Forests cover roughly 80% of the park, and lodgepole pine makes up nearly all of that canopy. Lodgepole pine, Engelmann spruce, subalpine fir, whitebark pine, and limber pine are found at higher elevations.
Douglas-fir forests occur at lower elevations, especially in the northern portion of the park. The thick bark of Douglas-fir trees allows them to tolerate low intensity fire. Some of the trees in these forests are several hundred years old and show fire scars from a succession of low-intensity ground fires. In contrast, lodgepole pine trees have very thin bark and can be killed by ground fires.
At higher elevations, such as the Absaroka Mountains, older forest is dominated by Engelmann spruce and subalpine fir, especially in areas that grow on andesite, a volcanic rock. These forests may have been dominated by lodgepole pine at one time, but have been replaced by Engelmann spruce and subalpine fir in the absence of fire and presence of non-rhyolitic soil (a non-volcanic soil). Engelmann spruce and subalpine fir can also be common in the understory where the canopy is entirely composed of lodgepole pine.
In rhyolitic soils (another volcanic substrate), which are poor in nutrients needed by fir and spruce, lodgepole pine remains dominant. At higher elevations such as the Absaroka Mountains and the Washburn Range, whitebark pine becomes a significant component of the forest. In the upper subalpine zone, whitebark pine, Engelmann spruce, and subalpine fir often grow in small areas separated by subalpine meadows. Wind and desiccation cause distorted forms known as krumholtz where most of the “tree” is protected below snow.
The lodgepole pine (Pinus contorta) is by far the most common tree in Yellowstone. Early botanical explorers first encountered the species along the West Coast where it is often contorted into a twisted tree by the wind, and thus named it Pinus contorta var. contorta. The Rocky Mountain variety, which grows very straight, is Pinus contorta var. latifolia. Some American Indian tribes used this tree to make the frames of their tipis or lodges, hence the name “lodgepole” pine.
Lodgepoles are the only pine in Yellowstone whose needles grow in groups of two. The bark is typically somewhat brown to yellowish, but a grayish-black fungus often grows on the shady parts of the bark, giving the tree a dark cast.
The species is shade intolerant; any branches left in the shade below the canopy will wither and fall off the tree. Lodgepoles growing by themselves will often have branches all the way to the base of the trunk because sunlight can reach the whole tree.
Like all conifers, lodgepole pines have both male and female cones. The male cones produce huge quantities of yellow pollen in June and July. This yellow pollen is often seen in pools of rainwater around the park or at the edges of lakes and ponds.
The lodgepole’s female cone takes two years to mature. In the first summer, the cones look like tiny, ruby-red miniature cones out near the end of the branches. The next year, after fertilization, the cone starts rapidly growing and soon becomes a conspicuous green. The female cones either open at maturity releasing the seeds, or remain closed—a condition called serotiny—until subjected to high heat such as a forest fire. These cones remain closed and hanging on the tree for years until the right conditions allow them to open. Within a short period of time after the tree flashes into flame, the cones open up and release seeds over the blackened area, effectively dispersing seeds after forest fires. Trees without serotinous cones (like Engelmann spruce, subalpine fir, and Douglas-fir) must rely on wind, animals, or other agents to carry seeds into recently burned areas.
Lodgepole pines prefer slightly acidic soil, and will grow quickly in mineral soils disturbed by fire or by humans, a road cut for example. Their roots spread out sideways and do not extend deeply—an advantage in Yellowstone where the topsoil is only about 6 to 12 inches deep, but a disadvantage in high winds. Lodgepole pines are vulnerable in windstorms, especially individuals that are isolated or in the open.
Besides reseeding effectively after disturbance, lodgepole pines can grow in conditions ranging from very wet ground to very poor soil prevalent within the Yellowstone Caldera. This flexibility allows the species to occur in habitat that otherwise would not be forested.
