Ecological dynamics
This Sierra lodgepole pine ecological site occurs primarily on glacial outwash plains, and is associated with soils that have a silica-cemented pan which perches water. This creates a high water table during the wet season, and a harsh, dry soil in the summer. This site is often found in basins and along cold air drainages where cold air tends to sink.
The presumed most successionally-advanced community phase is an open Sierra lodgepole pine (Pinus contorta var. murrayana) – western juniper (Juniperus occidentalis) forest. These are multi-aged stands of Sierra lodgepole pine interspersed with western juniper and an occasional white fir (Abies concolor) and Jeffery pine (Pinus jeffreyi). The presence of western juniper in the site indicates harsh summer conditions. Junipers are scattered throughout the open areas and range from very large old trees to young seedlings. The understory is sparse but has a diverse cover of native grasses, forbs and woody plants including tree regeneration. The diversity is due to its proximity to riparian corridors, where fingers of the dry-appearing Sierra lodgepole pine-western juniper forest interlace with lush riparian willow and marsh habitats. This site is also situated in a basin, which creates seasonally high water tables that can influence the area.
Sierra lodgepole pines can tolerate harsher conditions than many other conifers—especially cold, light, heat, drought, saturation, and hardpan soils (Kocher, 2005; Lotan and Critchfield, 1990). They are also moderately shade and competition tolerant. However, the thin bark and shallow root systems make the Sierra lodgepole pines susceptible to fire and windfall (Lotan and Critchfield, 1990). Prolonged drought and flooding can kill the trees or make them vulnerable to disease and pest outbreaks.
Historically, fire played an important role in thinning and renewing Sierra lodgepole pine woodlands. Studies on fire frequency in lodgepole pine forest vary from 20 to 200 years, depending on location, elevation, and precipitation (Cope, 1993; Murphy and Knopp, 2000). This area has moderate precipitation, is at mid elevations, and has seasonal wetness. The fire intervals for moderate and severe fires were most likely between 70 to 100 years, heavily correlated with mountain pine beetle (Dendroctonus ponderosae) infestations and the decline of the overstory trees. In addition to a natural fire regime, it is believed the Washoe Indians used fire to preserve meadow environments and to keep forests open (Murphy and Knopp, 2000).
Human disturbance regimes have altered the historic plant community and its natural cycles. Much of the Sierra lodgepole pine forest terrain was clear-cut during the Comstock era, from the mid-1870s to the mid-1890s, although small-scale logging occurred in sections from 1911 to the 1970s (Murphy and Knopp, 2000). Intense sheep and cattle grazing began as early as 1850, especially in the meadows. Then, in 1924, fire exclusion became national policy (Murphy and Knopp, 2000). This policy led to increased forest density and over- and understory fuel build-up. Consequently, this Sierra lodgepole pine community is younger and likely more overstocked than the pre-European settlement forest.
The reference state consists of the most successionally advanced community phase (numbered 1.1) as well as other community phases that result from natural and human disturbances. Community phase 1.1 is deemed the phase representative of the most successionally advanced pre-European plant/animal community including periodic natural surface fires that influenced its composition and production. Because this phase is determined from the oldest modern day remnant forests and/or historic literature, some speculation is necessarily involved in describing it.
All tabular data listed for a specific community phase within this ecological site description represent a summary of one or more field data collection plots taken in communities within the community phase. Although such data are valuable in understanding the phase (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), it typically does not represent the absolute range of characteristics nor an exhaustive listing of species for all the dynamic communities within each specific community phase.
Community 1.1
Mature forest
The most successionally advanced community phase was most likely open older stands of predominantly Sierra lodgepole pine with a few large, scattered western junipers, white fir, and Jeffery pines. Western juniper is usually favored during long, dry summers or cold winters with little moisture. The junipers are scattered throughout the open areas, and range from very large, old trees to young seedlings. The age for this community is estimated to have ranged from 60 to possibly more than 200 years old depending on local disturbances. Stand-replacing fires would have initially established an even-aged forest. However, minor disturbances, such as windfall and insect infestation would have eventually led to an irregular open, more multi-aged forest community. The understory cover was probably moderate in the openings with grasses, forbs, and scattered shrubs and trees including western juniper seedlings and saplings.
Forest overstory. The overstory in this multi-aged woodland is open, rarely exceeding 60 percent canopy cover. Old growth and mature lodgepole pines dominate with a cover range of 25 to 55 percent, with an average of 40 percent. Large old growth specimens of western juniper, white fir and Jeffrey pine are most likely present with low frequency and cover.
