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Research Project Summary: Understory recovery after burning and reburning quaking aspen stands in central Alberta

Gucker, Corey L., comp. 2005. Research Project Summary: Understory recovery after burning and reburning quaking aspen stands in central Alberta. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].

Source: Unless otherwise indicated, the information in this Research Project Summary comes from the following paper:

Quintilio, D.; Alexander, M. E.; Ponto, R. L. 1991. Spring fires in a semimature trembling aspen stand in central Alberta. Information Report NOR-X-323. Edmonton, AB: Forestry Canada, Northwest Region, Northern Forestry Centre. 30 p.

The study area was part of the Slave Lake Forest located 200 km north of Edmonton, Alberta, and just 6 km west of the small town of Hondo. The precise geographical description is northeast section 30, range 2, township 70, west of the 5th meridian (5006'N, 11408'W).

Sites occurred on well-drained, loamy sand mineral soils with coarse and fine sand under layers. Average elevation was 590 m. Study sites occurred on slopes of less than 10%, but topography was strongly undulated [2].

Vegetation in this study area is likely represented by the following vegetation classifications:

SAF 16 Aspen [1]
SRM 411 Aspen woodland [3]

Study sites were within the boreal mixed wood forest in almost pure quaking aspen (Populus tremuloides) stands that averaged 43 years old. Stands were surrounded by muskeg grasslands with scattered black spruce (Picea mariana). Quaking aspens made up 99% of the basal area and 98% of the stand density in the study area. Average quaking aspen height was 13 m, dbh was 11 cm, and basal area averaged 29 m/ha. Within burned sites, a few white spruce (P. glauca), jack pine (Pinus banksiana), and infrequent white birch (Betula papyrifera) clumps occurred. Several tall shrubs including mountain alder (Alnus viridis ssp. crispa), pin cherry (Prunus pensylvanica), and beaked hazelnut (Corylus cornuta) populated the quaking aspen stands. Common understory small shrubs, herbs and grasses included bunchberry dogwood (Cornus canadensis), American twinflower (Linnaea borealis ssp. americana), cream pea (Lathyrus ochroleucus), wild sarsaparilla (Aralia nudicaulis), dwarf red blackberry (Rubus pubescens), bluejoint (Calamagrostis canadensis), and rough bentgrass (Agrostis scabra).

1972 October 5/Low intensity
1972 October 5/Moderate intensity
1978 May 5 (reburn)/High intensity

The 1972 fire effects on the understory plant community were studied jointly, and in this summary fire characteristics are averaged for the 2 fires. The 1978 spring fire reburned areas that experienced moderate intensity fires in 1972.

The researchers indicate that quaking aspen trees leafed out following all fires, suggesting that sap flow began prior to and occurred during fires.

The objective of this study was to burn quaking aspen stands under a range of conditions. Researchers wanted to quantify the the behavior of these fires and study the subsequent effects of fire on the understory vegetation. This research project summary covers only a fraction of the fires that researchers burned in the area. Only those fires for which corresponding understory vegetation recovery was provided are presented here.

Temperature, relative humidity, and wind speed were lower during the spring reburn than the initial fall fire. Wind speeds were measured at a height of 1 m. The time since rainfall accumulation of 0.6 mm or more was greater on the reburned area than the fall burned sites. Presented below are the weather conditions when the 1972 and 1978 fires burned:

Time of fire Dry-bulb temperature
Relative humidity
Wind speed
Days since rain
(>0.6 mm)
1972 Oct. 5 22.5 24 13.4 4
1978 May 5 (reburn) 15.5 20 6.6 6

The Canadian forest fire indices indicate greater duff moisture and higher buildup index at the time of the 1978 reburn when compared to the 1972 fires.

Canadian fire weather indices*
1972 Oct. 5 92.5 17.5 30 11.9 17.5 15
1978 May 5 (reburn) 91.9 29 53 7.9 29 14
*For an explanation and discussion of Canadian forest fire weather indices see the Research
Project Summary Revegetation in a subalpine forest after logging and fire in central British

Litter mass in the study area averaged 0.3 kg/m, and the average mass of downed wood was 0.37 kg/m. Researchers considered forest floor weights low compared to other forest types in Canada.

