The ponderosa pine forest is a vegetation type that straddles the line between rangelands and forest. These woodlands have an overstory of ponderosa pine trees that under some conditions can grow so closely together as to create a dense canopy with few grasses and forbs in the understory, taking on the appearance and function of a true forest. Under other conditions, the pine trees can be widely spaced allowing a dense understory of herbs and shrubs, creating a park-like appearance that is classified as rangeland. The natural force that creates and maintains the savanna conditions of healthy ponderosa pine woodlands is fire. Wildfires were historically common in this forest type and natural and prescribed fires today reduce the density of trees, increase abundance of grasses and forbs, and reduce the risk of wildfire recurring for up to a decade after the fire.
Ponderosa pine (Pinus ponderosa) is the most widely distributed pine in North America and dominates many forested communities that cover vast areas in the West. The ponderosa pine woodlands are savannah-like ecosystems characterized by widely spaced ponderosa pine trees with an understory of grasses and shrubs. These western landscapes occur along the lower elevation of mountains extending from the Rocky Mountains in Canada to the Sierra Madre of Mexico. Theses pine savannahs occur in belts along the Sierra Nevada and Cascade Mountains in the west, in several areas throughout the Rocky Mountains, in the Laramie and Bighorn Mountains of Wyoming and in the Black hills of South Dakota. These zones of ponderosa pine woodlands vary from 8 to 40 km (5 to 25 mi) wide and from 1,515 m to 2,900 m (500’ to 9500’ feet) in elevation.
The Ponderosa Pine Woodlands are not uniform throughout their range. In moist forests, at northern latitudes, the ponderosa pine forest is a seral or subordinate type in forests dominated largely by Douglas fir (Pseudotsuga menziesii), and other conifers. In these moist forests, the ponderosa pine communities often dominates the more xeric south facing slopes. In the southern states and drier regions, the often forms the classic wide open grassy ponderosa savannas. Over its entire range, the ponderosa pine ecosystem covers about 15 million hectares (27 million acres).
The ponderosa pine woodlands are not uniform throughout their range. In moist forests at northern latitudes, the ponderosa pine forest is a seral or subordinate type in forests dominated largely by Douglas fir (Pseudotsuga menziesii) and other conifers. In these conditions, the ponderosa pine communities often dominate the more xeric south-facing slopes. In the southern states and drier regions, they often form the classic wide open grassy ponderosa savannas. Over its entire range, the ponderosa pine ecosystem covers about 15 million hectares (27 million acres).
Climate varies dramatically over the entire extent of the ponderosa pine’s range. Depending on latitude, the ponderosa pine can be found at elevations ranging from sea level to 3,050 meters (10,000 feet). Precipitation falls primarily in the winter as snow at the northern and eastern extent but falls more evenly between summer and winter at the southern extent. The amount of precipitation is also highly variable between the wet forests at the northern extent and the dry savannas at southern semi-arid extent.
Topography and Soils
The topography of ponderosa pine regions also vary significantly, from gently sloping landscapes on mountain foothills to steep mountainsides. Ponderosa pine forests grow primarily on medium- to coarse-textured soils. A well-developed A-horizon can be present in open stands with a significant understory of grass or other herbaceous plants.
As the name suggests, the ponderosa pine tree (Pinus ponderosa) is the prevalent and common feature of all ponderosa pine woodlands, and it is also one of the most widely distributed plants in North America. There are two sub-species of ponderosa pine: Pacific ponderosa pine (Pinus ponderosa var. ponderosa) and Rocky Mountain ponderosa pine (Pinus ponderosa var. scopulorum). Arizona pine or Arizona ponderosa pine (Pinus arizonica) is a closely related species of pine that was at one time a sub-species of ponderosa pine and is the dominant tree in much of the southwestern extent of ponderosa pine woodlands.
Several common western trees occur as co-dominants with ponderosa pine in specific regions of the ponderosa pine woodlands. These important trees include: Douglas fir (Pseudotsuga menziesii), grand fir (Abies grandis), western redcedar (Thuja plicata), quaking aspen (Populus tremuloides), and western juniper (Juniperus occidentale).
