Ecological dynamics

The Pothole ecological site is dominated by rough fescue (Festuca campestris) but can occur as mixtures with and Idaho fescue (Festuca idahoensis), needle and thread (Hesperostipa comata), and prairie Junegrass (Koeleria macrantha), and bluebunch wheatgrass (Pseudoroegneria spicata). While highly dominated by grass species, forb species do occur in high frequency but low cover predominantly old man’s whiskers (Geum triflorum). Associated species include blanketflower (Gaillardia aristata) and yellow penstemon (Penstemon confertus). Other associated montane forbs include yarrow (Achillea millefolium), western stoneseed (Lithospermum ruderale), largeflower triteleia (Triteleia grandflora), twin arnica (Arnica sororia), pointedtip mariposa (Calochortus apiculatus), stiff yellow Indian paintbrush (Castelleja lutescens), bluebell bellflower (Campanula rotundifolia), limestone hawksbeard (Crepis intermedia), hairy false goldenaster (Heterotheca villosa), yellow penstemon (Penstemon confertus) and spiny phlox (Phlox hoodii). Subshrubs including rosy pussytoes (Antennaria rosea).
The physiography feature of this ecological site being a closed depression, allows for increased moisture through run-in, and the depositional nature of the soils allows for a very productive vegetation community. This is particularly important as the surrounding landscape of flat outwash terraces tend to be drier, flatter and easily accessible, and therefore more prone to wildlife, livestock and human use. This can cause an influx of weed species invasion and decreased vigor of the native plant community. These closed depressions or potholes, which are difficult to access, can act as refugia for native plants if the surrounding flat drier terrace has experienced overuse or overgrazing.
The Pothole ecological site is most similar in regards to vegetation and soils (though not landform or landscape position) to the historic silty range site. This silty range site is now referred to as the loamy ecological site. A literature reference (Ross et al, 1974) for pristine Loamy ecological sites in the 15 to 19 inch precipitation range in western Montana (Rocky Mountain Area) is recorded as having the following percent composition by weight for species: rough fescue 17 to 92 percent, Idaho fescue trace to 40 percent basin wildrye 0 to 20 percent prairie junegrass trace to 5%, needle-and-thread grass 0 to 2 percent, western wheatgrass/slender wheatgrass/thickspike wheatgrass 0 to 2 percent, Richardson’s needlegrass 0 to 10 percent, Columbia needlegrass a trace to 1 percent, timber oatgrass 0 to 1 percent, perennial forbs 5 to 20 percent and shrubs a trace to 8 percent. Shrubs at this site include yellow rabbitbrush, shrubby cinquefoil, big sagebrush, threetip sagebrush and white sagebrush. In a comparison of non-grazed and grazed sites for the Loamy 15 to 19 inch precipitation range, Ross (1974) found that at four sites, rough fescue and basin wildrye composition was greatly reduced, shrubs were greatly increased and perennial forbs were reduced. Other grasses increased with grazing as rough fescue decreased. The particular species of increaser depended on what was present in the community before grazing including Sandberg bluegrass, Kentucky bluegrass, Richardson’s and Columbia needlegrasses, Idaho fescue, prairie Junegrass, timber oatgrass and threadleaf sedge.
This grassland is considered intermediate between the Pacific Northwest Bunchgrasses in the Columbia Basin and the Mixed Prairie of the Great Plains. Representative species of both of these vegetation communities occur. The rough fescue community is where the climate shifts from the Columbia Basin with lower summer but greater winter precipitation to the Great Plains with higher summer but lower winter precipitation. Pothole sites can have highly productive grassland due to the soils and climate of the area.


