Conventional wisdom on heat-stressed trees starts with water. Trees wilt and die in heatwaves mainly because drought tags along. Pull the drought out of the equation, the thinking goes, and most plants handle the heat just fine.
So a team of researchers did exactly that – removed the drought and cranked up the heat, keeping seven plant species well-watered while pushing temperatures as high as 104°F (40°C). The plants stayed alive.
But something unexpected showed up in the chemistry of their sugars. Even with water to spare, the leaves were already carrying a fingerprint of metabolic trouble.
A signal in sugar
Researchers wanted to know what happens inside a leaf when heat alone, not drought, is the problem. A team at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) built an experiment to isolate that one variable.
They picked seven, well-watered plant species and dialed the air temperature up in 5-degree steps from 50°F (10°C) all the way to 104°F (40°C). Humidity stayed constant. Leaf sugars told the real story.
After five days of acclimation at each temperature, Philipp Schuler, lead author of the study, and his colleagues sampled leaf tissue and tracked how each plant was breathing and photosynthesizing.
Plants exposed to heat alone
C3 plants – the category that includes most trees, wheat, rice, barley, and the great majority of the plant kingdom – handled temperatures up to about 86°F (30°C) just fine. Above that, the wheels started coming off.
Photosynthesis dropped. Respiration – the rate at which a plant burns its own sugar to stay alive – climbed steadily. The cellular machinery that converts sunlight into usable energy began failing above 86°F in nearly every C3 species tested.
An earlier review had shown that heatwaves cut carbon gain in trees. Until this study, nobody had cleanly observed what happens to a plant’s internal sugars when the only thing changing is the temperature.
When the math flips
At cool temperatures, leaves stored almost 14 percent of their dry weight as carbohydrates. More than half of that was locked up as starch – the plant equivalent of savings.
At hotter settings, the total pool dropped below 8 percent, and starch’s share fell to roughly one-fifth. The rest had been broken down into sugar, the ready cash for an overheated leaf that is burning fuel faster than usual.
This change is exactly what you’d expect from a plant scrambling to keep its respiration running. The puzzle is what gets left behind.
At 104°F (40°C), the experiment hit its limit. Most barley plants did not survive the heat. One tomato relative died too. But the sugar signal had already appeared well before the leaves gave out.
Heat caused change in plant sugars
Water and sugar molecules contain hydrogen and oxygen, and both elements come in slightly heavier and lighter versions known as isotopes. The ratio of heavy to light in a leaf’s sugar is normally pretty steady.
Heat broke that pattern in the experimental plants. As temperatures climbed above 86°F (30°C), leaf sugars became enriched with heavy hydrogen and depleted in heavy oxygen. Two opposite changes. Same trigger.
The most likely explanation, Schuler and colleagues suggest, is that as respiration accelerates under heat, lighter-hydrogen sugars appear to get consumed first, leaving the heavier kind behind. What exactly drives this is still being worked out.
C4 plants stayed steady in heat
Among the seven species, sorghum served as the lone C4 plant and the experiment’s control. C4 plants photosynthesize differently – they are more heat-tolerant. Examples include corn, sugarcane, and grasses of hot regions.
At 95°F (35°C) and beyond, it kept photosynthesizing steadily. Its hydrogen isotope ratio in leaf sugar didn’t budge. That alone told the team the C3 fingerprint was about heat-driven metabolic stress, not temperature on its own.
The split runs straight through the world’s food crops. Wheat, rice, barley, and most legumes are C3. Corn, sorghum, and sugarcane are C4. The grocery list has two very different responses to a warming planet built into it.
What tree rings remember
Sugars made in leaves eventually become wood. Tree rings lock in a year-by-year archive of a tree’s chemistry, including the hydrogen and oxygen isotopes inherited from those original leaf sugars.
Other research had hinted that hydrogen ratios in tree rings climb when a tree is in trouble – defoliated, stressed, or out of step with its environment. The new findings give that hint a mechanism.
If those same isotope swings get recorded in the wood of tree rings, the authors note, they could serve as a way to identify trees running an unfavorable carbon balance – essentially, burning more than they’re making.
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A new diagnostic tool
Climate scientists have long mined tree-ring chemistry to reconstruct past temperatures and rainfall. The assumption was that the chemical steps from leaf water to sugar stayed more or less constant across temperatures.
This study breaks that assumption. Above 86°F (30°C), the dual signature of rising hydrogen and falling oxygen leaves a clear trace of metabolic stress that older models did not account for.
For foresters and climate researchers, that opens a new diagnostic tool. Tree rings going back decades or centuries could now be reread for the chemistry of heat stress.
Silent damage that never made it to the bark or canopy may, all along, have been keeping a tidy ledger inside the sugar.
The study is published in npj Science of Plants.
NOTE – This article was originally published in Earth and can be viewed here
