Ecological dynamics

Reference Plant Community:

The Reference Plant Community of this site is characterized by an overstory dominated by lodgepole pine and a lush understory composed largely of shrubs, sedges, grasses and a diverse community of forbs. Minor trees that may be associated with these communities include white fir (Abies concolor), Engelmann spruce (Picea engelmannii), quaking aspen (Populus tremuloides) and ponderosa pine (Pinus ponderosa). Shrub and herbaceous species will vary depending on soil texture and depth to water table. Common shrubs may include Douglas spiraea, bearberry (Arctostaphylos uva-ursi), Lemmon’s willow (Salix lemmonii), Booth’s willow (Salix boothii) and bog blueberry. Sites within this concept with deeper water tables will support few obligate wetland species instead favoring shrubs such as bitterbrush (Purshia tridentate), bearberry (Arctostaphylos uva-ursi), and wax currant. Common herbaceous species may include Nebraska sedge (Carex nebrascensis), blue wildrye, bluejoint reedgrass (Calamagrostis canadensis), tufted hairgrass (Deschampsia caespitosa), widefruit sedge, strawberry (Fragaria spp.) and Baltic rush.

Ecology:

Most forests dominated by lodgepole pine are seral community phases that eventually yield to the dominance of other conifers (Agee 1993). Lodgepole pine forests on the pumice plateau however, represent a scenario where lodgepole pine will remain the overstory dominant through time due to topographic and edaphic characteristics that favor its competitive dominance. In the case of this site, low landscape positions and high water tables promote saturated soil conditions which the shallow-rooted lodgepole to

Disturbance:

Lodgepole pine forests on the pumice plateau were historically subject to occasional replacement severity fires and relatively frequent outbreaks of insects and disease and windthrow events. Fire regimes for pumice plateau lodgepole forests are classified by Landfire as group IV: 35-100+ year frequency, replacement severity (Landfire 2007). Overtime, these disturbance events, in addition to natural self-thinning processes, would have favored the development of mature lodgepole stands with open canopies. Lodgepole has thin bark and shallow roots which render the tree very susceptible to mortality from fire (Cope 1993). However, as a prolific seed producer with a relatively long seed viability, it is a highly aggressive post fire colonizer. While bitterbrush may re-sprout following low severity fire, high severity fire may lead to mortality and long-term response of bitterbrush to intact or altered fire regimes may vary (Agee 1993, Busse and Riegel 2009, Heyerdahl et al. 2014). With longer time between fire, increased development of understory fuels such as young lodgepole pine and woody fuels, along with the development of a closed canopy, may promote an increased frequency of stand replacing fires and insect outbreaks. Sites adjacent to ponderosa dominated stands may experience greater frequency of low severity fire (Heyerdahl et al. 2014). In riparian stands, many of the understory species in these communities are adapted to rebound following fire by re-sprouting from their crowns (Crowe et al. 2004).
Some sites without saturated soil conditions have been used for livestock grazing. Overgrazing may lead to loss of shrubs by compaction and trampling that impacts soil water availability and decreases plant vigor, and leads to increases in drier facultative species such as the non-native Kentucky bluegrass (Poa pratensis) and weedy forbs and increases in bare ground. At sustained levels of improperly managed grazing, some sites may experience bank erosion, channel incision and lowering of water tables, shifting plant community composition to dominance of plant species adapted to lower soil moisture conditions (Crowe et al 2004). Mountain pine beetle (Dendroctonus ponderosae) outbreaks are especially common in ponderosa and lodgepole pine forests and drought conditions and high stand densities may render stands more vulnerable to these outbreaks (Agee 1993). Large acreages of this site type have been subject to bark beetle outbreaks, which can influence subsequent fire severity and impacts (Agee 1993).

The state and transition model below represents a generalized and simplified version of forest change in response to fire in this ecological site. It does not attempt to display the complicated interactions between grazing, drought, channel morphology alteration, insects and disease on plant community change, many of which will vary at scales smaller than that represented by this site. Emerging evidence is suggesting that climate change is leading to hotter and drier conditions in western forests that will increase fire frequency and extent and lengthen fire seasons (Halofsky et al. 2020). When combined with the interacting impacts of fire suppression, drought, and insect outbreaks, it is possible that this ecological system will experience unpredictable ecosystem shifts and additional alternative states. As this site is updated in future iterations, descriptions will include more thorough treatments of disturbance and ecological change. The reference state of the current model is largely informed by Landfire biophysical settings model 0711670, Rocky Mountain Poor-Site Lodgepole Pine Forest (Landfire 2007).