These grasslands are adapted to a frequent fire regime that reduced the encroachment of woody shrubs and trees. Historically, the fire frequency is difficult to determine but is thought to average 20 to 30 years. These fires were generally from lightning strikes during the hot mid-to-late summer months. The bunchgrasses and perennial forbs of this vegetation community are tolerant of fire by resprouting and from underground roots and tubers. In general, the bunchgrasses and native perennial forbs complete their life cycles by the onset of summer drought and enter a dormant state (aestivation) which coincides with the fire season. These fires are less harmful since the plants are dormant. Fire is essential to this community as it prevents woody invasion and removes heavy accumulation of litter that suppresses biodiversity. Rough fescue is adapted to periodic burning by their growth form of dense, tufted bunch that insulates perennating buds located near the ground surface. It recovers from fire by tillering, sprouting from residual plants and from on-site and off-site wind-dispersed seed. Rough fescue is initially top-killed, but then recovers to pre-fire coverage usually in 2 to 3 years. There may be reductions in plant vigor following fires in the growing season more so than in dormant season burns. When fire suppression has occurred, rough fescue accumulates heavy litter within large diameter crowns and survival may be severely inhibited as crowns tend to continue to long after the passage of the flame front. If a hot fire occurs during the growing season and there is heavy litter accumulation, then fire effects can be severe. Spring burns can reduce seed production in rough fescue although fall burns have no effect. Fall burns may have reduced fire effects from elevated soil moisture, but may increase the chance of wind or water erosion, leaving rough fescue more susceptible to frost damage. Fires are most damaging when native bunchgrasses are actively growing in late fall, winter and early spring.

SPECIES DESCRIPTIONS OF DOMINANT GRASSES
Rough fescue is a native, cool-season, perennial bunchgrass that produces thick mats of persistent sheath and stem bases and culms that grow to 3.5 feet, and leaf tufts that grow to 16 inches in height (Cronquist, 1977). It has extensive fibrous roots to a depth of 4 feet, 73 percent of which are concentrated in the top 6 inches of soil (Coupland, 1953). Rough fescue regenerates from seed, tillers, and sometimes creeping rhizomes (Pavlick, 1984). It is well adapted to a short growing season by initiating growth following snowmelt, and completes growth before the onset of summer drought and can have fall regrowth. It is very productive and highly palatable to livestock and wildlife. Rough fescue is used by bighorn sheep, mule deer, elk, and bison (Lesica and Cooper, 1997). Rough fescue is highly palatable forage. It is prime winter forage: plants cure well on the stalk and retain high nutrient levels during dormancy. It is resistant to moderate grazing, but heavy grazing can result in severely decreased root depth and biomass (Aiken, 1990). Grazing can cause a general decline in rough fescue coverage, and it is one of the first species to decline (along with thickspike wheatgrass and Richardson’s needlegrass) with a concomitant increase of common increaser species, such as Idaho fescue, other needlegrass species, prairie Junegrass, prairie junegrass and Parry’s oatgrass (Mueggler, 1980).
Rough fescue and elk sedge are considered very resistant to human trampling due to its tough core, according to D. Cole of the USFS in his study of recreational human trampling effects on habitat types in western Montana. The majority of the loss of cover, a reduction of 50 percent, occurred in the first 400 passes. Thereafter, cover loss was stabilized from 400 to 800 passes. The community of rough fescue-timber oatgrass is considered very resistant to both light and heavy trampling (Cole, 1987).
Rough fescue is well adapted to periodic burning and resistant to light severity fire because of their dense, tufted habit. It sprouts from surviving residual plants and colonizes from off-site wind-dispersed seed. Fire may top-kill plants, but normal cover and production usually is attained in 2 to 3 years post-fire. Severe damage can occur by hot, mid-summer wildfires (Wright, 1982).

