Ecological dynamics

Ecological Dynamics
This ecological site is found within the same elevational zone as Douglas fir but in moister site locations.



State 1.0: Historic reference state.
There is no presence of weedy invasive species within the community phases of this state.

The 43B Montane Decidous Clayey Outwash Terrace ecological site is dominated by quaking aspen with an understory of tall-, medium-, and short-statured shrubs, diverse tall-statured forb and grass species, and low-growing herbaceous species. These mature aspen clones are adapted to the seasonal wetness in the soils of this site, and allow relatively high levels of light to penetrate which is characteristic of aspen groves that leads to a high diversity of forbs, grasses, and shrubs. There is an infrequent and very low cover of Engelmann spruce present at the site. The tall shrub serviceberry is present, along with the medium-height shrub common snowberry, and the low- to medium-height rose species shrubs. Infrequently, there can be moderate canopy cover of Utah honeysuckle, baneberry, prickly currant, russet buffaloberry, and creeping barberry. The understory has the tall forb cow parsnip, western sweet cicely, and the grass mountain brome. Low-growing herbaceous species include sticky geranium, western meadow-rue, and the nitrogen-fixing American vetch. Other grass and forb species also occur. There is a common but very low cover of conifers present at this site, including subalpine fir and Engelmann spruce.
This site relates to the Mueggler (1988) aspen community type of quaking aspen/Saskatoon serviceberry-mountain snowberry/tall forb type, and, less commonly, the quaking aspen/Saskatoon serviceberry-mountain snowberry/mountain brome type. Mueggler (1988) considered the quaking aspen/Saskatoon serviceberry-mountain snowberry/tall forb type type to be primarily a climax community type with very little conifer presence that will likely remain dominated by quaking aspen. It also relates to the Hansen Riparian and Wetlands of Montana classification system Quaking aspen/Western sweet-cicely habitat type. The wood production of this community type was somewhat below the average for all aspen community types found by Mueggler (1988), with an average basal area of 116 sq. ft. /acre (range is 88-144); the site index (the potential for forest trees to grow at a particular site i.e. the average age of dominate and/or codominant trees of an even-aged, undisturbed site at a base age) averaged 46 feet at 80 years (range is 37-55); and the volume growth at stand maturity was 34 cubic ft./acre/yr. (22-46). Our sites fall within the range of Mueggler’s (1988) values with a basal area of 80-180 sq. ft./acre, a site index of 50-70 feet at 80 years, and Cumulative Mean Annual Increment (defined as the mean annual growth per year a tree or stand of trees has experienced to a specified age) of 25-50 cubic ft./acre/yr. The understory is not only diverse, but Mueggler (1988) states it is fairly productive, with average annual air-dry production of 1,180 lbs. /acre (833-1,527 lbs. /acre). Half of this production was forbs, with a quarter to graminoids and a quarter to shrubs. We did not take production data and cannot compare these values.
Predominantly, the soils of this ecological site are well drained. Rarely, some sites occur that are poorly drained and within the taxonomic subgroup oxyaquic haplocryalfs and sometimes as a typic cryaquoll. Vegetatively, these sites are generally the same with the same species occurring in both, but sometimes in slightly different proportions. The poorly drained sites may have slightly higher canopy cover of rose species and common snowberry. Whereas, the well-drained sites may have higher cow parsnip and mountain brome. It relates to the Hansen classification of Montana Riparian and Wetland habitat types in Quaking aspen/ Western sweet-cicely habitat type. As well, these sites predominantly fall within the Mueggler classification system of quaking aspen in Quaking aspen/Saskatoon serviceberry-common snowberry/Tall Forb type, and less commonly Quaking aspen/Saskatoon serviceberry-common snowberry/mountain brome type.

