Northern Rockies Foothill Pine Wooded Steppe

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. It also occurs east across Idaho into the eastern foothills of the Montana Rockies. The system may also occur on the lower treeline slopes of the Wyoming Rockies. These wooded steppes occur at the lower treeline/ecotone between grasslands or shrublands and forests and woodlands, typically on warm, dry, exposed sites too droughty to support a closed tree canopy. This is not a fire-maintained system. The "savanna" character results from a climate-edaphic interaction that results in widely scattered trees over shrubs or grasses, and even in the absence of fire, a "woodland" or "forest" structure will not be obtained. 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 system can occur in association with cliff and canyon systems. It 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. These can also occur on areas of sand dunes, scablands, and pumice where the edaphic conditions limit tree abundance. Pinus ponderosa (var. ponderosa and var. scopulorum) and Pseudotsuga menziesii are the predominant conifers (not always together); Pinus flexilis may be present or common in the tree canopy. In interior British Columbia, Pseudotsuga menziesii is the characteristic canopy dominant. In transition areas with big sagebrush steppe systems, Purshia tridentata, Artemisia tridentata ssp. wyomingensis, Artemisia tridentata ssp. tridentata, and Artemisia tripartita may be common in fire-protected sites such as rocky areas. Deciduous shrubs, such as Physocarpus malvaceus, Symphoricarpos albus, or Spiraea betulifolia, can be abundant in more northerly sites or more moist climates. Important grass species include Pseudoroegneria spicata, Poa secunda, Hesperostipa spp., Achnatherum spp., and Elymus elymoides.

Pinus ponderosa (vars. ponderosa and scopulorum) and Pseudotsuga menziesii are the predominant conifers (not always together); Pinus flexilis may be present or common in the tree canopy. In interior British Columbia, Pseudotsuga menziesii is the characteristic canopy dominant. In transition areas with big sagebrush steppe systems, Purshia tridentata, Artemisia tridentata ssp. wyomingensis, Artemisia tridentata ssp. tridentata, and Artemisia tripartita may be common in fire-protected sites such as rocky areas. Deciduous shrubs, such as Physocarpus malvaceus, Symphoricarpos albus, or Spiraea betulifolia, can be abundant in more northerly sites or more moist climates. Important grass species include Pseudoroegneria spicata, Poa secunda, Hesperostipa spp., Achnatherum spp., and Elymus elymoides.

These wooded steppes occur at the lower treeline/ecotone between grasslands or shrublands and forests and woodlands, typically on warm, dry, exposed sites too droughty to support a closed tree canopy. The "savanna" character results from a climate-edaphic interaction that results in widely scattered trees over shrubs or grasses, and even in the absence of fire, a "woodland" or "forest" structure will not be obtained. 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 system can occur in association with cliff and canyon systems. It 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. These can also occur on areas of sand dunes, scablands, and pumice where the edaphic conditions limit tree abundance.

This is not a fire-maintained system. Periodic drought that limits tree establishment is the driving factor in this system. The concept is that of the climate-edaphic interaction that results in widely scattered trees over "shrub-steppe" of sage, bitterbrush, or sparsely distributed grasses. Tree growth is likely episodic, with regeneration episodes in years with available moisture. Tree density is limited in some areas by available growing space due to rocky conditions of the site. The tree canopy in this system will never reach woodland density or close due to the interaction of climate and edaphic factors, even in the absence of fire. This system burns occasionally, but the vegetation is sparse enough that fires are typically not carried through the stand. Fire frequency is speculated to be 30-50 years. This type usually has little surface fuel and replacement fires would be a function of extreme conditions, such as very high winds (LANDFIRE 2007a). Western pine beetle is a significant disturbance and especially affects larger trees, while parasitic mistletoe can cause tree mortality in young and small trees.

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has four classes in total (LANDFIRE 2007a, BpS 0911650). These are summarized with some modifications below:

A) Early Development 1 All Structures (10% of type in this stage): Dominated by bunchgrasses, mountain sagebrush and seed/sapling-sized Douglas-fir. Limber pine and ponderosa pine may be present in varying amounts.

B) Mid Development 1 Closed (tree-dominated - 2% of type in this stage): Tree cover is 31-100%. Relatively dense pole- and/or large-sized Douglas-fir. Limber pine and ponderosa pine may be present in varying amounts. Sagebrush has largely dropped out of the stand. Mixed-severity fire may open up the canopy; however, vegetation is generally too sparse to carry fire through stand and is more affected by drought.

C) Mid Development 1 Open (tree-dominated - 8% of type in this stage): Tree cover is 0-30%. Open poles of Douglas-fir with bunchgrass and sagebrush understory. Limber pine and ponderosa pine may be present in varying amounts. Surface fires may help maintain the open condition; however, vegetation is generally too sparse to carry fire through stand and is more affected by drought.

D) Late Development 1 Open (conifer-dominated - 80% of type in this stage): Tree cover is 0-30%. Widely spaced, open canopy of medium- to large-diameter Douglas-fir with bunchgrass and sagebrush understory. Canopy fuels are discontinuous. Limber pine and ponderosa pine may be present in varying amounts. Surface fires may help maintain the open condition; however, vegetation is generally too sparse to carry fire through stand except under extreme conditions.

LANDFIRE modeled fire regime as predominantly (70%) frequent, low-severity fires with an MFI of approximately 30 years. Mixed-severity fires occur with a typical frequency of 30-50 years primarily in dense stands (classes B and E). Native American burning may have occurred in many of these low-elevation forests. Limber pine may be affected by blister rust (LANDFIRE 2007a, BpS 0911650). However, this system generally has low surface fuels and is too sparse to carry fire through stand except under extreme conditions so fires are patchy.

Nutrient cycling, specifically carbon cycling, is an important ecological process within many ecological systems. 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).

This is not a heavily converted system, although some stands have been converted by various development activities, including suburban or rural expansion and road building. From WNHP (2011): The primary land uses that alter the natural processes of this system are associated with livestock practices, tree removal, exotic species, fire regime alteration, direct soil surface disturbance, and fragmentation. Excessive grazing disturbs the soil, opening the perennial herbaceous layers to the establishment of native disturbance-increasers and exotic annual grasses. Persistent grazing will further diminish perennial cover, expose bare ground, and increase exotic annuals. Any soil and bunchgrass layer disturbances, such as vehicle tracks or chaining of shrubs, will increase the probability of alteration of vegetation structure and composition and response to fire. Harvesting of tree species alters the structural characteristics of this system and, given the harsh environment, reestablishment of the trees typically occurs very slowly. Fire suppression has resulted in increased tree regeneration and thus a denser understory with young trees. Road development has fragmented many occurrences creating firebreaks.

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); but models also predict increases in extreme high precipitation over the next half-century, particularly around Puget Sound (Littell et al. 2009), which may provide freshwater pulses that are intermittent, less predictable; 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 climatic warming may stress host trees so mountain pine beetle outbreaks are projected to increase in frequency and cause increased tree mortality.

This system is found in the Fraser River drainage of southern British Columbia south along the Cascades into the Modoc Plateau of California, and the northern Rocky Mountains of Washington and Oregon. In the northeastern part of its range, it extends across the northern Rocky Mountains west of the Continental Divide into northwestern Montana and south to the Snake River Plain in Idaho. In Oregon, it is most common in south-central Oregon, in lands managed by the Lakeview District of the BLM, and by the adjacent Fremont and Deschutes national forests. It also occurs on the marginal lands coming south out of the Blue Mountains, on the edge of the northern Basin and Range.