Cover crops can affect winter temperatures

This aerial photo shows a mosaic of cropland to illustrate the different colours, and associated reflectivity, of the land surface related to management.  |  National Center for Atmospheric Research photo

Research finds the darker surface of a cover crop field absorbs more heat from the sun than does a snow-covered field

Cover crops have long been known to have ecological benefits, such as controlling runoff and erosion, weed control and increasing soil productivity.

But research now suggests that they can also produce warming temperatures in winter, perhaps influencing regional climate change, especially in the northern United States and southern Canada.

A new study by researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, has found that the darker surface of a cover crop field absorbs more heat from the sun than a snow-covered field, even when cover crops partially protrude above the snow. The study, which also involved scientists from the University of New Hampshire and New York’s Cornell University, showed that tall, leafy cover crops could influence the surrounding surface temperature by up to 3 C. Even crops partially covered by snow have an influence.

“I designed the study to look at characteristics of cover crops — tall or short, and leafy or not,” said Danica Lombardozzi, plant ecologist with NCAR and lead author of the research paper published in the journal Geophysical Research Letters.

She said researchers tested a low- and a high-end range of those cover crop characteristics and found the characteristics mattered, but only in regions that received snow in the winter, which in the study was defined as December, January and February, and where the average winter snow pack was less than about 30 centimetres deep.

She said in areas where crops are present, primarily the northern part of the central U.S. and southern part of central Canada, cover crop height and leafiness changes average winter temperatures compared to bare ground.

“In particular, the tall and leafy cover crops caused the most significant warming. The tall cover crops that were less leafy also increased temperature, though the increase was smaller (2 C) and the warming was much less widespread. The short cover crops, even when leafy, did not significantly change winter temperatures. This is because the shorter cover crops were more often buried by the snow,” she said.

Lombardozzi said the study did not look at specific types of cover crops, only the plant characteristics of the cover crops.

They tested three scenarios:

  • A tall and leafy scenario where cover crop height was 50 cm and leafiness was defined as four leaves per area of ground.
  • A tall and sparse scenario where cover crop height was 50 cm and leafiness was one leaf per area of ground.
  • A short and leafy scenario where cover crop height was 10 cm and leafiness was four leaves per area of ground.

She said that the cover crop height seemed to have the largest impact. When plants were tall enough to protrude above the snow, they absorbed more of the sun’s energy than the reflective snow.

“Both the scenarios with tall cover crops increased winter temperature, whereas the short cover crops, even when leafy, did not,” she said. “However, the leafiness did matter, since the leafy scenario increased temperature more than the sparse scenario.”

Previously, the impact of cover crops on winter temperatures has not been measured.

“This work illustrates that cover crops, if they stick above snow pack in the winter, can significantly increase winter temperatures,” she said. “However, the winter warming can be mitigated by selecting and planting short cover crops and/or mowing or grazing the cover crops before snowfall. Farmers can still get all the benefits of planting cover crops — soil health, weed suppression, increased cash crop productivity, etc. — and, if thoughtfully managed, can also reduce the impact on winter warming.”

She did not make specific recommendations but suggested that short crops, like clover, or other varieties that could be mowed or grazed before snowfall would still provide many benefits while minimizing the impact on winter temperatures.

She said understanding the physical and biological drivers of climate change can be difficult because often physical drivers have more localized and immediate impacts, whereas biological drivers occur over longer time periods and can have global impacts.

“We also need to better document when and where certain management practices are happening. For example, where are cover crops planted? Are they planted every year? Which types of crops are planted?

“Until we have this type of information, we cannot fully understand the impact of management practices on regional or global scales.”

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