• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

The crown of young trees is symmetrical and sharp-pointed, becoming irregular and rounded at the summit [162]. California white fir has a narrow, cylindrical, almost spire-like crown [185,186]. Rocky mountain white fir tends to have a broader crown [185]. White fir branches are short and stout, with leaves 1.2 to 2.8 inches (3-7 cm) long and generally curved upward. Branches are arranged in whorls of 4 or 5, which are repeatedly branched in one plane to form flat, horizontal sprays. Buds are blunt and resinous [71]. In Arizona white fir stands, braches reach nearly to the ground if the trees are widely spaced, and in more dense stands, half or more of the trunk is often bare [162].

The rooting habit of white fir is usually fairly shallow, but appears to be adaptable to local conditions: deep and intensive where soil conditions permit, to shallow and widespread where rocks or seasonal water tables limit effective soil depth. There is no strong tendency to maintain a single deep taproot although rapid taproot development is critical for survival of new germinants in a dry summer climate. White fir is susceptible to wind throw following partial cutting. Root diseases may contribute to lack of wind firmness. Root grafting between firs is common and is a factor in the spread of root rots [178]. Effects of mycorrhizal associations have been explored and appear to be important in white fir, especially for establishment and early growth on poor sites. It appears that bare mineral soils promote the association [22,178].

White fir is thought to be a slow growing species [157,178,219]. It can survive for exceptionally long periods as a suppressed tree and still respond to release by increasing growth dramatically. White fir may reach ages of 300-400 years. Old growth characteristics in Southwest are given by Stuever and Hayden [294]. Information on productivity of white fir is available [178,201,262].

The varietal differences in California and Rocky Mountain white fir are based on differences in morphological and chemical characteristics, such as needle tip shape and stomatal arrangement [40,133] and terpene content [124,178]. 

RAUNKIAER [253] LIFE FORM:

REGENERATION PROCESSES:

Cone and seed production: Cone and seed production vary with tree size, age and dominance. White fir trees can begin bearing cones at 40 years and continue beyond 300 years [178]. The best producers are mature, healthy dominants in the 12 to 35 inch (30-89 cm) d.b.h. range [120,178]. Cone production is higher on trees near openings [119] and on trees following release [178]. Immature trees can produce heavy seed crops, but their production is more erratic than that of mature trees [119,178]. Pole-sized trees in dense stands will not produce cones unless their leaders reach full sunlight. In mixed conifer forests of California only 4% of understory white fir between 3.6 and 7.5 inches (9.1-19 cm) in diameter produce cones [105]. In Oregon and California, heavy seed crops in white fir are borne on a 3- to 9- year cycle, with fair to good crops occurring every 2 to 5 years, and bumper crops every 5 to 9 years [178]. In the Rocky Mountains, medium to heavy seed crops are produced every 2 to 4 years [105]. Cone production patterns may be different on extreme sites [110,120,157,178]. Because cones are borne on the uppermost part of the crown, any top damage caused by insects, diseases or mechanical agents (e.g. wind and snow) directly reduces cone production. Cones produce about 185 to 295 seeds apiece [105,119]. Seed numbers can reach 600,000/acre (1.5 million/ha) or more where white fir is a site dominant [112,178], and as many as 220,000 where white fir is a minor overstory component [181].

Seed predation: Insects that feed on cones and seeds may seriously reduce yield. Seed chalids typically destroy 8 to 10% of white fir seeds and have destroyed up to 60% of a crop [108]. Cone moths, cone midges, and cone maggots also reduce yields [119]. In California and Oregon, the Douglas squirrel cuts and caches cones, but generally takes less than 1% of the cone crop [106].

Seed dispersal: White fir seeds are released and disseminated by wind as the cone disintegrates on the tree in the fall. Because white fir seed has a short, broad wing relative to its weight, if falls more rapidly and travels a shorter distance from the tree than many of its associated species. Downwind seed spread into an opening is about 1.5 to 2 times the height of the tree [119,178,203]. A small percentage of seeds may be transported greater distances by strong or gusty winds [218].

