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

IMMEDIATE FIRE EFFECT ON PLANT:

Young grand fir have thin bark and are easily killed by fire [109,112]. Trees under 4 inches (10.2 cm) diameter at ground level are most susceptible to direct fire mortality [89,92]. The bark thickens as trees age, and mature trees are moderately resistant to fire [12,34]. Ground fires burning into the duff injure shallow roots and may kill even mature trees [44,67,71,108,164].

Baker [71] found that grand fir seedlings in the laboratory were killed by exposing the stems to temperatures of 121 degrees Fahrenheit (49^oC) for 10 minutes.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

Because grand fir wood does not contain decay-inhibiting properties nor exude pitch over wounds, trees that survive fire are susceptible to the entry of decay fungi through fire scars and stimulation of dormant decay by fire injury. The problem is more serious east of the Cascade Range crest because of the ubiquitousness of Indian paint fungus in the eastern portion of grand fir's range. [7,8,9,14,16,108,62].

PLANT RESPONSE TO FIRE:

Grand fir regeneration is common after fire [9,155]. Seedlings establish on burns mostly from off-site seed sources [8,109,182]. Mature grand fir that survive a fire provide an on-site seed source [8]. Fire provides a favorable seedbed. When different substrates were compared, grand fir germination was best on ash or mineral soil [69,162]; however, seedling mortality may be higher on burned soils due to higher surface temperatures on blackened compared to unburned soils [159]. Seedlings often establish in the 1st few postfire years. For example, following a severe wildfire in a mature grand fir/queencup beadlily association in the Blue Mountains of Oregon, grand fir seedlings were 1st noted in study plots at postfire year 5 [109]. Following the Sundance Fire in northern Idaho, grand fir seedlings were 1st noted in postfire years 4 to 9, with time of 1st emergence varying among study plots. [176]. Because grand fir seedlings are not as drought tolerant as many conifer associates, grand fir establishment is sometimes slow or delayed by drought, but grand fir is usually established as component of seral vegetation by 20 to 30 years after fire [133,196]. Grand fir regeneration is also common after fire thins a dense overstory [9]. As a shade-tolerant tree, grand fir continues to establish until canopy closure in late succession [112].

Low-severity fire may have little effect on grand fir. A "light" fire in an early-seral grand fir/twinflower association the Oregon Blue Mountains killed the pole-sized overstory conifers (grand fir, Rocky Mountain Douglas-fir, and Rocky Mountain lodgepole pine), but their relative coverage remained similar to prefire levels during early postfire seedling establishment. Prefire coverage of grand fir was 5% compared to 3% coverage at postfire year 5 [109].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

In northeastern Oregon, 3 wildfire sites were selected to study fire's effects on late-seral grand fir/big huckleberry associations. Two sites were severely burned, and 1 site was lightly underburned. The severe fires killed all overstory and understory grand fir. The low-severity fire was continuous with fire scorching only the basal portion of the large-diameter (30-40 inch (76-102 cm)) trees. The low-severity fire reduced overstory grand fir coverage from 55 to 40%, and the understory was reduced from 10 to 5%. A thicket of grand fir saplings was reduced by 30% [109].

Fire may aid grand fir regeneration on most sites, but grand fir may regenerate poorly after fire on south-facing slopes or on dry sites [58,98]. In a grand fir/pachistima habitat in the Coeur d'Alene River drainage of northern Idaho, grand fir established readily on unburned sites following clearcutting, but required shade for regeneration on clearcut and burned sites [195].

FIRE MANAGEMENT CONSIDERATIONS:

Historically, low-severity surface fires and patchy, mixed-severity fires killed young grand fir and Douglas-fir in the understory while favoring early-seral, fire-tolerant tree species including Oregon white oak, ponderosa pine, Douglas-fir, and western larch [81,111,190,191]. Once open, park-like stands are being invaded by grand fir, other firs, and Douglas-fir, resulting in poor regeneration of early-successional trees [81,15,34,130,190,191]. Fuel loads have increased and produced very fire-prone communities with high probabilities of crown fires [91,92]. Fire exclusion has altered forest structure and affected understory vegetation [111]. Stands have developed understories or multiple canopy layers of grand fir and other shade-tolerant species [34]. These understories may be extremely dense, often thousands of stems per acre. Without fire, understory grand fir usually develop into thickets of stressed trees [15].

