Heat-stressed trees and the isotope fingerprint in leaf sugars

The usual story about heat-stressed trees begins with water. During heatwaves, trees often droop and die largely because drought arrives at the same time. Remove drought from the picture, the logic says, and most plants should cope with high temperatures without too much trouble.

To test that idea, a research team did precisely that - they eliminated drought and pushed heat upwards, keeping seven plant species fully watered while raising temperatures to as high as 40°C (104°F). The plants survived.

Yet an unexpected pattern appeared in the chemistry of their sugars. Even with ample water, leaves were already showing a tell-tale sign of metabolic strain.

A signal in sugar

The researchers set out to understand what happens inside a leaf when heat alone - rather than drought - is the stressor. Scientists at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) designed an experiment that held everything constant except temperature.

They selected seven plant species, all kept well-watered, and increased air temperature in 5-degree increments from 10°C (50°F) up to 40°C (104°F). Relative humidity was kept steady throughout. The clearest evidence came from the sugars in the leaves.

After five days of acclimatisation at each temperature step, Philipp Schuler, the study’s lead author, and colleagues collected leaf samples and monitored how each plant was respiring and photosynthesising.

Plants exposed to heat alone

C3 plants - a group that includes most trees, as well as wheat, rice, barley, and the vast majority of plant species - coped well up to roughly 30°C (86°F). Beyond that point, performance began to deteriorate.

Photosynthesis declined. Respiration - the process by which a plant burns its own sugars to stay alive - rose steadily. In almost every C3 species tested, the cellular systems that convert sunlight into usable energy started to fail once temperatures climbed above 30°C (86°F).

A previous review had already shown that heatwaves reduce carbon gain in trees. What had not been cleanly observed before this work was how a plant’s internal sugar balance shifts when temperature is the only changing factor.

When the maths flips

Under cooler conditions, leaves held nearly 14 percent of their dry mass as carbohydrates. More than half of that carbohydrate store was starch - essentially the plant’s savings.

At higher temperatures, the overall carbohydrate pool fell to under 8 percent, and starch’s share dropped to around one-fifth. Much of what had been stored was broken down into sugars, providing quickly usable fuel for a leaf that is consuming energy faster than normal.

That shift matches what you would expect from a plant trying to keep respiration running under heat stress. The more intriguing question is what the remaining sugars reveal.

At 40°C (104°F), the experiment reached its practical endpoint. Most barley plants failed to survive those conditions, and one tomato relative died as well. However, the sugar-based warning signal had emerged well before the leaves collapsed.

Heat caused change in plant sugars

Water and sugar molecules contain hydrogen and oxygen, and both elements occur in slightly heavier and lighter forms known as isotopes. In typical conditions, the balance between heavy and light isotopes within leaf sugars remains fairly consistent.

In the experimental plants, heat disrupted that stability. Once temperatures rose above 30°C (86°F), leaf sugars became enriched in heavy hydrogen while, at the same time, becoming depleted in heavy oxygen. Two opposing shifts, triggered by the same driver.

Schuler and colleagues propose that the most plausible explanation is linked to accelerated respiration in the heat: sugars containing lighter hydrogen seem to be used up first, leaving a higher proportion of heavier-hydrogen sugars behind. The precise mechanism responsible is still under investigation.

C4 plants stayed steady in heat

Of the seven species tested, sorghum was the sole C4 plant and effectively acted as a comparison point. C4 plants use a different photosynthetic pathway and tend to tolerate heat better; familiar examples include maize, sugarcane, and grasses from hotter regions.

At 35°C (95°F) and above, sorghum continued photosynthesising steadily. Its hydrogen isotope ratio in leaf sugar did not shift. For the team, that result indicated that the distinctive C3 pattern reflected heat-driven metabolic stress rather than temperature alone.

This divide runs through major food crops worldwide. Wheat, rice, barley, and most legumes are C3, whereas maize, sorghum, and sugarcane are C4. In other words, the shopping basket already contains two fundamentally different biological responses to a warming world.

What tree rings remember

Sugars produced in leaves are ultimately converted into wood. As tree rings form, they preserve an annual record of a tree’s chemistry, including hydrogen and oxygen isotope patterns inherited from those original leaf sugars.

Earlier studies had suggested that hydrogen isotope ratios in tree rings increase when trees are struggling - for example after defoliation, under stress, or when mismatched to their environment. The new results provide a potential mechanism behind that signal.

If similar isotope swings are written into the wood, the authors argue, they could help identify trees with an unfavourable carbon balance - effectively using more carbon than they are producing.

A new diagnostic tool

For decades, climate scientists have used tree-ring chemistry to reconstruct past temperature and rainfall. A key assumption has been that the chemical steps linking leaf water to sugars stay broadly consistent across different temperatures.

This study challenges that assumption. Above 30°C (86°F), the combined signature of increasing hydrogen and decreasing oxygen leaves a distinct marker of metabolic stress that older approaches did not incorporate.

For foresters and climate researchers, that points to a new diagnostic tool. Tree rings stretching back many decades - even centuries - could potentially be re-examined to detect the chemistry of heat stress.

Damage that never became visible in bark or canopy may, all along, have been recorded quietly in the sugars’ internal accounting.