Germination: After release in the fall, white fir seeds overwinter in or under the snow. This cold, moist stratification is required for germination [108]. Germination of white fir seeds occurs in the spring immediately following snowmelt. Where snowpack is deep, seeds may germinate in, on, and under the snow [157,178]. Of white fir seeds sown in November in white and red fir stands in the Sierra Nevada, 82 to 86% of all germination occurred by May 9, and 96 to 98% by May 22 [39]. Only a small proportion (20-50%) of seeds are viable [178]. Thus, germination is low, averaging about 37% . Under controlled conditions, white fir seeds may be stored for 5 or 6 years, but under natural conditions seeds do not remain viable over 1 year [108].

Seedling establishment and survival: White fir seeds that germinate in the snowpack, above the ground, rarely survive, therefore, seeds that fall before the permanent snow cover are more likely to produce seedlings [119]. Germination and early growth are best on bare mineral soil, though seedlings may establish in soils covered by a litter layer [71,100]. Root systems developed in mineral soil without organic layers are longer, heavier, and have more mycorrhizal root tips than those grown in soil with organic layers [22,178]. Seedlings generally establish best in partial shade, and can establish easily under a closed canopy in dense shade [178]. Shade favors seedling survival in white fir because seedlings are very sensitive to soil drying and heating. Damping off fungi, cutworms, drought, heat, trampling and browsing are responsible for most seedling mortality [39,119]. White fir seedlings are more susceptible to spring frost damage and deer browse than many associated species [157,178,219].

Growth: Once established, white fir grows best in full sun [178]. Shade-tolerant white fir saplings can, however, endure decades of suppression under a closed canopy or in dense brushfields. Leader growth is very slow under these conditions, and suppressed plants may be only 3 feet (0.9 m) tall at 50 years [91]. White fir dramatically increases in diameter and height growth when canopy openings are created, or when its height surpasses surrounding vegetation [100,181].

SITE CHARACTERISTICS:

Rocky Mountain white fir grows on high mountains with precipitation ranging from 20 to 35 inches (510-890 mm). California white fir grows in cold, high elevations and warm-to-hot low elevations with precipitation ranging from 35 to 75 inches (890-1900 mm), but grows best in the southern Cascades and western Sierra Nevada, where precipitation is between 39 and 49 inches (990-1240 mm) [178]. Within the mixed conifer forests of the Sierra Nevada, white fir tends to occupy the more mesic sites, such as northern exposures, at lower elevations, and the more xeric sites at upper elevations [111,262]. In Utah, white fir typically occupies cool and dry northern exposures [201,344]. In the Southwest, white fir occupies numerous topographical settings, and local conditions can vary from cold and moist to warm and dry [83]. Winter snowpack provides the majority of the moisture at high elevations, with fall and early spring rains providing most of the moisture at lower elevations [178]. The upper latitudinal limit of white fir may coincide with a mean maximum January temperature of about 30 to 32 degrees Fahrenheit (-1 to 0 °C) [201]. White fir is sensitive to both frost damage and, occasionally sun scald [178]. White fir is also moderately susceptible to ozone damage [157]. White fir is less tolerant of shade than associated true firs (except red fir), is slightly more tolerant than Douglas-fir, and is much more tolerant than pines or oaks [157,178,219].

California white fir occurs in a wide elevational range, as low as 3,000 feet (900 m) in the North Coast Ranges to over 10,000 feet (3000 m) in the San Bernardino Mountains and the Sierra San Pedro Mártir of Baja, California [186]. Pure white fir forests are common in Oregon and California and they occupy a narrow elevational band from about 4,600 to 5,250 feet (1400-1600 m) in the southern Cascades and from 5,400 to 5,900 feet (1650-1800 m) in the Siskiyou Mountains of northwestern California [111]. In the Sierra Nevada, white fir is a major component of mixed conifer forests occurring between 4,100 to 7,200 feet (1250-2200 m) [262]. Rocky Mountain white fir is found most frequently at elevations ranging from 6,900 to 8,900 feet (2100-2700 m) [178]. In the mountains of southern Arizona, it occupies the highest elevations [188]. Generalized elevational and precipitation ranges are as follows, by state:

White fir grows on a variety of slightly to strongly acid soils from almost every type of parent material [14,105,111,178,186,201]. It is generally tolerant of a wide range of soil conditions, nutrient availability and pH values. Growth and development are best on moderately deep and well-drained sandy-loam to clay-loam soils, regardless of parent material. California white fir is moderately sensitive to excess soil moisture [178]. White fir is usually found on frigid soil temperature regimes or the warmest of the cryic regimes [223,294]. In the arid Organ Mountains of southern New Mexico white fir occurs as a topo-edaphic climax on the cool upper eastern slopes [84].

White fir grows from canyon bottoms and ravines up to ridgetops on gentle, moderate, and steep slopes of all aspects. It develops best on gentle slopes and level ground [178]. In the Rocky Mountains, white fir, along with blue spruce and Douglas-fir, often replaces the dominant deciduous species near middle elevation streams passing through sheltered valleys or canyons [245].

Botanical associates of white fir that may affect its growth include snowbrush ceanothus, which contains allelopathic chemicals in its foliage that suppress radicle growth of white fir [66,178]. Mycorrhizal associations are thought to protect white fir roots from allelopathic chemicals produced by bracken fern [178].

SUCCESSIONAL STATUS:

The white fir series in the Southwest can have varying mixtures of white fir, with conifer associates dependent on moisture and temperature relationships of the site and stage of succession. The more successful reproduction of white fir is diagnostic of the white fir series [188,232]. Mosaics of contrasting successional stages are considered to be the result of both insects and past fires [223]. Within the mixed conifer type, white fir tends to achieve climax dominance on moist sites [111,327] and in localized areas with long fire-free intervals that give white fir the chance to mature to a point where it is moderately fire-tolerant. In mixed conifer forests with a natural fire regime of low-intensity surface fires, white fir is kept from attaining climax dominance because it is more fire sensitive than its coniferous associates [2,13]. Thus, many white fir habitat types are in mid-successional stages, with various seral species dominating the overstory and white fir dominating the reproductive size classes [40,111]. Seral associates that often dominate a site include ponderosa pine [83,115,185,223,294], Douglas-fir [83,115,185,294], southwestern white pine [185], Gamble oak, New Mexico locust [294], California black oak [113], quaking aspen [40,153,178,223,230,245], and lodgepole pine [245].

White fir will seed into the understory of ponderosa pine stands or in mixtures of ponderosa pine, Douglas-fir, quaking aspen, and southwestern white pine [185]. Many habitat types in the white fir series in the Southwest are dominated by Douglas-fir and ponderosa pine in mid-seral stages, with white fir steadily gaining dominance as succession proceeds [83,294]. Lodgepole pine is a common seral species in the white fir type, with seedlings of white fir in the understory that are less than a meter in height but in excess of 100 years old. Both aspen and ponderosa pine are present at sites where high intensity fires have occurred or where ground fires have slowed or prevented replacement by white fir and Douglas- fir [223]. With fire suppression, white fir is able to mature in the understory with a concurrent decrease in pine reproduction and eventually begins to replace the pines as they succumb to root diseases and bark beetles, resulting in a gradual change in structure and composition in white fir habitat types [27,40,113,115,184,186,257]. White fir will eventually dominate if the fire-free interval is sufficiently long to allow trees to grow to a fire-resistant size, unless another disturbance event gets them. In a Jeffrey pine forest in South Lake Tahoe, California, a disease outbreak killed the Jeffrey pine overstory, releasing the small white firs in the understory which went on to become the predominant species on the site. They then suffered extreme moisture stress form drought and succumbed to a fir engraver attack causing them to die rapidly [269].

