• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

FIRE ECOLOGY OR ADAPTATIONS:

The primary range of white fir is in the mid-elevation, mixed conifer and white fir zones in California and the central and southern Rocky Mountains. These forest types may be characterized by a mixed fire regime, with fires of variable frequency and severity [25], with some sites experiencing frequent surface fires [7], and others experiencing infrequent crown fires. Mean fire intervals are generally intermediate to intervals in understory and stand replacement regimes, ranging between 30 and 100 years [25]. Mean fire intervals in Sierra Nevada mixed conifer forests are estimated to range between 5 and 30 years, and varied in response to ignition source, fuel accumulations, fuel moisture and burning conditions [332]. Any given location within a mixed fire regime could experience some stand-replacement fires and some nonlethal fires along with a number of fires that burned at mixed severities, creating mosaic patterns of stand structure and fuels [25,215,304]. Low severity fires thin understory regenerating trees, while more severe crown fires may knock succession back to herbs and shrubs. Thus, past burn mosaics tended to increase the probability that subsequent fires would also burn in a mixed pattern. Complex mountain topography also contributed to variable fuels and burning conditions that favored nonuniform fire behavior [25]. After decades of fire exclusion, much of the landscape mosaic has aged and advanced successionally, and patches of late successional forests with large accumulations of dead and living fuels have coalesced, increasing likelihood of fires of unusual size and severity [25,54,169,209,285,286,287,315]. This shift toward landscape homogeneity may adversely affect biodiversity, and may also be perpetuated as the probability of large, high-severity fires increases with continued fire suppression [338]. Much of the living fuels in these forests are small white firs and other shade tolerant species, filling in the understories with dense thickets and increasing fuel continuity and fire ladders of resinous foliage, often in cylindrical crowns that may lead to crown fires when they do burn [178,186,223,287]. Fuel loadings in this type may vary widely due to stand history and site productivity [25,330].

There is evidence that a mixed regime may have been important for perpetuation of giant sequoia groves in the Sierra Nevada [25,295]. Giant sequoia groves burned every 2-10 years for the last 3000 years and have not burned in 100-130 years [288,289,295]. The more mesic, mid-elevation, mixed conifer forests of California formerly experienced low to moderate severity wildfires every15 to 30 years [287,332]. Other areas that may have had mixed fire regimes include the Marble Mountains of northern California [304]; the mixed conifer zone in the montane forests of the Madrean borderlands; the Animas Mountains of southwest New Mexico [296]; mixed conifer forests in the Jemez Mountains, New Mexico [307]; the white fir/Rocky Mountain maple habitat type in Arizona and New Mexico [232,294]; the high elevation, white fir/forest fleabane habitat type [294]; the lowest elevations of the subalpine forest in New Mexico [86,157,223]; the mixed conifer zone of the Sandia Mountains, New Mexico [36]; and the white fir zone in the central Siskiyou Mountains, Oregon [3].

White fir is also a component of drier ponderosa and Jeffrey pine habitat types that evolved with an understory fire regime. An understory fire regime is characterized by relatively frequent, low severity fires that result in open, uneven-aged stands consisting primarily of the more fire tolerant species. White fir was not a major component of these stands under this regime, and existed as scattered individuals or small groups that managed to survive to a fire resistant age. Open ponderosa pine, larch and Douglas-fir forests at lower elevations in the west have been extensively harvested and protected from fire resulting in a compositional shift to an unnaturally dense understory of Douglas-fir, grand fir white fir, or incense-cedar [27,185,234]. Areas where this fire regime was important include the ponderosa pine and mixed conifer forests of southern Arizona and New Mexico [33,35,68,115,296]; the Sacramento and White mountains of New Mexico [224]; and ponderosa pine stands in central Oregon [252,297]. Because of changes in fuels during the last century, these areas may now experience crown fires when they do burn, with high tree mortality [5].

White fir may also be a component in ecosystems with a stand-replacement fire regime such as western subalpine forests and Douglas-fir/western hemlock forests [25]. The more arid Jeffrey pine forests on the Mojave Desert side of the mountains in southern California may also have a stand-replacement fire regime due to the slow build up of fuels in the arid environment [287]. Evidence of a stand-replacement fire regime in a white fir-Jeffrey pine forest type in the Lake Tahoe Basin is presented by Russell and others [263]. Similarly, the subalpine forests are limited by cold and are also slow growing so fires are naturally infrequent and when they do burn it is usually a stand replacing fire in severe weather [5,287].

The following table provides some fire regime intervals for ecosystems in which white fir occurs:

Related categories for SPECIES: Abies concolor | White Fir