• 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:

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

Prescribed low-intensity summer surface fires in mixed conifer forests of Crater Lake National Park, Oregon, resulted in high mortality of small white firs [302]. Similarly, prescribed, low-intensity fall surface fires in a giant sequoia-mixed conifer forest in Kings Canyon National Park, California, resulted in mortality of 91% of trees less than 6 inches (15 cm) dbh, 39% of trees 6-12 inches (15-30 cm) dbh, and 5% of trees larger than 12 inches dbh [168].

Greater reduction in density is obtained in stands with higher prefire densities. In white fir stands with dense vegetation, consisting of both young stands of pure white fir and open stands of white fir with a shrub understory, fires burned at high intensity and killed most trees in the area studied in the Marble Mountains of California. In lower density forest stands of white fir with old-growth characteristics, fires were of low intensity and burned down rotten logs and standing snags with very little damage to the canopy trees [304]. Prescribed fires resulted in an increase in the density of giant sequoia at the expense of white fir [164,165,169].

Greater mortality of mature trees could be expected in stands with a deep litter layer, since smoldering of the duff for long periods after the fire has passed kills the moderately shallow roots of white fir [4].

PLANT RESPONSE TO FIRE:

Fire may encourage growth in white fir by eliminating competition. Evidence from a fire-scarred white fir stump in Oregon shows that after being scarred as a sapling-sized tree, it had growth release [3]. However, trees damaged or weakened by fire are also more susceptible to attack by insects and disease. Fire scars may allow a point of entry for a variety of disease and decay organisms [178,336], and fire-weakened trees that are attacked by insects can be killed within a few years [302].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

In the mixed conifer zone of the Sierra Nevada, white fir seedlings are often abundant under montane chaparral shrubs that form brushfields after high severity crown fires. Conard and Radosevich [65] found white fir reproducing abundantly on shrub-dominated sites, with a combined seedling and sapling density ranging from 445 to 4,453 per acre (1100-11,000/ha) where crown fires had occurred 38 to 62 years earlier. Densities of both snowbrush ceanothus and manzanita were higher on burned than on unburned and on logged than on unlogged plots [328]. Fire severity can be an important determinant of shrub response in white fir stands. Preharvest burning in a 70-year-old white fir stand in northeastern California resulted in 410,000 snowbrush ceanothus seedlings/ha in moderate-consumption burns, and 94% seed mortality in more severe burns - killing seeds to a depth of 10 cm in the soil [329,330].

Minnich [218] studied conifer reproduction on burned areas in mixed conifer forests in the San Gabriel and San Bernardino mountains of southern California. He found that seeds originating from outside the burned area were responsible for abundant white fir production on burns older than 10 years, but that reproduction was scant on burns less than 5 years old. Similarly, no white fir seedlings established within 5 years of a crown fire in the Sierra Nevada, even though several mature trees survived the fire and thus provided a seed source [65]. Another Sierra Nevada study found negligible white fir reproduction 17 years following a stand-replacing fire, even though a seed source was readily available [51]. Less than 1 year following underburning in a giant sequoia grove, white fir seedling establishment in June was abundant, with 39% survival in October [6].

FIRE MANAGEMENT CONSIDERATIONS:

Thinning: Prescribed burning in areas where white fir is not desired may be useful to control its abundance and promote the growth of more desirable seral species. Burning in some areas may create conditions favorable for suckering of aspen or Rocky Mountain maple [294]. Replacing conifer cover with aspen, shrub, or herbaceous vegetation can improve water yield [55]. Prescribed fire has been used to effectively eliminate fire-intolerant species such as white fir and Douglas-fir and to favor more fire-tolerant species such as ponderosa pine [27,98,115,227] and giant sequoia [163,168]. A dramatic decline in white fir basal area was observed following prescribed burning in the Sierra Nevada [233]. This may be useful in areas experiencing high levels of mortality due to stress from competition for water and resources in overcrowded stands and subsequent vulnerability to insects and disease [267]. Stephenson and Calcarone [287] developed a model to predict areas of montane conifer forest with overcrowded stand conditions. Areas predicted to be experiencing high stand densification meet all of the following criteria: vegetation type is a conifer forest, elevation is below 7,500 feet (2300 m), mean annual precipitation is greater than 25.6 inches (650 mm), canopy cover is greater than 60%, and slope is less that 60%. Fuel loads may be too hazardous to secure desired mortality of white fir while maintaining relatively low mortality of mature trees. Spring burning in old-growth ponderosa pine at Crater Lake National Park resulted in 30% mortality of ponderosa pine greater than 9 inches (22 cm) in diameter [297]. When fire prescriptions cannot ensure that young white fir will not ignite the crown of overstory trees, cutting all trees under a specified size before burning reduces this fire hazard [27]. In three different studies, white fir less than 9 inches (23 cm) dbh [170], 6 inches (15cm) dbh [222], and 11 feet tall [48] were felled before burning. This method can help to reduce fire damage from future wildfires, as well [5]. Cutting without subsequent burning is less effective [27]. A method of prescribed burning that decreases the probability of damage to mature white fir is given by Weatherspoon and others [331]. However, prescribed burning is not recommended as a thinning tool where true fir is the desired crop tree. In underburned white fir stands in southern Oregon, 36% of the residual white fir trees had sufficient scorch to cause partial cambial death that was associated with stained and decayed wood even 2 years after the burn [98].

