Ehsan Eyshi Rezaei and colleagues from the University of Bonn found that earlier heading caused by warmer springs can prevent exposure to extreme heat events around anthesis (flowering), the growth stage most sensitive to heat stress. This means that the intensity of heat stress around anthesis in winter crops in Germany may not increase under climate change, even if the number and duration of extreme heatwaves rise.

"There have been several other studies that concluded that increasing heat stress will affect wheat yield," said Rezaei. "But most of these studies used statistical models and did not take into account the acceleration in phenology of the plants. We believe that while climate change will affect wheat production, it will not be due to increased heat stress but rather due to a shorter growing period."

The group analysed temperature data from more than 1100 weather stations, and interpolated grids of monthly means of daily minimum, mean and maximum temperature at 1 km × 1 km resolution for 1951–2009. This information was combined with more than 80,000 observations of winter wheat day-of-heading (DOH) obtained from more than 5400 sites across Germany.

"We found that in the period between 1976 and 2009, the phenology of winter wheat in Germany has advanced by 14 days, meaning the anthesis starts in a cooler period and is less likely to be affected by heat stress," Rezaei told environmentalresearchweb. "The data also showed that the length of the period between emergence and heading has declined over the period 1976–2009. This would reduce winter-wheat yields because plant canopies have less time to intercept radiation, which is needed to produce biomass by photosynthesis."

While other studies have suggested that farmers need to adapt by growing cultivars with higher thermal requirements and later maturity, the Bonn researchers concluded that these adaptation strategies would expose the crop to more heat.

"While the accelerated phenology protected the winter wheat from heat stress in recent decades, it does mean that the potential to adapt to climate change by changing sowing dates and cultivars may be more limited than previously thought," said Rezaei. "We hope our work will be used to improve statistical models, and also be useful to producers of seeds as well as policy makers in the agricultural industry."

Rezaei and colleagues reported their work in Environmental Research Letters (ERL).

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