An Ontario Geological Survey map of documented forest rings in Northern Ontario. Note that the size of the rings on the map is to show the distribution of different-sized rings, they are not proportional to the map scale. (Courtesy Ontario Geological Survey)

It is a strange phenomenon: thousands of large, perfectly round “forest rings” dot the boreal landscape of northern Ontario.

From the air, these mysterious light-coloured rings of stunted tree growth are clearly visible, but on the ground, you could walk right through them without noticing them. They range in diameter from 30 metres to 2 kilometres, with the average ring measuring about 91 metres across. Over 2,000 of these forest rings have been documented, but scientists estimate the actual number is more than 8,000.

What causes these near-perfect circles in the forest?

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