Bluebunch wheatgrass is a native, cool-season, perennial grass that is densely tufted and is among the most drought-resistant native bunchgrasses. It is capable of an unusually broad range of osmoregulation, which helps it survive under a range of moisture conditions. It thrives best in the 14 to 17 inch precipitation zone in the Intermountain West, but can be found as low as the 10 inch precipitation zone. It requires excellent drainage and mostly full sun. It is considered one of the most important forage grasses for both livestock and wildlife, although it is not necessarily the most highly preferred species. It is also nutritionally sufficient for some animals for only part of the year. It is moderately grazing tolerant only during its non-growing season and extremely sensitive to defoliation during active growth. It is susceptible to competition from weedy invasives including diffuse and spotted knapweed, crested and desert wheatgrass. Bluebunch wheatgrass survives fires because its buds are protected by soil and/or plant foliage. It can be top-killed, but generally does not usually result in plant mortality. Burning stimulates flowering and seed setting. The timing of burning affects mortality in that more are killed in spring growing season and less in summer dormancy. Recovery post fire usually requires one to three years, with availability of soil moisture as an important factor determining time.

Idaho fescue is a long-lived native perennial cool-season bunchgrass. It is densely tufted with fine leaves. The root system is strong and can extend 16 inches deep (Hanson, 1959). In well drained soils, the root biomass is greatest at depths of 2 to 4 cm. Reproduction is from seeds and tillers, although seed production is variable (Stubbendieck, 1992). Idaho fescue is found in more mesic grasslands and is considered a climax species. It can survive fires of light severity, but usually is harmed by more severe fires (Smith, 1981). Rapid tillering of Idaho fescue occurs where root crowns are not suppressed and soil moisture is favorable. Plants may re-establish from seed after fire if the burn temperatures are low enough to allow for survival of seed in the soil. Idaho fescue can decrease with heavy grazing or severe fire and be succeeded by native and non-native increaser species including bluegrass and needlegrass grass species, sagebrush, lupine, phlox, and the invasive timothy (Phleum pratense) (Eckert, 1987). Idaho fescue is an important forage species for livestock (cattle, sheep, and horses) and wildlife species including elk and mule deer (Mueggler, 1980). It is particularly important in elk diets throughout the Rocky Mountain region.
Hansen et al. (1995) found that Idaho fescue is good forage for cattle, horse, and sheep: it has high energy value and medium protein values in the fall and winter. Sticky geranium is good sheep forage, but only fair for cattle and horses: it has low energy and protein values in fall and winter. Sticky geranium also is considered good food value for elk and whitetail and mule deer, but poor for antelope and for bird species. Old man’s whiskers (Geum triflorum) is considered fair to poor forage for cattle, sheep, and horses. It contains low energy and protein values in fall and winter. It has fair to poor food value for elk, whitetail and mule deer, and antelope, and also for bird species.

Prairie junegrass is a cool season perennial bunchgrass that is loosely-tufted, shallow-rooted and of small stature with long, mostly basal leaves. The leaves are drought resistant and persist under dry conditions. The roots are moderately long, 13 to 30 inches and root density decreases after 12 inches, with the greatest concentration in the upper 1.2 inches. Regeneration is from seed, which ripens late summer to fall and by sprouting from residual plants. It develops rapidly in early spring, flowers in Montana from May to July, and it avoids growth in driest summer months. Prairie junegrass occurs in numerous prairie and grassland habitats, at least in a small percentage. Preferred sites are cool, semi-arid or xeric infertile grasslands and rock outcrops with annual precipitation range from 16 to 21 inches. Livestock and several wildlife species utilize it as it provides good, early spring forage although due to its scattered distribution it is not a significant role in most wildlife species. Prairie junegrass sustains little to moderate damage from fire due to its small clump size and coarsely textured foliage which burn quickly and perennating buds near or below the soil surface which are insulated against fast moving fires. Damage is dependent on fire severity, physiological state of plant, soil moisture and season of burn. Survival strategy is through seed germination and residual plant survival.

