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Whitebark pine is a small to medium-sized native conifer. Tree height typically ranges from 40 to 60 feet (12-18 m) at maturity [30,64,114], reaching 5 feet (1.5 m) in diameter [64]. The largest recorded specimen is in the Sawtooth National Recreation Area of Idaho. It is 69 feet (21 m) tall with a 27.6-foot (8.4-m) circumference and a 47-foot (14-m) crown spread [6]. In pure, upper-subalpine stands, trees seldom exceed 50 (15 m) feet in height. Trees at the species' upper elevational limits grow in low shrub and krummholz forms that are < 3 feet (1 m) tall [47,87,194]. Trees may have a single-stemmed or a clumped, multi-stemmed habit [229,230]. Site differences and Clark's nutcracker seed-caching behavior may affect frequency of clumping (see Regeneration Processes below). A survey of whitebark pine at 10,000 to 10,200 feet (3,000-3,100 m) elevation on the Inyo National Forest, California, showed that multi-stemmed forms were more common: only 5.8% of sample trees were single-stemmed [47]. At 7,700 to 10,500 feet (2,300-3,200 m) on the Breccia Cliffs of Wyoming, 53% of whitebark pine was single-stemmed [115]. Several to all stems in a clump may be genetically distinct individuals [127].
Bark thickness is thin to moderate, seldom reaching over 0.4 inch (1 cm) [21]. Branches of mature trees are ascending [89]. Needles are in bundles of 5. They may reach 7 inches (18 cm) in length, or as little as 1.5 inches (3.8 cm). Mature female cones are 1.6 to 3 inches (4-8 cm) long [64]. They are located mostly at the tops of the upswept branches and are readily recognizable from the air, probably an adaptation to encourage seed foraging by Clark's nutcrackers. The purple cast of mature cones may further aid Clark's nutcrackers in finding ripe seed. Cone color is also the easiest way for humans to distinguish between whitebark and the morphologically similar limber pine [116,117]. A female whitebark pine cone contains an average of 75 seeds. Whitebark pine's wingless seeds are large and heavy for Pinus species: 7 to 10 mm in length and an average mass of 72 mg (+ 13 mg) per seed [21,89].
Tree stocking in whitebark pine communities can be low [110], but whitebark pine attains considerable cover on some sites. A stand on the Wenatchee National Forest of Washington supported 143 to 358 whitebark pine/acre with 41-214 feet2/acre basal area [126]. On the Okanogan National Forest of Washington, mean overstory cover in a whitebark pine/pinegrass community was 27% [230].
Whitebark pines at high elevations often attain extreme age. Stands in the Wind River Range, Wyoming, and Jasper National Park, Alberta, have been aged at > 600 and 700 years, respectively [133,194]. The oldest recorded specimen is on the Sawtooth National Forest of Idaho, over 1,270 years old [169].
Whitebark pine is extremely wind firm. High-elevation trees on shallow, undeveloped soils frequently endure near-hurricane-force winds [32].
Fire adaptations: Two strategies allow whitebark pine to survive in fire-prone ecosystems: survival of large and refugia trees, and postfire seedling establishment facilitated by Clark's nutcrackers. Mature trees usually survive low-, and sometimes moderate-severity surface fires. Bark thickness is moderate: thinner than ponderosa pine but thicker than lodgepole pine. Pole- and smaller-sized whitebark pine usually do not survive surface fires [100], but patchy fires resulting from fuel-limited whitebark pine habitats reduces whitebark pine mortality [2]. In western Montana, Arno [11] frequently found whitebark pine with multiple fires scars dating from 1600 to 1900, demonstrating ability to survive low- and moderate-severity surface fires [126,229].
Whitebark pine seedlings establish on open sites created by mixed-severity and stand-replacement fires [115,214,223]. Late-successional species dominate when fire-return intervals are long, but fires were historically likely to return before whitebark pine was successionally replaced [159]. Although whitebark pine recruitment is depressed in many areas, whitebark pine seedlings were historically highly competitive with other conifer species in the postfire environment. For example, 25 years after an 1892 stand-replacement wildfire on Mt. Adams in Washington, whitebark pine seedling establishment was equal to that of western hemlock (9% of total recruitment) and better than that of lodgepole pine and Engelmann spruce. Hofmann [85] noted that whitebark pine seedlings were fairly evenly distributed over the 80-acre (32 ha) burn, even though parent seed trees were at least a mile away.
FIRE REGIMES: Whitebark pine ecosystems have a mixed-severity fire regime of widely ranging fire intensities and frequencies [2,17,25,157]. Mixed-severity fires create complex landscapes of dead whitebark pine stands intermingled with live stands of different ages [18,38,97]. Whitebark pine stands also experience nonlethal surface fires and infrequent stand-replacement fires [2,10,97,159]. Under whitebark pine's highly variable fire regime, fire-return intervals range from 30 to 350+ years [17,25,157]. In a review paper, Agee [2] lists mean fire-return intervals of 29 to 300 years in whitebark pine habitats, with moderate-severity fire-return interval means of 25 to 75 years and stand-replacement fire-return interval means of 140+ years.
Whitebark pine wood is highly flammable even when green, and the dry, windswept upper slopes where whitebark pine grows are predisposed to lightning strikes. Whitebark pine and mixed-conifer communities with a whitebark component experience fire frequently; however, fire is usually unable to spread widely due to discontinuous canopies and sparse understory fuels [34,38,194]. Fire severity is low where surface fuels are sparse, and the resulting underburn kills mostly small trees and fire-susceptible overstory species, leaving live, mature whitebark pine. Understory species such as pinegrass and grouse whortleberry provide fine fuels that spread surface fires [164], which crown and consume conifers in dense patches. On 3 sites on the Bitterroot National Forest, Montana, Arno [10] reported minimum/maximum fire-return intervals of 2/68 (x̄ = 33), 4/78 (x̄ = 30), and 8/50 (x̄ = 41) years. Brown and others [38] found fires in the subalpine mixed-conifer zone of the Selway-Bitterroot Wilderness of Montana and Idaho were mostly mixed severity, with patchy, fine-grained patterns. Return intervals ranged from 25 to 60 years. Fires were still patchy and of mixed severity in pure whitebark pine stands at high elevations, but fire-return interval lengthened to a 115-year mean.
Infrequent stand-replacement fires are an important component of whitebark pine's fire ecology [11,97]. Occasional stand-replacement fire maintains whitebark pine as an early to mid-seral species in subalpine fir communities in Washington's Cascade Range [126] and elsewhere. Arno [11] found that in the Bitterroot Mountains, subalpine communities on moist, north-facing slopes were most likely to experience long return-interval, stand-replacement fires. He stated, "stand-replacement fire is essential to maintain whitebark on moist slopes because of the rapid rate of succession" [12].
Small, patchy fires are also important to whitebark pine regeneration, especially where whitebark pine is seral. Large and small fires create regeneration opportunities, recycle nutrients and biomass, and maintain whitebark pine on the landscape [158]. Fires historically started in summer and fall and burned over many weeks [97]. Extent of stand-replacement burns varies; ranges from 2.5 to 120 acres (1-50 ha) are typical [166,211]. Small-acreage fires were, and are, more common. For example, a 12-year study (1979-1990) in the Selway-Bitterroot Wilderness Area showed that 84% of wildland fires for resource benefit (prescribed natural fires) burned less than 4 hectares. A single year (1988) accounted for 39% of the area burned [38]. Large fires typically occurred in drought years, burned through lower-elevation plant communities as well the subalpine, and lasted from weeks to months [158].
