Alpine Solifluction Terrace Dryas octopetala (Arctostaphylos uva-ursi/Salix arctica)

Description

The growing season is very short, and vegetation is low- to very low-statured due to the wind and harsh environmental conditions. At the turf-banked terraces, the vegetation cover is lower to moderate, whereas lower downslope there are no stripes and the vegetation can be nearly continuous (75-90 percent). Species are highly adapted to this dry environment and grow in cushioned, matted, or succulent forms or grow as flat rosettes often with thick cuticles or dense covers of hairs. Many of these cushion plants are very long-lived and are adapted to the dry environment by their deep, fleshy taproots. Shrub species can be dominated by Dryas octopetala or can be a mixture with Arctostaphylos uva-ursi and Salix arctica as well. Dryas octopetala and Dasiphora fruticosa are nitrogen-fixing, as are other alpine leguminous species including Lupinus argenteus, Hedysarum sulphurescens, Astragalus alpine, Astragalus bourgovii, Oxytropis campestris, and Oxytropis sericea. Nitrogen aids in soil fertility. As well, litter can accumulate under the matted vegetation. Thus, these vegetated areas can facilitate additional species colonization within the mats. Many endemic and rare species are found in these alpine communities. Generally, the most wind-blasted areas are dominated by the very low-growing Silene acaulis, Minuartia obtusiloba, and Arenaria capillaris.

Nearly all species present at the site do not have affinities for wetland conditions, and have an upland, facultative upland or facultative wetland designation. Typical snow-loving tundra species are present including Arnica rydbergii, Arenaria capillaris, Astragalus bourgovii, Hedysarum suphurescens, Gentiana calycosa, Silene acaulis, Carex rupestris, Minuartia obtusiloba, Polygonum viviparum, Pedicularis contorta, Erigeron compositus, and Smelowskia calycina (Damm, 2001). Ubiquitous species occur as well, including Achillea millefolium, Galium boreale, Lupinus argenteus, Agoseris glauca, Poa alpina, and Trisetum spicatum. There are wetland facultative designated species present, but these are low in cover and in frequency. These include Anemone parviflora, Carex praegracilis, Polygonum bistortoides, Ranunculus eschscholtzii, Salix nivalis, and Vahlodea atropurpurea. Sedum lanceolatum occurs frequently at this site. Incidental species that occur infrequently, but in moderate to high cover when they do include Juniperus horizontalis, Carex scirpoidea, and Dryas drummondii.

Fire is a rare event in the Alpine Solifluction Terrace ecological site since it is at such high elevation, dry, and has moderate vegetation cover. The closest fire regime to this ecological site is that of the Northern Rocky Mountain Alpine and Subalpine Meadows and Grasslands community (Landfire, 2005), which has a fire interval of 350 years for replacement fire interval and 750 years for mixed fire interval, the average for all fires is 239 years (Agee, 1993). The National Park Service (NPS) generally regards the fire-return interval of tundra sites to be approximately at least 100-150 years, and fire would only carry in dry years such as 2003 and 2015 (personal communication, McClellan, NPS Fire Effects, 2016). Lightning strikes can occur, and fires of varying severity and extent can occur. Fires at the lower elevation whitebark pine-subalpine fir (Alpine Krummholtz Coniferous) ecological site can move into this ecological site. The whitebark pine-subalpine fir ecological site has a fire-free interval of 35 to over 300 years, with fires typically of low severity due to discontinuous fuels (Arno, 1979). Stand-replacement fires occur after intervals of more than 200 years, typically during drought conditions and brought up from severe wind-driven crown fires from lower elevations forests.

Other disturbances to this ecological site include high elevation mining, heavy recreational use, and grazing (Montana Natural Heritage Program website). Generally, grazing and human disturbance can be limited due to inaccessibility or low forage cover. Asebrook (2010) found that this site is in pristine condition with no invasion by exotic species and little invasion by tree species. Species of this site are fragile due to the extremely limited growing season and less soil development. Species generally are slow-growing and decrease in cover and vigor in areas of trampling or heavy grazing. However, the 1985 INT-350 paper by the US Forest Service (USFS) found that the Dryas octopetala alpine cushion community had a 50 percent reduction in cover, approximately, with 400 passes of human trampling, but this levelled to slightly less cover with 800 passes.

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Description

Climate change will impact the Alpine Solifluction Terrace ecological site, because the resident species are highly adapted to this site, and change could impact the distribution of peripheral species, endemics, and rare species. The Montana Natural Heritage Program has designated this ecosystem as a S5, and state that it is likely to increase as perennial ice and snow disappear. Desiccation and loss of alpine turf may also increase this ecosystems occupancy. Lesica (2014) found that in moist-turf (dominated by Dryas octopetala, Salix reticulata, and Carex scirpoidea) sites in Glacier National Park (GNP), there was a decline in arctic-alpine plants over two decades, with an approximate increase in temperature of 0.6 and 0.7 degrees Celsius than the previous four decades. There was a greater decline in dicots than monocots, and plants more restricted to high elevations declined more than species with broader elevational amplitude. Lesica’s paper documented upward migration of low-elevation species and declines in high-elevation species, and stated that the fluctuations were associated with increased temperature rather than declines in precipitation.

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