Underburning in grand fir stands reduces fuels and permits regeneration of pines and other fire-tolerant trees [136]. Several factors are considered in predicting and modelling mortality of grand fir and other conifers [154]. Underburning grand fir on steep ground generally results in high mortality [22]. Mortality is also dependent on bark thickness, stand structure, and duration of fire. Peterson and Ryan [144] present a model for predicting mortality of grand fir, subalpine fir, and Douglas-fir in the northern Rocky Mountains based on tree morphology, stand structure, and fire characteristics.

Fuels and fire behavior: Grand fir forests are usually highly productive, which leads to rapid fuel accumulation [85]. Mid-slope forests such as those occupied by grand fir are more prone to severe, stand-replacing fire than forests at lower or higher elevations [12,34]. Habeck [162] found that fuel loads in old-growth grand fir (> 250 years of age) in the Selway-Bitterroot Wilderness often exceeded 41 tons per acre (100 t/ha), with litter and duff layers averaging 5 inches (12 cm). Such highly productive sites are subject to reburn. Barrett [18] defines a reburn as a fire that burns in heavy downed woody fuel resulting from tree mortality in a previous fire, occurring when tree regeneration is in the seedling or sapling stage. Barrett [18] found that on the Clearwater National Forest, the driest aspects were most likely to reburn, but potential for reburn was also present on productive north slopes.

Average heat release of live grand fir has been summarized as follows [114]:

Wood		Bark		Twigs		Foliage
BTU/lb	(Mj/kg)	BTU/lb	(Mj/kg)	BTU/lb	(Mj/kg)	BTU/lb	(Mj/kg)
8,300	(19.31)	9,641	(22.43)	8,894	(20.69)	9,497	(22.09)

In a literature summary, Minore [148] reports that fire spread in fresh grand fir slash is intermediate compared to slash of 7 associated conifers. Fire spread in 1-year-old grand fir slash is slower than fire spread in 1-year-old slash of all associated conifers except western larch, in which fire spread is similar. Photo guides have been prepared for appraising slash fuels in grand fir forests of northern Idaho, and for downed woody fuels in grand fir, western larch, and Douglas-fir forests of Montana [67,119].

Fuel models: Brown [24] and Moeur [135] present equations for predicting crown width and foliage biomass of grand fir and associated conifers.

Keane and others [112] predict that decomposition rates of litter in grand fir-dominated forests are an order of magnitude less than in ponderosa pine or Douglas-fir forests.

Fire behavior models: Agee [4] provides models for predicting stand conditions that initiate crown fire in grand fir and other western forest types based on the critical surface fire intensity needed to initiate crowning (equation 1), and for identifying conditions that allow crown fire to spread (equation 2):

I_o = (Czh)^3/2   (equation 1)
where
I_o = critical surface intensity
C = 0.010 (constant)
z = crown base height
h = heat of ignition (largely a function of crown moisture content),     and

E = Rdh   (equation 2)
where
E = net horizontal heat flux, kW/m²
R = rate of spread, m/sec
d = bulk density of crown, kg/m³
h = heat of ignition, kJ/kg

Restoration: The general objective of restorative management is to develop open stands of seral conifers resembling stands maintained by historic fire regimes. Restoring presettlement stand conditions and fire regimes to grand fir habitats also reduces stand susceptibility to outbreaks of insect and fungi [37]. Because of dense understories of grand fir and other shade tolerant conifers, it is usually necessary to begin restoration with a "low thinning" treatment that removes excess understory and weak understory trees. Low-severity prescribed fire is then conducted to reduce fuel loadings, kill understory conifers, and promote herbaceous and shrub species in the understory. Once thinning and burning are accomplished, the stand can be maintained by periodic underburning alone, at 15- to 30-year intervals [15]. A selection cutting that retains many of the dominant overstory trees also helps maintain open-stand conditions when tree harvesting is an objective [15,38].

Related categories for SPECIES: Abies grandis | Grand Fir