Because lodgepole pines are dependent on sunny conditions for seedling establishment and survival, the trees do not reproduce well until the canopy opens up significantly. In the Yellowstone region, this allows the lodgepole pine forest to be replaced by shade-loving seedlings of subalpine fir and Engelmann spruce where the soil is well-developed enough to support either of these species. In areas of nutrient-poor soil, where Engelmann spruce and subalpine fir struggle, lodgepole pines will eventually be replaced by more lodgepole pine trees as the forest finally opens enough to allow young lodgepoles to become established.
Whitebark pine (Pinus albicaulis) occurs at high elevations a in subalpine communities in the northern Rocky Mountains and the Pacific Northwest. It often grows in areas with poor soils, high winds, and steep slopes that are inhospitable to other tree species. White bark pine is a key species in these upper ranges where it retains snow and reduces erosion, acts as a nurse plant for other subalpine species, and produces seeds that are an important food for birds, grizzly bears, and other wildlife. Whitebark pine produces wingless seeds and relies primarily on Clark’s nutcrackers (Nucifraga Columbiana) for seed dispersal.
Substantial mortality in whitebark populations has been documented throughout its range. Decreases are attributed to the introduced pathogen, white pine blister rust (Cronartium ribicola); native mountain pine beetle (Dendroctonus ponderosae); historic wildland fire suppression resulting in more frequent, larger, and hotter wildfires; and projected environmental factors associated with climate change. These agents, both individually and collectively, pose a significant threat to the persistence of healthy whitebark pine populations on the landscape.
A reported 14–16% of whitebark pine trees taller 1.4 meters tall are infected with blister rust in the Greater Yellowstone region. As of 2017, 1,502 of the more than 5,300 monitored trees had died, including 67% of those in the >10 cm in diameter size classes. (The mountain pine beetle prefers larger trees for laying their eggs; the larvae feed on the inner phloem of the bark.) In addition, the Greater Yellowstone Network has estimated that by the end of 2015, 26% of whitebark pine trees >1.4 meters tall had died.
Aerial surveys, which measure the spatial extent of mortality rather than the percentage of individual dead trees counted on the ground, have generally produced higher whitebark pine mortality estimates in the Greater Yellowstone Ecosystem. This could be because larger trees, which occupy more of the area in the forest canopy visible from the air, are more likely to be attacked by beetles. In 2013, an aerial survey method called the Landscape Assessment System was used to assess mountain pine beetle-caused mortality of whitebark pine across the region. Results of the one-time study indicate that nearly half (46%) of the GYE whitebark pine distribution showed severe mortality, 36% showed moderate mortality, 13% showed low mortality, and 5% showed trace levels of mortality.
Despite the high percentage of large trees that have died, there are trees that are still producing cones and regeneration is occurring. The network estimated an average growth of 51 small trees per 500 meters squared by the end of 2015.
Understory vegetation differs according to precipitation, the forest type, and the substrate. Lodgepole pine forest is often characterized by a very sparse understory of mostly elk sedge (Carex geyeri) or grouse whortleberry (Vaccinium scoparium). Pinegrass (Calamagrostis rubescens) occurs frequently under Douglas-fir forest but is also common under other forest types, especially where the soil is better developed or more moist. In some areas of the park, such as Bechler and around the edges of the northern range, a more obviously developed shrub layer is composed of species such as Utah honeysuckle (Lonicera utahensis), snowberry (Symphoricarpos spp.), and buffaloberry (Shepherdia canadensis).
Forest Insect Pests
The conifer trees of Yellowstone face six major insect and fungal threats. The fungus is an nonnative species, but the insects are native to this ecosystem. They have been present and active in cycles, probably for centuries. A scientist studying lake cores from the park has found some of their insect remains in the cores, indicating their presence even millions of years ago. However, in the last 10 years, all five insects have been extremely active, which may be due to the effects of climate change.
