Ecological dynamics
In its historical reference condition, a mature forest with an open stand structure was the most common expression of this ecological site. In this representative stage, widely spaced, older mature Douglas-fir (Pseutotsuga menziesii) and ponderosa pine (Pinus ponderosa) dominate the upper canopy layer, with mature grand fir (Abies grandis) and limited western larch (Larix occidentalis) in the mid-and upper layer(s) of the forest. Lodgepole pine (Pinus contorta) may also be found on this site, especially during early development phases following fire. Pinegrass (Calamagrosis rubescens) and elk sedge (Carex geyeri) dominate the herbaceous vegetation layer along lesser amounts of forbs that commonly include strawberry (Fragaria spp.), heartleaf arnica (Arnica cordifolia), common yarrow (Achillea millefolium), white hawkweed (Hieracium albiflorum) and tailcup lupine (Lupinus caudatus). Shrub species found on this site may include common snowberry (Symphoricarpos albus), creeping Oregon grape (Berberis repens), baldhip rose (Rosa gymnocarpa), birchleaf spirea (Spirea betulifolia), service berry (Amalanchier alnifolia) Scouler’s willow (Salix scouleriana) and mallow ninebark (Physocarpus malvaceus).
The modal plant associations that define this ecological site group concept are the widespread Douglas fir/Common snowberry and Grand fir/Pinegrass associations (as described in Johnson and Clausnitzer 1992, and Johnson and Simon 1987). Additional closely related plant associations include Douglas-fir/Pinegrass and Douglas-fir/Ninebark. Within MLRA 43C in Oregon, these communities exist in areas with greater moisture than pure ponderosa pine and ponderosa pine Douglas-fir forests and the warm and dry end of the climate range supporting grand fir forests. This site group corresponds to the USFS potential vegetation group “Dry Upland Forest” and plant association group “Warm Dry Upland Forest”.
Dry mixed conifer forests were historically subject to frequent surface fires primarily ignited by lightning strikes and Native American cultural burning practices. These fires would have likely occurred at less than 35-year intervals and approximated Landfire fire regime group 1 (0-35 year frequency, surface severity). Landfire estimated average fire return intervals for this forest type are 135 years, 110 years and 30 years for replacement, mixed and surface severity fires respectively. Frequent, low intensity fires would have decreased the density of young regenerating understory conifers, such as Douglas-fir and grand fir which are less tolerant of fire when young. Overtime, these fires, would have favored the development of mature, uneven-aged stands of Douglas-fir and ponderosa pine with open canopies. Less frequent mixed and surface severity fires would have increased the spatial and structural heterogeneity of these forests by creating patches of regeneration and removing portions of overstory components.
Fire-resistant ponderosa is well-adapted to fire, often surviving low severity fire when very young and developing increasing resistance with age by growing thick bark and self-thinning lower limbs. Douglas-fir is intolerant of fire until later developmental stages (~40 years) at which time it’s resistance is bolstered by very thick bark and deepening root habit. Western larch exhibits similar protective bark attributes, along with an open growing crown (i.e. low foliage volumes) that makes this species less vulnerable to consumption or scorch damage in the event of a crown fire. Grand fir is very intolerant of fire when young yet with advanced maturity gains some moderate resistance due to thickened bark and deep roots. Lodgepole pine is susceptible to mortality from fire with thin bark even with maturity, yet is well adapted to colonize following fire with prolific seed production and cones sometimes exhibiting serotiny. With longer time between fire, increased development of understory fuels such as young conifers, shrubs and down wood, along with the development of a closed canopy, can promote an increased frequency of stand replacing fires and insect outbreaks. Much of this site currently exists within this condition, as long-term fire exclusion has allowed for understory infill of shade tolerant Douglas-fir and grand fir and increases in fuel loads.
Understory species in these forests were adapted to respond to this fire driven disturbance regime. The dominant graminoids, such as elk sedge and pinegrass, will resprout from rhizomes and increase in cover following fire. Many shrubs found on site, including spiraea and snowberry, would also resprout from crowns or rhizomes following fire. These and other shrubs including serviceberry, may respond to high severity fire by forming dense shrubfields on sites where they were present prior to disturbance. Understory forbs, such as common yarrow and heartleaf arnica, may resprout from rhizomes while others, such as lupine, can effectively respond to post fire conditions that may delay the reestablishment of other species, such as low soil nutrient levels, by fixing their own nitrogen.
