Topic 12.1: Weed Control

Where growing trees is the primary silvicultural goal, weed control is often necessary.  Although a “weed” is often defined as a plant growing where it is not wanted, foresters often use the term to refer to plants (e.g., shrubs, vines, graminoids, bamboo, palms, or non-commercial tree species) that interfere with growth of desirable trees.  Depending on the type of weed to be controlled and the ease to which damage to future crop trees can be avoided, silviculturalists can chose from a wide variety of mechanical and chemical treatments or may opt to perform controlled burns.

Among the mechanical weed control methods available, roller chopping, disking, and other tractor-requiring treatments are generally only useful in young stands regenerating after clearcutting.  More often in managed natural forests, weeds interfering with future crop trees are cut with a machete, brush axe, motor-driven weed wacker, or chainsaw.  Although many weeds resprout vigorously after cutting, a well-timed mechanical treatment can promote growth of future crop trees that may then shade out light-demanding weeds.  Generally, mechanical control is effective early during the season of most active growth when most carbohydrates and other storage materials have been translocated to the above-ground parts that are removed.

Chemical weed control methods have improved a great deal during the last decade.  Compounds such as sodium arsenite and 2-4-5 T (contaminated with dioxins) posed serious environmental and health hazards and are now generally banned.  In comparison, herbicides such as glyphosate, hexazinone, and imazapyr have low toxicity to animals, brief residences in soil, and apparently safe breakdown products.  When used according to instructions, modern herbicides can be useful components of a forest manager’s toolbox.

Among the more common herbicides several modes of action are represented.  Glyphosate, for example, is a synthetic herbicide that mimics an amino acid  and is incorporated into proteins, thereby rendering them ineffective.  2-4-D, in contrast, mimics the effects of the plant hormone auxin and causes plants literally to grow themselves to death.

Selection of the appropriate herbicides, concentrations, and wetting agents (i.e., surfactants) depends on a number of biological factors.  Herbicide formulations vary a great deal in their effectiveness at controlling different sorts of plants.  Even with modern herbicides, there are still some environmental concerns; only a few herbicides, for example, are approved for use near water.

Herbicides are generally used in conjunction with one of a variety of wetting agents, penetrants, or surfactants.  With foliar application, in particular, compounds that enhance herbicide uptake through the cuticle are critical for effective weed control.

Herbicides are expensive on a per liter or per plant basis, but because successfully treated plants do not resprout, and repeated mechanical control involves substantial labor costs, herbicides are often cost effective.  Furthermore, systemic herbicides often kill entire clones of target plants even when only a portion of the connected stems are treated.  Many systemic herbicides are more effective when plants are actively translocating carbohydrates below-ground for storage, such as prior to a dry or a cold season.

Finally, because weed control with herbicides does not requires soil disturbance, subsequent problems with weeds germinating from buried dormant seeds can be avoided.  Mechanical treatments that disturb the soil are often problematic due to stimulation of weed seed germination.

Fire can be an effective silvicultural tool for weed control.  Where future crop trees are resistant to low intensity fires, controlled burns can be used to reduce weedy competitors.  Fire can also be used to prepare seed beds and reduce competition in even-aged stands before seed dispersal.   Controlled burns are generally appropriate in pyrogenic ecosystems, i.e., ecosystems in which fire has historically influenced forest structure and composition, but can be considered in other areas as well. Timing of weed control treatments can be critical to their effectiveness.  It has already been mentioned that some herbicides are generally most effective when photosynthates are being translocated from shoots to roots, and mechanical treatments work best when photosynthates have been translocated to above-ground structures (e.g., in the early growing season).  Silviculturalists also have to decide at what points during stand development weed control treatments are likely to be most cost effective, i.e., when future crop trees are likely to benefit the most and costs of application are the least.  This decision should be based on a thorough understanding of the modes of action of the herbicides to be used as well as of species-specific plant susceptibility to different herbicide formulations and application methods.  But the best approach is to do everything possible to avoid development of weed problems in the first place.  Maintaining canopy cover and avoiding soil disturbance generally contribute to avoidance of weed infestations, at least by light-demanding species.  Many of these species have buried dormant seeds that germinate after soil is disturbed and exposed to light with wavelengths with a high red-to-far red ratio or to the fluctuating soil temperatures characteristic of large canopy openings.

Where weed infestations have been allowed to develop, control efforts can be very costly.  Where weed problems are spatially heterogenous, which is usually the case, a cost-benefit analysis may reveal that certain badly infested areas with low stocking of future crop trees are not worth treating, at least if labor, machine, and/or herbicide costs are high and forest in need of silvicultural attention is abundant.

Woody vines, including climbing bamboo, pose a serious silvicultural problem in many tropical forests.  Vine infestations are especially common in logged forests, particularly those where uncontrolled logging by untrained workers was carried out.  Because many vines survive when their host trees are felled and sprout vigorously from fallen stems, many of the vines in logged areas have propagated vegetatively.  Pre-felling vine cutting, therefore, can have substantial post-felling advantages in addition to the advantage of reducing logging damage.  Furthermore, due to easier forest interior access prior to logging, pre-felling vine cutting is generally more cost effective than trying to control vines in vine-infested logged forests.  Finally, because vine leaves may constitute 25% or more of the total forest leaf area, vine cutting is analogous to carrying out a light shelterwood cut; tree seedling densities and growth rates may increase in response to vine removal.

The effects of weed control on wildlife needs to be considered.  Vine cutting, for example, may have serious locomotory impacts on non-volant arboreal animals.  Some of these animals are undoubtedly important seed dispersal agents; reducing their access to tree crowns may affect future forest composition. Where vines and other weeds have proliferated in response to uncontrolled logging and other human perturbations, their control seems easily justified from an environmental perspective.  But silviculturalists must always keep in mind that what is a weed from a commercial production perspective also provides homes and food for many animal species that will suffer greatly if forest domestication efforts are too intensive or too extensive.  Given the responsibility to manage forests in ways that maintain ecosystem integrity, leaving some areas untreated, in any event, seems reasonable.