Fire, conifer invasion, and disease are primary disturbances in this ecological site, along with grazing and browsing by wildlife and livestock. Aspen regenerates primarily through root suckering after mature stems die. Therefore, aspen seems to require disturbance from fire, flooding, landslides, or avalanches to maintain dominance and vigor (Shepperd et al. 2001, 2006). Recent studies, however, show that aspen can regenerate and be maintained without major disturbances. Kurzel et al. (2007) described four categories for aspen regeneration: 1.) Catastrophic, resulting from severe, coarse-scale disturbances; 2.) a fine-scale gap phase after small treefalls; 3.) Continuous, in which no new canopy openings are required; and 4.) Episodic, in which there are large pulses of regeneration unrelated to coarse-scale disturbance. Physiologically, aspen responds to the death of a mature stem by lowering a root-growth repressing hormone, thus allow root growth to occur (Bartos and Amacher, 1998). Aspen stands are a preferred area for conifer seedlings to establish due to moisture and a combination of shading and light. However, if the understory has high graminoid cover particularly rhizomatous grass species, then establishment of conifers would be difficult. Conifer infilling combined with a lack of disturbance can lead to the eventual dominance of conifer species through shading, which is not tolerated by aspen. If enough above-ground aspen ramets die off, the rootstock will also die and aspen may be lost from the site (Sheppard et al., 2006). Fire will kill conifers and regenerate aspen through root suckering. At this site there is sufficient disturbance to maintain and perpetuate aspen. If heavy, prolonged grazing of this ecological site occurred, then a decrease of tall forbs and graminoid and shrub species would follow. Mueggler (1988) states that this grazing disclimax site would have diminished shrub, tall forb, and tall grass species and could be found in the Quaking aspen/Saskatoon serviceberry-common snowberry/mountain brome, Quaking aspen/common snowberry/mountain brome, and Quaking aspen/mountain brome habitat types. Prolonged, severe grazing, browsing, and trampling of this site would lead to a significant decrease or elimination of the shrubs, forbs, and grass species that define the site, with an increase in increaser species and non-native species. Hansen also described a grazing disclimax phase with prolonged heavy or severe grazing, browsing and trampling that is very similar to that described by Mueggler (1995). Drought also is a contributing factor in aspen health and mortality in the western interior of Canada. After the severe regional drought of 2001-2002, Hogg (2008) found a net mean increment in living biomass that normally was 2.2 tons per hectare per year subsequently decreased to near zero. Productivity and biomass was positively related to multiyear values of climate moisture index and mineral soil silt content. Mortality/dieback was correlated with minimum annual climate moisture index. Strand (2009) found that marked declines of aspen likely are due to changes in fire regimes, herbivory, drought, and interspecific competition with conifer species. Wet microsites had significantly slower successional rates of conifer establishment than upland mixed aspen/conifer stands. In her studies of the Owyhee Plateau in Idaho, Strand found that fire return intervals for upland aspen-conifer areas of 50-70 years are desirable for maintenance of aspen. The sudden dieback and deterioration of mature aspen stands has occurred throughout North America. Shepperd (2008) describes sudden aspen decline as rapid (1-2 years) mortality of mature trees with a lack of new sprouting after overstory mortality. Frey et al. (2004) attributed this to insect defoliation, drought, and thaw-freeze events as factors initiating dieback in mature aspen stands. Pathogens, nutrition, and successional changes may be involved in the decline in vigor and may contribute to the dieback process. Within the western United States there has been a 50-96% decline in total aspen forest acreage since European settlement (Bartos, 2001). The USFS FIA unit in Ogden, UT suggests that within Montana and Idaho aspen acreages are down 64 percent and 61 percent, respectively. In a study by that FIA unit, in western Montana, it was found that Montana and northern Idaho appear to be experiencing aspen decline, though not sudden aspen decline. Factors attributing to this decline were stated as the absence of fire, advancing succession with increased conifer encroachment, and the aspen’s increased susceptibility to diseases and insects. Aspen stands often are invaded by conifer with the absence of fire, and the conifer will predominate after 80-120 years (Mueggler, 1994). Stands that have over 50% conifers will affect the below-ground root system of aspen (Sheppard et al., 2001). Heavy ungulate grazing detected where regeneration was present also resulted in stands unable to regenerate themselves. Drought could be a factor with climate change. East of the Continental Divide (CD), most stands (58 percent) were considered stable, 33 percent appeared to be expanding, and 8 percent were retreating or diminishing. As well, 78 percent of plots without conifers present were east of the Continental Divide, and these were recorded as not succeeding to some other tree species. The plots with severe conifer competition were on the west side of the Divide. The general condition of aspen stands in USFS Region 1 was found to be healthy. Plots east of the CD had a significantly greater proportion of aspen, with spike tops likely from sooty-bark canker and the greater incidence of wood borer damage east of the CD. The majority of live aspen trees had only minor crown dieback, except that plots east of the CD had a higher proportion of trees with moderate crown dieback. Dieback could be attributed to foliar insects, diseases, or late spring freezes. There was no evidence of sudden aspen decline in this report. Damages recorded included dieback, wood borer, defoliating insects, foliage disease, bark wounds, sooty bark canker, Cytospora canker, and Phellinus stem decay. Wood boring beetles including poplar borer, bronze poplar borer, and poplar dicera were the most common. Damage was significantly greater east of the CD and at plots at higher elevations and more northern locations. Defoliating insects included large aspen tortrix, aspen one leaf tier (caterpillar of moths), aspen two leaf tier, leafrollers, and forest tent caterpillars. The principal diseases were sooty-bark canker and Cytospora canker. Ink spot and Marssonina leaf blight were the common foliar diseases, and stem decay by Phellinus tremulae occurred. Root disease was rare but did occur in this study. Animal browsing and foliar diseases were the most damage-causing agents for aspen sprouts.