As a forested site with a productive herbaceous component in the understory, much of these forests were historically subject to livestock grazing, especially cattle and sheep, and use by native ungulates such as elk and mule deer. Often adjacent to climax grand fir sites with more dense understories and higher shrub cover, these sites may attract increased use. Prolonged historical use by these ungulates may have altered the composition of understory herbaceous and shrub communities. A decrease in preferred perennial grasses such as pinegrass as well as trampling impacts, may parallel increases in forbs such as heartleaf arnica, strawberry, vetch (Vicia spp.) and lupine, elk sedge and exotic annual grasses such as cheatgrass (Bromus tectorum), Medusahead rye (Taeniatherum caput-medusae), and North Africa grass (Ventenata dubia). Increases in exotic grasses have been associated with other impacts as well, including off highway vehicle use and proximity to human development. At high levels, the impacts of these invasions on nutrient cycling, wildlife habitat and fire cycles may be severe, yet this site may be less prone to widespread invasion than drier forest types.
In addition to direct impacts of wildfire, disturbance factors that resulted in the death of mature trees included injury from lightning strikes, wind events, weather extremes, and the collective influence of various biotic damage agents. Ponderosa pine is prone to injury or mortality from bark beetles, pine engraver, mistletoe, and other adapted insects and diseases. Douglas-fir is often killed or weakened by Douglas-fir beetle and other wood borers, tussock moth and western spruce budworm. These agents often cause “secondary effect” post-fire mortality when fire stresses, but does not outright kill, individual mature Douglas-fir. Endemic bark beetle results in patch mortality, whereas epidemic outbreaks typically cause larger scale mortality. Root disease, age, overstocking, and other biotic and abiotic stressors will also impact the health and well-being of individual Douglas-fir trees, often leading to mortality. Western larch is susceptible to damage from dwarf mistletoe, needlecast and various fungi. Larch casebearer, sawfly, spruce budworm and tussock moth are common defoliators of larch.
The state and transition model below represents a generalized and simplified version of forest change in response to major disturbance types in this ecological site. It does not attempt to model the potential effects of climate change on ecosystem function or process. 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 evidence increases, this model will likely undergo alterations and updates to reflect our emerging understanding.
State 1
Historical Reference
The historical reference state existed across the landscape as a spatially complex forest mosaic of plant communities in various stages of development and with varying composition. The most common expression was the mature open canopy stand, yet young open canopy stands were also common. Closed canopy conditions were less common due to site productivity and conditions favoring short intervals between mixed and low severity fire.
At the landscape scale, these historic stand structures were represented by a combination of patch openings, clumpy (dense or overstocked) tree groups which were often pole size or smaller, and as well-spaced mature overstory trees encompassing larger stand groups. A wide range of conifer establishment and expression was possible due to the influence of a mixed fire regime. Following a disturbance in which large representatives of individual conifers were eliminated, the key to conifer re-establishment in the reference state relied on the recruitment of seed from adjacent sites, or from the few remnant surviving seed bearing sources. In a replacement fire of an older, mature stand, the trees that did survive tended to be the mature and over-mature early seral species because they have the most resistance to fire induced mortality.
Community 1.1
Reference Plant Community, Mature, Open canopy
This community phase is a common representation of the Pre-European reference state. The class size of the overstory layer is very large, but overall canopy closure is low, and canopy gaps and patches are common. Sites can be single or multi-canopied. Ponderosa pine is well represented in the dominant overstory layer. Mature Douglas-fir and Western larch are common as dominant overstory species as well in mid-layer canopy positions. Grand fir regeneration can be found. Reproduction varies with disturbance history. Low intensity, non-lethal surface fires will limit mid and late seral conifer development and maintain the grass dominated understory and the open nature of the stand. In some instances, a mixed fire event may result in limited mid-sized openings which will undergo secondary succession. A sward of pinegrass, less competitive elk sedge, along with scattered shrubs commonly dominates the understory. Snowberry and wild rose are present but limited in extent due of the surface moisture competition of the rhizomatous grass species. Microsites with steeper slopes and protected aspects with higher soil moisture will support greater shrub composition including spiraea, common ninebark and serviceberry.
Community 1.2
Mature forest, Closed canopy
Large, mature trees dominate this community phase, which is a less common representation of the mature forest ecological site in the historic Pre-European context. Ponderosa pine, Douglas-fir and lesser amounts of Western larch occur in the upper canopy layer, with grand fir found in the mid-layer canopy positions. Closed stands rarely exceed 80% crown closure. This in turn increased the likelihood of a stand replacement fire due to the contribution of excessive ladder fuels. Fire suppression may exacerbate this condition within this state and at sustained levels may promote a transition to alternative state 2 or 3.
Community 1.3
Stand regeneration
Following a stand replacement fire or other wide-spread disturbance (i.e. epidemic outbreaks, etc.) resprouting species such as grasses, forbs and some shrubs will dominate. In some cases, dense shrubfields including species such as spiraea, serviceberry, Scouler’s willow and ninebark may develop. Post-disturbance older (relict) seed trees such as mature Ponderosa pine, western larch and Douglas-fir usually survive in sufficient quantities to provide seed sources for seedling establishment. In other cases, young patches of conifer seedlings and saplings, along with banked viable conifer seed in the surface of the soil, provide the source for initial conifer recruitment within the stand. A short fire return interval of repeat fires can eliminate virtually all young conifer reproduction when they are still relatively small and susceptible to damage (recruitment is most prone to fire caused mortality in the seedling/sapling stage). Successful conifer reestablishment will increasingly be hindered from the lack of seed sources and regeneration opportunities when successive burns occur. Long term persistent grass/shrub fields may develop in severe instances.