Common understory shrubs throughout the ponderosa pine woodlands include: mallow ninebark (Physocarpus malvaceus), snowberry (Symphoricarpos albus and S. Oreophilus), bearberry (Arctostaphylos uva-ursi), antelope bitterbrush (Purshia tridentana), woods rose (Rosa woodsii), dearbrush (Ceanothus integerrimus), and Gambels oak (Quercus gambelii).
Major native grasses and grass-like plants that create the grassy floor of many ponderosa pine woodlands include: pine grass (Calamogrostis candensis), elk sedge (Carex geyeri), Idaho fescue (Festuca idahoensis), bluebunch wheatgrass (Elymus spicatus), and blue grama (Bouteloua gracilis).
A large array of forbs are also present in the understory of ponderosa pine woodlands, though the forbs present vary significantly from region to region. Common forbs include: western yarrow (Achillea millefolium), arrowleaf balsamroot (Balsamorhiza sagittata), silky lupine (Lupinus argenteus), wild geranium (Geranium richardsonii and G. viscosissima), and prairie cinquefoil (Potentilla gracilis).
Current Plant Communities
The major plant communities that characterize ponderosa pine woodlands vary depending on the climatic conditions. Plant communities can be subdivided into several geographic regions.
Pacific ponderosa pine vegetation type occurs at the lower elevation slopes of the Sierra Nevada Mountains and the southern Cascade Range. Ponderosa pine can occur in dense, nearly solid, stands on the more mesic and productive sites. On dry or less productive sites, pine trees occur with a diverse understory, including a variety of fire-tolerant shrubs and herbaceous forbs. These shrubs include manzanita (Arctostaphylos spp.), deerbrush (Ceanothus integerrimus), coffeeberry (Rhamnus californica), and a variety of oaks
The east slope of the Cascades, the Blue Mountains, and the northern Rocky Mountains are home for ponderosa stands in eastern Washington, Idaho, western Montana, and southern British Columbia. Common grasses in the understory include bluebunch wheatgrass (Elymus spicatus), needle-and-thread (Stipa comate), Idaho fescue (Festuca idahoensis), and rough fescue (Festuca scabrella). Several showy wildflowers also occur in the understory. These include arrowleaf balsamroot (Balsmorhiza sagittata), woodlands star (Lithophragma inflatum), and silky lupine (Lupinus sericeus). The understory also include snowberry (Symphoricarpos albus), and antelope bitterbrush (Purshia tridentata).
The central and southern Rocky Mountains include a distinct ponderosa pine vegetation type with an understory of bunchgrasses. These bunchgrasses include mountain muhly (Muhlenbergia montana), Idaho and Arizona fescue (Festuca idahoensis and F. arizonica), blue grama (Bouteloua gracilis), and little bluestem (Schizachyrium scoparium). Dominant forbs include fringed sagewort (Artemisia frigida), western yarrow (Achillea millefolium), wild geraniums (Geranium spp.), milkvetch (Atragalous spp.), cinquefoils (Potentilla spp.), and pussy toes (Antennaria rosea). Shrubs in this vegetation type are not very abundant. However primary shrubs include bearberry (Arctostaphylos uva-ursi), true mountain mahogany (Amelanchier alnifolia), and Gambel oak (Qurecus gambelii).
Great Plains ponderosa pine woodlands and savannahs are primarily found in Montana. In the north-central part of the state, Douglas fir is intermixed with ponderosa pine, with bluebunch wheatgrass in the understory and few to no shrubs. In southeastern Montana, ponderosa pine dominates with juniper mixed in and on the edges of ponderosa pine stands. Shrubs commonly found are bearberry (Arcostaphylos uva-ursi), snowberry (Symphoricarpos spp.), chokecherry (Prunus virgiana), serviceberry (Amelanchier alnifolia), and ninebark (Physocarpus spp.). Grasses include big bluestem (Andropogon gerardii), sideoats grama (Bouteloua curtipendula), prairie junegrass (Koeleria macrantha), and western wheatgrass (Pascopyrum smithii). Also included are forbs like western yarrow (Achillea millefolium), arrowleaf balsamroot (Balsamorhiza sagittata), silky lupine (Lupinus argenteus), and prairie cinquefoil (Potentilla gracilis).