Fire regime examples: Fire histories of Yellowstone National Park show the mean fire-return interval in underburned whitebark pine stands ranged widely, from 66 to 204 years. Underburns were often patchy and restricted to 1 or several stands. Whitebark pine and mixed-conifer communities from 6,000 to 11,000 feet (2,000-3,300 m) experienced stand-replacement fire every 350 years or more. Slow fuel accretion and moist fuels restricted fire spread, and large fires occurred only in extreme fire weather years such as 1988 [25,181].
In the Sierra Nevada fuel loadings were historically light between 7,500 to 10,000 feet (2,300-3,050 m), and fires were usually of low severity. Large, stand-replacing fires occurred rarely but played an important successional role: fires created openings in which whitebark pine and the nonserotinous Sierra Nevada lodgepole pine established. In the absence of fire, red fir is successionally replacing the 2 pines at elevations below 10,000 feet. Fire records for Yosemite National Park from 1931 to 1978 show that most subalpine fires occurred in the lower, red fir zone (representing 10% of total Park area but 37% of total Park fires), where whitebark pine is seral. Although fires were not as common in the mid-subalpine lodgepole pine-mountain hemlock zone - where mature, cone-bearing whitebark pine is most prevalent - the fires burned over larger areas (14% of total fires, equaling 19,677 acres). The upper subalpine zone (> 10,000 feet) - where whitebark occurs in pure to nearly pure stands - represents 14% of total Park area and experienced no fires between 1931 and 1978. Fires were rare in the upper subalpine, but were "intense" when they did occur, especially in krummholz whitebark pine [34].
On the Cascade Range, fire regime of westside whitebark pine forests is typically stand replacement. Forests with a lodgepole pine component burn more frequently and severely; stands without lodgepole pine burn less frequently and have greater potential for creeping ground fire [126]. On the east side, whitebark pine forests appear to have the shortest fire-return interval of high-elevation forests [2]. Stand-replacement fires are rare on the east side [126].
Fire exclusion: Fire exclusion has favored shade-tolerant, late-successional species throughout whitebark pine's range [100,157,191]. Because fire-return intervals are often long where whitebark pine is climax, fire exclusion has affected high-elevation whitebark pine less than whitebark pine at mid-elevations, where whitebark pine was historically a highly productive seral species [100]. At the landscape level, however, fire exclusion in the upper subalpine has shifted succession away from whitebark pine to later-successional species [3]. Murray and others [164] suggest that livestock grazing in the 19th century reduced fire frequency even before fire suppression was practiced. Small, isolated mountain ranges may be most affected by anthropogenically altered FIRE REGIMES. Subalpine portions of the West Bighole Range, an isolated spur of the Bitterroot Range on the Montana-Idaho border, have experienced reduced fire frequencies and an 87% decrease in area burned since European-American settlement. From 1754 to 1873, actual fire rotation was 184 years; modeling predicts a fire rotation of 1,364 years based upon fire frequencies from 1874 to 1993.
The following table provides fire return intervals for plant communities and ecosystems where whitebark pine is dominant or common. 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) silver fir-Douglas-fir Abies amabilis-Pseudotsuga menziesii var. menziesii >200 grand fir Abies grandis 35-200 [13] mountain big sagebrush Artemisia tridentata var. vaseyana 15-40 [15,41,154] Wyoming big sagebrush Artemisia tridentata var. wyomingensis 10-70 (40**) [224,235] western larch Larix occidentalis 25-100 [13] whitebark pine Pinus albicaulis 50-300+ [2] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-300+ [11,13,181] Sierra lodgepole pine* Pinus contorta var. murrayana 35-200 Jeffrey pine Pinus jeffreyi 5-30 western white pine* Pinus monticola 50-200 Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47 [13] interior ponderosa pine* Pinus ponderosa var. scopulorum 2-30 [13,22,123] quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [13,74,153] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [13] coastal Douglas-fir* Pseudotsuga menziesii var. menziesii 40-240 [13,160,180] western redcedar-western hemlock Thuja plicata-Tsuga heterophylla > 200 mountain hemlock* Tsuga mertensiana 35 to > 200 [13] *fire-return interval varies widely; trends in variation are noted in the species summaryEffects of fire exclusion: Kendall and Keane [108] state "whitebark pine will continue to decline if fire is not allowed to periodically set back the successional clock." Secondary succession, accelerated by white pine blister rust and bark beetle outbreaks, results in rapid replacement of whitebark pine by shade-tolerant, fire-sensitive species such as subalpine fir and mountain hemlock. Without burning, genetically valuable seed produced by blister-rust resistant whitebark pine is wasted: no new openings are created where Clark's nutcracker can cache seed and seedlings can establish [97]. Based upon fire records from the U.S. Forest Service's Northern Region, Arno [12] estimated that less than one-half of 1% of the seral whitebark pine type had burned in 1970-1985. At that rate, he calculated a theoretical fire-return interval of 3,000+ years. Arno and other fire researchers caution that in reality, wildfire inevitably returns to fire-prone ecosystems [38]. Fuel build-ups resulting from long-term fire exclusion dictate that when fire does return, it burns more acreage at greater severity than was historical. Estimated loss of whitebark pine from the 1988 Yellowstone fires was 30% of cone-producing stands in the north of the Park, and 12% in the east. Total reduction of whitebark pine cover was estimated at 54% [111].
Wilderness: Across its range, the proportion of whitebark pine habitat that falls within Wilderness boundaries is greater than that of nearly any other tree species [187]. Whitebark pine in Wilderness Areas is not immune to decline: Kendall and Arno [107] estimated that as of 1990, 90% of whitebark pine in Glacier National Park, much of it in wilderness boundaries, had died from white pine blister rust. Fire management of whitebark pine is particularly problematic in small Wilderness Areas, where management-ignited fires are seldom an option [101,163]. Yet conservation of whitebark pine may be impossible without reintroduction of fire to Wilderness Areas [109]. Since firelines in Wilderness areas are costly, damaging, difficult to construct in remote areas, and often in violation of the Wilderness Act, wilderness fires for resource benefit present the best management option [97]. As of this writing (2002), studies are underway to determine fire histories and explore management options in small, isolated Wilderness Areas [163]. Renkin and Despain [178] summarized a 17-year trend (1972-1988) of fire occurrence under the prescribed natural fire program in Yellowstone National Park. They found that the high moisture levels in whitebark pine ecosystems did not favor crown fires in most years (1988 being an exception), and fire occurrence was less than expected (based on amount of unburned area available) in whitebark pine communities. In mixed-conifer forests where whitebark pine was a component of the vegetation, fire occurrence was greater than expected in subalpine fir-Engelmann spruce and old-growth lodgepole pine with a subalpine fir-Engelmann spruce understory, and less than expected in seral lodgepole pine. Keane [97] states "the most important management action for conserving and maintaining vital whitebark pine ecosystems is to allow fires to burn in wilderness areas and play a more natural role in the ecosystem."
Restoring whitebark pine with fire: Long-term outlook for whitebark pine is not without hope, but restoring whitebark pine ecosystems cannot be accomplished without returning fire to subalpine landscapes. Keane and Arno [100] state "maintenance of native FIRE REGIMES is the single most important management action to ensure conservation of whitebark pine." Whitebark pine will continue to decline in the short term, but natural selection will probably increase genetic blister-rust resistance in whitebark pine populations [108,198]. It is also likely that future disturbances, particularly large fires and mountain pine beetle attacks, will kill many of these genetically valuable trees. Despite the dangers of landscape-level fire to whitebark pine populations, returning fire to the landscape is best way to restore whitebark pine. Kendall and Keane [108] state "It is important to note that fire exclusion has a far greater negative than positive consequence for whitebark pine. In the absence of fire, atypical amounts of fuel accumulate that foster more fires that are lethal to mature whitebark pine trees." Reintroduction of stand-replacing fire fosters whitebark pine regeneration by providing open sites suitable for Clark's nutcracker caching and seedling establishment. It also reduces impacts of mountain pine beetle infestations by creating mosaics of mutiaged stands that are less conducive to beetle epidemics [78]. It is encouraging that 40% of the progeny of healthy trees in stands otherwise heavily infested with blister rust show some genetic resistance to blister rust [84]. Without intervention, it is likely that the small proportion of whitebark pine resistant to white pine blister rust will be killed in stand-replacement fires before they can reproduce [108].
