After a fresh snowfall has lain for a short while the wind gets up and plays "air quoits" - Benoulli's Rings? - around the tree trunks. Larger trunks have wider rings than smaller trunks - very small trunks have none at all.
This phenomenon must be yet another example of the Bernoulli Effect: air flowing around a (cylindrical) object speeds up and at the same time reduces its pressure. This in turn acts as a vacuum cleaner preventing deposit of snow and sucking up any lying immediately adjacent to the lower part of the object. Varying wind directions help ensure a nice uniform ring.
The change in colour half way up the exposed tree stumps shown in the photograph is a previous high water line that prevented richer growth below it.