Needle and thread grass is a cool-season, native, perennial bunchgrass that is moderately to highly drought resistant and recovers well from drought. It has small and widely spaced bunches that are shallow to medium-rooted and produce numerous fibrous roots. It reproduces by seed, which are long-lived, and tillers. It is common on dry hills and plains and on stony and sandy soils with slightly high pH, low water holding capacity, low clay percentage and high bulk density. In Montana, it grows best in the 10 to 18 inch precipitation zone. It is generally considered early or mid-seral species. It begins growth in spring and becomes dormant during hot weather. It can be important to livestock and wildlife, in Montana cattle, mule deer and pronghorn, especially in early spring. In summer, the fruit has a sharp awn that may injure grazing animals. It is considered severely damaged by fire, depending on the severity of fire. After fire, needle and thread grass sprouts from the caudex, if heat has not been sufficient to kill the underground plant parts. It recovers in 2 to 10 years from fire.

Columbia needlegrass is a native, cool-season, perennial bunchgrass that grows in dense, leafy tufts and is long-lived and drought tolerant, with slow to moderate seedling growth rate and medium herbage volume, with deep and fibrous roots. Columbia needlegrass reproduces by seed and tillers (though this has been disputed). Columbia needlegrass has a sharp pointed callus (a hard projection at the base of a floret, spikelet, or inflorescence segment) which aids in dispersal but causes it to be avoided for forage, which promotes stand replenishment. In Montana, it is good forage for cattle, horses and mule deer. Columbia needlegrass prefers well-drained, fine-textured soils with clay loam to sandy loam surface texture and has low fertility requirements and good heat tolerance, can grow on shallows soils, moderately tolerant of salinity, and can grow on dry, rocky infertile sites. Columbia needlegrass grows on a wide variety of middle and upper elevation sites. Columbia needlegrass begins growth during the early spring is most seriously injured by midsummer fires and less by late spring or fall burns. It is among the least fire resistant bunchgrasses due to its densely tufted stems but because it has relatively few culms per clump, it ranges only slightly to moderately damaged by fire.

Richardson’s needlegrass is a native perennial cool-season bunchgrass with fine stems. It is shallow-rooted and clay accumulation can restrict roots (Lackschewitz, 1991). Richardson’s needlegrass becomes dormant following the depletion of surface soil moisture during the latter part of the growing season (Nimlos, 1968). It reproduces by seed and is wind- and animal-dispersed (Tyser, 1990). Richardson’s needlegrass is considered a climax codominant species, meaning that while it is found in the climax community, it is usually co-dominant with rough fescue (Koterba, 1971). In general, perennial needlegrasses are among the least fire-resistant of the bunchgrasses, especially with midsummer burns: the accumulated dead culm and leaves makes them more susceptible to burning. Perennial needlegrasses often survive low-intensity fires as the heat is not transferred below the soil surface, only top-killing plants (Wright, 1965). Richardson’s needlegrass is an important forage species for livestock and wildlife especially deer, bighorn sheep, and elk.


EFFECTS OF LAND MANAGEMENT PRACTICES ON ECOLOGICAL DYNAMICS AND INVASIVE SPECIES
INVASION THEORY
There are threats to the mesic rough fescue grasslands include habitat fragmentation, habitat degradation from weed invasion, improper livestock grazing, and alteration of fire regime and herbicide drift (Hill and Gray, 2004). Habitat fragmentation is caused by development, roads, agriculture of other human activities that create small patches of the vegetation community. The effects of creating small patches of vegetation are to limit pollinators associated with plant species, limit genetic mobility of species with inbreeding depression and other genetic pressures associated with small population size. Habitat degradation is a decline in habitat that alters the structure, function, and composition of the habitat. Improper livestock grazing can cause changes to plant community through preferential grazing of certain species, changes to soil and hydrology function. A grazing induced change in vegetation community structure away from native bunchgrasses that have high canopy cover and therefore lower bare soil cover can increase soil erosion. Trampling of vegetation by livestock can also reduce plant vigor. Livestock can introduce non-native species into the native community. Variation in fire regimes from historical occurrence can cause vegetation community dynamics to change, particularly when fires occur in different season than was common under the historic frequency. Fire suppression can cause potentially devastating and severe fires due to litter accumulation after longer time between fires. Suppression can also allow for encroachment by woody shrub and tree species. An increase in fire cycles can also be detrimental to bluebunch wheatgrass-Idaho fescue grasslands by reducing the post-fire recovery time and native plants may be vulnerable to alien competition. As well, fires that are out of season to historic fire cycles, can cause higher mortality if occurring during the growing season. Herbicide drift from adjoining agricultural lands to mesic rough fescue grasslands can negatively impact the vegetation community of bunchgrasses and perennial forbs with lower vigor or mortality.