Management-ignited fires can be used for fire hazard reduction and whitebark pine restoration treatments [37]. Fire researchers emphasize that it is less important to reconstruct historic stand structures than to reintroduce fire to whitebark pine ecosystems. It is crucial to create open sites that are favorable for Clark's nutcracker caching and growth of natural and artificial regeneration. Six Demonstration Areas have been established in the Selway-Bitterroot Wilderness Complex of Idaho and Montana as part of the Restoring Whitebark Pine Ecosystems Project. Ongoing restoration treatments include prescribed fire and silvicultural cuttings. Since the research is ongoing, conclusions and recommendations are based on limited data, and further suggestions will be forthcoming as the project continues [100].
Large, stand-replacement fires are not recommended in areas where whitebark pine is in severe decline (for example, northern Idaho and northwestern Montana). Small-scale prescribed burning is recommended; otherwise, natural whitebark pine regeneration will be extremely slow [210]. Prescribed burning is best conducted in fall, after an early frost (<25oF (-4oC)) kills herbaceous plants and shrub foliage. Such foliage quickly cures and can propagate fire. In other seasons whitebark pine ecosystems are usually too wet to burn, or in extreme fire years, downslope vegetation is so dry that spotting may ignite fire in lower elevations. Aids for conducting stand inventories, prioritizing whitebark pine habitat for prescribed fire, designing and implementing treatments, and posttreatment monitoring and available [100]. Follow-up thinning treatments, especially of subalpine fir, are usually needed to encourage whitebark pine growth [210]. Augmenting natural regeneration with blister-rust resistant seed sources is recommended in areas where whitebark pine seed sources are absent or greatly reduced [84,210].
Unfortunately, restoration treatments may increase bark beetle predation on whitebark pine. On the Beaver Ridge Demonstration Area in northern Idaho, Six [190] found that Pityogenes fossifrons beetles were the most serious posttreatment pest species: they preferred young, apparently healthy whitebark pine in Clark's nutcracker openings, but also attacked a few fire-damaged mature trees. Ips spp. colonized slash heavily and attacked a few fire-damaged mature trees, but mostly left healthy trees alone. Mountain pine beetle numbers, which are rising in the study area, rose on the treatment sites but did not significantly respond to treatments. To reduce Pityogenes fossifrons infestation, Six [190] recommended spraying high-value whitebark pine in Clark's nutcracker openings with carbaryl for 2 posttreatment years (refer to Fire Case Studies).
Fuels: Information on tree biomass is useful in determining fuel loads and predicting potential fire behavior. Moeur [155] provides a model for estimating crown widths and foliage weights of whitebark pine and other northern Rocky Mountain conifers. Regression equations [35,36] and tables [193] are available for estimating whole-tree weight and weights of boles, branches, branchwood with foliage, and live and dead crowns of whitebark pine and other western conifers. Van Wagtendonk and others [220,221,222] provide models for calculating weight, depth, heat content, and other fuel properties of whitebark pine and other Sierra Nevada conifers.Climate: Whitebark pine grows in cold, snowy, and generally moist climates. On semiarid ranges it is most common on cold, moist sites, whereas it is most common on warm, dry sites on moist ranges [139]. Whitebark pine is common on ridges and near timberline, where trees are exposed to strong, desiccating winds. Hurricane-force wind velocities (> 73 mi/h (117 km/h)) occur every year on most whitebark pine sites and are especially common on ridgetops. Precipitation in whitebark pine communities ranges from 24 to 63 inches (600-1,600 mm) per year [226], 2/3rds of which is snow [17]. Frost and snow are possible throughout the growing season, which is about 90 days [17,87,202]. Whitebark pine is more tolerant of wind and ice damage than any other high-elevation conifer except alpine larch. Whitebark pine sites often experience summer drought, especially in southern latitudes of the tree's distribution. Whitebark pine is more drought tolerant than any other high-elevation species [126,173,183].
Topography: Whitebark pine is most common on rocky, well-drained sites [87,126]. Best development occurs on sheltered, north-facing slopes and basins [139]. In the southern Sierra Nevada, whitebark pine is confined to moist north slopes. Topography is rolling to rough with moderate to steep slope [75,126]. Slopes on the Blue Mountains ranged from 5 to 60% [87]. Whitebark pine occurs on all exposures but is most common on south- and west-facing slopes [20,75,105,126,194,230].
Soils: Soils in whitebark pine communities are classified as cryochrepts [76]. Soils are moderately to poorly developed and well drained. Coarse fragments are well represented [75,126]. Whitebark pine soils are nutrient poor and usually derived from granite or basalt [76,87,126,230], although whitebark pine occasionally grows on sedimentary soils [76,185]. Soil pH is usually strongly acidic, although whitebark pine also occurs on basic soils [65,171,227]. Whitebark pine occurs on serpentine soils in the Blue, Klamath, and Siskiyou mountains [75,184]. Soils textures include coarse sands, sandy loams, and loams [75,230]. Rooting depth is typically shallow; for example, maximum rooting depth at 1 site on the Okanogan National Forest of Oregon was < 12 inches (30 cm) [230]. Krummholz or matted whitebark pine grows mostly on high-elevation sites where glacial scouring has eliminated most of the soil [182].
Elevation: Whitebark pine occurs from 4,300 to 12,100 feet (1,300-3,700 m) elevation [64]. Ranges by state or province are as follows:
Location Elevation CA 7,000 to 9,500 feet (2,100-2,900 m) in the CascadesWhitebark pine is a valuable source of food and cover for wildlife [147,212]. Bears, rodents, and birds consume the seeds [66,90,92,143,203]. The trunks provide nesting sites for cavity nesters including northern flickers and mountain bluebirds [107,148]. Blue grouse use the branches for roosting and escape cover [107,137].
Bears: Whitebark pine ecosystems provide critical habitat for grizzly and black bears. Agee and others [5] found that grizzly bear sightings in North Cascades National Park, Washington, were more frequent than statistically expected in whitebark pine/alpine larch habitats. Whitebark pine seeds are a high-quality bear food [141,143,152]. Red and Douglas' squirrels provide an important ecological link between whitebark pine and bears by making the seeds more readily available. Grizzly and black bears rarely harvest whitebark pine cones from trees; they raid squirrel middens laden with whitebark pine seeds [105,143,204]. Bear consumption of whitebark pine seed peaks just before hibernation in late October and early November. Bears feeding on whitebark pine seeds tend to feed on nothing else, and a good supply of seeds increases bear fecundity. In Yellowstone National Park, female grizzly bears were less likely to abort, and more likely to have larger litters (3 cubs compared to 1-2 cubs) in good conecrop years. Most importantly, grizzly bear death rate was nearly double when bear consumption of whitebark pine seeds was low [142]. Grizzly bears and humans have fewer troublesome encounters when whitebark pine seeds crops are large. In Yellowstone National Park, grizzly bear summer and fall movement is related to availability of whitebark pine seed. In good crop years, grizzly bear tend to congregate in remote whitebark pine communities. When seed is limited, they tend to forage in more populated, lower-elevation sites. Yearling cubs and females with cubs-of-the-year are most likely to be displaced to lower elevations in poor conecrop years [33,141,142].