The Black Hills region is an area of higher elevation that rises above the surrounding plains of the Missouri Plateau, characterized by granite outcrops, including the stone in which four U.S. Presidents are carved in Mount Rushmore. The Black Hills contain diverse forests dominated by ponderosa pine that vary from dense stands with little vegetation under the trees to open stands of ponderosa pine with a grassy understory. The central core of the Black Hills is a more mesic area with an understory dominated by bearberry (Arctostaphylos uva-ursi) and snowberry (Symphoricarpos albus). Lower elevations and more xeric stretches of the Black Hills include pine stands with understory of herbaceous plant including sun sedge (Carex heliophila), poverty oatgrass (Danthonia spicata), Kentucky bluegrass (Poa pratensis), little bluestem (Schizachyrium scoparium) and western wheatgrass (Agropyron smithii). Throughout the Black Hills there are also areas dominated by aspen trees (Populus tremuloides) with shrub understories.
Many unique animals reside in and migrate through the ponderosa pine woodlands. These include birds like the Williamson’s sapsucker, white-headed woodpecker, and pygmy nuthatch (which require pine trees), and other conifers that provide food and shelter. Mammals such as Abert’s squirrel and porcupines are common and depend on the trees in the ponderosa pine forest for their habitat. Also common throughout the ponderosa pine forest are black bears, which can damage pine trees by peeling back the bark in the spring to eat the sugar-rich sapwood.
The dead standing trees of ponderosa pine, called snags, are important to a great variety of wildlife species. Several insects, including carpenter ants and termites, can reside in snags where they create colonies and inadvertently become food for other animals such as woodpeckers and bears. Brown creepers and nuthatches also eat insects that inhabit snags and often nest in cavities found in these dead trees. Bats settle into the crevices behind the bark of ponderosa pine trees for daily roosting sites.
Large ungulates including deer and elk are year-round inhabitants of ponderosa pine woodlands. These grazers are often attracted to recently burned areas where forage quality is increased for a year or two after a fire.
Ponderosa pine woodlands and savannas are maintained by frequent fires. Low intensity ground fires reduce the understory vegetation, remove accumulated biomass, and remove ponderosa pine seedlings and other trees. This results in an overstory of tall pine trees with few branches near the ground.
The ponderosa pine forest was historically dominated by mostly mature ponderosa pine (Pinus ponderosa) growing in a relatively widely-spaced fashion with canopies of branches not touching or overlapping. With the reduction of wildfires during the 1900s, the density of pine trees is estimated to have increased significantly throughout the range of the ponderosa pine forests and woodlands. For example, a study in northern Arizona reported the density of pine trees rose more than eleven-fold from 26 trees/acre in 1883 to 291 trees/acre in 1995 (Denton et al. 2011).
Many reasons have been given for the reduced frequency of fire in ponderosa pine and other dry mixed-conifer forests. Logging activity was significant throughout the ponderosa pine forest in the 1900s, which reduced the fuel loads in western forests and likely kept fire frequency low.
Grazing throughout the ponderosa pine region was also very heavy in the early part of the 1900s. By all accounts, the number of grazing animals on lands administered by the U.S. Forest Service was four to five times lower in the 1970s than at the turn of the century. This heavy grazing decreased herbaceous grasses and forbs which likely reduced the frequency of low intensity ground fires through the ponderosa pine range and allowed young pine trees to become established. Both of these effects would have resulted in ponderosa pine forests with a greater density of pine trees than was observed historically.