Invasion of weedy species into native vegetation communities requires an understanding of the processes and mechanisms by which an invasion occurs. Weedy species degrade native habitat by altering its structure, composition and function. Weedy species often outcompete, invade and displace native plant communities. Weedy invasions reduce canopy cover of large perennial plants and increase the cover of bare ground. Bare ground can result in increased soil erosion resulting in changes in soil structure and chemical composition and alter microclimate. Weedy species can also change ecological processes of the native community such as productivity, soil water, nutrient dynamics, community successional patterns and disturbance cycles. Resistance and resilience of the native community are essential elements in predicting the success of the invasion. There are two counter point theories on invasive species. The driver theory considers the invasive species to be driving species decline while the passenger model sees the invading species as filling in empty niches left by habitat alteration (Didham, 2005). The passenger model suggests that disturbance is the cause and if stopped, invasion can be reversed. Potential mechanisms of invasion include theories such as novel weapons, enemy release, competitive superiority, and manipulation of environment. “Weedy species can outcompete native species if they have rapid germination and growth, relatively short maturation times, high reproductive and seed output, a large seedbank and efficient dispersal mechanisms that can facilitate their spread”. Novel weapons include biological weapons or associations with micro-organisms that allow the invader species to either access new resources or steal them from indigenous plants (Tannas, 2011). Specifically, arbuscular mycorrhizal fungi may provide a substantial competitive advantage to spotted knapweed by carbon parasitism (Carey, 2004). In these cases, the invader uses these weapons to drive the invasion process. Enemy release describes the concept that once invader species are released from their native predator species or chemical warfare within their original community, they are more aggressive in their new community (Blumenthal 2006, Callaway and Aschelhoug 2000). The invader species may have characteristics that allow it to be more competitive than resident plant species such as grazing resistance, adaption to a harsh environment or another competitive ability (Tannas, 2011). Invading species can manipulate the environment to their advantage through resource competition. Mechanisms include modifying light interception, water uptake efficiency or change in soil water holding capacity, nutrient uptake and cycling (D’Antonio and Vitousek, 1992). The final outcome of invasion is establishment of the invading species which occurs as either dominance, coexistence, or exclusion from the indigenous plant community (Seabloom, 2003). D’Antonio and Vitousek (1992) stated grass invasions are of particularly concern because they are actively moved by humans, exotic grasses compete effectively with native species, they may change nutrient cycling, modify regional microclimates and can alter fire dynamics.

