Question
Six years ago, we planted a grove of six 10' tall quaking aspen trees.  They seemed healthy, except one had some branches that died last summer.  This winter, during a fairly strong wind, that tree, which was about 8" in diameter at the base, broke off at ground level and toppled.  The rest of the trees seem ok.  The wind didn't break the trunk.  It appears to have rotted.  What would cause a healthy-appearing tree to die so quickly?  Thank you.

Answer
The tree probably did not die quickly.  It was probably in slow decline and over the previous several months or year.  It is possible that it is a fungal canker problem.  Quaking aspen can be infected by several different types of fungi which cause cankers.  The result is that the fungi girdle the trunk(s) of one or more trees and cause it to die.  Sometimes only one tree in a stand is affected while in other cases, the entire stand may become infected.  

Here is some more information from the US Forest Service.  You can review the images and determine if any of your trees may be afflicted by this problem.  Hypoxylon is a good possiblity.
http://www.na.fs.fed.us/spfo/pubs/fidls/q_aspen/q_aspen.htm