Moisture content was lower for the fermentation and humus layers on the reburned sites, and the depth of burn and forest floor reduction were greater on reburned sites. Fuel moisture conditions and forest floor reductions are summarized below:


Moisture content (%) Depth of burn
Reduction in forest floor
litter layer fermentation and humus layers
1972 Oct. 5 8.9 161 1.5 36
1978 May 5 (reburn) 11.2 84 3.5 76

The 1972 and 1978 fires behaved quite differently. Researchers suggest that the 1972 fires would have been rated "fairly easy to control." Fuel consumption was low as was fireline intensity during the fall fires of 1972. The increased fireline intensity during the spring reburn was due to increased fuel loads. The 1972 fires caused mortality in the quaking aspen stands that contributed to the increased fuel load on reburned sites. The researchers indicated that the 1978 fire resulted in greater "bole heating" and "downward heat transfer to the root zone." Researchers allowed the flame front to move 10 m before measuring. The amount of fuel consumed, rate of fire spread, and intensity of the fires are summarized below.

Fire Fuel consumption
Energy/unit area
Rate of head fire spread
Fireline intensity
1972 Oct. 5 0.4 7,457 1.7 236
1978 May 5 (reburn) 3.4 57,261 4.6 4,392

The effects of burning on quaking aspen were analyzed in detail for the low- and moderate-severity fires of 1972.  Sites burned with low intensity had "little to no effect on the quaking aspen overstory." The average mortality for quaking aspen with small diameter stem size (2.5-10 cm) was 79% on sites burned with moderate intensity. Mortality was less than 35% for quaking aspen stems between 10 and 20 cm, and there was no mortality of stems 20 cm or greater on moderate-intensity sites. Tree mortality increased when sites were reburned. Quaking aspen with stem sizes 2.5 to 17.5 cm in diameter suffered 100% mortality when reburned in 1978. For quaking aspen trees greater than 17.5 cm in diameter, mortality ranged from 90% to 97%. Quaking aspen mortality was not restricted to only burned sites, however. On unburned control sites, there was 100% mortality of the very smallest diameter trees (2.5-5 cm), but the average mortality of quaking aspen trees greater than 5 cm in diameter was only 15%. The researchers also noted that postfire sprouting in quaking aspen was "minor" following the 1972 fire and the 1978 reburn. This result was unexpected and may relate to low postfire soil temperatures or depleted root reserves late in the growing season following fall fires. For a more complete review of quaking aspen's recovery following these fires, see the Fire Case Study Overstory mortality after repeat spring prescribed fire in central Alberta.

Fire effects on the understory vegetation were highly variable. Some species were sensitive to any fire, while others were sensitive to only repeated fire. Other species appeared only on burned sites, increased on burned sites, and/or increased on repeatedly burned sites.

Fire-adapted species: The following species increased on, remained the same, or were restricted to burned sites. Many species showed the greatest increases in both frequency and cover on the reburned sites. The following species had greater coverage and/or frequency on burned sites than unburned sites: prickly rose (Rosa acicularis), American red raspberry (Rubus idaeus), lingonberry (Vaccinium vitis-idaea ssp. minus), wild sarsaparilla, bluebell bellflower (Campanula rotundifolia), fireweed (Chamerion angustifolium), Virginia strawberry (Fragaria virginiana), western showy aster (Aster conspicuus), northern bedstraw (Galium boreale), tall bluebells (Mertensia paniculata), arctic sweet coltsfoot (Petasites frigidus var. palmatus), dwarf red blackberry, Lindley's aster (Symphyotrichum ciliolatum), cream pea, limber honeysuckle (Lonicera dioica var. glaucescens), American vetch (Vicia americana), bluejoint, and rough bentgrass (Agrostis scabra).

Fire-sensitive species: The number of species that decreased following fire were fewer than those that increased. A higher proportion of fire-sensitive species were shrubs. The following species had lower coverage and/or frequency or were removed from the community on burned sites: mountain alder, Saskatoon serviceberry (Amelanchier alnifolia), beaked hazelnut, pin cherry, chokecherry (Prunus virginiana), common snowberry (Symphoricarpos albus), velvetleaf blueberry (Vaccinium myrtilloides), twinflower, Canada mayflower (Maianthemum canadense), sidebells wintergreen (Orthilia secunda), and Canadian white violet (Viola canadensis).

