The natural system is complex, with a wide variety of
species,
energy pathways, and nutrient translocation pathways. Coarse woody debris
provides a critical reservoir of moisture and nutrients, and in many
systems has been described as the site of nitrogen fixation by
cyanobacteria. Actinomycetes in the nodules on the roots of species like
red alder (Alnus rubra hosting Frankia) also "fix" nitrogen,
converting gaseous
atmospheric nitrogen into a nitrate form useable by plants.
A diverse array of organisms, ranging from anadromous fish in the stream
to insects and birds in the air contribute to a healthy flow of energy in
the system.
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Simplicity is the key word when it comes to describing the hybrid
poplar stand. Anthropogenic selection focuses as much of the available
soil resources (nutrients and water) on a single species (in this case a
highly productive hybrid) as possible. Doing this can produce high
biomass yields. However, a monoculture is ecologically less stable than a
diverse, natural system, subject to pest outbreaks, disease, and
degradation of the soil. In this system, nutrient replacement processes
are inhibited, discouraged, or eliminated.
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