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.

The lethal temperature of the cambium layer of woody plants is about 70-75 C. Fire, of course, is much hotter than that. Grass fires produce temperatures of 400-500 C or hotter and oak leaves burn at about 200 C. (See this link for oak leaf temperature measurements.) 

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

An understanding of how fire works offers a convincing case that herbicide use is essential to eradicate invasive shrubs. Fire and herbicide act as collaborative agents in the eradication of invasive woody plants. Although herbicide can be used without fire, it is much easier to kill shrubs after they have experienced fire. The small resprouts that arise from the top-killed shrubs are excellent targets for herbicide. A spring fire will set the stage for spraying.

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.

Buckthorn resprouts in a burned area. There are at least 3 shoots, each from a dormant bud

 

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.