I came across this on a hike today. In bonsai, this is referred to as a raft — when a tree falls and branches of the fallen tree grow to mature trees in their own right.
In most rafts, as I am sure is true for this example, roots emerge along the fallen trunk to support the new trees. An interesting phenomenon to be sure.
Bonsai Iterate 
Reblogged this on Wolf's Birding and Bonsai Blog.
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