Ecological dynamics
This site occurs on nearly level lava plains overlain by eolian pumice sands and supports a juniper woodland plant community. These woodlands are characterized by an open stand of old growth western juniper, which are over 150 years old and express old growth morphological and structural characteristics. The interspaces between the trees (openings) are dominated by antelope bitterbrush and mountain big sagebrush over a diverse understory of grass species. Idaho fescue is the dominant grass on this site, with dense stands under the trees and within influence of the tree crowns. Interspaces between the trees express a more diverse mix of grasses including Idaho fescue, Thurber’s needlegrass, western needlegrass, and Indian ricegrass.
Woodland ecological sites existing on these eolian-volcanic sands collectively represent the most extensive old growth western juniper woodlands within the range of the species (Waichler et al. 2001). Trees in this region have been aged at over 1,600 years old and standing snags have been dated back to 100 BC. When compared to younger trees, old growth trees have a more rounded canopy shape with limited leader growth, deep vertical furrows in their bark, rot pockets, cavities, abundant arboreal lichen cover, and changing branch structure, including large basal branches. These woodlands have evolved over centuries and are relatively stable, resulting in the accumulation of dead wood in the form of snags, dead branches in live trees, logs, and weathered stumps (RMRS-GTR-403, pg 57). It also predominately stays aloft throughout the decay process, deteriorating more through abiotic weathering than biotic decomposition; greatly limiting contributions to soil organic matter and nitrogen pools (Waichler et al. 2001).
Western juniper is highly susceptible to fire, so old growth juniper woodlands are often isolated in fire resistant locations such as “rock outcrops, knolls, ridges, and/or soils that are shallow, coarse, rocky, and often high in clay or sand” (RMRS-GTR-403, pg 57). This site occurs on basalt lava plains overlain with moderately deep to deep pumice ash, primarily derived from Mt. Mazama. While soils on this site are deeper and have fewer course fragments than is typical of other old growth juniper sites, the soils on this site are geologically young; less than 7,000 years old, and primarily course pumice sands. Limited pedogenic development of these sandy soils in a low precipitation environment (10-12 inches annually) limits resources and results in a plant community that expresses large gaps between individual plants and limits overall understory production. Distance between trees and a discontinuous understory breaks up fine fuels and limits ladder fuels, typically containing fire events to one or several trees thereby creating the conditions for old growth woodlands to develop. Stand replacement fires and mixed-severity fires were historically rare (return intervals measured in centuries) (Tech Bulletin 152, 2005, pg 21 and RMRS-GTR-403, pg 112); meaning climatic conditions, such as severe drought, are the primary influence on tree mortality and establishment and understory dynamics.
In addition to influencing fire behavior, soil characteristics on this site drive species composition and production in the understory. While soil textures are coarse, the soil particles themselves are porous and can hold water in addition to the capillary water of the profile. Some of this water is plant available, greatly increasing the available water capacity (AWC) of the soil compared to non-pumice soils with similar textures. (Ecological Provinces, SR 990, pg 89). These pumice soils also have high albedo. The light color on the surface absorbs less sunlight and thus keeps the soil cooler than soils with darker A horizons. The porous nature of these soils also makes them very insulating, moderating subsurface temperatures, and keeping them cooler longer into the growing season. These unique characteristics result in a diverse species composition in the understory community that is typically more common on cooler, wetter, higher production sites. Spatial distribution of these understory species is further influenced by the size of the pumice sands. Redistribution of the pumice ash by wind resulted in varying surface textures across the site. The courser sands favor the needlegrasses, while the finer sands favor other species like Idaho fescue and bluebunch wheatgrass.
