Black Hills White Spruce Forest

This uncommon system is limited to relatively high-elevation outliers of montane environments in the northwestern Great Plains. Best known areas of this system are small portions of the Black Hills of Wyoming and South Dakota and the Cypress Upland of southern Alberta and Saskatchewan. These highland areas have a cooler climate than surrounding mixedgrass prairie. In the Black Hills, these woodlands occur as small or large patches within the ponderosa pine matrix, from about 1740 to 2135 m (5700-7000 feet); at lower elevations, they are restricted to north-facing slopes. At the higher elevations, they are found on level or gently sloping areas. In other locations, this woodland system is limited to sideslopes and depressions, likely adjoining riparian zones, where snow is well-retained. Soils vary widely from deep to quite shallow. Picea glauca is the characteristic conifer, but other trees can include Pinus ponderosa, Populus tremuloides, and Betula papyrifera. Undergrowth shrubs typically include Arctostaphylos uva-ursi, Juniperus communis, Linnaea borealis, Symphoricarpos albus, and Vaccinium scoparium. Disturbance regimes are not well-documented for this system, but likely include periodic windthrow as well as fire spreading from adjacent, lower elevation woodlands and grasslands.

This system is limited to relatively high-elevation outliers of montane environments in the northwestern Great Plains of the U.S. and southern Canada. These highland areas have a cooler and more mesic climate than surrounding mixedgrass prairie. In the Black Hills, these woodlands occur as small or large patches within the ponderosa pine matrix, from about 1740 to 2135 m (5700-7000 feet) elevation; at lower elevations, they are restricted to north-facing slopes. At the higher elevations, they are found on level or gently sloping areas. In other locations, this woodland system is limited to sideslopes and depressions, likely adjoining riparian zones, where snow is well-retained. Geology is generally dominated by limestone, granite, slate and schist. Soils vary widely from deep to quite shallow. In the Cypress Hills of Alberta and Saskatchewan, the elevations where this system is found range up to 1466 m; generally these woodlands occur on north-facing slopes or near small springs and seeps.

Disturbance regimes are not well-documented for this system, but likely include periodic windthrow as well as fire spreading from adjacent, lower elevation or drier woodlands and grasslands. There is some debate about whether mixed-severity fire would have occurred in this type based on tree-ring and historical evidence; estimated at a 100-year return interval (Landfire 2007a). Stand-replacing disturbances are primarily associated with climatic fluctuations and include fire and insect (in late-development classes only, mountain pine beetles create larger patch sizes; Ips beetles create smaller patches). Snowbreak and windthrow events may occur. The majority of the insect outbreaks generally occur in late-development stands but in periods of drought (such as that which the forest is currently experiencing), tree mortality is occurring in ponderosa pine that are less than 18 cm (7 inches) dbh. Surface and stand-replacing fire events occur in this system. Stand-replacing fires were likely most common in higher elevation and northern slopes that were primarily dominated by spruce, with surface fires occurring most often in the moist ponderosa pine.

Conversion of this type has commonly come from logging or clearing for rangeland. This is not a heavily converted ecosystem. Historically, without fire suppression, it is expected that there would have been much less spruce than what currently exists on the landscape today in the U.S. part of the system's range Landfire (2007a). It is also expected that there is a greater canopy cover of conifer species (ponderosa pine and spruce) and less canopy cover of hardwoods (such as aspen and birch) and grassland openings (refer to historical photos from 1874 (Graves 1899) to current photos of the same areas). With denser canopies of conifers it is generally expected that there is less herbaceous understory growth than occurred historically with a less dense canopy. Other stressors include livestock grazing, mining, logging, oil and gas development in the adjacent foothills, fragmentation by roads, outdoor recreation, and tourism activity (Marriott et al. 1999). The Black Hills are considered the most productive timber source in the region, with harvesting occurring throughout; gold mining has also been a pervasive activity since the 1880s (Marriott et al. 1999). Recreational use in the Black Hills is also heavy; similar usage probably occurs in the Cypress Uplands. These highland "island forests" in the midst of the Great Plains grasslands are attractive to people because of their scenic and wildlife values, and valuable contributions of water to the surrounding lowlands. Invasive exotics are likely to occur in this system, but no documentation of such was found; probable species would include perennial (pasture) grasses associated with relatively mesic sites such as Poa pratensis and Bromus inermis.

Over the century to 2100, climate scenarios for Saskatchewan suggest (Henderson et al. 2002, Barrow 2009): a warmer climate - temperatures may generally rise 2° to 4°F; a longer growing season, but drier, despite an increase in precipitation. This is a result of increased summer temperatures and increased evapotranspiration. Expect more frequent and more intense extreme events (e.g., heavy precipitation or drought). Droughts will likely increase in intensity and frequency.

Henderson et al. (2002) project that by the 2050s natural regeneration of aspen, lodgepole pine or white spruce is very unlikely to be possible outside of very localized sites within the Cypress Hills. The future landscape is likely to be one of small patches of stressed woodland persisting only in the most favorable sheltered sites. By the 2080s it is very possible that there will be no regeneration of spruce or lodgepole anywhere in the hills. Alternatively, a few sheltered coulee slopes may remain moist enough to prevent complete extirpation of extant tree species. Lodgepole stands will be increasingly vulnerable to mountain pine beetle attack. It is widely believed that periods of very cold winter weather act as an effective control on mountain pine beetle outbreaks. Periods of sanitizing cold are already less frequent. Spruce budworm attack is also possible, which could affect white spruce stands. There is a great and increasing risk of catastrophic fire. It is possible that post-burn forest regeneration would be slow and patchy even under today's climate conditions, as conditions are already drier than those under which the existing forest developed. Regeneration will be ever more difficult in future.

This system is limited to relatively high-elevation outliers of montane environments in the northwestern Great Plains. Best known areas of this system are small portions of the Black Hills of Wyoming and South Dakota and the Cypress Upland of southern Alberta and Saskatchewan. It may also occur in very small stands of the Bighorn Mountains of north-central Wyoming and south-central Montana.