Large ungulates: Whitebark pine is a minor browse species for big game, but whitebark pine understories often provide valuable forage. Rocky Mountain mule deer consume trace amounts of whitebark pine [113]. Productivity in whitebark pine understories is highly variable. Stands with grassy understories are usually most productive. Herbage production on the Wenatchee National Forest averaged 204 lbs/acre in whitebark pine/pinegrass communities and 115 lbs/acre in whitebark pine/green fescue communities. In contrast, a whitebark pine/grouse whortleberry/smooth woodrush community site showed herbage production of 22 lbs/acre [126,229].
Birds: Many bird species use whitebark pine ecosystems. Tomback and Kendall [212] provide a list of year-round and neotropical species that nest in or otherwise use whitebark pine ecosystems.
Whitebark pine provides ecologically critical linkage between Clark's nutcracker and lower-elevation, Clark's nutcracker-dependant pines (i.e., limber pine and pinyon pines (Cembroides)). When the seed crop of 1 pine species is insufficient, Clark's nutcrackers migrate up- or downslope to harvest species with more bountiful seed crops. Loss of whitebark pine, their preferred species, reduces Clark's nutcracker populations, and may have negative consequences for other pine species with seeds dispersed by Clark's nutcrackers [212].
Palatability/nutritional value: Whitebark pine seeds are highly nutritious. They are especially high in lipids. Content of seed collected from the Gallatin National Forest was 52% fat, 21% carbohydrate, 21% protein, 3% ash, and 3% water. Major minerals present were copper, zinc, iron, manganese, magnesium, and calcium [121]. Energy content of fresh, mature whitebark pine seed collected on the Bridger-Teton National Forest ranged from 5,526 to 7,308 calories/g (x̄ = 6,800 calories/g) [89,115]. Tomback [204] reported similar energy values (x̄ = 7,716 calories/g) for whitebark pine seed from the Sierra Nevada.
Cover value: Wildlife and livestock use whitebark pine/shrub communities for shade and bedding cover [126]. In Silverbow County, Montana, elk primarily used whitebark pine with a subalpine fir component as fall cover. Female mule deer used whitebark pine communities 15% of their time in summer and 3% in fall. Male mule deer used whitebark pine communities 4% of their time in summer; insufficient data precluded estimates of their fall usage [131]. Whitebark pine ridgetops are prime calving habitat for woodland caribou. Additional fire-created openings in whitebark pine ecosystems might be an asset to caribou reproduction [186].
Although its role in the plant community is changing
(see Management Considerations),
whitebark pine historically dominated many of the upper subalpine plant
communities of the western United States. Whitebark pine was a major component of subalpine forests in the
northern Rocky
Mountains, the northern Cascades, the Blue Mountains, and the Sierra Nevada. It
comprises 10 to 15% of total forest cover in the northern Rocky Mountains. It was a minor component of subalpine forests
in British Columbia and Alberta, and showed scattered occurrence on the Olympic
Peninsula, the southern Cascades and other ranges of southern Oregon and upper
northern California, and in northern Nevada [17]. At high elevations, krummholz whitebark pine
communities merge into alpine vegetation. At
mid-elevation, whitebark pine communities merge into mixed-conifer forests
[17,30].
Several
other conifer species may share dominance with whitebark pine. At mid-elevation, whitebark pine throughout its range
is associated with lodgepole pine (Pinus contorta) [75,87,126,185,230].
Whitebark pine is not commonly perceived as a mid-elevation species, but stand
reconstruction studies show that whitebark pine was an important historical component of mid-elevation forests.
On the Bitterroot National Forest of western Montana, whitebark pine dominated 14% of mid-elevation
(6,500-7,500 feet (1,950-2,250 m)) stands from the 1700s to 1900 [18,77]. Where not
dominant, it was a common component of mid-elevation Rocky Mountain lodgepole pine
(P. c. var. latifolia) forest. By the time of the study
(1991), whitebark pine dominated none of the mid-elevation
study sites. At its lowest elevations throughout its range, whitebark pine overlaps with Douglas-fir
(Pseudotsuga menziesii) [194]. In the coastal states, it associates with mountain hemlock (Tsuga mertensiana), with increasing
codominance of the 2 species to the south [87].
Subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea
engelmannii)
co-occur in northwestern states; subalpine fir is the most frequent codominant
where its range overlaps with that of whitebark pine [54,126,185,230]. In the
Northwest, whitebark pine types between 6,600 and 7,800 feet (2,000-2,400 m) are often found
adjacent to alpine larch (Larix lyallii) communities. The 2 species tend to be
complementary rather than competitive in distribution, with whitebark pine occupying dry south and west
aspects and alpine larch
restricted
to more mesic sites [9,16,50,126].
Sadly, whitebark pine communities of the northern Cascades [126],
eastern Washington [229], Idaho, and Montana [171] are
often characterized as "ghost forests" of whitebark that have been dead for decades, with little regeneration
[130].
Washington: Whitebark pine communities in the Cascade Range are often mixed with or
adjacent to mountain big sagebrush (Artemisia tridentata ssp. vaseyana) or mountain grassland communities. On some
sites, whitebark pine patches form "islands" within shrubland or grassland communities:
these mixed communities form highly diverse mosaics. At lower elevations (~ 6,230
feet (1,900 m)),
whitebark pine grades into subalpine fir, or sometimes (at ~ 5,720 feet), coast Douglas-fir
(Pseudotsuga menziesii var. menziesii) communities. Depending on elevation, subalpine fir,
Engelmann spruce, lodgepole shore pine (Pinus contorta var. contorta), and coast
Douglas-fir are common components of whitebark pine communities; subalpine fir
is the most common co-dominant. Shrubs typically show low cover; Oregon boxwood
(Paxistima myrsinites) is the only
constant shrub associate. The herb layer is often diverse.
Whitebark pine forms
fringe forests and woodlands at timberline. It is an important component of alpine
larch communities occurring below ~ 7,330 feet (2,230 m), and persists as krummholz
in
higher-elevation alpine larch communities [126].
In Mt. Rainier National Park, krummholz whitebark pine/common juniper (Juniperus
communis) forests
dominate high, rocky ridges above Yakima Park [69].
In eastern Washington and northern Idaho, whitebark pine is a seral component
of subalpine fir communities and dominates the highest peaks and ridges (> 6,000 ft
(1,800 m)). Understory
cover is typically discontinuous on these high-elevation sites. Engelmann spruce,
Rocky Mountain lodgepole pine, and Rocky Mountain Douglas-fir (Pseudotsuga menziesii
var. glauca) may associate,
especially on mid-elevation sites [54,229].
Grouse whortleberry (Vaccinium scoparium) is the most widespread and constant dominant shrub in
whitebark pine communities throughout the Rocky Mountains [17]. Pinegrass (Calamagrostis
rubescens) is the most common and constant herb; smooth woodrush
(Luzula hitchcockii) and Drummond's
rush (Juncus drummondii) also occur in and sometimes dominate whitebark pine
understories [229]. Oregon boxwood and
common juniper are characteristic shrubs [54,229].
Oregon: In the Blue Mountains of eastern Oregon and Washington, whitebark
pine codominates with subalpine fir between 7,600 and 8,500 feet (2,300-2,550
m). Whitebark pine assumes increasing dominance with elevation; it is the only tree on the highest sites. Rocky Mountain lodgepole
pine, Engelmann spruce, and Rocky Mountain Douglas-fir co-occur. Alpine larch assumes dominance on
cold, moist sites. Elk sedge (Carex geyeri) is usually
dominant on the ground layer; it is the most constant herb across sites [75,230]. In the Cascade Range yellow sedge (C. pensylvanica) and
Wheeler bluegrass (Poa nervosa) are dominant ground layer species [17].