The primary cause of tree mortality in the Yellowstone is native bark beetles. The beetles damage trees in similar ways: their larvae and adults consume the inner bark. If the tree is girdled, it dies. Their feeding activity can girdle a tree in one summer, turning the crown red by the following summer. The needles usually drop within the next year, leaving a standing dead tree. Isolated pockets of red-needled trees are scattered throughout the park.
The severity of insect-caused tree mortality has been considerable throughout the West for over a decade, and the insects have spread to previously unaffected plant communities. Several native bark beetle species in the Scolytidae family have altered extensive areas within Greater Yellowstone. Forest structure, tree health, and climate are the major factors in determining whether an outbreak expands; drought and warmer temperatures can make forests more vulnerable to infestation.
Recent evaluation has shown decreases in infection and infestation rates since 2001, suggesting that resistance may be slowly increasing. Although activity by both Douglas-fir beetle and Engelmann spruce beetle has declined to endemic (natural to Yellowstone) levels since 2000, other forest insects of ecological significance remain active. Mountain pine beetle activity was largely confined to the northwest portion of the park, in high-elevation whitebark pine and lower-elevation lodgepole pine, peaking in 2009 with annual decreases in mortality since then. Defoliation of Douglas-fir and Engelmann spruce by the western spruce budworm is present in the park throughout the lower Lamar and along the Yellowstone and Lamar river valleys, but has spread considerably less in recent years. These trends appeared to continue in 2011, when the park was only partially surveyed.
Future of Insect Outbreaks in Yellowstone
Landscape-scale drought and the availability of suitable host trees have contributed to the initiation and persistence of insect outbreaks. Healthy trees can defend themselves from beetle attack by “pitching out” adult females as they try to bore into the tree. Extreme winter temperatures can kill off overwintering broods, and wet summer weather impedes the insects from invading additional trees. Insect activity also decreases as the larger and more susceptible trees are killed off. Spruce beetles have declined because they have killed almost all of their preferred food source (spruce trees more than 10 inches indiameter).
Recent and ongoing studies supported by the National Park Service are investigating the interaction between insect infestations and wildfire. Researchers have focused on how bark beetle epidemics may affect fire behavior in lodgepole-dominated forests and are comparing the resulting fire hazard with that in Douglas-fir forests.
Amman, G.D. and K.C. Ryan. 1991. Insect infestation of fire injured trees in the Greater Yellowstone Area, Edited by US Department of Agriculture, Forest Service, Intermountain Research Station.
Christof, B., D. Kulakowski, and T.T. Veblen. 2005. Multiple disturbance interactions and drought influence fire severity in Rocky Mountain subalpine forests. Ecology 86(11):3018–3029.
Despain, D.G. 1990. Yellowstone vegetation: Consequences of environment and history in a natural setting. Boulder, CO: Roberts Rinehart Publishing Company.
Fleming, R.A., J.-N. Candau, and R.S. McAlpine. 2002. Landscape-scale analysis of interactions between insect defoliation and forest fire in central Canada. Climatic Change 55(1–2):251–272.
Furniss, M. M. and R. Renkin. 2003. Forest entomology in Yellowstone National Park, 1923–1957: A time of discovery and learning to let live. American Entomologist 49(4):198–209.
Hagle, S.K. et al. 2003. A Field Guide to Diseases and Insects of Northern and Central Rocky Mountain Conifers. U.S. Forest Service Report R1-03-08.
Hicke, J.A. et a.. 2006. Changing temperatures influence suitability for modeled mountain pine beetle outbreaks in the western United States. Journal of Geophysical Research 111:G02019.
Johnson, P.C. and R.E. Denton. 1975. Outbreaks of the western spruce budworm in the American northern Rocky Mountain area from 1922 through 1971, Edited by US Department of Agriculture, Forest Service, Intermountain Forest and Range Experimentation Station. Ogden, UT.
Logan, J.A. et al. 2003. Assessing the impacts of global warming on forest pest dynamics. Frontiers in Ecology and the Environment 1:130–137.
