Western larch is a fast growing, long-lived, deciduous conifer native to alpine and subalpine forests of the northwestern United States and adjacent Canada [4,45,115,154]. Trees over 900 years old have been reported [80,115,116].
One of the world's largest larches, western larch typically grows 100 to 180 feet tall (30-55 m) but can be over 200 feet (60 m) tall [50,57,71,72,115,157], with diameters up to 6 feet (2 m) [50,62,72,157]. Basal area increases rapidly to about age 40, then decelerates and nearly levels off after age 100 [115]. A deep, spreading root system stabilizes these large trees [57,72]. In a synthesis of literature on northwestern trees, Minore [93] ranked western larch root depth in the middle category of 5 categories.
Bark in mature trees is thick and furrowed into large, flaky plates [50,71,115,157]. At age 50, basal bark thickness ranges from 5 to 10 inches (12-25 cm), and at age 100 bark is 10 to 18 inches (25-45 cm) thick [12]. Western larch trunks are usually bare for a half to a third of the height when in stands, while trees in the open may have branches to within a few feet of the ground [71,72,157]. Crowns are generally short, open, and pyramidal with nearly horizontal branches, though branches may droop in the lower crown of older trees grown in the open [50,57,71,72]. Crown length, width, and density were all ranked low in Minore's [93] synthesis of literature on northwestern trees.
Branches are stout and brittle, changing from pubescent to glabrous with age. Buds are small, rounded, and hairless [71,72,157]. As trees mature, clustered epicormic branches replace older branches, beginning with the lower portion of the crown. Eventually, epicormics, which grow from dormant buds at the base of first order branches, comprise the entire crown [83].
Clusters of 15 to 30 slender, soft, spirally-arranged needles 1 to 2 inches (2.5-5.0 cm) long arise from dwarf twigs [71,72,157]. Western larch foliage is replaced annually [55,57].
Male western larch cones are 0.4 inch (1 cm) long [71,157]. Ovulate cones are papery, 1 to 1.5 inches long (2.5-3.5 cm) and 0.5 to 0.6 inch (1.3-1.6 cm) wide with long subtending bracts [12,50,71,72,157]. Seeds are 0.1 inch (3 mm) long with 0.2 inch (6 mm) wings [50,71,72,157].
The preceding description provides characteristics of western larch relevant to fire ecology and is not meant to be used for identification. Keys for identifying western larch are available [40,70,82].
Western larch is considered the most fire-resistant tree in its range [10,17,24,51,89,137]. Fire is an important part of western larch's ecology; without fire or other stand replacing disturbance, western larch will not regenerate successfully and will eventually be replaced by more shade-tolerant species [116].
Fire adaptations: Western larch has many adaptations that enhance its ability to either survive fire or to quickly colonize recently burned areas. While seedlings, saplings, and poles are somewhat susceptible to fire, trees that are 150 to 200 years old or older are able to survive all but the most severe fires [24,116]. It is common for a handful of mature western larch trees to be the sole survivors after fire [24]
Surviving fire: Western larch's extremely thick basal bark protects its cambium from overheating [10,24,48,49,92,116,126,143,155]. Low resin content and light lichen growth also decrease flammability [10,116,143]. Western larch's characteristic high, open crown; open stand habit; and self-pruning lower branches minimize ladder fuels and risk of crown fire [10,24,48,49,59,116,143]. Its deep roots are protected from surface and ground fires [24,49,59,143]. In a synthesis of the literature on northwestern tree species, Minore [93] ranked western larch's bark in the most fire resistant category and its foliage in the least resistant category. He ranked western larch the most fire resistant tree in British Columbia, Washington, Oregon, and Idaho.
Needles of western larch are less flammable than other species' due to their small size. Because they are never more than 5 months old, they maintain a higher water content than other conifers' needles that are replaced every 2 or 3 years [10,24,49,116,143]. Since western larch replaces its needles annually anyway, it is better adapted to defoliation than other conifers. In fact, after defoliation early in the season western larch trees often will produce a 2nd set of needles from heat-resistant woody buds and epicormic branches [10,17,36,48,89]. The small needles also minimize accumulation of surface litter at tree bases [24].
Postburn colonization: Western larch survivors quickly reseed burned-over areas; on mineral soil seedlings develop rapidly and outgrow competitors [10,48,58,85,116,126]. Fire-killed trees may contribute to seeding if fresh cones in the burned crown mature and disperse seed [10]. Seeds are very light and long-winged, allowing trees in nearby stands to reseed even if no onsite seed source is present [10,48]. Since western larch is a very long-lived and fire-resistant species, a potential seed source remains in the area for centuries once it has established [13].
