Plant control; Fire


Tools that vegetation managers can use to manipulate vegetation fall into 4 general classes: chemical, mechanical, biological, cultural

Chemical (herbicidal)

Herbicides are seldom prescribed to kill all vegetation; dosage rates and timing are chosen to selectively reduce survival or growth of a particular group of species

used to kill or reduce existing vegetation

contact herbicides (kill tissue on contact)--rarely used because of non-selectivity

growth-regulators --> usually selective

sensitivity = f(phenological stage)

in general, plants are most susceptible when photosynthates are being translocated to roots

some species are very susceptible when resprouting following fire or mechanical top-removal

an alternative to spraying is injection (injector, axe)

(+) very effective, very selective

(-) very high labor cost


economical, safe, convenient compared to mechanical

can be integrated with many different objectives via herbicide selectivity


high skill level required

timing is critical

drift, residues

social reaction


Most grazing animals prefer herbs > deciduous browse > coniferous browse

Therefore, livestock grazing can be used to reduce interference faced by conifers

Foresters often fear trampling damage

usually insignificant except in SE pine plantations

single-wire electric fences fixes problem in SE

Livestock grazing can be timed to favor specific group of vegetation


In forests, soil disturbance associated with timber harvest is often adequate to reduce competition

Scalping--hand clearing individual spots when planting

must remove roots for best control

large scalps (> 1 x 1 m) usually required to be effective

Use of heavy equipment

dozer with or without brush blade




Removal treatments


excellent control of target species

timing not crucial


soil compaction and removal of nutrients

very expensive

Constraint: heavy equipment can not be used on steep slopes (> 35%)

Cultural (Fire)


Piles, windrows after mechanical treatment


perceived as ecologically most appropriate --> socially acceptable

fire historically prevalent in North American ecosystems

minimal soil compaction, may accelerate nutrient cycling

can be used on steep slopes



high skill, narrow prescription windows


Fire has become the most common means of plant control on forests and rangelands, so we will spend the remainder of today and another day talking about it; bear in mind that fire is also the most difficult tool to apply

Wildland fires require:

adequate fuel to carry a fire

dry season to lower the fuel moisture

ignition source

No two fires are alike--response varies widely between and within species

Diversity and biomass are commonly measured attributes of communities, so we will compare various large-scale community types with respect to these two attributes

Diversity and biomass are strongly affected by fire frequency and fire season; fire behavior (area and intensity) is much less important


Well-adapted to periodic fire

herbaceous production originates from growing points or seeds at or below the soil surface

nutrients and carbohydrates move rapidly between above- and belowground biomass

aboveground biomass is well-aerated, fine fuel

Historically, frequency decreased with increased aridity and/or increase in plants with caespitose growth form

Fire is particularly detrimental during peak growth

Humid grasslands

high frequency (every 1-3 yr)

increased post-fire productivity

individual species may be reduced, but fire is best viewed as a "regenerative" event

these are nitrogen- and light-limited communities

removal of litter --> light increases

soil temperature increases--> microbial activity increases--> decomposition of organic matter into available N increases

i.e., total N decreases, but available N increases

Semi-arid grasslands

lower frequency (every 5-25 yr)

wider range of pre-fire conditions and post-fire response

fire intensifies plant water stress by increasing evapotranspiration

light-limited communities maintain or increase production

non-light-limited communities maintain production in years with average precipitation, but decrease production in years with 80% or less of average precipitation

Herbaceous dicots usually increase following fire

Arid grasslands

lowest fire frequency of grasslands (>25 yr)

post-fire production maintained only when precipitation is above average

fire produces long-term changes in community structure


Shrubs within a grassy matrix (e.g., South Texas mixed-brush, Arizona mesquite):

Fire regime controlled by amount and distribution of fine fuel

Shrubs usually resprout

Absence of fire --> dense overstory --> fine fuel decrease--> fire no longer possible --> species diversity and herbaceous biomass decrease

Heath (scrub, chaparral, fynbos)

Fire regime controlled by species composition and biomass accumulation within the shrub layer

Biomass accumulates rapidly post-fire

Fire maintains species diversity


Structurally diverse

Herb production inversely related to tree density/cover, therefore directly related to fire frequency

Fire frequency increase--> shrubs replace non-sprouting trees

Humid woodlands (e.g., southeastern pine savannas):

1-5 yr fire frequency

fire exclusion --> deciduous shrubs increase and fine fuel decrease--> flammability decrease

Semi-arid woodlands (e.g., ponderosa pine savannas):

10-30 yr fire frequency

trees poorly adapted to fire as juveniles

fire exclusion --> conifers increase--> flammability increases

Additional Information (also see assigned readings):

Jordan, G.L. 1981. Range seeding and brush management on Arizona rangelands. Arizona Agricultural Experiment Station Extension Bulletin T-81121.

Vallentine, J.F. 1989. Range Development and Improvements, 3rd edition. Academic Press, San Diego.

The Nature Conservancy's Wildland Invasive Species Program

Ffolliott, P.F., DeBano, L.F., Baker, M.B., Jr., Gottfried, G.J., Solis-Garza, G., Edminster, C.B., Neary, D.G., Allen, L.S., and Hamre, R.H. (technical coordinators). 1996. Effects of fire on Madrean province ecosystems: a symposium proceedings. USDA Forest Service Rocky Mountain Experiment Station General Technical Report RM-289, Fort Collins, Colorado.

Fire Effects Information System

Johnson, E.A. and Gutsell, S.L. 1994. Fire frequency models, methods and interpretations. Advances in Ecological Research 25:239-287.

Rasmussen, G.A., G.R. McPherson, and H.A. Wright. 1988. Economic comparison of aerial and ground ignition techniques for rangeland prescribed fires. Journal of Range Management 41:413-415.

USDA Forest Service Pacific Northwest Research Station fire tools

Wright, H.A. and Bailey, A.W. 1982. Fire Ecology. John Wiley & Sons, New York.