Woody Actinorhizal PlantsBy Douglas Barnes
don't eat dogs. At least normal, healthy dogs don't eat dogs. So, if
anyone tells you "It's a dog-eat-dog world," smile submissively and
slowly back away – you are dealing with a sociopath. The world is, on
the whole, a symbiotic dog-help-dog world. Why, even dogs help dogs!
Nitrogen-fixing plants are one example of the general pattern of
Plants release an average of 40% of their
photosynthates (the products of photosynthesis) out their roots. They
don't do this out of inherent inefficiency. These chemical compounds
are doing tasks such as sending signals to call in mycorrhizal fungi
and feed those fungi, to share with beneficial nitrogen-fixing and
other bacteria, or to make soil nutrients more soluble and available
for uptake by the roots.
Among woody nitrogen-fixing plants, there are two varieties: those associating with the bacteria Rhizobia
; and those associating with the topic of this article, the actinomycetes Frankia
Actinomycetes are a type of bacteria that grow in long chains of
filaments resembling the hyphae, or hair-like roots, of fungi.
the woody nitrogen-fixers in temperate regions, actinorhizal plants are
an important group. These pioneering plants are able to grow in poor
soils, enriching them with nitrogen and organic matter. This makes them
very valuable in repairing disturbed or damaged soils.
diagram below shows the 7 families of woody actinorhizal plants and
their 23 genera (I left out the one herbaceous family, the Datiscaceae
). A higher, 1 MB resolution image is available on request.
rapidly revegetate a damaged landscape, be sure to include plenty of
these species to help quickly build up the soils. In areas of very
problematic soil, such as arid, tropical and subtropical regions, make
90% of your initial planting of trees nitrogen fixing, pioneer species
(associating with either Frankia
and 10% of species your long-term canopy overstory species. When the
system reaches maturity, the proportions will be reversed with 10%
nitrogen-fixing, support species and 90% canopy species. The same
formula could be followed for temperate regions, but the soils in these
area are not so fragile and can stand a lower percentage of nitrogen
fixers. A 70/30 or even lower may suffice in these areas, as the
seasonal cycles of death and regrowth feed these soils well.
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