FIRE REGIMES: Wildfires have occurred in western larch forests for over 10,000 years [10,30]. Barrett and others [20] suggest 2 FIRE REGIMES for western larch forests: 1) 25-75 year intervals between mixed-severity fires, and 2) 120-350 year intervals between primarily stand-replacing fires. While the species is primarily associated with these regimes, frequent surface FIRE REGIMES can also support western larch populations [8]. Frequency and severity of fires vary with elevation, aspect, and habitat type.
Frequent understory fires: Warm, dry sites at the lower elevations of western larch's range in western Montana have been characterized by frequent, low-intensity surface fires occurring at 10 to 30 year intervals. These habitat types include Douglas-fir and grand fir. Stand replacing fires occurred in some of these stands at 150- to 400-year intervals [7,10,60].
In ponderosa pine-western larch habitat in Pattee Canyon near Missoula, Montana, fire scars indicate a mean fire return interval of 7.1 years from 1557 to 1918. Fire occurred an average of every 5 to 10 years from 1750 to 1850, and in 10 to 20 year intervals from 1850 through 1900. After 1900, intensity and frequency of fires were reduced until the late 1900s, when high intensity fires swept through north and south slopes of the canyon [60].
In the Flathead National Forest of western Montana, underburning occurred on average every 20 to 30 years in even-aged ponderosa pine-western larch stands before 1900, with stand replacing fires occurring at 150- to 400-year intervals. From 1735 to 1900 in the grand fir habitat type of western Montana, an average fire return interval of 17 years (range 3-32) maintained western larch as the most abundant tree followed by lodgepole pine and Douglas-fir. Western larch was also found on 3 Douglas-fir habitat type sites with average intervals of 7 (range 2-28), 16 (range 4-29), and 19 (range 2-48) years [15].
Arno's [8] literature review reported that understory FIRE REGIMES prior to 1900 in ponderosa pine-mixed conifer habitat types of western North America favored western larch and other fire resistant species such as ponderosa pine and Jeffrey pine. From 1600 to 1900 in several relict habitat types where western larch occurs in western Montana, fire return intervals averaged 27 (range 17-35) years in the Douglas-fir-big huckleberry (V. membranaceum) type, 25-30 years in the Douglas-fir-dwarf huckleberry (Gaylussacia dumosa) type, and 24 (range 9-42) years in the subalpine fir-queen cup beadlily (Clintonia uniflora) type [7].
Mixed-severity fires: Much of the northern Rocky Mountains are characterized by 30- to 100-year-interval fires of varying severity, which favor open stands of western larch and Douglas-fir in Douglas-fir, western larch, and lodgepole pine habitat types [8,14]. In the Bob Marshall Wilderness, Montana, western larch-Douglas-fir-lodgepole pine and ponderosa pine forest types were historically maintained by mixed severity fire regimes. Many live western larches in this area had 1 to 3 fire scars, and 1 was found with 4 scars. Fire return intervals in this area are nearly twice as long as historic mean intervals [14].
In western larch-Douglas-fir forests of the North Fork of Glacier National Park, Montana, mean fire frequency from 1650 through 1935 was 36 years in relatively dry sites and 46 years in relatively moist sites. In the drier areas, up to 7 understory fires occurred between stand-replacing fires, which occurred at a mean interval of 141 years. Only 1 or 2 understory fires occurred between the less frequent stand-replacing fires (186-year mean intervals) on moister sites [20].
On dry subalpine fir and cool, moist Douglas-fir habitat types that were codominated by western larch, lodgepole pine, and Douglas-fir, average fire return intervals ranged between 30 and 75 years. Severity varied from understory burns to stand-replacing fires [10].
Infrequent stand-replacement fires: In western larch-Douglas-fir forests of northwestern Montana, average fire return intervals from 1735 to 1976 were 120 years in valleys and montane slopes and 150 years for subalpine slopes. Most fires were small and of moderate intensity with occasional patches of high intensity. Though some stands had as many as 6 fires during the period studied, most stands had only 1. A trend of decreasing mean frequency with increasing elevation was noted, and fires on north aspects were more intense and less frequent. Multiple burns occurred primarily on south-facing slopes. In these forests, single burns of low to moderate intensity thinned the overstory and tended to favor regeneration of mixed conifers with patches of seral species, while single intense burns resulted in even-aged forests. Intense, repeated burns (fire return interval <50 years) created shrubfields or homogeneous stands, usually of lodgepole pine [35].