Idaho: Limber pine, subalpine fir, and/or Rocky Mountain lodgepole pine
co-occur in whitebark pine communities. Elk sedge, Ross' sedge (C. rossii), or cushion plants such as
subalpine fleabane (Erigeron peregrinus) and rosy pussytoes (Antennaria
rosea) dominate the sparse understory
of high-elevation whitebark pine/barrengrounds [185]. On lower-elevation
sites (< 9,000 feet (2,700 m)), grouse whortleberry, common juniper, pink
mountainheath (Phyllodoce empetriformis),
Oregon boxwood, Idaho fescue (Festuca idahoensis), and/or smooth woodrush
are understory dominants [50,194].
Wyoming: Whitebark pine occurs in the Absaroka, Teton, and Wind River ranges.
Best development of whitebark pine habitats occurs on the relatively dry
Wind River Range, where whitebark pine is at the edge of its distribution. Rocky
Mountain lodgepole pine is seral in this type. Rocky Mountain
Douglas-fir, subalpine fir, and Engelmann spruce occur occasionally, but rarely
reproduce well. Grouse whortleberry, heartleaf arnica (Arnica cordifolia), Ross' sedge, and
Wheeler's bluegrass (Poa wheeleri) are common dominant understory components;
common juniper and russet buffaloberry (Shepherdia canadensis) are occasional
dominants [194].
California: Pure or nearly pure whitebark pine
communities occur at treeline in the Sierra Nevada and Cascade Range. Western
hemlock typically codominates with whitebark pine in the Cascades and northern Sierra
Nevada but is
increasingly replaced by Sierra Nevada lodgepole pine (P. c.
var. murrayana) from the central Sierra
Nevada southward. In upper montane and subalpine forests (6,000-11,000
(1,830-3,350 m)), whitebark pine is common in mixed stands with Sierra Nevada
lodgepole pine, mountain hemlock, and/or foxtail pine (P. balfouriana)
[87,136,182].
At lower elevations (7,500 feet in the north and 9,000 feet in the south),
whitebark pine
merges with mixed Sierra Nevada lodgepole pine, red fir (Abies magnifica),
and/or Jeffrey pine (P. jeffreyi) forest. Krummholz whitebark pine
merges
into alpine fell-fields at high elevations (9,500-11,100 feet (2,900-3,4900 m), depending on
latitude) [17,32,87,202].
In the Warner Mountains, whitebark pine co-occurs with Sierra Nevada
lodgepole pine, Washoe pine (P. washoensis), white
fir (A. concolor), and Jeffrey pine [136,179].
Klamath Mountain associates in whitebark pine/oceanspray (Holodiscus
discolor)
communities include mountain hemlock, Shasta red fir (A. m. var. shastensis), western white pine (P. monticola), Jeffrey pine, foxtail pine, and Sierra Nevada lodgepole
pine [156,184].
Nevada: Limber pine is the primary codominant [17]. Limber pine dominates the lower subalpine zone
(8,000-9,000 feet (2,400-2,700 m)) of the Ruby and Humboldt mountains, while whitebark pine
dominates vegetation in the upper subalpine zone (8,550 to 10,600 feet
(2,610-3,230 m)) [125]. In the Ruby Mountains, whitebark pine forms subalpine communities
with
Great Basin bristlecone (P. longaeva) and limber pines; it is the only area where
the 3 Strobus species
overlap [117,125].
Due to inaccessibility and previously low interest in managing whitebark pine
types, whitebark pine communities are not well described compared to other subalpine types. There is
agreement in the literature that whitebark pine understories are diverse, and more whitebark pine types exist than have been
classified [50,54,194,195].
Accurate descriptions of whitebark pine communities are further confounded by loss of the
overstory dominant to insects and disease, moving successional pathways onto new
trajectories. The following classifications present preliminary descriptions of whitebark pine
plant communities.
Whitebark pine is a keystone species in upper subalpine communities [174]. Tomback and others [209] and the authors below have identified several critical roles whitebark pine plays in subalpine ecosystem function. Some items listed below are explained in other sections of this summary.
Wood Products: Mainly due to the species' inaccessibility, whitebark pine wood is not considered commercially valuable. Large-diameter whitebark pine in mixed stands were harvested in the past. Whitebark pine is classified as a soft-wood pine, and its wood has bending, compression, and shearing properties similar to eastern (P. strobus) and western white pine. Wood density is slightly higher than most white pines, and is similar to Douglas-fir [55,96].
Timber: Whitebark pine reforestation techniques are still in the trial stage. Whitebark pine sites are not recommended for timber production at this time [126,229].
Grazing: A short growing season, drought, and commonly shallow, rocky soils make whitebark pine habitats slow to recover from grazing [126,229]. Whitebark pine/grouse whortleberry habitats seem more tolerant of grazing than whitebark/bunchgrass types [194]. "Moderate overgrazing" on whitebark pine sites may increase lupines (Lupinus spp.), luina (Luina nardosima), and other unpalatable herbs at the expense of bunchgrasses. "Severe overgrazing" can create large, highly erosive patches of bare soil [229]. Willard [228] provides guidelines for assessing range condition in whitebark pine ecosystems, including plant species indicators and soil condition indicators.
Moderate grazing on subalpine mixed conifer-meadow ecotones may encourage invasion of whitebark pine and other conifers into meadows. Conifer invasion into meadows on the Wind River Mountains of Wyoming began about 1890, concurrent with cattle grazing. Tree invasion ceased in 1963, concurrent with cessation of cattle grazing. Dunwiddie [58] suggests that moderate cattle grazing favors whitebark pine and other conifers by reducing competition with meadow vegetation.
Wilderness: Half of whitebark pine's distribution lies in designated Wilderness Areas or Parks [97]. See Fire Management Considerations/Wilderness for further information.
Other values: As a long-lived species, whitebark pine tree-ring chronologies are a valuable source of long-term climate information. Perkins and Swetnam [169] have correlated patterns of tree growth and climate for more than 1,000 years from whitebark pine stands in the Sawtooth Mountains of Idaho.
Whitebark pine is valued for its beauty, and whitebark pine ecosystems are popular backcountry recreation sites [49,149,209].
Whitebark pine is planted worldwide as an ornamental [162].
Whitebark pine seeds are a traditional Native American food [216]. The easternmost population of whitebark pine, isolated in the Sweetwater Mountains of Montana, may have originally been planted by Native Americans as a food source [17].
Male and new female cones are initiated from mid-July to mid-September, just prior to winter bud formation. They resume growth in April or May. Pollen disperses from male cones and 1st-year female cones open for pollination from May to mid-August, varying with latitude, elevation, and temperature [57,86,144,146]. Second-year female cones enlarge during June and July. Seeds of 2nd-year female cones ripen in mid-August to mid-September [144,146]. Germinants emerge after June snowmelt through early September [146]. Seeds hand-planted on the Gallatin National Forest in fall began emerging in late June and stopped emerging by late July [147]. Phenology for whitebark pine on Bachelor Butte, in the Cascades Range of central Oregon, follows. Data were collected at 7,710 to 8,300 feet (2,350-2,530 m) elevation [134].