It is widely believed that Native Americans set fires throughout the ponderosa pine range to promote an understory of herbaceous plants, providing forage for game species such as elk and deer. Cattlemen and sheep herders in the early 1900s were also commonly recognized for setting fires in the ponderosa pine forest in the fall as they brought their herds and flocks to lower elevation for the winter. These low-intensity fires would have resulted in killing pine tree seedling and promoting herbaceous plants without causing damage to large mature pine trees.
The ponderosa pine ecosystem can look very different depending on the elevation and precipitation regimes in which it is growing. The appearance and composition of the ponderosa pine woodlands varies by the density of trees and the type and amount of understory plants. In the cooler and higher precipitation reaches of ponderosa pine savannas, the pine trees tend to form rather dense stands and are seral to other trees including Douglas fir, grand fir and western red cedar. The climax forest in these more moist regions is generally a forest dominated by fir trees with ponderosa pine on the drier, rockier sites and in regions that experience frequent wildfires.
As precipitation decreases, ponderosa pine trees begin to form dense climax pine forests as found throughout the Central Rocky Mountain Region and Cascade Mountains. Continued decreases in precipitation and longer, hotter summers lead to a decrease in ponderosa density, creating woodlands and wide, open savannas. The savannas have an understory of low-growing shrubs and grasses with the lower branches of pine 1.5 n to 3 m (5’ to 10’) above the ground.
In the savannah form of ponderosa pine forests the vegetation in the understory can be dominated by either shrubs or grasses depending on the soil and local climatic conditions. In more mesic areas the understory is generally dominated by mountain shrubs including bitterbrush (Purshia tridentate) and snowberry (Symphoricarps albus). Drier ecosystems generally result in savannas with groundcover dominated by rhizomatous grasses, like pine grass (Calamagrostis rubescens), or sedges, like elk sedge (Carex geyeri).
At the lowest elevations of ponderosa pine, the trees become less and less dense until just a few lone trees occur in the surrounding rangeland types which include mountain shrublands, intermountain grasslands, sagebrush steppe, pinyon-juniper woodlands, or mixed prairies.
Ponderosa pine is highly adapted to frequent fires. The cracked orange bark is thick enough to protect the cambium from most fires. The trees naturally drop their lower branches (i.e., trees are self-pruning), resulting in trees where the lowest branch may be far above the ground and above any threat from a surface fire. The pine needles are dry and brittle, carpeting the ground after a few years, facilitating fast-moving low-severity surface fires. The seedlings also grow fast, in an attempt to grow beyond the influence of the next surface fire. A lack of fire can allow seedlings to grow up above the risk of fire, resulting in over-thickening of ponderosa trees. Despite all the fire adaptations of ponderosa pine trees they do not have serrotenous cones.
Wildfires over the entire ponderosa ecosystem generally occur between 5 and 25 years. This fire return interval varies with climate, and some isolated areas have had fire naturally excluded for much longer. The fires prevent invasion of other vegetation types and maintain the savannas and woodlands characteristic of the ponderosa ecosystem.
Recent fire suppression is often cited for causing an increase in ponderosa densities, turning savannas and woodlands into overcrowded forests. The result is a change from few trees with branches high above the surface, to a mixed-age forest with branches extending from the crown to the surface. These ladder fuels allow the next surface fire to turn into a crown fire, which can kill all the ponderosa in the area.
Ponderosa pine trees have deep tap roots and thick bark which persists in areas with high temperatures and low precipitation that preclude other coniferous trees. Drought conditions are common throughout the ponderosa pine woodlands. While mature ponderosa pine trees are well-adapted to drought conditions, seedlings are not able to survive extended periods of low moisture. Thus, one of the most obvious results of drought is limited recruitment of young pine trees. Another significant effect of drought is that it makes trees more susceptible to insect damage by insects such as that caused by mountain pine beetles.
Overuse by domestic livestock has caused undesirable changes in vegetation. With continued overgrazing, mountain muhly and fescues are replaced by plants that are more grazing–resistant, including blue grama, bluegrasses, and fringed sagewort. High numbers of both sheep and cattle grazed the ponderosa pine forest in the 1920s and 1930s before grazing management practices were instituted by the U.S. Forest Service. Heavy grazing during this period resulted in removing the fine flashy fuels from these landscapes, virtually “fireproofing” the ecosystem and preventing wildfires that are naturally widespread. Fire suppression by humans also occurred extensively during the 1900s, creating forests that are older and denser than they would have been with a natural fire regime. Although livestock numbers have been greatly reduced in the last 90 years, grazing continues to be an important use of ponderosa pine forests.