From 1650 to 1935, relatively moist western larch-Douglas-fir forests in Glacier National Park had stand replacement fires at mean intervals of 140 to 340 years [20]. In subalpine fir and Engelmann spruce habitat types in the Middle Fork Drainage of Glacier National Park, Montana, lodgepole pine and western larch stand-replacement intervals were generally 150 to 300 years but as short as 25 years [19], and in grand fir habitat in the Swan Valley of Montana, stand-replacing fires occurred in average 150-year intervals, ranging from less than 20 to more than 300 years [2].
Moist sites of grand fir, subalpine fir, western redcedar, and western hemlock habitat types, which were dominated by western larch, lodgepole pine, Douglas-fir, and Engelmann spruce, burned primarily as stand-replacement fires with average fire return intervals of 120 to 350 years [10].
In western redcedar-hemlock (Tsuga) forests of northern Idaho, fire free intervals ranged from 50 to 100 years with varied intensity. In subalpine fir habitat type, low to medium intensity fires occurred at intervals greater than 150 years [9].
FIRE REGIMES for plant communities and ecosystems where western larch is a common associate are summarized below. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) grand fir Abies grandis 35-200 [8] Rocky Mountain juniper Juniperus scopulorum 103] western larch* Larix occidentalis 25-350 [2,4,6,7,8,9,10,14,15,19,20,35,60] Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to > 200 [8] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-300+ [8,113] interior ponderosa pine* Pinus ponderosa var. scopulorum 2-30 [8,18,87] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [8,11,15] Oregon white oak Quercus garryana < 35 western redcedar-western hemlock Thuja plicata-Tsuga heterophylla > 200 mountain hemlock* Tsuga mertensiana 35 to > 200 [8] *fire return interval varies widely; trends in variation are noted in the species summaryFire is an important management practice for maintaining western larch [10,30,60,91]. Ideally, prescribed burns should expose well-distributed patches of mineral soil and reduce sprouting potential of competitors [67,114,123]. However, even areas with very little burned surface result in significantly better western larch regeneration than unburned sites [10,130]. In good seed crop years, overstocking may result in mesic habitats where too much mineral soil is exposed, and thinning may be necessary to facilitate a vigorous stand [37,67,114,123]. Harsh sites with poor regeneration potential may require planting after burning [10,37].
Norum [67,97] provides detailed recommendations for prescribed burning in western larch-Douglas-fir forests. Based on studies of fire and harvest regimes, Antos and Shearer [2] make recommendations for management practices on grand fir-queencup beadlily habitat type in northwestern Montana.
Timing and Site Conditions: The timing of prescribed burns is important for western larch site preparation; large fuels should be dry and soil moisture low in order to expose mineral soil [10,37,97,99,123]. Norum [98] reported that 10 to 17% water content in small diameter (<4 inch (<10 cm)) fuels is a safe and effective range for burning in western larch-Douglas-fir habitat. Spring and early summer fires usually burn only the surface of the duff layer, while late summer or early fall fires after dry summers tend to be more effective at exposing enough mineral soil for larch regeneration. August or early September, before the fall rains, are the best times for burning north-facing slopes, but on other aspects, there is more flexibility for timing a successful burn [10,37,97,99,123]. Timing of seed dispersal should also be considered when planning fall fires; burning before seedfall is preferable [37]. Depending on site conditions, removing duff from bases of western larch trees to prevent cambium and root damage and/or thinning understory to reduce ladder fuels may be necessary prior to burning [16,67,123].
Fire Intensity: An adequate seedbed for western larch usually results from moderate intensity fires in dry duff. High intensity fires may expose too much mineral soil and result in overstocking [10,37]. Prescribed burning after clearcutting or shelterwood cutting is sometimes used to mimic the effects of severe wildfires on western larch habitat [10,148].
While western larch seedlings usually establish best on severely burned sites [109], underburning may lead to consistent successful natural regeneration but requires careful attention to fuel and site conditions [10,96,104,114]. Harvey and others [69] found burning to remove slash reduced ectomycorrhizal activity after partial cuts in western larch-Douglas-fir forests of northwestern Montana. They recommend against burning to remove slash on harsh sites where understory competition may limit conifer germination or where soil organic matter is low. They suggest underburning is better suited for areas where excessive regeneration is expected or where understory vegetation is desired, especially if burn conditions are chosen to limit duff reduction, which in turn will limit conifer (including western larch) germination.
