In the areas with no radiation, 70 to 90 percent of the leaves were gone after a year. But in places where more radiation was present, the leaves retained around 60 percent of their original weight. . . . Because [the researchers] had so many bags placed in so many different locations, they were able to statistically control for outside factors such as humidity, temperature and forest and soil type to make sure that there wasn't anything besides radiation levels impacting the leaves' decomposition.