposite, with photosynthesis all but stopping but water use increasing.
“Our dynamic global vegetation models, particularly those that simulate the exchange of CO2 and water vapour between land and the atmosphere, will need to be revisited in light of these findings,” Professor Tjoelker said.
Then there is The Caveat we’ve come to expect. Good climate news always has a bad news rider:
He said it was a “good news-bad news story”, suggesting that scientists had underestimated gum trees’ resilience but over-estimated their carbon fixing capacity.
Since the trees kept on growing after the heatwave, any loss of carbon fixation measured in days or hours, seems pretty minor in the planetary scheme of things.
The temperature of leaves during an extreme experimental heatwave in the Austral Spring-Summer. Leaf temperature (Tleaf) measured in the upper canopy of each tree was strongly correlated with air temperature (Tair; a). Points reflect 15-minute averages of data collected during high light conditions (PPFD > 500 μmol m-2 s-1) from 2016-10-31 through
2016-11-03. The dashed line shows 1:1, the solid black line was fit to the data: (Tleaf = 1.38 + 1.02 * Tair – 0.0012 * Tair 2, P< 0.001, r2 = 0.8), and the solid red line shows the predicted leaf temperatures by the photosynthetic model. Histograms of Tleaf in high light conditions (PPFD > 500 μmol m-2 s-1, 12-16 hrs) in control and heatwave treatments (b-c) were constrained to values below T50. The T50 values are shown as the vertical dashed lines (b-c; blue = control or
before heatwave T50, red = peak heatwave T50). Note that some leaves in the heatwave treatment exceeded the T50 value observed pre-heatwave (dashed blue line) but did not exceed the T50 value observed during the heatwave (dashed red line; c).
These trees dig deep for water, but other types of trees with shallow roots need other solutions to cope with a month of no rain and 4 days of extreme heat. Apparently they have tricks like reflective leaves or they use heat-shock proteins to limit the damage. Obviously nature has dealt with hot temperatures before.
Thanks to Lance for help.
REFERENCE
Drake et al (2018) Trees Tolerate an Extreme Heatwave via Sustained Transpirational Cooling and Increased Leaf Thermal Tolerance, DOI: 10.1111/gcb.14037