Insects and Disease
Ponderosa pine trees are susceptible to damage from a host of insects and diseases. More than 100 insects are known to cause damage to ponderosa pine trees. The most damaging of these are Pine Beetles of the Dendroctonus genus.
Dwarf mistletoe (Arceuthobium vaginatum ssp. A. vaginatum) is the most widespread disease affecting ponderosa pine. Trees infected by mistletoe often respond by growing additional branches at the site of attack creating a broom-line branching pattern. Mistletoe can reduce growth and cause death in as much as a third of the trees in an infected stand.
A host of diseases can affect roots, such as black stain rood disease. Other rots are “heart rots” that affect the wood of ponderosa pine, including western red rot which causes damage to wood and reduces its value for lumber. There are also diseases that attack foliage including Elytroderma deformans which causes needle cast and reduces growth.
Though insects and diseases can all cause death individually as a primary effect, they can also weaken trees, making them more susceptible to damage and death for other insects or diseases. Damage from these agents can also make trees more susceptible to death from drought or fire.
The many values and uses of ponderosa pine Forests include wood products, grazing, watersheds, recreation, and development. These many uses create challenges and opportunities for multiple-use management.
Fire suppression by humans, heavy grazing and other factors have resulted in dense forests in which fire does not regularly occur. Many ponderosa pine forests have lost their savanna-like character with lower branches far above the ground allowing cool fires to burn through the understory without killing the overstory trees. Restoration techniques including commercial and non-commercial tree thinning, prescribed fire and spraying are often used to create and maintain the open native canopy of the ponderosa pine forests.
Many invasive herbaceous plants cause ecological and economic problems in ponderosa pine ponderosa pine forests, depending on location and climatic conditions. Herbaceous plants such as houndstongue (Cynoglossum officinale), sulfur cinquefoil (Potentilla recta), dyer’s woad (Isatis tinctoria), spotted knapweed (Centaurea stoebe), and Dalmatian toadflax (Linaria dalmatica) are particularly problematic throughout the ponderosa pine range. Several invasive grasses including annual grasses such cheat grass (Bromus tectorum) and perennial grasses such as smooth brome (Bromus inermis). Herbaceous plants also increase after prescribed burning or wildfire, competing with native plants and pine seedlings. After a burn, many areas are susceptible to cheat grass which creates an easily ignited fuel reducing the time between fires. These invasive plants require additional vigilance and potential weed treatment after a burn.
At higher elevations and in the moist regions of the ponderosa pine Forest, conifer trees such as Douglas fir and grand fir can move into stands and outcompete the ponderosa pine trees. Under drier conditions, trees such as Western Juniper can compete with ponderosa pine trees.
The steady crawl of urban expansion is a threat to many of our open spaces and especially to ponderosa pine forests. Population growth in the western U.S. has fueled urbanization and encroachment into the forests. Mountainous forest terrain has become highly sought for building homes and communities to escape cities. This exurban development has created areas with many roads and buildings that fragment landscapes. Human development also brings non-native plants and pets which change native plant communities.
A healthy ponderosa pine stand benefits watersheds by providing a healthy understory that prevents erosion. A reduced overstory pine canopy or degraded understory vegetation with excessive bare ground allows more runoff from precipitation to occur. This can put the forests at greater risk of erosion and damage the overall water quality of rivers and streams with increased amounts of silts and minerals.
Ponderosa pines are a very important tree species in carbon sequestration because they account for about 22% of western U.S. timberland. Older stands of ponderosa can store about twice the amount of carbon that a young stand can, so it is vital to keep mature stands of these trees across the west.