It appears that white fir may be an early colonizer of disturbed sites, gradually increasing in dominance over time [34,139,262]. In northeastern California, overgrazing in big sagebrush- steppe communities allowed for the invasion and establishment of white fir in an unusually xeric setting for white fir [257]. White fir may also be a pioneering species in upper elevation meadows within its type where it has been observed to invade by growing near older lodgepole pine [111,178]. White fir is an early seral species at the lowest elevations of the subalpine forest in New Mexico [223].

Following overstory removal (logging or stand-replacement fires) in mixed conifer or white fir forests of the southern Cascades and Sierra Nevada, sites are often dominated by montane chaparral shrubs, primarily ceanothus and manzanita, but also mountain whitethorn, currant and gooseberry (Ribes spp.), chinquapins, and some oaks [65,105,111,149,178]. Seeds of some species may lie dormant in the forest floor for as long as 300 years and germinate following removal of overstory. Sierra mountain misery and grasses may also assume significant roles [178]. Given a nearby seed source and absence of further burning, white fir seedlings can establish under shrubs within about 10 to 20 years [65,218]. A 30 year delay in tree recruitment was observed after a stand-replacement fire in the Lake Tahoe Basin [263]. Given the continued absence of fire, white fir will eventually overtop the shrubs and dominate the site, creating pure stands in otherwise mixed conifer areas [65].

Following overstory removal in mixed conifer or white fir forests in the Southwest, herbaceous species usually dominate the vegetation for the first few years of succession, and diminish in late seral conditions as shading inhibits their growth [83,101,134,157,223,294]. Seral shrubs and trees that follow herbs include aspen, New Mexico locust, Rocky Mountain maple, bush oceanspray, and Gambel oak, and may dominate the site for the next 40 to 100 or more years [83,101,134,153,157,223]. Gradually tree seedlings, including white fir, become established, although growth is slow under the canopy. Scattered conifers emerge above the shrubs after 50 to 100 years. This stage persists for another 50 or more years, with a closed stand of replacement conifers not fully developed until 100-200 years after the fire, the time depending on numerous factors such as shrub density, climatic conditions, availability of tree-seed sources, soil conditions, livestock, wildlife, and human use. Old growth is characterized by conifer overstory and low shrubs in the understory, with possible sparse herbaceous cover [134,157,223]. White fir may establish quickly on mesic sites and dominate early seral stages, while conifers such as southwestern white pine, limber pine and ponderosa pine may dominate the early seral stages on xeric sites [13,225,294].

Crane [69] presents successional trends of white fir habitat types in Colorado that appear reasonable for Utah types as well. In Utah, white fir becomes the dominant climax tree on characteristically cool and dry sites that are usually northern exposures [344]. These sites are apparently too warm and dry for subalpine fir and Engelmann spruce. Although white fir reproduces most abundantly under shade, it also invades open slopes within the mountain-brush zone [201]. White fir and Douglas-fir are replacing brush species on some northern exposures, and, in the absence of fire, these sites may develop into conifer forests [63].

SEASONAL DEVELOPMENT:

In New Mexico, white fir growing with Douglas-fir completed bud elongation by May 4, and most of its flush extension by the middle of June, and exhibited about twice the flush growth of Douglas-fir over a 5-year period [56]. Flowering of Rocky Mountain white fir may extend into July at higher elevations. Female cones reach full size in late summer and turn brown when mature. The seed matures in September, up to 3 weeks before seedfall [178,235].

Radial growth begins before height growth in white fir, and lasts longer. Height growth in white fir begins 1 to 1.5 months later than in associated conifers and lasts about 6 weeks [178]. On the west slope of the Sierra Nevada at 5,200 feet (1585 m) within the Stanislaus National Forest, white fir leader growth began on June 24 and was completed by mid-August. New needles emerge from fascicles after height growth begins [104]. Old needles are shed primarily in the fall and winter. In California, most needles drop in October and November [46].

Related categories for SPECIES: Abies concolor | White Fir