Models: Reinhardt and Ryan [109,110] present a model for predicting postfire mortality of western larch and 6 other western conifers using bark thickness and percent crown volume. Desired levels of mortality can be predicted using tree species, diameter, height and crown ratio, and maximum allowable flame length. FIRE-BCG simulates fire succession on coniferous forest landscapes of the northern Rocky Mountains, including western larch habitat [78], and FIRESUM models tree establishment, growth, mortality, fuel accumulation, fire behavior, and fuel reduction in ponderosa pine/Douglas-fir forests of the inland northwest [77].
Western larch occurs in mountain valleys and lower slopes, often in somewhat swampy areas [50,71]. It needs well-lighted areas for maximum development, so it performs best in open stands [72]. Western larch is usually found at elevations of 1,500 to 5,500 feet (460-1,700 m) in the northern portions of its range and may be found at elevations over 7,000 feet (2,100 m) in the southern parts of its distribution [50,72,117]. Latitude and elevation affected genetic variation patterns of western larch populations in the Rocky Mountains. Populations from more northern areas and from high elevations had lower growth potential, lower resistance to disease, and lower survival [108]. Elevational ranges for some states and 1 province in western larch's range are:
Montana 3,000-7,200 feet (900-2200 m) [12,79] Oregon 3,500-6,500 feet (1000-2000 m) Washington 2,000-5,500 feet (600-1700 m) [79] British Columbia 2,000-5,550 feet (600-1700 m) [12]Climate: Western larch occupies relatively cool, moist climatic zones. Its upper elevational range is limited by low temperatures, while the lower extreme is limited by low precipitation [45,46,115,117].
Average climatic conditions for western larch's range are [115]:
Temperature 45ð Fahrenheit (7ð C) Maximum temperature 84ð Fahrenheit (29ð C) Minimum temperature 15ð Fahrenheit (-9ð C) Growing season temperature 60ð Fahrenheit (16ð C) Frost free days 60-160 days Annual precipitation 28 inches (710 mm) Growing season precipitation 6 inches (160 mm) Snowfall 103 inches (2620 mm)Climatic conditions for 4 forest habitat types where western larch occurs are [45]:
Douglas-fir grand fir western redcedar-Soil: Western larch is found on a wide variety of soil types, most of which are derived from bedrock or glacial till, but some are of loessial or volcanic ash origin. Deep, porous soils, such as those of mountain slopes and valleys are ideal, and growth is related to soil depth [46,72,115,116,117]. Western larch is also quite dependent on mineral soil or burned seedbeds, more so than any associated tree species including lodgepole pine [46,115]. Western larch is adapted to medium and coarse textured soils with a pH of 6 to 7, and has no salinity tolerance [154].
Topography: Western larch occupies valley bottoms, benches, and mountain slopes. It is found on all exposures but is more common on north and east aspects. South and west exposures are often too severe for seedling establishment [115,116]. This trend is more pronounced in the southern parts of its range, where it is found almost exclusively on north- and east-facing slopes [45,46].
Harsh environments: Western larch has moderate to high resistance to wind throw because its root system provides good anchorage. It is adapted to a wide range of temperatures, but since buds open earlier than associated conifers, hard frosts in late spring may result in cone crop loss [117]. Frozen seed cones are associated with nighttime air temperatures of 25ð Fahrenheit (-4ð C) or less [27,130]. Snow and ice are generally not threats to western larch survival. Wet snow when needles are present (early spring or late fall) may cause snow bend, but rarely results in permanent damage [117].
Mature western larch trees are more fire-resistant than any other species in their range. (Refer to Fire Ecology or Adaptations for more information.) Unless a smoldering surface fire or ground fire girdles boles or the buds are killed by torching, mature western larch trees will survive all but the most severe fires [10,48,61]. Peterson and Ryan [105] found that death of dormant buds on burned western larch occurred 20% lower on trees than foliage death.
Seedlings and saplings of western larch are readily killed by fire [61]. They are less tolerant than those of ponderosa pine [17,89], but may tolerate low-severity underburning better than white fir (A. concolor), lodgepole pine, or Douglas-fir [155].
Larch forests provide food and cover for a wide range of fauna. Rodents eat seeds and seedlings, birds forage for insects and nest in western larch, and squirrels often cut and cache cones. Deer, elk, and moose browse larch, though probably only as a last resort, and black bears forage on sugars that are concentrated in the sap layer in the spring [117,131].
