The tree is broadly speaking bio-mass
zones. These are the stem, the crown, the detritus,
humus and the root associates.
Forests are dependent on a constant
input and output of nutrients, the tree has shed its
weight many times over to earth and air and has built
much of the soil it stands in where micro-organisms
fix the nutrients by interacting with the mosses, lichens,
fungi, ferns, the litter on the forest floor, the soil
and the breakdown of bed rock.
Fungi play multiple roles in ecosystems everywhere. They are parasites, mutualists and decomposers. Without them, terrestrial life would have a completely different evolutionary trajectory.
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The living tree stands in
a zone of decomposition much of it transferred,
acts as a filter system,
water production or is re-incarnated into
mosses, fungi, bacteria, insect life, birds
and mammals etc.