INVASIVE SPECIES DESCRIPTIONS
Specifically, scientific literature on invasions by Kentucky bluegrass, smooth brome, spotted knapweed, leafy spurge and Canada thistle into rough fescue grasslands in Canada and Montana will be reviewed. Species in bold are on the Montana State listed Noxious Weeds List (Montana Department of Agriculture, 2003): spotted knapweed (Centaurea stoebe), leafy spurge (Euphorbia esula), and Canada thistle (Cirsium arvense). Kentucky bluegrass invasion into rough fescue grasslands can take multiple pathways. Heavy grazing of rough fescue which reduces litter amount combined with timing of defoliation, winter versus growing season and abiotic factors like seasonal variation in soil moisture content can make native grasslands less resistant to invasion (Douwes, 2012, Tannas, 2012). Resilience of the native grassland is dependent on vigor and density of rough fescue and restoration establishment is more successful with cuttings and plugs than seeding (Tannas, 2011). Although, seeding rough fescue as a monoculture is effective (Sherritt, 2012). A study of grazing effects on a rough fescue at Stavely grassland, a Canadian research station, found that heavy grazing pressure by cattle resulted changes in plant species composition to an increase in shallow rooted species, less productive overall, but more resistant to grazing (Dormaar, 1990). In a study of seasonal biomass changes, Willms (1996) found that with grazing intensity the vegetation community composition shifted from one dominated by rough fescue to one dominated by parry oatgrass-Kentucky bluegrass in moderately grazed pastures to Kentucky bluegrass-sedge species in heavily grazed pastures. The rough fescue dominated community had the greatest forage value compared to communities resulting from moderate, heavy and severe grazing (Willms, 1996). More than 20 years of drastically reduced stocking rates were required to enable recovery (Willms, 1985). Soils associated with heavy grazing were transformed to a soil more characteristic of a drier microclimate (Johnston, 1962 and 1971), by reducing the thickness of Ah horizon, reducing percent organic matter and soil moisture and increasing soil temperature with grazing intensity. Heavy grazing also reduced the fertility and soil water holding capacity (Dormaar, 1998). Soil organic matter, and nutrient cycling differed between grazed and ungrazed rough fescue grasslands (Willms, 1988). At a watershed scale, heavy grazing lead to larger summer storm and spring snow melt runoff compared to watersheds with less grazing (Chanasyk, 2002). The quantity and quality of surface runoff from these watersheds showed that grazing posed little risk of nutrient contamination of adjacent streams (Mapfumo, 2002). There was less snow accumulation in heavily and moderately grazed watersheds (Willms, 2006). A study on the effects of grazing on germinable seeds found that soil disturbance in fescue grassland is more likely to lead to a seral community dominated by annual broad-leafed plants, than a rough fescue dominated grassland (Willms, 1995). Skim grazing (light, once-over-spring defoliation) by cattle was not conducive to rough fescue conservation (Moisey, 2005). Rough fescue tolerated light winter-early spring elk grazing but not heavy grazing (Thrift, 2013). A rough fescue grassland in Rumsey Block, Alberta Canada tolerated moderate grazing which resulted in a community co-dominated with shortbristle needle and thread while heavy grazing and/or moderate to major oil and gas disturbance crossed a threshold requiring complete eradication of species and reseeding (Desserud, 2014). A study of effects of human caused disturbance in rough fescue grasslands in Manitoba Canada, found it depends on invasive species introduction history (Gifford, 2013). Kentucky bluegrass tolerates grazing and can increase in abundance after heavy grazing. Therefore, Kentucky bluegrass resided in historically grazed areas, while smooth brome occurred along roads. In a study of smooth brome on rough fescue grasslands in Saskatchewan Canada, found that it is likely the combination of traits of smooth brome (higher productivity, abundant production of lower quality litter, clonal growth, and greater nutrient uptake capability) that allows it to invade native prairie (Piper, 2015). Smooth brome had a consistent negative impact on community structure and function across 8 grasslands in Alberta Canada with the impact on native species richness higher in species rich areas, while impact on native biomass was larger in productive, warmer and more variable sites (Stotz, 2016).
The noxious weed spotted knapweed was found to strongly reduce the final biomass and reproduction of native Idaho fescue grasslands. An insect biocontrol agent had little effect on spotted knapweed, while a native fungal pathogen killed it in a common garden experiment in Missoula Montana (Ridenour, 2003). Invasion of grasslands by spotted knapweed are mediated by root exudation of catechin, a potent phytotoxin Perry (2005). Catechin resistance was positively correlated with mean seed mass for eight species identified as resistant: Mountain brome, curlycup gumweed, needle and thread grass, basin wildrye, cicer milkvetch, boreal sweetvetch, common blanketflower, and alfalfa. Perry (2005) further found that residual soil catechin may interfere with reestablishment of native grassland species even after spotted knapweed populations are controlled.