Mean timing for whitebark pine phenological and animal interactions on the Bridger-Teton National Forest [89]:
Event Date Total seedcrop harvested red squirrels begin foraging mid-July 4% Clark's nutcrackers begin foraging early August 5% 1st germinable seed early August 10% Clark's nutcrackers begin caching mid-August 18% cones mature early Sept. 35% chipmunks begin harvesting late Sept. 65% Clarks nutcrackers recache seed from 1 site to another mid-October 90%Seedling establishment: Whitebark pine establishes from seed on open mineral soil seedbeds created by mixed-severity and stand-replacement fires. Clark's nutcrackers prefer open sites with mineral soil for caching, and readily cache seed on large openings created by stand-replacement fire and in smaller openings created by mixed-severity fire. Most seed is cached in the 1st good conecrop year following fire, but Clark's nutcrackers may continue to build up the seed bank for decades as long as the site remains open and whitebark pine seed is available. Cone-bearing trees close to burns are usually the parent trees, but some Clark's nutcrackers collect and transport seed from distant trees [206]. Because whitebark pine shows delayed germination and subalpine climates are often unfavorable for germination and growth, good establishment may not occur in the 1st few years after caching. Given a good seed bank, good seedling establishment usually occurs within the 1st decade after fire. Following a wildfire in the Bob Marshall Wilderness of western Montana, whitebark pine showed no establishment at postfire years 1 or 2, but seedling density was 264/acre at postfire year 3 [20]. In Yosemite National Park, a 2-hectare, mixed-severity August wildfire killed the majority of krummholz whitebark pine; a few trees survived or were missed. At postfire year 4, a total of 54 whitebark pine seedlings was counted on burn transects. Only 3 whitebark pine seedlings were found on transects in the adjacent, unburned control. Seedling heights ranged from 0.4 to 5 inches (x̄ = 1.5 + 1.0 in.) (1-13 cm (x̄ = 3.9 + 2.6 cm)). Most seedlings were near objects such as rocks, logs, and bases of burned trees. Such objects help Clark's nutcrackers remember where cached seed was stored, but when the seed is not retrieved, such placement may aid whitebark pine establishment by shading germinants. Tomback [205] noted that postfire whitebark pine establishment was still underway at the Sierra Nevada study site. At postfire year 4, she observed Clark's nutcrackers collecting seed from lower-elevation, erect-form whitebark pine and caching it on the burn.
Postfire growth: Few studies have been conducted on postfire growth rates of whitebark pine. Sund [199] found that on the Sleeping Child Burn described below, whitebark pine seedling heights at postfire year 26 ranged from 0.4 to 94 inches (1-238 cm), with seedling height highly correlated (p < 0.001) with seedling age. Seedling height was greatest on ridges (x̄ = 10.3 in. (36.2 cm)), followed by south and north slopes (x̄ = 12.0 and 11.9 in. (30.6 and 29.7 cm)), and was least on roadside plots (x̄ = 7.6 in. (19.2 cm)).
Effects of fire size and other disturbance agents: Large stand-replacement fires can favor whitebark pine over wind-dispersed conifers if cone-bearing whitebark pine are nearby. For example, in 1961 the lightning-ignited Sleeping Child Fire burned over 27,900 acres (11,300 ha) on the Bitterroot National Forest, Montana. Blister rust infection in the area was light, so whitebark pine seed sources were not limited. Clark's nutcrackers primarily collected whitebark pine seed from trees in the adjacent unburned forest; however, some birds traveled 5 miles (8 km) or more to collect seed. Conifer re-establishment was assessed at postfire year 26 (1987). Whitebark pine and Rocky Mountain lodgepole pine dominated study plots, with whitebark pine showing dominance on north slopes and lodgepole pine showing dominance on south slopes and ridgetops. Lodgepole pine was absent from many upper subalpine plots. Although the oldest conifers in the burn were 25 years of age, the oldest whitebark pine were 21 years old, demonstrating both whitebark pine's tendency to delay postfire establishment and its ability to compete with other conifers despite the delay. Typical of species with wind-dispersed seed on large burns, subalpine fir showed good establishment at the burn's perimeter, and poor establishment in the burn's interior [205].
The combination of mountain pine beetle attacks and blister rust infection followed by large, stand-replacing fire is harmful because whitebark pine seed sources are severely reduced. The Sundance Fire in northern Idaho provides an example. A history of bark beetle infestation and high incidence of blister rust in the area had already reduced whitebark pine seed sources prior to the wildfire. A survey conducted at postfire year 25 revealed a 27% blister rust infection rate in mature whitebark pine adjacent to the burn. Most whitebark pine in unburned plots were snags with mountain pine beetle galleries. On unburned plots, mean density of live whitebark pine > 0.4 inch (1 cm) in diameter was 0.008 tree site (single tree or cluster)/m2. In contrast, seed source density for the Sleeping Child Burn was 0.064 tree site/m2,and the Sleeping Child Burn had more whitebark pine regeneration.Comparison of whitebark pine regeneration on the 2 burns is given below. Data are mean densities (1 standard deviation, range) [210].
Burn area Burn year Study year Density (tree site/m2) Sundance 1967 1992 0.0077 (0.0131, 0-0.0800) Sleeping Child 1961 1987 0.0700 (0.1041, 0-0.5120)Tomback and others [210] state that whitebark pine seed production near the Sundance Burn was so low that Clark's nutcrackers were not caching many seeds. Without active management including artificial regeneration, the long-term outlook for whitebark pine in the area does not look good. Twenty-nine percent of the seedlings on the Sundance Burn show symptoms of blister rust; actual rate of seedling infection is probably higher.
Fire scorching may increase whitebark pine's susceptibility to mountain pine beetle attack [62,72]. A 1990s investigation of 2nd-order fire effects following the 1988 wildfires in Yellowstone National Park revealed the following trends in whitebark pine mortality at postfire year 7 [175]:
Tree condition Percent green (survived fire) 36.1 fire killed 59.7 postfire insect kill (mostly mt. pine & pine engraver beetles) 2.8 unknown mortality 1.4Breeding system: Most genetic diversity is harbored within populations; between-population diversity is low in whitebark pine. Gene flow is facilitated by wind dispersal of pollen and bird dispersal of seed [39,40,57]. Probably due to long-range movement of Clark's nutcrackers dispersing whitebark pine seeds over time, genetic diversity of whitebark pine is low compared to other North American pine species [39,40].
On a fine scale, genetic structure of whitebark pine consists of clusters of close relatives. As a consequence of Clark's nutcracker's habit of planting seeds from a parent tree in the same cache, individuals within clusters often cross- or self-pollinate. This results in inbreeding. Trees within clumps are usually related as half-siblings, full siblings, or selfed. Neighboring clumps (probably planted by a different bird and/or collected from a different parent tree) are not closely related to each other [61,127,208,213].
Seed production: Cone production requires 2 years, as is typical for pines (Pinus spp.). Cones are 1st produced at 20 to 30 years of age on good sites. Trees do not reach full cone production until 60 to 100 years of age on most sites [125,146]. Peak cone production extends for another 250 years, then gradually declines. Some 1,000-year-old trees still reproduce [207]. Cone production is characterized by frequent years of small cone crops and less frequent years of moderate to heavy crops [104]. In the Greater Yellowstone area, moderate or large whitebark pine conecrop years occurred 2 or 3 times a decade (1980-1990) [159]. Best reproduction occurs when day/night July temperatures are above 68/39 degrees Fahrenheit (20o/4o C), and there is no summer water stress [226].
Factors limiting reproduction: A number of agents reduce natural regeneration in whitebark pine. White pine blister rust, fire exclusion, bark beetles, animals, and fungal diseases reduce ability of mature trees to reproduce. White pine blister rust is the greatest threat to whitebark pine regeneration [28]. In blister rust-infected trees, branch die-off 1st occurs on the ends of large, cone-producing branches. Although tree mortality may not occur for decades, infected trees rapidly loose ability to produce seed [17]. By reducing the gene pool, genetic consequence of white pine blister rust is inbreeding depression (expression of maladaptive or lethal genes) [84,232]. However, other factors also contribute to poor regeneration and decline (see Other Management Considerations). Using historical stand reconstruction studies on the Bitterroot National Forest, Arno and others [18] determined that whitebark pine dominated 14% of the landscape in 1900. By the end of that century, combined effects of fire exclusion and white pine blister rust had reduced whitebark pine to the point that whitebark pine longer dominated any of the study sites. Remaining stands with cone-bearing whitebark pine were one-half their former size. Mountain pine beetle epidemics can depress whitebark pine regeneration for decades by killing mature, cone-bearing trees [23]. On the Sundance Burn in northern Idaho, scant whitebark pine regeneration has been attributed to mountain pine beetle attacks prior to large-scale wildfire coupled with blister rust damage to whitebark pines on the burn's periphery [210].
