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
a key habitat, which is suitable for niches
A decaying log that provides habitat for many plants, animals and microorganisms.
A nurse log is a fallen tree trunk that nourishes new life on the forest floor as it decomposes. It provides a rich, moist habitat for ferns, lichens, mosses, and tree seedlings.
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.