Several studies have investigated the importance of western larch forests to woodpeckers. McClelland [90] found that pileated woodpeckers, a sensitive species dependent on old-growth western larch forests, used 17 times more western larch trees than Douglas-fir even though Douglas-fir trees were 5 times more abundant. Hadfield and Magelsson [62] reported all western larch trees in their 5-year postburn study showed signs of woodpecker foraging, and most feeding occurred in 1st year after tree death. After stand replacing fires in conifer forests of the northern Rocky Mountains, Hutto [74] found evidence of woodpecker foraging on 64% of western larch trees larger than 3.9 inches (10 cm) d.b.h. compared with 81% of ponderosa pine, 48% of Douglas-fir, 2.3% of Engelmann spruce, and 0.2% of lodgepole pine.
Palatability/nutritional value: Western larch appears to be unpalatable to most big game animals, but it is eaten as emergency food [115,117]. Its seeds are palatable to small birds and mammals, although larger seeds are preferred [131]. Larch needles provide a major source of food to several species of grouse [12].
Nutrient values for western larch needles, twigs, and other tree parts have been reported from 2 sites in western Montana [140]. Whole tree values have also been published [139]. Western larch needles at two locations in eastern Washington contained 2.0% and 1.7% nitrogen, respectively [57]. Green needles from Lubrecht Experimental Forest in western Montana, had a mean ash content of 5.8% with a range of 3.47 to 8.16%, and those from Coram Experimental Forest, Montana, had mean ash content of 5.3% with a range of 4.9 to 8.9%. The following table summarizes nutrient values for needles from these 2 sites [140].
Lubrecht mean Lubrecht range Coram mean Coram range Calcium (õg/g) 3,031 2,000-4,800 2,213 1,980-2,390 Copper (õg/g) 8.3 5.0-15.2 15.5 10.7-35.2 Iron (õg/g) 86.8 41-173 126 109-218 Potassium (õg/g) 6,405 2,800-9,760 4,958 4,390-5,388 Magnesium (õg/g) 1,098 692-1,592 1,083 1,005-1,113 Manganese (õg/g) 216 81-405 181 160-239 Nitrogen (õg/g) 13,518 9,730-15,540 23,320 17,920-28,923 Sodium (õg/g) 61.4 24.4-123.0 56 45-125 Phosphorus (õg/g) 2,343 1,678-3,189 2,960 1,894-3,269 Zinc (õg/g) 15.8 6.0-35.6 24.6 21.1-27.7Cover value: Woodpeckers and other cavity nesters utilize western larch. Around its decaying interior, a dead western larch tree retains a protective layer of sapwood, which provides nesting, roosting, and feeding opportunities. Flying squirrels, woodpeckers, owls, and various songbirds nest in rotting western larch cavities. Snags are used by osprey, bald eagles, and Canada geese for nesting [12], and raptors may nest in brooms of trees infected with dwarf mistletoe [25].
Except when it is young, western larch is rarely found in pure stands. Its most
common tree associate is Douglas-fir (Pseudotsuga menziesii), and on
low-elevation dry sites it is found with ponderosa pine (Pinus ponderosa).
Common associates in warm, moist forests include grand fir (Abies
grandis), western hemlock (Tsuga heterophylla), western redcedar
(Thuja plicata), and western white pine (P. monticola). In cool,
moist, subalpine forest types Engelmann spruce (Picea engelmannii),
subalpine fir (A. lasiocarpa), lodgepole pine (Pinus contorta), and mountain
hemlock (Tsuga mertensiana) are more common.
Hardwoods that occur with western larch include paper birch (Betula papyrifera),
black cottonwood (Populus balsamifera ssp. trichocarpa), and quaking aspen
(P. tremuloides) [72,115,116,126,132].
Major understory associates include common beargrass (Xerophyllum tenax), huckleberry
(Vaccinium spp.), thimbleberry (Rubus parviflorus), menziesia (Menziesia ferruginea),
ninebark (Physocarpus malvaceus), serviceberry (Amelanchier spp.),
Oregon boxwood (Paxistima myrsinites), and bearberry (Arctostaphylos uva-ursi).
Western larch is not considered a climax species, but it is a
long-lived early successional species. Refer to Successional Status
for more details [34,53,88,116,156]. Classifications describing
plant communities in which western larch is an important seral species include the following:
Idaho: [33,34,64,144]
Montana: [65,66,106]
Oregon: [51,63]
Washington: [34,51,63]
Wood Products: Western larch is one of the most important timber-producing species in the western United States and western Canada. It has the densest wood of the northwestern conifers and is also very durable and moderately decay-resistant. Its high heating value makes it one of the best fuel woods in the region. The wood is also used commercially for construction framing, railroad ties, pilings, mine timbers, interior and exterior finishing, and pulp, and burned snags are often used to make shakes [12,50,72,115,154]. High sugar content of western larch makes it undesirable for concrete forms because the sugars react chemically with the concrete [117]. Faurot [43] describes methods for estimating total volumes of western larch wood, wood residue, and bark.
