Thaw Circles in the Snow

As winter gives way to spring, you might notice that the snow melts away around tree trunks first. These are thaw circles and can be found in deciduous forests where snow accumulates, and they are caused by heat: As the sun warms the forest, the dark trunks of the trees absorb more heat than the reflective snow. The absorbed heat radiates outwards from the trunks, melting away the snow in a ring around the tree.

Sometimes the small-scale topography of a patch of land can also create uneven melting, forming circles without trees at their center.

And this can give other plants an early start as they can grow in these open patches of land. Plants on the forest floor have just a short period of time to peek above ground and soak up sunlight before trees start growing leaves, creating shade that hinders vegetation growth below. For plants adapted to grow before the forest’s upper canopy darkens the ground, these snow circle open patches of land can provide a head-start on the growing season, and a reason to cluster around tree trunks.