Leafy spurge has an extensive rhizomatous root system, a deep root system up to 30 feet that can access water sources not available to many native plants, potential allelopathic properties and all parts contain high starch latex which seals wounds and is a possible deterrent against insect attacks. Areas with leafy spurge invasion that have been treated with herbicide application and mechanical removal still had higher bare ground area, significantly lower soil arthropod densities and lower plant species richness and cover (Pritekel, 2006). Jordan (2008) found that invasive plants, specifically leafy spurge, smooth brome and crested wheatgrass, are capable of modifying soil microbiota to facilitate further invasion by conspecifics and other invasive species. These soil alterations have the potential to impede restoration of native communities after removal of an invasive species. Successional management may require repeated treatments to achieve a desired outcome. Pokorny (2009) found that while broadleaf herbicide applications decreased hoary cress, Canada thistle and undesired forbs within a leafy spurge invaded site, the results were temporary and seeding was necessary for native species establishment. Leafy spurge has been effectively controlled and managed with biological control agents, particularly black and brown flea beetles. Butler (2006) found that black flea beetles significantly reduced the foliar cover of leafy spurge with black flea beetle release in a study in northwestern Dakota and southeastern Montana. Grass and grass-like plant cover increased but forb cover did not reach the non-infested levels. In another study in southeastern Montana, (Butler, 2010) found that the native vegetation did not recover to the extent assumed possible. While leafy spurge foliar cover was significantly reduced with use of the biological control agent black flea beetle, non-native Poa species became dominant while the native species did not recover completely.
Use of biological control agents on leafy spurge have been successful in Montana although the recovery of the native vegetation community has been mixed. Lesica (2004) found in a study of black fleas controlling leafy spurge that the response of the weed and the native vegetation community depended on abiotic factors and previous herbicide use. In all areas, the black fleas resulted in a decrease of aboveground leafy spurge biomass, the difference between areas were size in reduction of the weed and the proportion of vegetative to flowering stems of leafy spurge. Areas that were more stressful (poor soils) had greater reductions than areas less stressful (good soils). Areas with good soils that also had previous herbicide use, when treated with black fleas, produced more vegetative and less flowering stems in leafy spurge and slowed the recovery of species diversity compared to control areas which had experienced an increase of diversity over the same time period. The opposite effect was found in areas with poor soils and no previous herbicide use, with more flowering stems produced by leafy spurge and an increase in species diversity as compared to control plots.
Butler (2010) found that black flea beetles released in western Montana resulted in very large reductions of leafy spurge, but the native vegetation community did not regain its diversity compared to areas that were non-infested. The infestations of leafy spurge have been reduced significantly but then Kentucky bluegrass replaced leafy spurge in dominance. Functional groups that were able to persist during the leafy spurge invasion continued to be present after it was reduced. Non-infested areas were dominated by native species. This has been theorized by Carson (2008) as the associated species factor in why native vegetation does not recover fully after invasive species have been controlled. The co-occurring non-native species quickly invades that area previously occupied by the initial invasive weed species.
Butler (2006) found that in southwestern Montana, released black and brown flea beetles differed in their ability to quickly establish and reach their maximum population size within two years. The black flea beetles outcompeted the brown flea beetles. The cover of leafy spurge was significantly reduced. Concomitant with the reduction in leafy spurge, grass and grass-like species increased cover while forb species did not reach the non-infested area cover. Livestock tend to avoid high stem density leafy spurge infestations. Therefore, once stem densities were reduced, livestock grazed the area and remaining grass species and caused a slight decline in cover. Forb cover did not achieve non-infested area covers even after leafy spurge cover was reduced. Leafy spurge has a strong filtering effect on the resident vegetation community and forbs are more heavily impacted than graminoid species.
