Weeds and Their Effect on the Ecosystem
We
first observed the significance of the effects of weeds on a
native ecosystem when the Santa Margarita Lake watershed area was
aerially seeded with Blando Brome (Bromus hordeaceus) after a
wildfire in 1985. (This wasn't the worst part; the seed was
contaminated with other major weeds of California, such as Yellow
Star Thistle, or Centaurea solstitialis.) Over the next decade we
saw the area degrade terribly, the weeds spread, the fires
increase, and the water-holding capacity of the land decrease. All
the explanations for these observations are found in the reference
section. (That is another book in itself!)
First, I want to tell you about the interactions in the chaparral plant community. The plants in the chaparral depend heavily on underground microorganisms to survive in the Mediterranean climate and soils of California, just as the microorganisms depend upon the plants. There is movement of nutrients (mostly nitrogen and phosphorus), food (carbohydrates) and water between the plants themselves and between the plants and the microorganisms. The microorganisms depend upon the plants for carbohydrates and the plants receive nutrients from the microorganisms. The water moves from plant to plant via the microorganisms The most important point is that the major role of the microorganisms (in association with the mature plants) is to control pathogens and alien species in the rhizosphere.
The
chaparral is a plant community that has evolved with fire. Fire
usually happens in the dry season of summer. Fire normally occurs
every 60-120 years. A fascinating succession takes place in this
plant community after a fire. Well, at this critical period, both
the plants and the microorganisms are vulnerable. All the plants
that you see above ground are gone, burnt up, and so they cannot
transport food (that is made by their green leaves with the help
of the sun), to the microorganisms. The microorganisms need this
food so they can survive, and can live only several months without
the plants help. These tiny microorganisms also move water and
nutrients from plant to plant, and to themselves when needed. So,
to repeat; after a fire, this transfer of nutrients and food and
water stops temporarily, and the plant community is in a critical
period, until this transfer can resume. The soil is bare; no
plants shade the soil or protect the soil and the area looks like
a moonscape. Well, the eyes do deceive us; It is not a moonscape.
Immediately the rains begin in late fall, and the tiny
fire-follower annuals germinate and cover the ground so thickly
that you can not see any soil between them; they connect to the
underground organisms, and start sending food to the
microorganisms. In turn, the microorganisms start transporting
nutrients to the burnt-up plants so that they can regrow from the
ground up. Many of the shrubs, in the chaparral resprout from the
base of the plant. They were not, in fact, dead. Just the tops had
been burnt off. The root system and the associated microorganisms
were still very much alive.
Weeds that are wholesale seeded onto this vulnerable chaparral plant community after a fire, replace the fire-followers, cripple the microorganisms and, in turn, the native shrubs, block succession, lower the water-holding capacity of the soil, increase runoff, increase erosion, increase fire frequency,etc.
Next,
there is the deleterious effect of weeds on oak trees. When
removing the weeds, the oak trees stop dropping large dead
branches, and new growth starts appearing and the whole tree has a
different appearance, much healthier-looking, lots of shiny, green
leaves, with hardly any yellowish or dead leaves, whereas before
the leaves were dull green, with many yellow and dead leaves, and
many twigs and branches were dead. And all this happened, just by
removing the weeds!! The litter layer of dead oak leaves that drop
to the ground seasonally or periodically, depending upon it being
an evergreen oak or a deciduous oak, is critical to the health of
the oak tree. The weeds block the litter layer from forming and
take the water and nutrition from the oak, eventually helping to
kill the tree. AT the interface of the soil and the litter layer
is where specific microorganisms live, extracting nutrients from
the litter, and also preventing other organisms from extracting
nutrients from the oak leaves. This method of acquiring nutrition,
for the oak tree via the litter, via the microorganisms, is
blocked when weeds appear under the oak trees. The same holds true
for pine trees.
Third, the weeds can spread throughout an area via early germination and vigorous unchecked growth (no natural checks and balances), displacing the native annual wildlflowers, and turning a beautiful wildflower meadow with many kinds of flowers into a mass of annual weeds of one or two kinds, thickly covering the ground, and filling the spot with dry, dead tissue in the summer, that does not disintegrate, and poses a fire hazard.
Fourth,
some of the worst weeds are the favorite food of ground squirrels,
and mice, which in turn help to disseminate the seed throughout an
area. So, the weedy field then becomes a breeding ground for
rodents, and you end up with an unnaturally high population of
rodents AND a weedy field, instead of a wildflower meadow!
The Message: Remove the weeds!!!