Quaking aspen (Populus tremuloides) is a native deciduous tree that has a shallow root system with wide-spreading lateral roots and vertical sinker roots descending from the laterals (Jones, 1985). It forms clones connected by a common parent root system. Clones can be either male or female. Stems can live to be 150 years old in the West. Aspen is not shade–tolerant, and it does not tolerate long-term flooding or waterlogged soils. It regenerates from seed and by sprouting from the roots. Sprout development is suppressed by apical dominance. Once the stem is removed by cutting, burning, girdling, or defoliation, the apical dominance is lost and sprouting begins. Seedling establishment is less common in the West due to dry periods following rainfall that kill newly germinated seedlings. Germination and seedling survival require a moist mineral seedbed with adequate drainage, moderate temperatures, and freedom from competition. Aspen is adapted to fire and is highly competitive on burned sites (DeByle, 1987). It has thin bark with little heat resistance, and the root systems of top-killed stems sprout rapidly and profusely for several years after fire. Following a fire, a new even-aged aspen stand can develop within a decade. It is self-thinning, which develops a mature forest of healthy trees. Quaking aspen stands tend to have high plant productivity, and have been linked to greater soil nutrient availability because aspen litter has greater nutrient content and faster decomposition than conifer needles, which increases the aspen’s nutrient inputs and cycling rates (Preston et al, 2009). Aspen stands also tend to have significantly lower leaf area index values than conifer-dominated stands. This increases light penetration and snow accumulation, which results in greater light and water availability. Climate change may effect this ecological site due to aspen’s physiological sensitivity to temperature extremes and drought (Hogg et al., 2008), but could be offset by increased fire cycles, which favor aspen and cause conifer mortality, which would increase aspen regeneration. Mature conifers tend to be more tolerant to climatic extremes than aspen and may shift the competitive advantage towards conifers in mixed aspen-conifer stands. Conifer seedlings establish under aspen due to increased moisture under aspen trees and moisture holding soil and litter compared to the hydrophobic needles of conifers (St. Clair, 2013).
Quaking aspen is subjected to various diseases and insect pathogens. Some of the more common and serious cankers are the sooty-bark (Encoelia pruinosa), black (Ceratocystis fimbriata), Cryptosphaeria (Cryptosphaeria populina), and the Cytospora canker (Cytospora chrysosperma). These cankers enter through wounds in the bark and create abnormal growth and blackish cankers. The gravest canker, the sooty-bark type, can kill a tree in just 1-10 years. These are natural and essential to aspen ecology, creating openings in the canopy and understory, and regenerating new cycles of aspen as a mature tree dies. Aspen is prone to the root disease Armillaria species, which can weaken or even kill trees and cause windthrow. Boring insects and beetles can attack aspen, leaving wounds which later can become entry points for stem cankers. Foliage diseases occur, such as ink spot (Ciborina whetzelii), aspen tortrix (Choristoneura conflictana), and western tent caterpillar (Malacosoma californicum). White trunk rot fungus (Phellinus tremulae) also is a problem for aspen because it decays the base of the tree (Ostry and Walters, 1983).

