Northern Rockies Ponderosa Pine Woodland

This inland Pacific Northwest ecological system occurs in the foothills of the northern Rocky Mountains in the Columbia Plateau region and west along the foothills of the Modoc Plateau and eastern Cascades into southern interior British Columbia. These woodlands and savannas occur at the lower treeline/ecotone between grasslands or shrublands and more mesic coniferous forests typically in warm, dry, exposed sites. Elevations range from less than 500 m in British Columbia to 1600 m in the central Idaho mountains. Occurrences are found on all slopes and aspects; however, moderately steep to very steep slopes or ridgetops are most common. This ecological system generally occurs on glacial till, glacio-fluvial sand and gravel, dune, basaltic rubble, colluvium, to deep loess or volcanic ash-derived soils, with characteristic features of good aeration and drainage, coarse textures, circumneutral to slightly acidic pH, an abundance of mineral material, rockiness, and periods of drought during the growing season. In the Oregon "pumice zone" this system occurs as matrix-forming, extensive woodlands on rolling pumice plateaus and other volcanic deposits. These woodlands in the eastern Cascades, Okanagan and northern Rockies regions receive winter and spring rains, and thus have a greater spring "green-up" than the drier woodlands in the central Rockies. Pinus ponderosa (primarily var. ponderosa) is the predominant conifer; Pseudotsuga menziesii may be present in the tree canopy but is usually absent. In southern interior British Columbia, Pseudotsuga menziesii or Pinus flexilis may form woodlands or fire-maintained savannas with and without Pinus ponderosa var. ponderosa at the lower treeline transition into grassland or shrub-steppe. The understory can be shrubby, with Artemisia tridentata, Arctostaphylos patula, Arctostaphylos uva-ursi, Cercocarpus ledifolius, Physocarpus malvaceus, Purshia tridentata, Symphoricarpos oreophilus or Symphoricarpos albus, Prunus virginiana, Amelanchier alnifolia, and Rosa spp. common species. Understory vegetation in the true savanna occurrences is predominantly fire-resistant grasses and forbs that resprout following surface fires; shrubs, understory trees and downed logs are uncommon. These more open stands support grasses such as Pseudoroegneria spicata, Hesperostipa spp., Achnatherum spp., dry Carex species (Carex inops), Festuca idahoensis, or Festuca campestris. The more mesic portions of this system may include Calamagrostis rubescens or Carex geyeri, species more typical of Northern Rocky Mountain Dry-Mesic Montane Mixed Conifer Forest (CES306.805). Mixed fire regimes and surface fires of variable return intervals maintain these woodlands typically with a shrub-dominated or patchy shrub layer, depending on climate, degree of soil development, and understory density. This includes the northern race of Interior Ponderosa Pine old-growth (USFS Region 6, USFS Region 1). Historically, many of these woodlands and savannas lacked the shrub component resulting from 3- to 7-year fire-return intervals.

Pinus ponderosa (primarily var. ponderosa) is the predominant conifer; Pseudotsuga menziesii may be present in the tree canopy but is usually absent. In southern interior British Columbia, Pseudotsuga menziesii or Pinus flexilis may form woodlands or fire-maintained savannas with and without Pinus ponderosa var. ponderosa at the lower treeline transition into grassland or shrub-steppe. The understory can be shrubby, with Artemisia tridentata, Arctostaphylos patula, Arctostaphylos uva-ursi, Cercocarpus ledifolius, Physocarpus malvaceus, Purshia tridentata, Symphoricarpos oreophilus or Symphoricarpos albus, Prunus virginiana, Amelanchier alnifolia, and Rosa spp. common species. Understory vegetation in the true savanna occurrences is predominantly fire-resistant grasses and forbs that resprout following surface fires; shrubs, understory trees and downed logs are uncommon. These more open stands support grasses such as Pseudoroegneria spicata, Hesperostipa spp., Achnatherum spp., dry Carex species (Carex inops), Festuca idahoensis, or Festuca campestris. The more mesic portions of this system may include Calamagrostis rubescens or Carex geyeri.

This ecological system within the region occurs at the lower treeline/ecotone between grasslands or shrublands and more mesic coniferous forests typically in warm, dry, exposed sites at elevations ranging from 500-1600 m (1600-5248 feet). These woodlands receive winter and spring rains, and thus have a greater spring "green-up" than the drier ponderosa woodlands in the Colorado and New Mexico Rockies. In eastern Washington, precipitation varies from 36-76 cm (~14-30 inches) with most occurring as snowfall (WNHP 2011). It can occur on all slopes and aspects; however, it commonly occurs on moderately steep to very steep slopes or ridgetops. This ecological system generally occurs on most geological substrates from weathered rock to glacial deposits to eolian deposits (e.g., glacial till, glacio-fluvial sand and gravel, dunes, basaltic rubble, colluvium, to deep loess or volcanic ash-derived soils) (WNHP 2011). Characteristic soil features include good aeration and drainage, coarse textures, circumneutral to slightly acidic pH, an abundance of mineral material, and periods of drought during the growing season. Some occurrences may occur as edaphic climax communities on very skeletal, infertile and/or excessively drained soils, such as pumice, cinder or lava fields, and scree slopes. In the Oregon "pumice zone" this system occurs as matrix-forming, extensive woodlands on rolling pumice plateaus and other volcanic deposits. Surface textures are highly variable in this ecological system ranging from sand to loam and silt loam. Exposed rock and bare soil consistently occur to some degree in all the associations.