Non-wood uses: Native Americans used western larch for treatment of cuts and bruises, tuberculosis, colds and coughs, sore throats, arthritis, skin sores, cancer, and for blood purification [68,153]. They also made syrup from the sap, ate the cambium, and chewed solidified pitch as gum [68]. Arabinogalactan, the gum from the tree, is used for lithography and in food, pharmaceutical, paint, ink and other industries. The most desirable sources of this gum are waste butt logs. Oleoresin from western larch is used to produce turpentine and other products [117].
Western larch's active growth occurs from May through August [115,154]. Vegetative development of western larch proceeds as follows:
Stage of Development Timing Buds begin to appear early fall [115] Buds swell, then open late March and April [121] Needle growth declines mid-May Diameter growth begins mid-May Diameter growth peaks, needle growth ends, height growth begins mid- to late June Height growth peaks mid-July Height growth complete mid-August [132]Reproductive development in western larch proceeds as follows:
Stage of Development Timing Notes Cone initiation early summer [101,115] ---- Buds appear early fall [115] ---- Pollen and seed cone buds develop prior to winter dormancy [101,115] ---- Pollen and seed cone development begins late March and April [121] ---- Pollen and seed conelets appear mid-April to mid-May ---- Pollination late April to early June [115] ---- Fertilization June to July [102,115] Fertilization occurs 6-8 weeks after pollination. Cones mature mid- to late August Cones mature faster during warmer summers. Cone opening begins late August and early September [115,117] Long periods of cool or moist weather may delay opening [115]. 80% of seeds dispersed mid-October [115,117,121,125] Seeds dispersed later usually have lower viability [125]. Cones fall winter Some cones may remain on trees through the next summer [115]. Germination late April through early June [115] Germination roughly coincides with snowmelt and occurs 1-2 weeks before that of associates. Germination occurs earlier on lower elevation or exposed sites and later at upper elevations or in sheltered areas [117].Survivors: Young larch that are wounded by surface fires often heal and survive for centuries [10]. Trees defoliated by May slash burning may produce new needles 1 month later, and appear completely recovered within 2 years [49]. One year after underburning shelterwood units in Idaho, western larch overstory mortality was 7% [136]. After prescribed underburning of Douglas-fir-western larch forest in western Montana, western larch's radial growth was reduced in the 1st year postfire, but increased over the following 7 years, suggesting that decreased competition may have enhanced western larch growth [109].
Early Regeneration: Fires that expose mineral soil and reduce competition, especially on north-facing slopes, favor rapid and abundant western larch regeneration and dominance [2,114]. Western larch usually establishes in the 1st season after fire [2], and as much as 5 inches (13 cm) of 1st year postburn seedling growth has been reported after spring, summer, and fall burning of white fir sites on Wallowa-Whitman National Forest, Oregon [104]. In the Flathead National Forest of northwestern Montana, western larch began colonizing both wildfire and prescribed burn sites during the 1st year postfire [133,145]. Burned seedbeds from underburning in shelterwood units in Idaho produced 3 to 7 times more western larch seedlings than unburned seedbeds [136]. After clearcutting in northern Idaho subalpine fir-Engelmann spruce-menziesia type, western larch stocking was 20% on clearcuts that were burned and scarified, compared to 8% stocking on unburned clearcuts [23]. After fires in 1910 and 1919 in Coeur d'Alene National Forest, Idaho, western larch had restocked up to 200 seedlings per acre (500 seedlings/ha) on the north aspect of the study area by the fall of 1923. Western larch seedlings accounted for 83% of conifer seedlings present on all slopes and 88% of those on the north-facing slope [84]. Overstocking may result on some sites if too much mineral soil is exposed [37,67,114,123].
Old skid trails often support high densities of western larch seedlings, but the compacted soil does not allow trees to grow as well as on other sites. Good sites for potential for western larch establishment decrease as regeneration of a burned site progresses [2].
Latham and others [86] found that in general, fires that resulted in open sites, relatively free of vegetation, with full sun, moving shade, and a mineral soil seed bed favored the development of western larch forests. In these conditions, western larch seedlings were generally able to establish quickly and grow taller than other vegetation. Where tree establishment was delayed, however, shrubs were able to establish and suppress western larch.