Animal seed predation on whitebark pine seed is high. Except following good conecrop years, whitebark pine seedling establishment is probably incidental due to high rates of seed predation [214,229]. Even Clark's nutcracker harvesting of whitebark pine seed, often presented as a classic example of animal-plant mutualism [204], may be detrimental on some sites. Although individual Clark's nutcrackers only remove seeds that they plant themselves [147], researchers fear that in areas of high blister rust infection, whitebark pine seed will become so rare that Clark's nutcrackers will consume most of the seed they cache, leaving few seed reserves for regeneration [215]. Clark's nutcrackers were the most efficient harvesters of whitebark pine seed on the Bridger-Teton National Forest of Wyoming, showing a 97% forage success rate (measured as time spent harvesting/seeds collected). Other important predators that harvested directly from whitebark pine cones included pine grosbeaks (92% success rate), ravens (79%), red squirrels (60%), and chipmunks (35%) [89]. Similarly, vertebrates harvested 100% of mature whitebark pine seeds on the slopes of Bachelor Butte in the Cascade Range of Oregon. Most successful seed collectors were Clark's nutcrackers, Douglas' squirrels, least chipmunks, and golden-mantled ground squirrels, respectively [134]. Mammalian and bird seed predation reduced the amount of soil-cached seed significantly (p<0.01) on the Gallatin National Forest of Montana. Northern pocket gophers were the most important seed predator [147].
Little is known of insect cone predators and their possible effects on whitebark pine regeneration. Further studies are needed in this area. Anderton and Jenkins [8] have documented whitebark pine seed predation by seed bugs (Leptoglossus occidentalis) and larch cone flies (Strobilomyia macalpinei) on the Bitterroot National Forest, Montana. Insect damage ranged from 0.4 to 7.1% of total seed crop in their study. A study across California, Oregon, Washington, Idaho, and Montana found that seed bugs were the most serious insect pest (27% of total whitebark pine seedcrop destroyed), with fir coneworms (Dioryctria abietivorella) damaging up to 13% of whitebark pine seeds [104].
Seed dispersal: Because cones are indehiscent, seed caching by Clark's nutcrackers is the only important means of dispersal [89]. Clark's nutcrackers break through the cone scales with their beaks to remove the seeds, then bury the seeds in shallow caches for use as future food [203,204]. In good conecrop years, the birds cache many more seeds than they recover for food [204]. Hutchins and Lanner [89] estimated that 1 Clark's nutcracker caches 98,000 seeds in a good conecrop year. Many unretrieved seeds germinate and produce new trees [89,116,120]. The birds prefer burns and other open, disturbed areas as cache sites, although they also select closed, shady sites that are unfavorable for whitebark pine regeneration [203,204]. Norment and Conner [166] found that Clark's nutcrackers are most abundant on small (0.1- to 2-ha), disturbed patches or nonforested patches. Approximately 40% of caches on plots in the Sierra Nevada were on sites favorable for whitebark pine regeneration [204].
Germination: Germination and the 1st few weeks of seedling life may be the most critical stages of whitebark pine's life history. Seedlings do not emerge until (a) embryonic development has occurred and (b) the seedbed is moist [212]. Clark's nutcrackers often cache whitebark pine seeds before they are fully ripe and developed [117]. Embryonic development continues after planting and requires stratification and weathering of the seedcoat before germination occurs [124]. Germinants typically emerge 2 or more years after caching, when embryos are mature and seedbeds are moist long enough for seeds to fully imbibe (> 4 days under laboratory conditions) [124,208]. Some germination occurs in fresh seed the 1st growing season after caching. Germination of 1st-year, mature seed collected on the Bridge-Teton National Forest, Wyoming, ranged from 6.7 to 56.7% [89]. Above-average precipitation may favor emergence. On the Gallatin National Forest, seeds that were hand planted in 1988, a dry year, showed reduced 1st-year emergence compared to seeds planted in 1989, a moist year. Emergence is best on burned or other mineral soils compared to soils with litter [147]. Light-severity burns do not prepare as good a seedbed as more severe burns [147,225]. Because they are relatively free from competition, seedlings on burns have the best chance of growing into mature trees [145].
Seed banking: Whitebark pine appears to be the only North American pine (Pinaceae) with a seed bank. Due to seed caching by Clark's nutcrackers and delayed seed germination, whitebark pine may show good seedling establishment even if the previous year's cone crop was poor. Studies conducted after the 1988 fires on the Gallatin National Forest and Yellowstone National Park found that germination rates of natural regeneration were greatest 2 years after good cone crops. Some seeds germinated the spring after Clark's nutcracker planting, while others germinated in the 3rd (and last) year of the study. Synchronous germination occurred in both seedling clusters and single germinants. As of 1995, mean survivorship of seedling clusters > 1 year of age was 25%. The role of precipitation was unclear, but favorable precipitation was positively correlated (r=0.935) with good seedling establishment on the Yellowstone site [208]. Clark's nutcrackers have been observed caching seed as far as 13 miles (22 km) from parent trees [223]. They sometimes relocate cached seed to new sites [89], so actual dispersal distances may be greater. Longer travel distances may translate to fewer seedlings, however. Seedling density on the Sleeping Child Burn of western Montana decreased significantly (p > 0.05) as distance from seed source increased [199].
Seedling establishment and growth: Due to delayed germination and Clark's nutcracker caching habits, good seedling establishment requires many years. Clark's nutcrackers continue to cache seeds on burns and other disturbed sites as long as sites remain open and soils are bare. Burns where fire was exceptionally hot may not show good establishment for several postfire decades [11]. For example, the Sleeping Child and Saddle Mountain burns of western Montana 1st showed whitebark pine establishment 5 and 7 years after fire, respectively, with best establishment occurring 2 or more years after favorable summer rains promoted cone production [214].
Whitebark pine seedlings are generally considered hardy after their 1st few weeks of life [17,208]. Seedlings rapidly grow deep roots and thick, drought-resistant stems [40], enabling whitebark pine seedlings to better survive drought compared to their more sun-intolerant conifer associates. Even so, droughty, coarse-textured soils may reduce whitebark pine establishment. Light shade improves seedling survivorship; however, McCaughey [147] found that heavy shade increased drought-related seedling mortality on the Gallatin National Forest. He suggested that in dry years, increased cover might intercept critical precipitation. Shrub nurse plants may increase whitebark pine seedling survivorship, but herbaceous species with abundant fibrous roots appear to inhibit establishment. Based upon relative species abundance, whitebark pine seedlings on the Sleeping Child and Saddle Mountain burns were most frequently associated with grouse whortleberry, and seldom associated with smooth woodrush and beargrass (Xerophyllum tenax) [199,214].
Whitebark pine survivorship is generally considered best on burns [147]; however, given open conditions and mineral soil, seedlings may show good survivorship on a variety of sites. In Yellowstone National Park, whitebark pine seedlings showed best establishment on moist, moderately to severely burned sites compared to moist, unburned sites and dry burned/unburned sites. On the Gallatin National Forest, however, seedling establishment was similar on burned and unburned sites with similar moisture regimes [208].
