Trees
For me trees are one of natures great examples of abstract art, especially in winter when the branches are bare.
The examples shown here are varied -- stark branches against snow and sky in Yosemite and Lancashire in winter -- palm fronds in the breeze at twilight in Kauai -- silhouettes in winter fog in Oxford -- sun and shadows in California -- and aspens in Aspen (of course!).
You will also find some additional examples of images of trees in my Infra-Red portfolio.