Lynch, H., R. Renkin, R. Crabtree, and P. Moorcroft. 2006. The influence of previous mountain pine beetle (Dendroctonus ponderosae) activity on the 1988 Yellowstone fires. Ecosystems 9(8):1318–1327.
McCullough, D.G., R.A. Werner, and D. Neumann. 1998. Fire and insects in northern and boreal forest ecosystems of North America. Annual Review of Entomology 43(1):107–127.
Pauchard, A. and Alaback, P. 2006. Edge types defines alien species invasions along P. contorta burned, highway and clearcut forest edges. Forest Ecology and Management 223: 327–335.
Renkin, R.A. and D.G. Despain. 1992. Fuel moisture, forest type, and lightning-caused fire in Yellowstone National Park. Canadian Journal of Forest Research 22(1):37–45.
Ryan, K.C. and G.D. Amman. 1994. Interactions between fire-injured trees and insects in the Greater Yellowstone Area. In D. G. Despain, ed., Plants and their environments: Proceedings of the first Biennial Scientific Conference on the Greater Yellowstone Ecosystem, 259–271. Yellowstone National Park, WY: US Department of the Interior, National Park Service, Rocky Mountain Region, Yellowstone National Park.
Schmid, J.M. and G.D. Amman. 1992. Dendroctonus beetles and old-growth forests in the Rockies. In M.R. Kaufmann, W.H. Moir and R.L. Bassett, ed., Old-growth forests in the Southwest and Rocky Mountain regions, proceedings of a workshop, 51–59. Portal, AZ: USDA Forest Service, Rocky Mountain Forest and Range Experimentation Station.
Schoennagel, T., T.T. Veblen, and W.H. Romme. 2004. The interaction of fire, fuels, and climate across Rocky Mountain forests. BioScience 54(7):661–676.
Simard, M. et. al. 2011. Do mountain pine beetle outbreaks change the probability of active crown fire in lodgepole pine forests? Ecological Monographs 81(1): 3–24.
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Resources (Whitebark Pine)
Gibson, K. 2006. Mountain pine beetle conditions in whitebark pine stands in the Greater Yellowstone Ecosystem, 2006. Missoula, MT: USDA Forest Service, Forest Health Protection, Missoula Field Office.
Greater Yellowstone Whitebark Pine Monitoring Working Group. 2017. Monitoring whitebark pine in the Greater Yellowstone Ecosystem: 2016 annual report. Natural Resource Data Series NPS/GRYN/NRDS—2017/1453. National Park Service, Fort Collins, Colorado.
Macfarlane, W.W., Logan, J.A., and Kern, W.R.. 2013. An innovative aerial assessment of Greater Yellowstone Ecosystem mountain pine beetle-caused whitebark pine mortality. Ecological Applications 23(2):421–37
Mahalovich, M. F. 2013. Grizzly Bears and Whitebark Pine in the Greater Yellowstone Ecosystem. Future Status of Whitebark Pine: Blister Rust Resistance, Mountain Pine Beetle, and Climate Change. US Forest Service. Report Number: 2470 RRM-NR-WP-13-01.
Shanahan, E., K. M Irvine, D. Thoma, S. Wilmoth, A. Ray, K. Legg, and H. Shovic. 2016. Whitebark pine mortality related to white pine blister rust, mountain pine beetle outbreak, and water availability. Ecosphere 7(12)
Shanahan, E., K. Legg, and R. Daley. 2017. Status of whitebark pine in the Greater Yellowstone Ecosystem: A steptrend analysis with comparisons from 2004 to 2015. Natural Resource Report NPS/GRYN/NRR-2017/1445. National Park Service, Fort Collins, Colorado.
Tomback, D.F., S.F. Arno, and R.E. Keane. 2001. Whitebark pine communities: Ecology and restoration. Washington, DC: Island Press.
Last updated: October 29, 2019