nurse log

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A large fallen tree on which other trees and plants sprout as the wood decays, with the roots of the seedlings often straddling the log as they grow toward the soil.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a large decomposing tree trunk that has fallen, usually in a forest; the decaying wood provides moisture and nutrients for a variety of insects and plants