The combined effect of season and repeated fire makes it difficult to determine whether a species was sensitive to repeated fire, a particular fire season, or a combination of these.

Below are the percent frequencies and coverages for the understory species on unburned, fall burned, and spring reburned sites. Postfire measurements were made in August of 1972 and 1978 [2].

Common name Scientific name


Fall 1972

Spring 1978

Shrubs Frequency
mountain alder Alnus viridis subsp. crispa 28 16.8 20 20.8 4 0.4
Saskatoon serviceberry Amelanchier alnifolia 20 2.4 ---- ---- 4 1.6
bunchberry dogwood Cornus canadensis 72 19.2 76 18 64 12
beaked hazelnut Corylus cornuta 48 61.2 16 31.2 4 4
American twinflower Linnaea borealis subsp. americana 40 6.8 16 2.8 12 1.2
pin cherry Prunus pensylvanica 20 8.4 ---- ---- ---- ----
chokecherry Prunus virginiana 24 4.8 12 6.4 ---- ----
prickly rose Rosa acicularis 56 20.4 84 30 60 16
American red raspberry Rubus idaeus 8 2 16 5.2 12 2
common snowberry Symphoricarpos albus 20 4 12 1.6 4 0.4
velvetleaf blueberry Vaccinium myrtilloides 16 4 4 0.8 4 0.8
lingonberry Vaccinium vitis-idaea subsp. minus ---- ---- 4 1.6 ---- ----
squashberry Viburnum edule 20 3.6 4 0.4 20 4.8
wild sarsaparilla Aralia nudicaulis 56 11.2 56 14 72 14.8
bluebell bellflower Campanula rotundifolia ---- ---- ---- ---- 12 1.6
fireweed Chamerion angustifolium 8 2.4 32 9.2 80 23.2
marsh horsetail Equisetum palustre 8 0.8 4 0.4 8 1.2
western showy aster Eurybia conspicua ---- ---- 12 2.8 16 3.2
Virginia strawberry Fragaria virginiana 4 1.2 20 2.8 4 1.2
northern bedstraw Galium boreale 8 1.2 20 8 20 3.2
Canada mayflower Maianthemum canadense 40 5.2 16 1.6 8 0.8
tall bluebells Mertensia paniculata ---- ---- ---- ---- 8 4
sidebells wintergreen Orthilia secunda 12 1.6 4 0.8 8 1.2
arctic sweet coltsfoot Petasites frigidus var. palmatus ---- ---- 8 1.6 ---- ----
dwarf red blackberry Rubus pubescens 40 7.2 40 8 64 13.6
Lindley's aster Symphyotrichum ciliolatum ---- ---- 4 0.4 ---- ----
Canadian white violet Viola canadensis 4 0.4 ---- ---- ---- ----
cream pea Lathyrus ochroleucus ---- ---- 44 10 68 30.8
limber honeysuckle Lonicera dioica 4 0.4 4 0.4 12 1.6
American vetch Vicia americana ---- ---- 4 0.4 44 14.4
rough bentgrass Agrostis scabra 8 1.2 28 5.6 20 2.4
bluejoint Calamagrostis canadensis 8 2.4 44 13.2 48 7.2

Researchers successfully studied the in-depth fire behavior of many individual spring fires that burned during a range of conditions. Their collection of empirical fire behavior data was extensive. The measurement of vegetation on unburned, fall burned, and reburned sites allowed for an evaluation of the postfire recovery and fire tolerance of the understory vegetation.

Burning in this area resulted in a change in species dominance. Shrub coverage was reduced on burned sites, while forb and grass species increased following fire. The coverage of shrubs was drastically reduced on reburned sites, which became forb and grass dominated. The decrease in shrub cover may affect small mammal and bird populations in the area as fruit production may have suffered a setback. However, the increase in grasses and likely new shrub growth may increase the use of burned areas by livestock and/or native ungulates.


1. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. [905]
2. Quintilio, D.; Alexander, M. E.; Ponto, R. L. 1991. Spring fires in a semimature trembling aspen stand in central Alberta. Information Report NOR-X-323. Edmonton, AB: Forestry Canada, Northwest Region, Northern Forestry Centre. 30 p. [19243]
3. Shiflet, Thomas N., ed. 1994. Rangeland cover types of the United States. Denver, CO: Society for Range Management. 152 p. [23362]

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