Community 1.4
Young forest, Open canopy
Canopy closure does not occur for a number of reasons (as described in community pathway 1.3B). In this community phase, fuels are discontinuous, resulting in a low probability of a stand replacement fire. Shade intolerant species remain dominant components in the subcanopy. Dominant understory plants include elk sedge, pinegrass, common snowberry and wild rose. Low intensity ground fires may occur on a regular basis, thereby maintaining the mid development (open) expression of the community phase. These events without altering the basic structure of the overall stand at this stage.
Community 1.5
Young forest, closed canopy
Abundant seed source(s) and successful establishment results develop a closed, mid-development successional stage stand. Even though this is a “closed” canopy, closure rarely exceeds 80%. Developing small and medium size ponderosa pine and western larch (to a lesser extent) begin to dominate the upper canopy layer. Mid-seral Douglas-fir and late seral grand fir will begin to establish as shade levels increase. These species will often develop to occupy the understory mid-layer canopy position as the general stand ages. Lodgepole pine can be found on colder micro-sites across the landscape. This state may be at higher risk of a stand replacement fire due to the contribution of excessive ladder fuels. Fire suppression may exacerbate this condition within this state and at sustained levels may promote a transition to alternative state 2 or 3.
Pathway 1.1B
Community 1.1 to 1.2
In the absence of low/mixed severity fire events or disease/insect outbreaks regeneration and recruitment will continue to occur in the stand eventually leading to densely stocked closed canopy conditions.
Pathway 1.1A
Community 1.1 to 1.3
Stand replacement fire shifts the stand to the early development phase. Secondary succession is initiated.
Pathway 1.2A
Community 1.2 to 1.1
Mixed fire events or endemic insect/disease impacts shift the stand structure to the mature, late development open, mature structural phase.
Pathway 1.2B
Community 1.2 to 1.3
Stand replacement fire transitions the stand to the early development plant community phase; Secondary succession is initiated. Re-establishment of the conifer portion of the stand is dependent on successful seedling recruitment and establishment, similar to community pathway 1.1B
Pathway 1.3B
Community 1.3 to 1.4
The absence of larger scale fire disturbance for ~ 40 years, along with a scattered seed source of adapted conifers in the establishment phase, develop over time to the open mid-development community phase. Early seral species such as Ponderosa pine, and lesser amounts of Douglas-fir and other adapted conifer species result in low levels of overall conifer occupancy (spotty recruitment and low-density development of conifers may also occur due to impacts of other on-going disturbance events). Under these conditions, the stand develops to an open young forest phase.
Pathway 1.3C
Community 1.3 to 1.5
Absence of larger scale fire disturbance for ~ 40 years, with abundant conifer recruitment (with low levels of other disturbance events), results in the progression to the closed mid-development structure phase.
Pathway 1.4A
Community 1.4 to 1.1
The canopy density remains low as the stand grows towards maturity, with limited levels of additional seedling recruitment of early seral species. Regular surface fires also help to keep young tree recruitment in check.
Pathway 1.4B
Community 1.4 to 1.3
A large-scale stand disturbance (such as a stand replacement fire, which would be a rare event given the nature of the fuels in plant community 1.4) shifts the stand back to community phase 1.3 (early development). The re-establishment of the conifer portion of the site is again dependent on the factors described in phase 1.3.
Pathway 1.5A
Community 1.5 to 1.2
In the absence of large-scale stand replacement or mixed fires for a short number of decades, the stand continues to develop towards the late-development, closed community phase. Surface fires can maintain an open understory, but in the relative absence of surface fires late seral grand fir and other shrub and grass species will increase in abundance.
Pathway 1.5B
Community 1.5 to 1.3
A large-scale stand disturbance (such as a stand replacement fire shifts the stand back to community phase 1.3 (early development). The re-establishment of the conifer portion of the site is dependent on the factors described in phase 1.3.
Pathway 1.5C
Community 1.5 to 1.4
Mixed fire events or endemic insect/disease impacts shift the stand structure to the young forest open canopy structural phase. In order for this to occur, the impacts of the disturbance would have to by patchy and small scale in nature.
