For almost a quarter-century, a greenhouse in the Alaska Arctic has been the setting of an experiment in trying to speed up the effects of global warming. Researchers have watched as the soil within the greenhouse has become more hospitable to woody shrubs than tundra plants. It was expected the change would result in a net loss of carbon into the atmosphere, which would contribute to further global warming, but so far the scientists have documented a complex process that indicates otherwise.
According to [University of California Santa Barbara doctoral student] Seeta Sistla, the answer to this mystery might be found in the finer workings of the ecosystem. ... The warmer temperature – on average 3.6 degrees Fahrenheit in the air and 1.8 degrees in the soil to the permafrost – has caused the increase plant productivity, which has increased plant litter inputs to the soil. The scientists found that the soils in the greenhouse experiment developed warmer temperatures in the winter, while the summer warming effect decreased.
“These changes reflect a complicated feedback,” Sistla said. “Shrubs trap more snow than the lower-lying vegetation, creating warmer winter soil temperatures that further stimulate both decomposers and plant growth. Shrubs also increase summer shading, which appears to have reduced decomposer activity in the surface soil by reducing the greenhouse effect during the summer.”
Also, from Ars Technica: Carbon in Alaska soils stays stored despite warming