Paleobotany and Climate:
Western juniper first arrived in its current geographical range in Central Oregon between 4,800 to 6,600 years ago, during the mid Holocene era. Cool and moist conditions 4,000 to 3,000 years ago favored tree growth, cone production and seedling establishment, resulting in rapid expansion; Western juniper reached its prehistoric maximum across most of its present-day range during this period. A subsequent warm period 2,500 to 3,000 years ago caused severe droughts, major fires, and regional declines in western juniper stands. With the onset of the Little Ice Age conditions again became cooler and wetter and pollen records indicate juniper again began to gradually increase to its current range. (RMRS-GTR-403, pg 102-103). Under a natural disturbance regime, climate is the primary driver of juniper distribution and persistence, particularly in juniper woodlands. It directly influences seed production, seedling establishment, and plant mortality while also influencing other disturbances such as fire, competition, insects, and disease. (RMRS-GTR-403, pg 123)
Cool, moist conditions favor juniper expansion, while severe or extended drought and warmer conditions cause tree stress and mortality (RMRS-GTR-403, pg 102). Some mortality is the direct result of drought stress, while some trees succumb to secondary disturbances due to reduced vigor. Old trees are susceptible to heart-rots, which target the heartwood of the trunk and large limbs. Heart-rots rarely kill the tree but can structurally weaken them. Stressed trees are also more susceptible to mistletoe infestations and insects. In spite of vulnerabilities, western juniper is very resistant to many pressures. Overall, tree mortality in old growth juniper woodlands is typically low (RMRS-GTR-403, pg 42 to 48); estimated at less than one percent per century (RMRS-GTR-403, pg 111).
Western Juniper Woodland Infill:
While old growth juniper woodlands are relatively stable, they are not static. They have historically experienced cycles of infill and mortality in response to climatic conditions. With the end of the Little Ice Age in the 1850s, evidence suggests these woodlands were slowly expanding and infilling. Rate of infill throughout the Great Basin greatly accelerated in the late 1800s, peaking in the early 1900s. Infill rates slowed with the onset of widespread severe droughts starting in the 1920s. This acceleration coincides with a significant rise in settlement throughout the Intermountain West and is attributed to a combination of factors including climate, grazing, altered fire regimes, and increased CO2 levels (RMRS-GTR-403, pg 83, 104).
Woodland infill, precipitated by the end of the Little Ice Age, coincided with a time of rapid settlement and introduction of livestock grazing in the Great Basin, altering understory dynamics and fire behavior. Wetter periods, as were experienced during this timeframe, typically result in the accumulation of fine fuels (RMRS-GTR-403, pg 83). In addition to effects increased understory vegetation has on fire cycles, paleobotany literature suggests competition with herbaceous vegetation may limit tree seedling establishment and/or result in thinning. This indicates a robust understory can provide some competition to juniper establishment, limiting infill (RMRS-GTR-403, pg 112). Large numbers of domestic livestock grazing during this period prevented fine fuel accumulation and areas of heavy grazing would have resulted in understory plant stress. Without fire or competition, young juniper trees were able to rapidly establish.
Carbon dioxide (CO2) levels have also increased since the end of the Little Ice Age. Increased CO2 can increase water use efficiency in conifers relative to herbaceous species, resulting in faster tree growth and denser canopies (RMRS-GTR-403, pg 119). As climate conditions became warmer and drier in the early 1900s, expansion and infill slowed but still exceeded historic rates under similar conditions. This suggests CO2 concentrations and reduced competition from perennial grasses could be playing a significant role in current woodland expansion (RMRS-GTR-403, pg 121 and 123). These factors have resulted in altered stand structure in many persistent woodlands.
Infill trees compete with understory vegetation for water and other resources. Shrubs decline as trees increase in dominance, with sagebrush being highly sensitive to tree competition. Bitterbrush and Idaho fescue will persist longer as infill progresses, in part due to their superior shade tolerance relative to other species but will also eventually be removed from the site. (RMRS-GTR-403, pg 52; Miller et al., 2000) Herbaceous species are more resistant to tree competition, but also decline with increased tree dominance. Microclimate conditions under old growth trees shelter Idaho fescue, so interspace species are the first to decline. Understory decline is accelerated when plants are stressed by other disturbances such as drought, heavy grazing, or recreational use (RMRS-GTR-403, pg 48 to 55).
Non-Native Species:
With increased native plant understory stress and mortality, there is greater opportunity for establishment of non-native species and native increasers. This site is susceptible to cheatgrass, an invasive annual grass, and introduced annual forbs like pale madwort (Alyssum alyssoides). Medusahead and Ventenata dubia are not a concern on this site, as they typically do not establish in these coarse textured pumaceous soils. With repeated disturbance, sprouting shrubs (e.g. rabbitbrush species) and forbs (e.g. granite prickly phlox, Linanthus pungens and tansymustard Descurainia sp.) also increase and fill open spaces in the understory.