State 2
Long Term Fire Exclusion
Conditions favorable to the development of this alternative state began to occur within the Reference State around the turn of the twentieth century. The impacts of fire exclusion, a management goal of post-European settlers and land managers, allowed many stands to progress without the natural occurrence of any fire, including frequent surface fires. The ecologic benefits of the low intensity fires were lost. Fire suppression shifted the age expression and density of the younger stands and changed the composition of understory vegetation, leading to reduced spatial variation. Fuel levels and fuel stratum layers increased, shifting the fire regime/condition class toward a greater likelihood of stand replacement fire episodes.
The overall consequences of the changes to the forest structure and function due to the combined management actions in the last century are:
INCREASED
• Stand Density
• Shift towards mid and late seral species
• Amount of understory and secondary stand levels of conifers
• Fuel loads and risk of catastrophic high severity fires
DECREASED
• Large old pine and other fire adapted early seral species
• Regeneration of early seral species
• Habit for species of open stands of old pine forests
• Decreased levels of snags and large organic debris
• Reduction in soil quality due to loss of soil wood and organic matter
• Decrease in genetic variation of early seral species
State 3
Catastrophic Fire
State 3 represents conditions immediately following a catastrophic, stand replacement fire. These types of fires, when they occurred within the historic context of the reference state, transitioned the stand to this alternative state if and when the vast majority of the cone producing conifers were eliminated by the fire, and when other sources of conifer recruitment are also absent (i.e. resulting in unstocked stand conditions). State 3 would infer a much longer post-fire stand recovery period compared to situations that normally existed in the reference state (where older relict, seed producing early and mid-seral seed sources existed). The basic natural resource values (especially soil quality) were generally preserved or quickly restored in these instances.
State 3 could also result from catastrophic stand replacement fires that originate from conditions found in alternative state 2.1 and 4.1 (as shown on the state and transition diagram by the T2A and T4A transition symbols). The destructive heat generated impacts of these events are much greater than naturally occurring replacement fires, and in these instances the basic natural resource values (plants, animals, hydrology and especially soil quality elements) have been degraded and are very slow to recover. Natural sources for conifer recruitment are absent.
State 4
Loss of Conifers from High Levels of Root Disease
This state may seem to mimic the conditions of Alternative state 3.1 in that forest stocking is virtually non-existent, but the underlying cause leading to the unstocked condition and the recovery options are vastly different. In this state, immediate restoration by planting is not feasible because the root mass is still active in the soil, and young developing conifer seedlings will succumb to root disease mortality in a short period of time. Poorer quality sites are at greater risk of root disease occurrence and impacts, and species such as grand fir and Douglas-fir are most susceptible. Note that Alternative State 4.1 is at risk of catastrophic wildfire while fuel levels are in excess, which would transition the site to Alternative State 3.1 (by way of T4A.).
Transition T1A
State 1 to 2
Long term fire exclusion (50-100+) years (resulting in Alternative State 2).
Transition T1B
State 1 to 3
A wide spread stand replacing fire event occurs as a natural event in any community phase of the reference state. A vast majority of the cone producing conifers are eliminated, but site quality values remains relatively intact.
Restoration pathway R2A
State 2 to 1
Treatment practices that reduce excessive ladder fuels (low thinning) and reduce overstory bulk density and continuity (crown and selective thinning) may provide immediate benefits in terms of reducing the potential of catastrophic wildfire. Other practices which reduce overstocking, or which shift species towards early seral species (pre-commercial and commercial thinning, tree planting), as well as introducing understory prescribed burning as a maintenance practice, can contribute to increased resiliency and will improve the ecologic function of the stand if done properly.
Transition T2A
State 2 to 3
Fuel build-up in Alternative State 2 results in a catastrophic wildfire, similar to that of T1B, but the likelihood of replacement fire and the intensity and detrimental impact(s) exceed historic norms.
Transition T2B
State 2 to 4
In the absence of catastrophic fire, the long-term site occupancy of mid to late seral Douglas-fir and grand-fir leads to increased levels of root disease, especially on poor quality sites, eventually excluding all conifer species from the site over time.
Restoration pathway R3A
State 3 to 1
Ponderosa pine and Douglas-fir are planted in order to overcome the virtual lack of adequate seed source of surviving Ponderosa pine, larch, or Douglas-fir of any size or age class. Natural recovery will be extremely long without tree planning efforts, up to many 100’s of years as Ponderosa pine and associated Douglas-fir and western larch slowly re-establish perimeter areas and migrate inwards by natural reproduction and under favorable circumstances. It is likely that persistent brush or grass/brush cover would exist for hundreds of years if un-planted. Soil quality is slow to respond to pre-fire levels, especially with the lack of soil organic wood input and other contributors to soil health.
Restoration pathway R4A
State 4 to 1
Reforestation, after the underground root infestation has receded to threshold levels, is applied. Site preparation may be necessary to control competition from brush and grass species
Transition T4A
State 4 to 3
Widespread catastrophic fire occurs similar to that of T2A, with varying levels of live and dead conifer individuals, along with abnormal levels of native brush species.