Fuel reduction: Heavy fuel loadings and well-developed understories of shade tolerant conifers like white fir set the stage for stand-replacing crown fires [5,349]. By leaving the largest trees and treating fuels, fire tolerant forest conditions can be created, so that fire severity can be reduced. These treatments are sensible where low-severity fire regimes are now supporting high severity fires due to fuel build ups, but not in areas with stand-replacement fire regimes, where weather is the driving factor in fire severity [5]. Prescribed, low-intensity fires will kill large numbers of small white fir and reduce fuel loading, helping to reduce this threat [4,159,164,171,286,302]. Not all forest floor fuel is consumed in an initial prescribed burn [265], and much of the initial volume reduction may be replaced by material killed but not consumed in the initial fire [4,55,86,164,168,302]. The fuel ladder is generally broken by the 1st fire, so that a 2nd fire is generally easier to control, 5-10 years after the first [4,171]. Fire effects monitoring in Sequoia and Kings Canyon national parks reveal an average initial reduction in fuel load by 71% (93% duff and 56% woody), 1 year after prescribed burning, an increase in total fuel load from the 1-year postfire to the 5-year postfire inventory and a total fuel load exceeding prefire levels after 10 years, with woody fuels nearly double their prefire levels and duff at 28% prefire levels [163,164]. Tree density was reduced from 498 prefire to 295 post; white fir was 60% pre and 56% 1 year post; 10 years post was at 51% and giant sequoia had increased from 7% to 23% [163,254]. Between 80 and 90% of shrubs on a site will be top-killed by fire. Those capable of sprouting will do so based on season of burn and degree of duff consumption or fire severity. Sprouting shrubs are most susceptible to fire kill under dry conditions in late spring and early fall when fuel consumption is highest [4,159,160]. Summer and fall burning more effectively reduce white fir and giant sequoia fuels than does spring burning [7,167]. Fire used for fuel reduction must be handled carefully to avoid escape and stand damage [349]. Fuel accumulations are site specific but can be estimated using prediction equations based on stand basal area, tree height or diameter or with depth of forest floor and stand overstory age. Info on production and estimation of fuels and fuel characteristics are available for Sierra Nevada mixed conifer [312,313,314], giant sequoia [265,334], ponderosa pine and mixed conifer in the Southwest [264], and for mixed conifer in Arizona [122]. Fire behavior models may be used to predict how silvicultural and fuels treatments will affect fire behavior [286]. In areas with high fuel loadings, dense multistoried stands, and smoke restrictions, it may be necessary to use silvicultural tools that include prescribed fire [123,221,246].

Preharvest underburning/site preparation: Underburning before timber harvesting with the shelterwood method in mixed conifer forests can be used to aid natural regeneration, and reduce shrub seed reserves in the soil [55]. Prescriptions developed in the Blue Mountains of northeastern Oregon recommend felling all understory trees less than 6 inches (15 cm) in diameter before burning. The combination of cutting and burning removes all advanced regeneration, thus sanitizing the site of heart rot which is present in many 5- to 6-inch diameter (12.5 -15 cm) white fir (these trees are white fir x grand fir hybrids). Following harvest, seedling establishment of all conifers was abundant [222]. In some locations preharvest underburning is not recommended because it stimulates dormant shrub seeds to germinate [328]. In the Siskiyou Mountains, soil erosion can be a problem following fires that remove duff layers on granitic soils [30]. Severe fires can eliminate dwarf mistletoe by destroying infected stands [69]. In the Blue Mountains of Oregon, prescribed burning was initiated to sanitize the sites of heart rot and stagnated understory, prepare the site, and encourage natural regeneration [221,246].

Fire can enhance range and wildlife habitats by rejuvenating forage and browse species [55,221,237,246]. Use of prescribed fire for fuel management in California spotted owl habitat is suggested to reduce the threat of stand replacement fires [273,332]. Prescriptions may include leaving snags and larger size fuels for wildlife habitat [221,246]. Additionally, moisture stored in these logs may expedite forest recovery by providing important refuges for roots and associated mycorrhizal fungi of pioneering vegetation [23].

Restoring fire to its natural role in Sierra Nevada forests by prescribed burning at lower and middle elevation types and by allowing lightning fires to burn in higher elevation forests is suggested [169]. Returning fire and natural process to areas that evolved with mixed fire regimes requires a deliberate approach based on descriptions of the mosaic of aggregations that constituted the presettlement forest communities, a determination of the kinds of vegetation changes that it is feasible to make, and the degree of success to be expected from different vegetation management techniques, keeping in mind that that the initial restoration program will determine the character of the vegetation mosaic for centuries to come [52]. Structural maintenance objectives may be biologically infeasible, thereby restricting management to the pursuit of process maintenance objectives [53]. This is complicated by the need to return the forest community to prefire exclusion structure before reintroducing fire to the ecosystem. Managers need region specific fire regime data to develop process-based management schemes [301], and/or determine reference conditions for key ecosystem functional and structural components that may be used in an ecosystem management context [115]. Characteristics for mixed conifer forest reference stands at Sequoia are given by Riegel and others [256]. Tools available to fire managers for prescribed burning planning include PREFEX, a small expert system for managing fire effect information [99], and the Fire Monitoring Handbook [254]. Smoke production and air quality must be considered [58]. Water quality was not adversely affected by prescribed burning in a ponderosa pine-mixed conifer watershed in east-central Arizona [121].

Related categories for SPECIES: Abies concolor | White Fir