Over time, this can create fuel continuity that is atypical of old growth woodlands, making this site more prone to fire. Fires become more frequent and widespread. Higher tree densities from juniper infill also creates ladder fuels, making old trees more susceptible to fire, leading to more stand-replacing fires. (RMRS-GTR-403, pg 115). Recurring fires favor re-establishment of annual and sprouting species over native perennial grasses and shrubs, thereby perpetuating the fire cycle.
Resilience Management:
Within the natural range of variability under a normal distribution regime this site is resistant to disturbance, but has low resilience when disturbance occurs. Physiographic characteristics including proximity to rock outcrops and adjacent shallow soils as well as low fine fuel loads and minimal ladder fuels make this site very fire resistant. The site is also capable of withstanding extended drought with limited plant mortality. Disturbances outside of shifts in climate are not common resulting in a very stable site.
When disturbance occurs outside the normal range of variability the plant community shifts quickly and is very susceptible to invasion by non-native annual species. This low resilience is a result of the site’s limited resources (arid soil moisture regime and sandy soils) and warmer climate (mesic soil temperature regime).
State 1
Reference
The Reference state is representative of the natural range of variability under pristine conditions. No introduced species occur in this state and disturbance has not fundamentally altered soil quality or plant community composition. This state represents a persistent old growth juniper woodland and has two general plant communities; a grass – shrub dominant community and a grass – forb dominated community. Both support an open stand of old growth juniper.
Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These include the presence of all structural and functional groups and low fine fuel loads. Plant community phase changes are primarily driven by climate. Warm, dry cycles resulting in severe drought stresses plants and increases susceptibility to insects and disease. Cooler, wetter cycles that are conducive to tree establishment may result in woodland infill, and fine fuel accumulations may result in single tree or small area fires. Fire return interval is centuries.
Characteristics and indicators. • Lack of introduced plant species • Presence of all functional and structural groups • High plant vigor and reproductive capacity in most years
Resilience management. Management should focus on preservation of old growth juniper and prevention of non-native species establishment. Removal of infill trees to maintain pre-European infill rates can prevent transition to State 2.
Community 1.1
Old Growth Juniper, Bitterbrush, Mountain Big Sagebrush, and Deep Rooted Perennial Grasses
This community is characterized by an open stand of old growth juniper. Occasional younger trees of varying ages are present in the understory or as a limited part of the overstory. The interspaces between the trees (openings) are dominated by antelope bitterbrush and mountain big sagebrush over a diverse understory of grass species. Idaho fescue is the dominant grass on this site, with dense stands under the trees and within influence of the tree crowns. Interspaces between the trees express a more diverse mix of grasses including Idaho fescue, Thurber’s needlegrass, western needlegrass, and Indian ricegrass. This community is very stable and the most common community in this state.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
465 |
590 |
700 |
| Shrub/Vine |
160 |
200 |
250 |
| Tree |
45 |
70 |
90 |
| Forb |
30 |
40 |
60 |
| Total |
700 |
900 |
1100 |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Community 1.2
Old Growth Juniper, Deep Rooted Perennial Grasses, and Forbs
Juniper overstory has experienced isolated mortality as the result of small, single tree or small area fire; or drought and disease. This community is limited to the extent of the disturbance (usually a small, isolated area). Site is still characterized by an open stand of old growth juniper. Young trees (<150 years old) and/or shrubs are reduced or removed from the site. Perennial forbs and grasses expand.
Pathway 1.1A
Community 1.1 to 1.2
Insects (e.g. aroga moth), drought stress, and/or small isolated fire reduces shrubs and young juniper trees. Disturbance from drought and disease may be more widespread than fire.
Pathway 1.2A
Community 1.2 to 1.1
Time and favorable climatic conditions (cooler/wetter climatic cycles) facilitate increase in shrub overstory.
State 2
Current Potential
This state is similar to the reference state yet some amount of introduced species occur and/or infill rates are exceeding pre-European settlement rates. Ecological function is largely intact, however the resiliency of the state has been reduced. This state represents a persistent old growth juniper woodland and has two general plant communities; a grass – shrub dominant community and a grass – forb dominated community. Both support an open stand of old growth juniper. Non-natives may increase in abundance but will not become dominant within this State.
Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These include the presence of all structural and functional groups and low fine fuel loads. Positive feedbacks decrease ecosystem resilience and stability of the state. These include the non-natives’ high seed output, persistent seed bank, rapid growth rate, residual dry matter accumulation, and adaptations for seed dispersal.
Plant community phase changes are primarily driven by climate. Warm, dry cycles resulting in severe drought stresses plants and increases susceptibility to insects and disease. Cooler, wetter cycles that are conducive to tree establishment may result in woodland infill, and fine fuel accumulations may result in single tree or small area fires. Fire return interval is centuries.
Characteristics and indicators. • Presence of all functional and structural groups • High plant vigor and reproductive capacity in most years
Resilience management. Management should focus on preservation of old growth juniper and prevention of non-native species expansion. Removal of infill trees to maintain pre-European infill rates can prevent transition to State 3; or restore to Reference (State 1) if non-native species are not present.
Community 2.1
Old Growth Juniper, Bitterbrush, Mountain Big Sagebrush, and Deep Rooted Perennial Grasses
This community is characterized by an open stand of old growth juniper. Occasional younger trees of varying ages are present in the understory or as a limited part of the overstory. The interspaces between the trees (openings) are dominated by antelope bitterbrush and mountain big sagebrush over a diverse understory of grass species. Idaho fescue is the dominant grass on this site, with dense stands under the trees and within influence of the tree crowns. Interspaces between the trees express a more diverse mix of grasses including Idaho fescue, Thurber’s needlegrass, western needlegrass, and Indian ricegrass. Annual non-native species may be present but not dominant. This community is very stable and the most common community in this state.
Community 2.2
Old Growth Juniper, Deep Rooted Perennial Grasses, and Forbs
Juniper overstory has experienced isolated mortality as the result of small, single tree or small area fire; or drought and disease. This community is limited to the extent of the disturbance (usually a small, isolated area). Site is still characterized by an open stand of old growth juniper. Young trees (<150 years old) and/or shrubs are reduced or removed from the site. Perennial forbs and grasses expand. If annual non-native species are present, they may increase with disturbance but will not become dominant.
Pathway 2.1A
Community 2.1 to 2.2
Insects (e.g. aroga moth), drought stress, and/or small isolated fire reduces shrubs. Disturbance from drought and disease may be more widespread than fire.
Pathway 2.2A
Community 2.2 to 2.1
Time and favorable climatic conditions (cooler/wetter climatic cycles) facilitate increase in shrub overstory.
State 3
Western Juniper Infill
In this state western juniper dominates the overstory and site resources. Young juniper (<100 years old) is co-dominant with old growth juniper and increasing. Bitterbrush, big sagebrush, and perennial bunchgrasses may still be present, but they are no longer controlling site resources. Soil moisture, soil nutrients and soil organic matter distribution and cycling have been spatially and temporally altered.
Increased pace of tree recruitment results in a multi-age class overstory. As tree density increases, shrub mortality increases, and deep-rooted perennial grasses are reduced. Bare ground interspaces are large and connected, allowing for establishment and expansion of cheatgrass, annual forbs, mustards, and granite prickly phlox.
Characteristics and indicators. • Western juniper infill trees are co-dominant with old growth trees or dominant in the overstory • Presence of all functional and structural groups, though diversity may be reduced • Bare ground patches are large and connected
Resilience management. Management should focus on preservation of old growth juniper and promoting native understory vigor and expansion. Removal of infill trees to release resources and reduce competition for native understory vegetation can help maintain State 3; or restore to Current Potential (State 2) if all functional groups and understory species are present.
Community 3.1
Western Juniper, Bitterbrush, Mountain Big Sagebrush, and Deep Rooted Perennial Grasses
Western juniper dominates the overstory and site resources. Young trees are actively growing with noticeable leader growth. Shrubs and bunchgrasses are still present in the understory, but production has been reduced. Gaps between perennial plants have increased. Annual non-native species and perennial increasers are present, particularly in the interspaces.
Community 3.2
Western Juniper (At Risk)
Western juniper dominates the site. Trace amounts of bitterbrush and sagebrush may be present however dead skeletons will be more numerous than living brush. Deep-rooted bunchgrasses may or may not be present in the interspaces. Idaho fescue may still be present under the canopy of large trees. Bare ground interspaces are large and connected. Soil redistribution may be increasing, especially by wind . Annual non-native species and perennial increasers are present and increasing, particularly in the interspaces .