This ecological site has high value for livestock and wildlife grazing, browsing, bedding and cover. They are important big game winter and summer grounds. Deer and elk browse young quaking aspen suckers and occasionally feed on the bark of older trees. Use by elk in spring, fall and winter is often moderate to heavy (Kufeld, 1973). It is also a preferred species by moose (Costain, 1989). Beaver can heavily use this site. Common nesting and feeding bird species include flickers, chickadees, sap suckers and woodpeckers (Flack, 1976). Quaking aspen also stabilizes stream banks and provides overhanging cover. This site can be altered due to heavy grazing, browsing, and trampling by livestock and wildlife. Quaking aspen suckers can be significantly decreased with heavy browsing, thereby affecting the regeneration of quaking aspen. Mueggler (1988) found that if this site is grazed by sheep, then grasses such as blue wildrye (Elymus glaucus), mountain brome (Bromus marginatus), and Kentucky bluegrass (Poa pratensis) increase, whereas tall and low shrub species are likely to decrease under prolonged heavy grazing. Black raspberry (Rubus occidentalis), Lathyrus species, and tall ragwort (Senecio serra) tend to increase substantially if grazed by cattle. Soil compaction can occur with heavy use. Introduction of non-native weedy species can occur with livestock grazing.

The following is a summarization by species of wildlife interpretations found in the appendices of Hansen (1995):
Quaking aspen is a facultative upland (FACU) on the wetland indicator classification status, meaning these are nonhydrophyte plants and that it usually occurs in non-wetlands, but may occur in wetlands. It has good sheep forage palatability (highly relished and consumed to a high degree), and fair palatability for cattle and horses (Hansen, 1995). It has medium protein and energy value during fall and winter, which means that it retains usable energy and digestible protein values moderately well. It provides good cover value for elk, mule and whitetail deer, upland game bird, small non-game bird, and small mammals, but gives poor cover for waterfowl. It has only fair food value for elk and mule and whitetail deer, and is poor for antelope. It has good food value for bird species, except for waterfowl.

Western serviceberry is a FACU wetland-designated species that has moderate energy and protein values in the fall and winter. It has good sheep forage palatability, but is poor value for cattle and horses. It has good food value for mule and whitetail deer and antelope, but is rated poor for elk. It has poor food value for waterfowl, but has fair value for all other bird species. Its cover value is fair for mule and whitetail deer, but provides poor cover for elk. It has good cover value for waterfowl, but is rated poor for all other bird species.

Common snowberry is given a wetland indicator designation of FACU, has medium energy value and medium protein value. It has fair forage palatability for cattle and sheep, but is rated poor for horses. It is considered fair in food value for elk, mule and whitetail deer, and antelope. It has fair food value for upland game birds, waterfowl, small non-game birds, and small mammals. It has fair cover value for mule and whitetail deer, but is rated poor for elk, which means it is rarely or never utilized for cover when available. It provides good cover for upland game birds, waterfowl, small non-game birds, and small mammals.

Cow parsnip is a FACU wetland-designated species with low energy and protein value in the fall and winter seasons. It has good forage palatability for cattle, sheep, and horses. It has good food value for elk, mule and whitetail deer, but is rated poor for antelope. It provides fair food value for upland game birds and waterfowl, but has poor value for small non-game birds and small mammals.

Western sweet cicely is a FACU wetland-designated species with low energy and protein value in the fall and winter seasons.

Community phase 1.1: Reference state
This phase has mature, tall, straight round topped aspen trees. The understory is fairly open.
Community Phase 1.2: Herbaceous Phase: Immediately Post-Fire Disturbance
Post-fire regeneration of aspen root suckers with understory plant cover may be high. Site has full sunlight. Few older aspen may survive a mixed severity fire, but generally only dead skeleton trees remain.
Community Phase 1.3: Aspen Intermediate Aged Forest
Competitive exclusion phase of forest succession with thick, dense, pole-sized ASPEN trees.
Community Phase 1.4: Mature Aspen Forest
Forest proceeds from the thick, dense, pole-sized forest in phase 1.3 with small gap dynamics of singe tree death and regeneration, to a more open stand on tall, straight mature aspen trees.

