Abundance of seeds on snow

Observation by Lars Flora and Mike Brubaker:

Former a period of several weeks the snow in Anchorage was covered with a blanket of what we took to be seed from birch and other trees. Neither of us recall seeing this much before. In the city trail systems, the snow was mostly brown in some areas. We are wondering if this actually is unusual, and what combination of factors would cause this to occur.

LEO says:

This observations has been sent to our friends at UAF for review and comment. A consultation is pending.