WordNet
n. 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
Wikipedia
A nurse log is a fallen tree which, as it decays, provides ecological facilitation to seedlings. Broader definitions include providing shade or support to other plants. Some of the advantages a nurse log offers to a seedling (as enumerated by Daniel Mathews) are: water, moss thickness, leaf litter, mycorrhizae, disease protection, nutrients, and sunlight. Recent research 1 into soil pathogens suggests that in some forest communities, pathogens hostile to a particular tree species appear to gather in the vicinity of that species, and to a degree inhibit seedling growth. Nurse logs may therefore provide some measure of protection from these pathogens, thus promoting greater seedling survivorship.
Usage examples of "nurse log".
He had climbed back to the mossy nurse log, hidden his face behind it, and lost himself to dry sobs.
Sometimes the nurse log rotted away completely, leavmg a colonnade of six or eight trees rising on roots like bowlegs.