Competition and Succession: Following fire, western larch must establish rapidly. Insufficient sunlight or exposed mineral soil will delay western larch establishment, allowing development of shrubs or more shade-tolerant tree species [2,148]. If the area does not burn again, shade will prevent western larch regeneration, and other species will eventually replace western larch [85,116]. Generally, stand-replacing fires favor western larch over its competitors because western larch is most likely to survive and postfire survivors will provide an onsite seed source, while less fire-resistant competitors must rely on offsite sources or unburned islands [1,10,135]. Low to moderate intensity fires thin out competitors [10,30]. The species may dominate the area for 150-350 years in the absence of fire [10,135].
Western larch and lodgepole pine are early seral species that often compete in the same recently burned areas. In general, lodgepole pine performs better on drier or more exposed sites [135]. Due to western larch's later age of 1st seed production and longer lifespan, it may be favored over lodgepole pine on sites that burn less frequently [138]. Western larch-lodgepole pine stands in grand fir sites of northwestern Montana with as little as 10% western larch overstory can eventually be dominated by western larch [1]. In Coram Experimental Forest in northwestern Montana, single high intensity burns in western larch-Douglas fir habitat thinned the overstory and favored regeneration of western larch, Douglas-fir and lodgepole pine, while multiple severe burns tended to promote lodgepole pine [138]. Western larch benefits from periodic surface fires that kill competing shade-tolerant conifers [15].
Absence of fire: Prior to 1900, fire maintained western larch as a dominant seral species in various habitats [3,7]. Lack of periodic fires may limit western larch regeneration [37]. Fire suppression in last century has favored thickets of suppressed shade-tolerant conifers [4,7,10], which result in a decline in the vigor of all trees [10]. These sites are at risk of high-intensity wildfires [3,4,60]. Large areas in and around the western larch habitat type are now characterized by such crowded and stagnant stands [10], and fire suppression has been linked to the decline of western larch habitat in Idaho, Montana, Oregon, and Washington [10,30,60,91].
In Bear Creek Canyon of the Bitterroot Mountains, Montana, where the old western larch are prevalent and younger ones less abundant and dwarf mistletoe has infected most trees, the species is near extinction due to lack of fire or other disturbance [91]. Remaining old-growth ponderosa pine-western larch habitat in Pattee Canyon near Missoula, Montana, has a thick understory of Douglas-fir saplings and pole-sized trees. This understory could provide ladder fuels, resulting in a crown fire [60]. If fire does not occur before the remaining trees die in these areas, or if ladder fuels create a crown fire that burns intensely enough to kill the remaining trees, the western larch seed source may be eliminated. However, if a seed source remains after fire, western larch may thrive in the postfire mineral seedbed with reduced competition.
Breeding system: Western larch is monoecious with both staminate and ovulate cones distributed throughout the crown [115,116].
Pollination: Western larch pollen is distributed by wind and is less abundant than that of other conifers. Owens [101] described the physiological details of pollination in western larch.
Seed production: Western larch cone production may begin as early as age 8 though it is unusual on trees less than 25 years old. Heavier crops usually begin at approximately 40 to 50 years of age and continue for 300 to 500 years [116,130]. Trees usually produce cones annually, but crop size varies with year and location; heavy cone crops occur every 5 years on average, with fair to poor crops in other years [115,121]. Shearer and Carlson [121] reported an annual average of 1,393 potential cones per tree over a 6-year period. Because western larch cones are borne throughout the crown, the size of the crop generally corresponds to the size of the crown [115,116,117]. Shearer and Kempf's [132] literature review indicated that the number of cones also increases with increased spacing of trees.
Western larch seeds are small and light, just 137,000 to 143,000 per pound (301,00-315,00/kg) [12,115,154]. On average each mature cone produces 39 seeds, but some may contain as many as 80, and mature stands of western larch may produce more than 0.5 million seeds per acre (1.2 million seeds/hectare) [115]. Roe [111] described a method for estimating the size of western larch seed crops up to 1 year in advance.
Viability of seeds typically increases with crop size and decreases with tree age [115]. Inviable seed results from lack of pollination, inviable pollen, lack of fertilization, later ovule abortion, or embryo abortion [102]. Over a 6-year period, Shearer and Carlson [121] found that 4% of potential seeds at the time of bud burst matured as filled seeds.
Seed dispersal: Most of western larch's small, light, long-winged seeds are distributed within 328 feet (100 m) of the parent. However, depending on wind conditions, they may be dispersed up to 820 feet (250 m) or more [116]. This distance is comparable to that of Engelmann spruce seeds, but is longer than Douglas-fir and subalpine fir [115]. Seed spread rate is considered moderate [154].