Summer drought and frequent, low-severity fires create woodlands composed of widely spaced, large trees with small scattered clumps of dense, even-aged stands which regenerated in forest gaps or were protected from fire due to higher soil moisture or topographic protection. Closed-canopy or dense stands were also part of the historical range of stand variability but under natural disturbance regimes are a minor component of that landscape. Mixed fire regimes and surface fires of variable return intervals maintain these woodlands typically with a shrub-dominated or patchy shrub layer, depending on climate, degree of soil development, and understory density. Historically, many of these woodlands and savannas lacked the shrub component resulting from low-severity but high-frequency fires (2 - to 10-year fire-return intervals). Some sites, because of low productivity, naturally lacked a dense shrub understory. Mixed-severity fires had a return interval of 25-75 years while stand-replacing fire occurred at an interval of >100 years (Arno 1980, Fischer and Bradley 1987). The latter two intervals only occurred on 20-25% of stands within the landscape while surface fires were the dominant fire regime on over 75% of stands (Landfire 2007a). Presettlement fires were triggered by lightning strikes or deliberately set fires by Native Americans.

Pinus ponderosa is a drought-resistant, shade-intolerant conifer which usually occurs at lower treeline in the major ranges of the western United States. Establishment of ponderosa pine is erratic and believed to be linked to periods of adequate soil moisture and good seed crops as well as fire frequencies, which allow seedlings to reach sapling size.

Western pine beetle is another significant disturbance and especially affects larger trees. Bark beetle outbreaks are highly related to stand density. Denser stands in relation to site capacity will favor outbreaks, which will decrease as trees are thinned (Landfire 2007a). Mistletoe can cause tree mortality in young and small trees. Fires and insect outbreaks resulted in a landscape consisting of a mosaic of open forests of large trees (most abundant patch), small denser patches of trees, and openings (Franklin et al. 2008). White-headed woodpecker, pygmy nuthatch, and flammulated owl are indicators of healthy ponderosa pine woodlands. All these birds prefer mature trees in an open woodland setting (Jones 1998, Levad 1998 Winn 1998, as cited in Rondeau 2001).

LANDFIRE developed several state-and-transition vegetation dynamics VDDT models for this system across its range and dry or mesic conditions. This model is typical of much of the range and has five classes in total (LANDFIRE 2007a, BpS 1910530). These are summarized as:

A) Early Development 1 Open (5% of type in this stage): Fire-maintained grass/forb and/or seedlings and saplings. Seedling/sapling size class would be less than 5 inches in diameter. There would be no large patches (10-100 acres) of large or old-growth trees due to poor site conditions and abundance of rock outcroppings. However, dispersed large-diameter fire-remnant ponderosa pines and snag trees could be present. These large-diameter trees would have a density of less than one tree per acre. Grass species are the dominant lifeform in this class attaining maximum heights of 3 feet and patchy in distribution (25-75% cover).

B) Mid Development 1 Closed (tree-dominated - 10% of type in this stage): Tree cover is 41-60%. Closed ponderosa pine pole and medium-diameter stand; may have Douglas-fir as incidentals. Larger, old-growth trees may be present in this class, though the pole and medium-diameter class (5-21 inches) occurring between these large trees is most abundant and characteristic of this class. May see large-diameter snags, dead and downed trees present. High-density stunted pole stands are counted here; may see insect/disease here.

C) Mid Development 1 Open (tree-dominated - 20% of type in this stage): Tree cover is 0-40%. Open ponderosa pine pole and medium-diameter stand that may have Douglas-fir as incidentals. Larger, old-growth trees may be present in this class, the pole and medium-diameter (5-21 inches) trees are characteristic for this class. These patches have probably had recent fire or are drier so they retain a more open condition.

D) Late Development 1 Open (conifer-dominated - 55% of type in this stage): Tree cover is 0-40%. Fire-maintained open, park-like ponderosa pine; nearly any fire maintains; Douglas-fir may be seen as incidentals or in patches, but not a major component of the overstory. The overstory is characterized by large and very large ponderosa pine and isolated Douglas-fir. Understory is dominated by grasses and is relatively open. Seedlings are very infrequent, with <10% cover and usually occurring in patches.

E) Late Development 1 Close (conifer-dominated - 5% of type in this stage): Tree cover is 41-60%. High-density, multi-storied ponderosa pine stand; Douglas-fir regeneration on some sites. Thickets of various size classes distributed within the class and may be interspersed with large snags.

