• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

IMMEDIATE FIRE EFFECT ON PLANT:

Western yarrow's rhizomes and mycorrhizae are usually only slightly damaged by fire [10,38,60], although western yarrow is susceptible to fire-kill and reduction by severe fire [51]. Western yarrow is not highly flammable. Out of 14 species commonly found in boreal forests, western yarrow has the lowest potential ignitability based on chemical characteristics measured on live stem, live leaf and dead leaf tissues. These rankings rely primarily on total ash, silica-free ash and energy content [40]. Ignitability is measured as time to ignition.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

No entry

PLANT RESPONSE TO FIRE:

Fire results in fragmentation of western yarrow's rhizomes stimulating regeneration [15]. Cover and frequency of western yarrow generally increase 1 to 2 years after fire but not with any consistent pattern [4,13,14,32,40,56,71]. After initially increasing in cover, western yarrow may decrease to unburned levels as early as 3 years after fire [17,37,65,75]. Production doubled within 3 to 4 years postfire near Missoula, Montana [6] and other ponderosa pine/mountain grassland ecosystems [32,69]. In another study of fire effects in ponderosa pine, western yarrow increased by 0.37 stem/m in 6 years, a negligible amount [55]. Western yarrow is responsive to season of burning. Late spring burning usually reduces western yarrow [4,12,66].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

The initial surge of western yarrow is probably caused by 
extensive rhizome sprouting; mineral soil exposure and the 
resulting favorable seedbed; less competition from tree, 
grass and shrub cover; and nutrient release [28,53].


A burn was conducted each April for at least 24 years on a 
rough fescue(Festuca scabrella) grassland in a 
quaking aspen parkland in east-central Alberta. Average 
frequency and canopy cover for western yarrow were as
follows [3]:

            % Frequency               % Cover              
            burned  unburned         burned   unburned
            36         23                   3.0         1.1

Density and crown area of western yarrow 
(per 180,000 in²)following an August wildfire 
of moderate severity in a northeastern California range 
dominated by bitterbrush (Purshia tridentata) 
and various perennial bunchgrasses were as follows [23]:

                             Number of plants     Crown area (in²)
Unburned plots      99                             153
postfire yr 1             3                              29
postfire yr 2             9                            101
postfire yr 3           88                            531
postfire yr 4          269                           252
postfire yr 5            48                         1391

Productivity values (kg/ha) of western yarrow before and after 
a late August fire in western Wyoming quaking aspen communities 
are listed below for plots of different burn intensities [9]:

Before burning:  14 kg/ha 
After a "light" burn:  40 kg/ha 
After a "moderate" burn:  16 kg/ha 
After a "heavy" burn:  14 kg/ha

FIRE MANAGEMENT CONSIDERATIONS:

Western yarrow's good resprouting ability, high germination percentages, and competitive seedlings result in a remarkable persistence under fire disturbance. Western yarrow often appears in the first stages of succession [15,63]; however, no consistent trends relative to age of burns seem evident for the western yarrow [4,57].

Western yarrow has low ignitability, and can be used as a fire barrier, created by replacing highly flammable vegetation with species that are less likely to burn [41]. Planting less-flammable vegetation in fire-prone areas, or around property and fire-sensitive areas, may help prevent ignition or slow fire spread [40].

Related categories for SPECIES: Achillea millefolium | Western Yarrow