Fire and herbicide as collaborative agents in the eradication of invasive woody plants
The control of woody invasive plants is one of the most
important uses of fire in restoration ecology . It is important to understand
that in most situations fire does not actually “kill” invasive shrubs. What it
does is kill the cambium layer and associated xylem and phloem, thus preventing
vertical transport between the stems and the roots. This effect is analogous to
girdling.
The roots, protected from the fire, are unharmed. In the
root collar region of most hardwood trees and all shrubs are living dormant
buds whose growth is suppressed by hormones coming from the stems. With the plant’s
vertical transport system destroyed, the hormone supply to the roots is gone, so
one or more of the dormant buds in the root collar is released.
The above is a complicated way of saying that fire top-kills
a shrub but does not get rid of it, because it will resprout from the base.
Thus, even a low-intensity savanna or oak woodland fire is hot
enough to kill the stems of any shrub it passes near. The dormant buds in the
root collar, now released from hormonal inhibition, grow. Thus, fire cannot eradicate
well-established invasive shrubs.
Typical low-intensity fire line with oak leaves fuel |
Patch of woody shrubs after the fire has moved through |
Spring is an ideal time to spray after burning because the
native spring vegetation is small or still underground, and the shrub resprouts
are also small and can easily be found.
Fire should still be used even if herbicide use is not an
option. If a site is burned annually, the shrubs will be top-killed annually and
should never be able to become dominant again. However, in this situation, you
can’t miss a year.
There is, however, one situation in which fire by itself will
actually eradicate woody plants. Newly grown shrub seedlings are killed by
fire. This is because they have not had time to make a substantial root system,
so dormant buds are not present. Thus, one might be able to eradicate a patch
of buckthorn seedlings with fire (prescribed burn or a propane torch). However,
in most restorations, the invasive shrubs are well past the seedling stage.
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