Northern Rockies Foothill Streamside Woodland

This ecological system of the northern Rocky Mountains and the east slopes of the Cascades consists of deciduous, coniferous, and mixed conifer-deciduous forests that occur on streambanks and river floodplains of the lower montane and foothill zones. Riparian forest stands are maintained by annual flooding and hydric soils throughout the growing season. Riparian forests are often accompanied by riparian shrublands or open areas dominated by wet meadows. Populus balsamifera is the key indicator species. Several other tree species can be mixed in the canopy, including Populus tremuloides, Betula papyrifera, Betula occidentalis, Picea mariana, and Picea glauca. Abies grandis, Thuja plicata, and Tsuga heterophylla are commonly dominant canopy species in British Columbia, western Montana and northern Idaho occurrences, in lower montane riparian zones. Shrub understory components include Cornus sericea, Acer glabrum, Alnus incana, Betula papyrifera, Oplopanax horridus, and Symphoricarpos albus. Ferns and forbs of mesic sites are commonly present in many occurrences, including such species as Athyrium filix-femina, Gymnocarpium dryopteris, and Senecio triangularis.

Alluvial soils along perennial and intermittent streams. Valley type is an important variable, as riparian woodlands are mostly found in V-shaped, steep valleys with many large boulders and coarse soils or U-shaped gullies formed by glacial processes. These systems can also be found in broad unconfined reaches with deeper soils and more complex geomorphic surfaces. Narrow and steep (i.e., confined) occurrences have minimal to no floodplain development, whereas less steep and wider valley bottoms (i.e., unconfined) occurrences are often associated with substantial floodplain development (Gregory et al. 1991).

Natural disturbance regimes are the primary influence on riparian system characteristics. Maintained by the complex interaction of hydrological and geomorphological processes which influence periodic flooding and hydric soils, riparian systems are the most dynamic of all forested, woodland and shrub systems. Hydrogeomorphology determines the form, composition and function of riparian woodland and shrub systems. Typically occurring in watersheds with snow-dominated hydrological processes, sometimes mixed rain and snow, these riparian systems are further influenced by the variability of inter-annual and seasonal weather patterns. Typical flow regimes of British Columbia's central interior plateau and mountains are snow- (nival) dominated. Precipitation falls as snow and is stored for long periods of time, resulting in low winter flows, and peak flows following snowmelt in May to July (depending on annual temperature variations and snow depth). Glacial snow regimes are similar to nival, except that high flows may continue until August or September (Eaton and Moore 2010). Periods of peak flow have greatest influence on channel morphology and vegetation dynamics. Large woody debris is important for affecting channel morphology.

Beaver can be important hydrogeomorphic driver of montane riparian systems, especially along unconfined reaches. The direct, local presence of beaver creates a heterogeneous complex of wet meadows, marshes and riparian shrublands and increases species richness on the landscape. Naiman et al. (1988) note that beaver-influenced streams are very different from those not impacted by beaver activity by having numerous zones of open water and vegetation, large accumulations of detritus and nutrients, more wetland areas, having more anaerobic biogeochemical cycles, and in general are more resistance to disturbance.

Conversion of this type has commonly come from agricultural development, roads, dams and other flood-control activities that drown reaches under reservoirs or dewater streams through upstream diversions, as well as grazing of domestic animals, urban and industrial development. Historic and contemporary land-use practices have impacted hydrologic, geomorphic, and biotic structure and function of riparian areas. Human land uses both within the riparian area as well as in adjacent and upland areas have fragmented many riparian reaches which has reduced connectivity between riparian patches and riparian and upland areas. Adjacent and upstream land uses also have the potential to contribute excess nutrients into riparian areas. Reservoirs, water diversions, ditches, roads, and human land uses in the contributing watershed can have a substantial impact on the hydrologic regime, reducing high flows and augmenting low flows (Eaton and Moore 2010). Management effects on woody riparian vegetation can be obvious, e.g., removal of vegetation by dam construction, roads, logging, diverting and blocking waterflow, or they can be subtle, e.g., removing beavers from a watershed, removing large woody debris, or construction of a weir dam for fish habitat. In general, excessive livestock or native ungulate use leads to soil damage and increased invasion of non-native species, less woody cover and an increase in sod-forming grasses particularly on fine-textured soils. Undesirable forb species, such as stinging nettle and horsetail, increase with livestock use. Non-native plants or animals, which can have wide-ranging impacts, also tend to increase with these stressors. All of these stressors have resulted in some riparian areas being incised and down cut, which alters riparian plant communities (changes their successional state, for example from woody dominated to herbaceous dominated), and also may result in an increase in non-native species (WNHP 2011).

Climate change effects are likely to most profoundly affect natural disturbance regimes (Haughian et al. 2012, Wiensczyk et al. 2012) and the effects on riparian systems likely more rapidly than climate change effects on more stable systems with less frequent natural disturbance regimes. Average temperature has already increased roughly 1.5°F compared to the 1960-1979 baseline period in the southwestern U.S., including the southern Rocky Mountains (Karl et al. 2009, Wiensczyk et al. 2012). Predictions are for 3.5-5.5°F increase in average temperatures by mid-century (Karl et al. 2009). Predictions also suggest an increase in probability of droughts, and that droughts will be exacerbated by warmer temperatures. Increased temperatures will drive declines in spring snowpack and Colorado River flow (Karl et al. 2009). For the higher elevations, in areas where it snows, a warmer climate means major changes in the timing of runoff: streamflow increases in winter and early spring, and then decreases in late spring, summer, and fall. This shift in streamflow timing has already been observed over the past 50 years (Peterson et al. 2008), with the peak of spring runoff shifting from a few days earlier in some places to as much as 25 to 30 days earlier in others (Stewart et al. 2004). This trend is projected to continue, with runoff shifting 20 to 40 days earlier within this century. Reductions in summer water availability are expected to see reductions of about 10% in colder regions such as the Rocky Mountains (Karl et al. 2009). Moreover, increased flood risk in the southern Rocky Mountains is likely to result from a combination of decreased snow cover on the lower slopes of high mountains, and an increased fraction of winter precipitation falling as rain and therefore running off more rapidly (Knowles et al. 2006). The increase in rain on snow events will also result in rapid runoff and flooding (Bales et al. 2006).

Potential climate change effects could include: a shift away from cottonwood-dominated reaches due to shift in timing of high flows and seed distribution of Populus spp., as riparian Populus species do not survive in the seed bank for more than 2 weeks to 1 month (Schreiner 1974); lower streamflows in late summer and early fall leading to earlier senescence of vegetation, which may shift species composition to more drought-tolerant and heat-tolerant species such as tamarix (which may move north with warmer climates (Kerns et al. 2009); lower groundwater tables due to less recharge and lower streamflows, which may result in loss of deep-rooted riparian species; and higher flooding and greater sedimentation may result in increase in woody species over herbaceous species (Stromberg et al. 2010b).

This system is found in the northern Rocky Mountains.