Community Phase Pathway 1.1A: Post fire return of the aspen stand to the herbaceous and shrub stage.

Community Phase Pathway 1.1B: Post fire succession with growth of resprouting aspen to saplings.

Community Phase Pathway 1.1C: Further post fire succession with growth of aspen saplings to pole sized trees.

Community Phase Pathway 1.1D: Transition of pole sized and large trees to a mature, more open forest of aspen trees.


State 2.0
Current reference state, include a minor amount of weedy invasive species present in any community phase.
COMMUNITY PHASE 2.1: Quaking aspen/Saskatoon serviceberry-common snowberry/common cowparsnip-western sweetroot/mountain brome.
Overstory of clones of older aspen with maximum height for the site, with straight, clear stems with short, high rounded crowns. Understory vegetation is a lush, diverse mixture of shrubs, grasses and tall forbs. Our sites were considered reference in the field, although the actual ages from the tree corers show that they are slightly younger than what scientific literature consider reference (i.e. 60+ vs. 70+ years old). Our reference sites generally have all age cohorts filled to a degree including tree, pole, saplings, and seedlings. The tree heights are generally shorter than stated in references for other areas and this is due to high winds east of the Continental Divide, we postulate. The overstory heights for aspen were in height class 7 (40-80 feet tall) or height class 6 (13-40 feet tall). Canopies were moderately to highly closed, ranging from 45-60% total canopy closure. Our reference sites did not fall within any recent fire perimeters (1967-2007, NPS shapefile). Only half of the reference sites fell within the NPS stand age map for fire history and these were in the 1834-1859 age cohort meaning that there has been approximately 150 years since fire. This coincides with the age of the majority of aspen acreage throughout the western United States (N. DeByle et al 1987). It is generally accepted that a fire frequency of 100 to 300 years is appropriate for regenerating and maintaining western aspen forests (DeByle and Winokur 1985). The only fire on the east side of the Continental Divide that burned areas with the NPS Vegetation classification of mixed conifer-deciduous community were the Red Eagle fires of 1998 and 2006. The other sites were not covered in the stand age shapefile either. Our sites would fall in the aspen fire regime classification postulated by Shinneman et al (2013) as either class 1=fire –independent, stable aspen that rarely experience fire significantly into their stands because they are located within hydrologic or edaphic niches or topographically isolated sites. These have multiple ages of cohorts. Class 2= Fire-influenced, stable aspen that do burn occasionally but are not dependent. Fires are mixed-severity or occasionally crown replacing with right conditions resulting in mixed or even-aged cohorts. Kurzel et al (2007) found that aspen may persist without fire by episodic regeneration in which decadent stands may be replaced by regenerating stands. Aspen individuals rarely last more than 120 years before giving way to younger cohorts. Gap dynamics in which treefalls create small openings in the canopy, also occur and facilitate regeneration. The understory is lush and diverse with shrub, grass and forb species (canopy cover data, 8 sites, all sites regardless of Mueggler designation). Frequently occurring species include the tall shrub serviceberry, the medium statured shrubs common snowberry and white spirea, the tall forbs cow parsnip, western meadowrue and false green hellebore and the lowest layer has mountain brome and vicia species.

Community Phase 2.2: Herbaceous Phase: Immediately Post-Fire Disturbance
Post-fire regeneration of aspen root suckers with understory plant cover may be high. Site has full sunlight. Few older aspen may survive a mixed severity fire, but generally only dead skeleton trees remain.

Community Phase 2.3: Aspen Intermediate Aged Forest
Competitive exclusion phase of forest succession with thick, dense, pole-sized aspen trees.

Community Phase 2.4: Mature Aspen Forest
Forest proceeds from the thick, dense, pole-sized forest in phase 1.3 with small gap dynamics of singe tree death and regeneration, to a more open stand on tall, straight mature aspen trees.

