What is a distinguishing characteristic of barchan dunes?

Barchan dunes are indeed crescent-shaped formations that are primarily created by the action of wind in arid environments. These dunes typically develop when a limited supply of sand is present and the wind direction is consistent. The shape of the barchan, with its horns pointing downwind, allows it to migrate across the landscape as the wind continually shifts sand from the stoss side (the windward side) to the leeward side. This adaptation to wind patterns makes barchan dunes particularly interesting geological features.

The other options do not correctly describe barchan dunes. They are not formed by water erosion, are not typically found in evergreen forests, and do not consist of volcanic ash. Each of these incorrect options refers to various other geological or ecological processes and environments that do not pertain to the specific characteristics of barchan dunes.