Frequent, non-lethal surface fires were the dominant disturbance factor, occurring every 3-30 years (Arno 1980, Arno and Petersen 1983, Fischer and Bradley 1987). Three-year fire-return intervals are likely very localized and associated with Native American burning. However, there is some disagreement as to the extent of Native burning. More median fire-return intervals were likely about 15 years. Mixed-severity fires likely occurred about every 50 years, again, depending on the vegetative state. Stand-replacement fires likely occurred in stands and small patches on the order of a few hundred acres every 300-700 years depending on the vegetative state. Some authors note that little information is available regarding the exact nature of stand-replacement fire severity in this BpS (LANDFIRE 2007a, BpS 1910530). Western pine beetle can attack large ponderosa pine in any canopy density (LANDFIRE 2007a, BpS 1910530).

Nutrient cycling, specifically carbon cycling, is an important ecological process within many ecological systems. However, biological decomposition in ponderosa pine forests is more limited than biological production, resulting in accumulation of organic materials, especially in the absence of fire (Harvey 1994, Graham and Jain 2005).

Conversion of this type has commonly come from rural and urban development. Since European settlement, fire suppression, timber harvest, livestock grazing, introduced diseases, road building, development, and plantation establishments have all impacted natural disturbance regimes, forest structure, composition, landscape patch diversity, and tree regeneration (Franklin et al. 2008). Timber harvesting has focused on the large, older trees in mid- and late-seral forests thereby eliminating many old forest attributes from stands (Franklin et al. 2008). Overgrazing may have contributed to the contemporary dense stands by eliminating grasses in some areas thereby creating suitable spots for tree regeneration as well as reducing the abundance and distribution of flashy fuels that are important for carrying surface fires (Hessburg et al. 2005, Franklin et al. 2008). Road development has fragmented many forests creating firebreaks. With settlement and subsequent fire suppression, occurrences have become denser. Presently, many occurrences contain understories of more shade-tolerant species, such as Pseudotsuga menziesii and/or Abies spp., as well as younger cohorts of Pinus ponderosa. These altered occurrence structures have affected fuel loads and alter fire regimes. With fire suppression and increased fuel loads, fire regimes are now less frequent and often become intense crown fires, which can kill mature Pinus ponderosa (Reid et al. 1999). Longer fire-return intervals have resulted in many occurrences having dense subcanopies of overstocked and unhealthy young Pinus ponderosa (Reid et al. 1999). With vigorous fire suppression, longer fire-return intervals are now the rule, and multi-layered stands of Pinus ponderosa and/or Pseudotsuga menziesii provide fuel "ladders," making these forests more susceptible to high-intensity, stand-replacing fires. The resultant stands at all seral stages tend to lack snags, have high tree density, and are composed of smaller and more shade-tolerant trees (WNHP 2011). Mid-seral forest structure is currently 70% more abundant than in historical, native systems, and late-seral forests of shade-intolerant species are now essentially absent (WNHP 2011). Early-seral forest abundance is similar to that found historically but lacks snags and other legacy features.

In the Pacific Northwest, regionally downscaled climate models project increases in annual temperature of, on average, 3.2°F by the 2040s. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), and some models project wetter autumns and winters and drier summers. Warmer temperatures will result in more winter precipitation falling as rain rather than snow throughout much of the Pacific Northwest, particularly in mid-elevation basins where average winter temperatures are near freezing. This change will result in: less winter snow accumulation, higher winter streamflows, earlier spring snowmelt, earlier peak spring streamflow and lower summer streamflows in rivers that depend on snowmelt (as do most rivers in the Pacific Northwest) (Littell et al. 2009). Potential climate change effects could include: reduction in freshwater inflows through the further reduction in summer flows (Littell et al. 2009); drop in groundwater table; increased fire frequency due to warmer temperatures resulting in drier fuels, the area burned by fire regionally is projected to double by the 2040s and triple by the 2080s (Littell et al. 2009); and additionally, likely warming may stress host trees so mountain pine beetle outbreaks are projected to increase in frequency and cause increased tree mortality.

The ways in which the climate in the region where this system reaches its eastern limit is likely to change, and the effects of those changes on the structure and function of this system are all hard to predict, and only broad generalizations can be made (Rice et al. 2012). Average annual temperature likely will increase by 1.7°C by 2050 and by 1.1° to 5.5°C by the end of this century. Annual precipitation may increase by 10%, with wetter winters and drier summers, but less certainty can be assigned to possible precipitation changes than temperature changes. Climate changes will also affect the ecological system indirectly, through bark beetle populations and other ecological agents. Changes in the extremes of temperature and precipitation likely will have a stronger effect than will changes in annual averages, and the patterns of these extremes are especially hard to predict. Climate changes almost certainly will disrupt the composition, structure, and function of this ecological system, in ways that can only be very generally anticipated.

This system is found in the Fraser River drainage of southern British Columbia south along the Cascades and northern Rocky Mountains of Washington, Oregon and California. In the northeastern part of its range, it extends across the northern Rocky Mountains west of the Continental Divide into northwestern Montana, south to the Snake River Plain in Idaho, and east into the foothills of western Montana.