Ponderosa pine forests produces valuable forest products, second only to Douglas fir in volume of trees cut. In addition, ponderosa pine-bunchgrass ranges have been important livestock-producing areas since early introduction of domestic stock by the Spanish settlers in the Southwest.
As tree numbers increase, forage production and timber quality decreases. Restoration to regain the preferred tree stand poses economic and technical difficulties. Selective cutting by age groups may provide a solution. Similar problems exist in other forest types where fire exclusion has resulted in overly thick tree stands and invasion of formerly productive mountain meadows by trees. In many national parks, naturally occurring fires are now allowed to burn where lives, structures, and private lands are not endangered, and prescribe burning is now being recommended for a number of forest and rangeland types.
Grazing Management Practices
Forage Sources and Seasonal Use
The herbaceous plants that dominate the understory of ponderosa pine woodlands create important forage sources for livestock and wildlife throughout western North America. Grazing in ponderosa pine ecosystems is generally restricted to the growing season because these forests are covered by snow during the winter throughout their range. The grazing season in ponderosa pine forests occurs between May to November depending on how much snow accumulates during the winter. These regions have historically been used in the spring and fall throughout the mountainous regions where they occur.
These woodlands can be grazed sustainably with a recommended utilization level (i.e., proper use factor) of 30 to 40%. Repeated heavy utilization exceeding 50% of forage plants can lead to reduced livestock productivity (i.e., rate of gain), and subsequent growth of herbaceous plants.
These woodland ecosystems have been historically and are currently grazed by both cattle and sheep. It is generally agreed that some sort of rotational grazing program is an important tool to maintain ecological health in these ecosystems. However, there is no specific grazing system that is uniformly superior. Deferred rotation, rest rotation, and management-intensive grazing have all given desirable results when appropriately applied.
Because ponderosa pine ecosystems often occur between lower elevation grasslands and higher elevation forests, they are often grazed in the early and late summer as part of a seasonal suitability, or “follow-the-green,” grazing program. In these grazing programs, forage plants are grazed early in the season, before they become highly susceptible to damage by grazing, or late in the season as they become senescent and less sensitive to herbivory. Thus, sustainable livestock grazing is often observed in areas with seasonal suitability grazing.
Animal Nutrition and Forage Quality
The nutritive value of plants in the ponderosa pine woodlands for livestock and wildlife varies by season and forage type. Regions of the ponderosa pine woodlands where the understory is dominated by shrubs provide important high quality forage for deer, sheep and goats. The shrubs found in ponderosa pine ecosystems are generally good forage plants that provide protein and nutrient levels higher than grasses, particularly in the late growing season into the dormant season.
Woodlands with a grassy understory are particularly valuable to elk and cattle as forage in the spring. While grasses are actively growing they are good sources of both energy and protein. As the season advances, the protein level of grasses declines, though they provide good sources of digestible energy until the snow flies.
The specific nutritional value of grasses, forbs, and browse depends on the season and specific plants available. As with many grazed ecosystems, the plants occurring in woodlands that are in good ecological condition are of higher nutritive value than those in degraded condition.
Pine Needle Abortion (PNA) is a condition caused when cows eat the needles off ponderosa pine trees late in their pregnancies, resulting in premature parturition or “abortion” of their calves. Both green and dry needles can induce PNA, causing substantial economic loss to the cattle industry through the loss of calves
Brush and Weed Control
Brush and weed control are achieved primarily by prescribed burning, spraying, commercial thinning and heavy monitoring. Because of suppressed fire in these areas brush has grown unchecked for many years, and as a result needs to burned or cut back. Fire, although effective, also puts the area at risk of invasive herbaceous plants like smooth brome and houndstongue.
Ponderosa pine is extremely important for lumber production in the United States. It ranks third behind Douglas fir and western hemlock for annual production. Annually1.3 billion board feet of ponderosa pine lumber is produced out of Oregon, the largest lumber supplier in the US. It’s primarily used for construction. Ponderosa pine is also highly valued for cabinetry and making furniture because of its clear, knot-free wood that is low-resin and resistant to splitting.