Pathway 3.1A
Community 3.1 to 3.2
Time allows for maturation of the tree community. Additional disturbances such as recreational vehicle use, repeated heavy grazing, and pressure from the urban interface that stress understory species accelerate this pathway.
Pathway 3.2A
Community 3.2 to 3.1
Removal of disturbances such as recreational vehicle use, repeated heavy grazing, and pressure from the urban interface may allow understory species to increase vigor and expand in the understory if infill trees are not utilizing all site resources. Mechanical treatment of infill trees (non-old growth juniper) can also facilitate this pathway.
State 4
Annual
In this state, perennial bunchgrasses and non-resprouting shrubs are greatly reduced or missing on the site. The understory has been replaced by non-native annual grasses and forbs, sprouting shrubs, and native increasers. Competition with the tree overstory and/or repeated disturbances such as fire, heavy grazing, and recreational use has allowed non-resprouting shrubs and perennial grasses to be replaced by cheatgrass, an invasive annual grass, and introduced annual forbs like pale madwort (Alyssum alyssoides). Forbs (e.g. granite prickly phlox , Linanthus pungens and tansymustard, Descurainia sp.) and sprouting shrubs (e.g. rabbitbrush species) also increase and fill open spaces in the understory.
This state may still support a western juniper overstory. If the site has not had a stand replacement fire, it will be a mix of old growth and young juniper. Following a stand replacement fire, the tree overstory will be temporarily lost. Time without disturbance will allow trees to reestablish. However, fuel continuity typical of annual dominated systems causes fires to become more frequent and widespread making replacement of old growth trees unlikely. Reestablishment of old growth trees would take centuries without disturbance (primarily fire), and feasibility of this pathway is currently unknown.
Characteristics and indicators. • Understory is dominated by cheatgrass and introduced annual forbs • Perennial grasses are greatly reduced or missing
Community 4.1
Old Growth Western Juniper and Annuals
Annual non-native plants, native increaser forbs, and sprouting shrubs dominate this site. This community has a western juniper overstory that still supports old growth trees. Young trees (<100 years old) are co-dominant or dominant. Non-resprouting shrubs may be present in trace amounts.
Community 4.2
Annuals
This community occurs after stand replacement fire. Annual non-native plants, native increaser forbs, and sprouting shrubs dominate this site. Scattered juniper or patches of juniper may persist, but the western juniper overstory has been removed.
Community 4.3
Young Western Juniper and Annuals
Annual non-native plants, native increaser forbs, and sprouting shrubs dominate this site. Western juniper has re-established in the overstory following a stand replacing fire.
Pathway 4.1A
Community 4.1 to 4.2
Stand replacement fire.
Pathway 4.2A
Community 4.2 to 4.3
Time and lack of fire allows for sprouting shrubs such as rabbitbrush to increase, and potentially bitterbrush and sagebrush to establish. The probability of bitterbrush/sagebrush establishment is extremely low. Young juniper reestablishes.
Pathway 4.3A
Community 4.3 to 4.2
Fire
Transition T1A
State 1 to 2
Introduction of non-native species, altered fire regimes, repeated heavy grazing by domestic livestock, and/or increased CO2 levels.
Context dependence. The combination of multiple triggers increases likelihood of this transition.
Restoration pathway R2A
State 2 to 1
Mechanical treatment of infill trees (non-old growth juniper); non-native species are not present.
Context dependence. Non-native species are not present.
Transition T2A
State 2 to 3
Time and favorable conditions for tree expansion allow trees to become dominant.
Restoration pathway R3A
State 3 to 2
Mechanical treatment of infill trees (non-old growth juniper).
Context dependence. Depleted understories may be slow to recover. Understory recovery may not be possible if anthropogenic disturbances are still stressing understory species. Restoration to State 2 is possible from Community 3.1 when all functional groups are still present, and site has maintained understory species diversity. Restoration from Community 3.2 is unlikely due to depleted understory.
Transition T3A
State 3 to 4
Time allows for maturation of the tree community and loss of understory species; or stand replacement fire.
Constraints to recovery. Additional disturbances such as recreational vehicle use, repeated heavy grazing, and pressure from the urban interface that stress understory species accelerate this transition.