Forest understory. Historically, the understory cover was most likely moderate with a range of 5 to 20 percent. The open canopy would have allowed for a diversity of herbaceous and graminoid species. Data is unavailable for this historic community, but the species were most likely similar to the ones listed in young open lodgepole woodland community.
Table 5. Ground cover
| Tree foliar cover |
25-55%
|
| Shrub/vine/liana foliar cover |
1-10%
|
| Grass/grasslike foliar cover |
6-9%
|
| Forb foliar cover |
1-15%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0%
|
| Litter |
1-2%
|
| Surface fragments >0.25" and <=3" |
1-10%
|
| Surface fragments >3" |
0-1%
|
| Bedrock |
0%
|
| Water |
0%
|
| Bare ground |
1-20%
|
Community 1.2
Stand initiation
Historically, regeneration would occur in the event of a severe canopy fire. Today, a carefully planned harvest and prescribed burn could imitate a canopy fire and initiate forest regeneration.
Native cover is generally low, ranging from 5 to 20 percent, leaving the remaining area vulnerable to non-native and noxious weed infestations. These seeds require moist soil and full sun to establish in the first season. Sierra lodgepole pine also readily germinates from seed in the mineral soil exposed by fire and can survive in the frost pockets that define these sites, where other tree seedlings cannot. Dense groves of even-aged lodgepole pine seedlings can establish and grow well following stand replacing fires. Remnant overstory trees may be present in limited numbers.
Forest overstory. A small number of older Sierra lodgepole pines may have survived the disturbance to become a seed source for lodgepole pine regeneration. These lodgepole pines would have low cover and little impact on the understory.
Forest understory. Ross’ sedge (Carex rossii), squirreltail (Eymus elymoides) and many other species of grasses and forbs such as grassy tarweed (Madia gracilis), and common yarrow (Achillea millefolium) regenerate from seed after fire and will dominate this community for several years.
Community 1.3
Young forest
Figure 7. Community Phase 1.3
Young Sierra lodgepole pine forests will develop from community phase 1.2. Western juniper and white fir may germinate during this time in the shade of the young Sierra lodgepole pines and small shrubs. The age period for this community is estimated to be from 10 to 60 years. Historically, surface fires, insect outbreaks, and possibly deer browsing maintained this open forest.
Forest overstory. In the young forest phase, the overstory is dominated by sapling and pole-sized trees, maturing to tall small-diameter lodgepole pines within 20 to 60 years. A few western junipers, white firs and Jeffrey pines will also establish. Canopy cover ranges from 20 to 45 percent.
Forest understory. The understory is dominated by grasses and grasslike species, with sparse forbs and shrubs. Understory cover ranges from 5 to 20 percent.
Community 1.4
Dense young forest
This community phase is characterized by a very dense overstory of Sierra lodgepole pine, with canopy cover of up to 80 percent, and increasing cover of western juniper and white fir in the understory. This phase is highly susceptible to severe canopy fire and to insect infestation.
Community 1.5
Dense mature forest
This community phase is a dense mature forest co-dominated by western juniper, white fir and Sierra lodgepole pine. Canopy cover is high, at up to 90 percent, and trees are 60 to over 100 years old. This phase is highly susceptible to severe canopy fire and to insect infestation.
Community 1.6
Mountain pine beetle epidemic
Standing dead forests can result following mountain pine beetle epidemics, sometimes in combination with prolonged drought or flooding. Large patches of forest remain standing dead for many years until fire or manual treatment remove the dead trees and surface fuels. If surface fuels are not too high, grasses and forbs may grow in the understory and in openings.
Pathway 1.1a
Community 1.1 to 1.2
In the event of a severe canopy fire, or a clear-cut with or without a prescribed burn, phase 1.1 would quickly develop into phase 1.2, the stand initiation phase. Sierra lodgepole pine is susceptible to death from fire at any age because of their thin bark and shallow root systems (Kocher, 2005).
Pathway 1.1c
Community 1.1 to 1.5
This pathway occurs with fire suppression.
Pathway 1.1b
Community 1.1 to 1.6
Plant community phase 1.1 may develop into phase 1.5 with the infestation of pests. The primary threat to Sierra lodgepole pines from pest invasion is from the mountain pine beetle (Dendroctonus ponderosae). Infestations can lead to a high mortality rate, sometimes leaving a stand of dead trees, and causing high fuel loads. Natural outbreaks of variable severity tend to occur every 20 to 40 years (Cope, 1993).
