Scientists warmed Minnesota forest trees by 3.4°C for years expecting respiration to surge 23%; the trees adapted and erased nearly 80% of that increase

The open-air study was designed to mimic future warming while maintaining natural outdoor conditions. Researchers established plots in both open and closed-canopy forest environments and planted seedlings representing 11 tree species.

In a Minnesota forest, researchers turned up the heat, not for days or weeks, but for years. Beneath the trees, heating cables warmed the soil, while infrared heaters raised the temperature around the plants. And the goal was to see what happens when forests experience conditions resembling a warmer future. What they found challenged a key assumption about how trees respond to climate change.

What did the researchers find?

According to the study titled “Boreal and temperate trees show strong acclimation of respiration to warming,” researchers studied juvenile trees representing 10 North American species growing under forest conditions. In this experiment, some trees were exposed to temperatures about 3.4°C above ambient conditions, while others experienced normal temperatures. The focus was leaf respiration, the process through which plants release carbon dioxide as they use stored energy. Because respiration generally increases as temperatures rise, scientists have been concerned that global warming could cause forests to release substantially more carbon dioxide, potentially creating an additional climate feedback.Surprisingly, the warmed trees showed a much smaller increase than expected. Trees grown and measured under the 3.4°C warming treatment had leaf respiration rates averaging just 5% higher than trees under ambient conditions. Without this physiological adjustment, researchers estimated the increase could have been about 23%; thermal acclimation effectively eliminated nearly 80% of the expected increase.

A long-term experiment in Minnesota

The findings do not mean warming has little effect on forests. Instead, they indicate that trees may possess a greater capacity to adjust their physiology to sustained temperature changes than previously assumed.

The research was conducted as part of the Boreal Forest Warming at an Ecotone in Danger, or B4WarmED, experiment in northern Minnesota. The open-air study was designed to mimic future warming while maintaining natural outdoor conditions. Researchers established plots in both open and closed-canopy forest environments and planted seedlings representing 11 tree species. The experimental plots were warmed to different target levels: ambient temperature, approximately 1.7°C above ambient and approximately 3.4°C above ambient. Infrared heaters warmed the plant canopies, while buried resistance cables heated the soil. During the 2009–2011 growing seasons, the researchers achieved warming close to their targets, including temperatures elevated to depths of at least one metre in the soil.

Why the findings matter for climate models

Plant respiration represents an enormous movement of carbon dioxide between forests and the atmosphere. If warming caused respiration to accelerate substantially, forests could potentially release more carbon and weaken their ability to offset atmospheric carbon dioxide. The study suggests that long-term exposure changes that response. The trees adjusted to both experimental warming and natural seasonal temperature changes. The acclimation observed in the field was also stronger than responses commonly reported in shorter laboratory experiments.The findings do not mean warming has little effect on forests. Instead, they indicate that trees may possess a greater capacity to adjust their physiology to sustained temperature changes than previously assumed. If similar acclimation occurs widely across terrestrial ecosystems, future increases in plant respiration, and the resulting carbon feedback from forests could be smaller than many climate projections have suggested.Images Courtesy: istock