Most seedlings gain rapid root growth, acquiring top-growth more slowly. First-year germinants on the Gallatin National Forest showed root lengths ranging from 2 to 7.1 inches (5-18 cm) [146]. In Yosemite National Park, mean top-growth rate of seedlings at 10,000 feet (3,050 m) elevation was 0.9 inch (2.3 cm)/year, while seedlings at 10,810 (3,295 m) gained an average 0.7 inch (1.7 cm) per year [204].
A number of agents may damage or kill seedlings. Heat damage to unshaded stem tissue is the common cause of death. Browsing animals also kill seedlings. Northern pocket gophers cause highest mortality on whitebark pine seedlings on the Gallatin National Forest, although browsing elk, chipmunks, and birds - including Clark's nutcrackers - also consume seedlings [146]. Tomback [204] found that 2 years after emergence, survivorship of natural whitebark pine regeneration on 2 Sierra Nevada sites averaged 41 and 65% of 1st-year cohorts.
Most growth occurs in mid-summer [87]. Growth on cold sites may be very slow [229], taking as long as 17 years to produce a 5-inch-long (12-cm) branch [197]. Tree-ring data from the central and southern Sierra Nevada show that best growth occurs following warm, wet winters, and slowest growth occurs after cool, dry winters [70].
Asexual regeneration: Whitebark pine reproduces by layering where long-lasting snowloads bend lower branches and thin, flexible stems onto soil. Layering is most common in krummholz whitebark pine [17,146]. Krummholz whitebark pine rarely sets seed and when it does, the seed often shows poor germination. Krummholz patches usually originate from lower-elevation seed transported into the upper subalpine by Clark's nutcrackers. Once krummholz is established, layering is its primary method of patch expansion [205]. Except in the upper subalpine, layering is not an important method of whitebark pine regeneration [17].
Arno [14] characterizes whitebark pine as a generally minor seral species in lower subalpine communities, a major seral species in the upper subalpine, a co-climax species in lower timberline, and a climax species in upper timberline. Whitebark pine tolerates open, sunny to moderately shady sites [122,171,183]. It is typically the 1st tree species found on sites where fire or another deforestation event has occurred [93,126]. In Canada and the northwestern United States, many subalpine whitebark pine communities are seral, and subject to successional replacement by shade-tolerant conifers [212]. After whitebark pine cover is established, shade-tolerant species often establish in the shelter of established whitebark pines [43,126,183,229]. Callaway [43] found that in the Bitterroot Mountains of western Montana, whitebark pine facilitated establishment and growth of later-successional subalpine fir on high-elevation sites. Seedling and sapling subalpine fir were highly aggregated around mature whitebark pine on upper subalpine sites (> 8,580 ft (2,600 m)), but not at relatively low-elevation subalpine sites (7,260 ft (2,200 m)). On upper subalpine sites, mature subalpine fir adjacent to living or dead mature whitebark pine showed more rapid growth rates compared to mature subalpine fir growing in the open. Whitebark pine is most productive on sites where it is seral [108]. On landscapes across the West, whitebark pine is increasingly becoming displaced by later-successional species. Murray and others [165] estimated that since 1753, 50% of 6 subalpine watersheds on the Idaho-Montana border have shifted to late-successional subalpine fir. Only 3% of the subalpine landscape has shifted to seral whitebark pine.
Whitebark pine in the Cascade Range occupies a seral role in subalpine parklands and forests [2,69,126]. Early to mid-seral conditions in these associations are mostly maintained by occasional stand-replacement fire. In the absence of fire, subalpine fir usually forms closed stands of mature trees [126]. In high-elevation whitebark pine communities, stands are typically open even in "near-climax" conditions [4,67]. Subalpine fir is sometimes present in the understory, suggesting eventual replacement in even these high-elevation types. However, successional patterns in high-elevation ecosystems are largely undocumented and difficult to predict [4]. On Mount Rainier, whitebark pine and subalpine fir are invading subalpine meadows simultaneously [68].
In Crater Lake National Park, Oregon, whitebark pine is the dominant tree on Wizard Island. Sierra Nevada lodgepole pine is invading the island and appears to be replacing whitebark pine in importance [93].
The scientific name of whitebark pine is Pinus albicaulis Engelm.
(Pinaceae) [53,64,79,95,114]. Whitebark pine is the only
stone pine (subgenus Strobus, section Strobus, subsection Cembrae)
native to North America [53,118].
Whitebark and limber pine (P. flexilis) have been crossed in the
laboratory, and putative hybrids between the 2 species have been identified on
the Rocky Mountain Front of Montana. Such entities are rare and
apparently infertile [51,59].
Restoration: Increasing mortality from successional replacement, white pine blister rust, bark beetles, and other agents foretell that whitebark pine will not remain an important component of subalpine communities without long-term, active management intervention [12,100]. Whitebark pine researchers recommend the following:
1. Assess the local extent, successional status, and vigor of whitebark pine. If it appears that cone crops will dwindle in the future [12],
2. inventory stands to document tree age, stand structure, cone-production potential, and projected time frame of successional replacement [12,18,19].
3. Apply and evaluate management-ignited and wildland for resource benefit fires designed to kill late-successional trees and favor whitebark pine (see Fire Management Considerations).
4. Conduct seed trials with blister rust-resistant stock in areas where natural whitebark pine seed sources have disappeared [12].
Artificial regeneration: Whitebark pine can be grown in the nursery from seed [94], and there is considerable interest in outplanting nursery-grown, blister-rust resistant stock to replace dead and dying mature whitebark pine [119]. Transplanted whitebark pine has shown fair survivorship rates; further studies are needed to determine which habitat/aspect/elevational combinations are best for artificial regeneration. Whitebark pine appears to tolerate broad, possibly regional seed transfer. Transfer guidelines are available [135,218]. Planting seed may be a good restoration option, as natural dormancy of whitebark pine seed may help ensure germination under conditions favorable for establishment (see Regeneration Processes). On the Gallatin National Forest, seedlings established from hand-planted seed showed mean 1st-year survivorship rates of 73% in a drought year (1988) and 90% in a wet year (1989) [147]. Kendall [106] recommends developing a cold-stored genetic seed bank for whitebark pine, emphasizing that collections from small, isolated populations should be a priority. Authorities from the U.S. Forest Service Nursery in Coeur d'Alene, Idaho, [42] provide guidelines for collecting whitebark pine seed in the field, growing whitebark pine in the greenhouse, and transporting seedlings to planting sites.
Genetic considerations: Hoff and others [83,84] provide advice on managing whitebark pine in the field to promote genetic resistance for blister rust. Experts on whitebark pine genetics have raised the possibility of establishing "seed orchards" similar to those successfully used for western white pine, an important timber species that has also been decimated by blister rust. Seed orchard trees are started from seed collected from parent trees showing blister-rust resistance. The seedlings are planted on favorable sites (sometimes even in greenhouses). Given ideal growth conditions, age of reproduction of parent trees can be greatly reduced. (Some whitebark on moist sites on the Kootenai National Forest have produced female cones at 10 years of age (pers. observ.); trees in greenhouses may set seed as early as 7 years). Natural and artificial cross-breeding of symptomless whitebark parents will enhance genetic selection for blister-rust resistance and provide opportunities for restocking blister rust-decimated landscapes with orchard progeny, many of which will inherit mechanisms for blister-rust resistance [84].
Yanchuk [233,234] provides guidelines for prioritizing conifer species for genetic conservation programs in British Columbia. Selection is based on species protection status, conservation breeding programs already in place, and relative capacity for regeneration. As of this writing (2002), whitebark pine was rated the species in greatest need of genetic conservation management.
Due to its clumping habit, whitebark pine has an unusual genetic population structure (see Breeding system). Planting whitebark pine without regard for its natural habit of growing in clumped family groups may have long-term consequences on natural selection processes operating on whitebark pine [207].