Pathway 1.2a
Community 1.2 to 1.3
Community phase 1.2 will naturally transition to phase 1.3 if given time without severe disturbances. This pathway is followed with a natural fire regime. Reports vary on the natural fire return interval, but this pathway assumes that surface fires were relatively frequent with 20 to 40 year cycles (Cope 1993). Manual thinning with prescribed burns can imitate the natural cycle and lead to the same community phase.
Pathway 1.2b
Community 1.2 to 1.4
This pathway occurs with fire suppression that prevents thinning necessary for the natural patchy structure of this ecological site. Lack of ground fire allows white fir to establish and gain maturity in the understory.
Pathway 1.3a
Community 1.3 to 1.1
The natural shift for this phase is to grow and develop into community phase 1.1. This pathway evolved with a historic fire regime of occasional surface and moderately severe fires, with occasional pest outbreaks that can lead to partial tree death. Manual thinning or prescribed burning can be implemented to replace the natural disturbances that kept this forest relatively open.
Pathway 1.3b
Community 1.3 to 1.2
In the event of a canopy fire, this phase would quickly return to phase 1.2
Pathway 1.3c
Community 1.3 to 1.4
This pathway occurs with fire suppression that prevents thinning necessary for the natural patchy structure of this ecological site. Lack of ground fire allows white fir to establish and gain maturity in the understory.
Pathway 1.3d
Community 1.3 to 1.6
This phase can transition to phase 1.6 with the infestation of pests. The primary threat to Sierra lodgepole pines from pest invasion is from the mountain pine beetle (Dendroctonus ponderosae). Infestations can lead to a high mortality rate; leaving a forest of standing dead trees, and causing high fuel loads. Natural outbreaks of variable severity tend to occur every 20 to 40 years (Cope, 1993).
Pathway 1.4c
Community 1.4 to 1.2
In the event of a severe canopy fire or clear-cut/scarification, phase 1.2 would develop.
Pathway 1.4d
Community 1.4 to 1.3
A naturally occurring moderate or surface fire in this forest is unlikely due to the high fuel load. Considerable management effort would be needed to create the open forest conditions that should exist in this forest with a natural fire regime. Manual treatment or prescribed burns could thin out dense Sierra lodgepole pine, western juniper and white fir. This would shift this forest back to its natural state of open, patchily distributed young Sierra lodgepole pine forest (Community phase 1.3).
Pathway 1.4a
Community 1.4 to 1.5
With continued fire suppression, this phase will transition to a dense mature forest.
Pathway 1.4b
Community 1.4 to 1.6
This phase can transition to phase 1.6 with the infestation of pests. The primary threat to Sierra lodgepole pines from pest invasion is from the mountain pine beetle (Dendroctonus ponderosae). Infestations can lead to a high mortality rate; leaving a forest of standing dead trees, and causing high fuel loads. Natural outbreaks of variable severity tend to occur every 20 to 40 years (Cope, 1993).
Pathway 1.5c
Community 1.5 to 1.1
A naturally occurring moderate or surface fire in this forest is unlikely due to the high fuel load. Considerable management effort would be needed to create the open forest conditions that should exist in this forest with a natural fire regime. Manual treatment or prescribed burns could thin out dense Sierra lodgepole pine, western juniper and white fir. This would shift this forest back to its natural state of open, patchily distributed Sierra lodgepole pine forest (Community phase 1.1).
Pathway 1.5a
Community 1.5 to 1.2
In the event of a severe canopy fire, or a clear-cut with or without a prescribed burn, this phase would quickly develop into phase 1.2, the stand initiation phase. Sierra lodgepole pine is susceptible to death from fire at any age because of their thin bark and shallow root systems (Kocher, 2005).
Pathway 1.5b
Community 1.5 to 1.6
This phase can transition to phase 1.6 with the infestation of pests. The primary threat to Sierra lodgepole pines from pest invasion is from the mountain pine beetle (Dendroctonus ponderosae). Infestations can lead to a high mortality rate; leaving a forest of standing dead trees, and causing high fuel loads. Natural outbreaks of variable severity tend to occur every 20 to 40 years (Cope, 1993).
Pathway 1.6a
Community 1.6 to 1.2
After a prolonged period, this phase will progress to phase 1.2. Severe fire will accelerate this transition. Fire is the natural disturbance at this point in the Sierra lodgepole pine cycle and will allow for the regeneration of the lodgepole pine forest seen in phase 1.3. Mechanical removal of the dead trees with partial scarification of the surface or a prescribed burn in the understory can also induce stand regeneration.