Seed banking: Western larch seeds are viable only until the year following fertilization [117].
Germination: Seeds of western larch germinate well on a variety of seedbeds and aspects [117], but Stoehr [147] found that germination and survival was greatest in mineral soil. In a synthesis of literature on northwestern trees, Minore [93] ranked western larch germination and survival in the highest category for mineral seedbeds, in the middle category for burned seedbeds, and in the lowest category for organic seedbeds. The ideal temperature for germination is 80ð Fahrenheit (27ð C), but germination can occur at temperatures as low as 65ð Fahrenheit (18ð C). Germination occurs at or above the soil surface. Natural stratification during winter results in rapid, complete germination. Spring-sown western larch seeds without stratification germinate slowly and erratically; some do not germinate until the following season [115]. Oswald [100] reported seed predation and shade both had negative effects on germination rates of western larch, though shading was not a significant factor. Shade appears to be more important as seedlings develop.
Seedling establishment/growth: On average 1 western larch seedling will establish for every 53 seeds produced and dispersed [127]. Seedlings grow rapidly and vigorously [115,154], averaging 2 inches (5 cm) of growth during the 1st season and 12 inches (30 cm) per year over the next 4 years. Western larch seedlings grow faster than all major associates except lodgepole pine, and the species grows faster than any other Rocky Mountain conifer until 100 years of age [12,115].
Site variation: Site requirements for establishment and growth of western larch seedlings are more specific than those for germination. Seedlings are well adapted to the mineral soil and sunlight of exposed seedbeds, such as those created by burning or mechanical scarification. They do not thrive in areas with undisturbed litter, humus, sod, or heavy root competition [12,54,115,116,117,123]. Overly dense stands slow growth. For the 1st few years shaded seedlings usually grow faster than those in full sunlight; thereafter, seedlings in full sunlight outgrow shaded seedlings. North, northwest, and northeast exposures and gentle to flat topography are best for seedling survival; high surface temperatures on south and west exposures may kill many seedlings [115].
Mortality: Seedling mortality is usually highest in the 1st season; losses after year 3 are minimal [115]. Biotic factors, such as fungi, birds, and rodents, cause the most 1st year seedling deaths early in the season, but drought is more detrimental after mid-July [115,117]. Newly germinated seedlings were killed by high soil-surface temperatures (>130ð Fahrenheit (54ð C)) in Montana, and these effects were most severe on western and southern exposures [117,122].
Asexual regeneration: Western larch does not reproduce by sprouts, but propagation by cuttings has been successful [115].
Western larch is the least shade tolerant conifer in its range [11,45,46,48,116]. As such, it is a seral species whose populations have been historically maintained by disturbances such as wildfire and glacial retreats [45,46,116,126] and is therefore usually found in even-aged stands [116]. It is an aggressive pioneer species after fire or other major disturbance [11,46,61,88] and competes best on moist sites [48,61]. In drier environments where fires are frequent, western larch may form a "fire climax" [152].
Western larch uses nitrogen more efficiently than evergreen trees, reducing its dependence on soil for nitrogen and increasing its effectiveness as a pioneer in disturbed, infertile habitats [56,149]. This aggressive pioneer quickly colonizes disturbed areas and grows rapidly, remaining taller than its associates for approximately 100 years [45,46,116,126]. Western larch's rapid height growth may indicate allocation of resources to early growth rather than early seed production, which would explain the species' relatively advanced age of 1st reproduction compared to other early successional species. Western larch extension growth was significantly greater than that of 6 other northwestern conifers. This characteristic and low shade tolerance were both associated with early successional species studied [152].
In the absence of disturbance, shade tolerant associates form understories that shade out future generations of western larch seedlings [116]. However, western larch's long lifespan and resistance to damage from fire and pathogens accounts for the presence of relict trees in late-successional stands that can repopulate the stand if fire or other disturbance removes competition and opens the canopy [45].
The currently accepted scientific name for western larch is Larix
occidentalis Nutt. (Pinaceae) [50,76,154].
Natural hybridization of western larch and alpine larch (Larix lyallii) has
been documented in the Carlton Ridge Research Natural Area and in the
Cabinet Mountains and Bitterroot Range of Montana, where the species are
sympatric. Usually, however, western larch and alpine larch are isolated by
elevation [26,27,31]. Carlson and Ballinger [28] reported that 1st generation western larch-alpine
larch crosses are viable. Carlson and others' [27] literature review reported successful crosses
of western larch with European larch (L. decidua) and with Japanese larch
(L. kaempteri).
