Occurrence  Various mechanical and biological processes contribute to the breakdown of lignin in fallen trees, resulting in the formation of niches of increasing size, which tend to fill with forest litter such as soil from spring floods, needles, moss, mushrooms and other flora. Mosses also can cover the outside of a log, hastening its decay and supporting other species as rooting media and by retaining water. Small animals such as various squirrels often perch or roost on nurse logs, adding to the litter by food debris and scat. The decay of this detritus contributes to the formation of a rich humus that provides a seedbed and adequate conditions for germination. Nurse log harboring a young western hemlock
Occurrence Nurse log harboring a young western hemlock Various mechanical and biological processes contribute to the breakdown of lignin in fallen trees, resulting in the formation of niches of increasing size, which tend to fill with forest litter such as soil from spring floods, needles, moss, mushrooms and other flora. Mosses also can cover the outside of a log, hastening its decay and supporting other species as rooting media and by retaining water. Small animals such as various squirrels often perch or roost on nurse logs, adding to the litter by food debris and scat. The decay of this detritus contributes to the formation of a rich humus that provides a seedbed and adequate conditions for germination. 
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Nurse log harboring a young western hemlock
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Nurse log harboring a young western hemlock