Community Phase Pathway 2.1A: Post fire return of the aspen stand to the herbaceous and shrub stage.

Community Phase Pathway 2.1B: Post fire succession with growth of resprouting aspen to saplings.

Community Phase Pathway 2.1C: Further post fire succession with growth of aspen saplings to pole sized trees.

Community Phase Pathway 2.1D: Transition of pole sized and large trees to a mature, more open forest of aspen trees.

Mueggler Aspen/tall forb sites. Community Phase 2.1 overstory and understory species with constancy and average cover values, 5 sites. Species with high constancy occur often, those with low constancy are rare. The average canopy cover is the average of the values for which it occurred. Therefore, species that are rare (only occurred once) show the canopy cover value for the one time it was found. Minimum and maximum canopy cover show the range of cover that the species was found. The most frequently occurring species in this canopy cover dataset include red baneberry, serviceberry, mountain brome, cow parsnip, western sweetroot, white spirea, common snowberry, western meadowrue. Other tall forb species encountered include green false hellebore, arrowleaf ragwort, Richardson’s geranium, sticky purple geranium, western showy aster and Lyall’s angelica.

Mueggler Aspen/mountain brome type sites. Community Phase 2.1 overstory and understory species with constancy and average cover values, 3 sites. Species with high constancy occur often, those with low constancy are rare. The average canopy cover is the average of the values for which it occurred. Therefore, species that are rare (only occurred once) show the canopy cover value for the one time it was found. Minimum and maximum canopy cover show the range of cover that the species was found. Species occurring with high frequency include serviceberry, mountain brome, strawberry species, northern bedstraw, sweet cicely, rose species, common snowberry, western meadowrue, vicia species and alpine leafybract aster.

Forest Production Summary of community phase 2.1.
FOREST OVERSTORY
Forest canopy Canopy cover range 35-60%
Average basal area:
Quaking aspen 80-180 Total ft2/acre
Lodgepole pine 50 Total ft2/acre
Site Index: 50-70 feet at 80 years
CMAI: 25-50 cubic feet per acre per year

Table 5. Foliar cover summary of community phase 2.1, at 4 sites.
TOTAL ANNUAL FOLIAR COVER OVERSTORY =40-100%
TOTAL ANNUAL FOLIAR COVER UNDERSTORY =79.1%
Foliar cover is high (94%) and ground cover is predominantly litter (91%) with soil underneath. The ground cover also has very low cobbles (2%), gravel (3%) and moss (2%) and a trace of stones. Species with highest foliar cover are aspen, common snowberry, white spirea, serviceberry and western meadowrue. This is a multi-storied canopy structure with moderately tall trees (aspen 953 cm or 375 inches tall and subalpine fir 450 cm or 177 inches tall). A tall shrub and forb layer (30-40 inches tall) includes alderleaf buckthorn, blue wildrye, Lyall’s angelica, western sweetroot and common snowberry. The next layer is 20-30 inches tall and includes white spirea, serviceberry, thimbleberry and western showy aster. The next layer is 10-20 inches tall and includes pinegrass, Rocky mountain maple, fireweed and Geyer’s sedge. The lowest layer up to 10 inches tall includes heartleaf arnica and viola species.

State 3.0
This State is a result of severe grazing. It is not possible to return to State 1 without human intervention. Ceasing grazing will not restore the vegetation community. Successful reproduction of quaking aspen will not occur due to browsing. Native shrub and forb species are severely limited due to grazing. There is an increase in non-native and increaser species.
State 4.0
This State is a result of conifer encroachment.
Transition T1A from State 1 to State 2: The presence of weedy plant species has changed the Historic Reference State to that of the Current Reference State.
Transition T2A from State 2 to State 3: This would occur with severe grazing and browsing. If the grazing and browsing pressure were removed for a significant amount of time, then a restoration pathway could occur.
Restoration R3A from State 3 to State 2: Cessation of severe grazing and browsing.
Transition T2B from State 2 to State 4: This would occur with severe grazing and browsing that allowed conifer encroachment and establish to give conifers the competitive advantage over aspen.
Restoration R4A from State 4 to State 2: Cessation of severe grazing and browsing and human intervention to reduced and suppress conifer establishment.