Forest rings are large, circular patterns of low tree density in the boreal forests of northern Canada. These rings can range from 50 metres (160 ft) to nearly 2 kilometres (1.2 mi) in diameter, with rims about 20 metres (66 ft) in thickness. The origin of forest rings is not known, despite several mechanisms for their creation having been proposed. Such hypotheses include radially growing fungus, buried kimberlite pipes, trapped gas pockets, and meteorite impact craters.
It is hypothesized that bacteria, such as Geobacter, might be contributing to the ring formation. Their capability to form conducting nanowires or pili and to perform directional electron transport might contribute to a circular charge transfer from the electron-rich center of the ring towards the ring edges. The so-catalyzed oxidation processes increase acidity and dissolve carbonates, thus creating a depression, and, accordingly, the ring edge. Recent microbiological investigations indeed show a high abundance of Geobacter at the ring edge.