Butler (2004) evaluated vegetation communities in Theodore Roosevelt National Park in southwestern North Dakota. The vegetation communities in which leafy spurge infestation occurred were divided into control and infested plots. Infested plots on the whole had 61% less species diversity than control plots. Species were rated as either sensitive or persistent to infestations. Thirty species that were common in the control plots of the various vegetation communities were absent in the infested plots. Forbs were far more sensitive than graminoid species to leafy spurge infestations leading to the conclusion that leafy spurge has a strong filtering effect on resident native vegetation communities. Overall, species richness on infested plots were significantly lower than on control plots throughout the eleven vegetation communities sampled.
Carson (2008) described factors that could cause a biological control agent released onto infestations to fail to recover the native vegetation community as indirect or direct effects of the invasive plant: native source limitations, novel weapons, static competitive hierarchy, trophic shifts, invasive engineering and associated invasive species. The first three are direct effects, the next two are indirect effects of the invasive species and the last can be attributed to either. Native source limitations refer to the inability of the native species to reproduce effectively to outcompete the invasive species due to lower relative abundance of seedbank or individuals to disperse seed. As well, numerous invasive species produce copious amounts of seed, effectively store seeds for long periods and form monotypic stands which are dominate the soil beneath it with its own seed. Biocontrol agents would need to target the invasive species’ most vulnerable life form to effectively reduce numbers i.e. like reducing seed set. Novel weapons are generally exudates from the invasive species that facilitates its invasion and persistence to the detriment of the native vegetation and its soil microbial community. Biological control agents that sufficiently negate these allelopathic chemicals can have success. Static competitive hierarchy refers to an invasive species that is a superior competitor for resources compared to native species. The biological control agent would need to reduce the invasive species ability to dominate resource acquisition and allow native species to become superior in order for it to be successful recovery. In the indirect effects factor of trophic shifts, the invasive plant changes the trophic levels (relationships) within an ecosystem to the disadvantage of the native community. Trophic levels include predators, parasitoids, mutualists, pathogens and herbivores. The relationship between soil microbiota and plants is host specific, therefore a monotypic stand of an invasive species can change the soil microbiota even after it has been reduced in aboveground biomass. This change in microbiota can make it difficult for native species to re-establish in the area. As well, a dominant invasive species may either become preferential food for native pollinators thereby lowering their use of native species or an invasive species may change the pollinator species composition. Lastly, invasive species can change herbivore behavior. In invasive engineering theory, the invasive species changes the abiotic environment to such a degree that the native species cannot dominate even after reduction of the invasive species. This can occur in wetlands in which the invasive species has change the hydrology. Therefore, a manipulation of the hydrology must occur as well as a reduction or elimination of the invasive species for the native community to recover. The last scenario of biological control failure occurs when a co-occurring invasive species to the dominant invasive, takes over an area after the dominant species is removed or reduced. The native vegetation cannot recover in the face of the secondary invader.
Steinger (1992) studied the effects of two biological control insects, a moth and a weevil, on spotted knapweed. As well, they tested if differences in nitrogen levels and graminoid competition affected the outcome of herbivory. Spotted knapweed responded to the root herbivory by compensatory root growth and therefore lower shoot growth. This was especially prevalent in low nitrogen levels with herbivory by the weevil in which shoot growth was reduced 60%. The weevil caused changes in the shoot to root allocation of spotted knapweed in response to its herbivory, shoots decreased but not roots, as well as more nitrogen concentration in roots than in shoots. Compensatory allocation to roots with herbivory was observed. Competition with grass resulted in lower shoot and root growth and smaller leaves. This competition was more detrimental to plant growth than herbivory or nutrient supply. Lower nitrogen affected spotted knapweed’s ability to compensate for root herbivory with additional root growth. There was greater reduction in spotted knapweed with herbivory and reduced nitrogen availability. Root herbivory greatly affected the physiology of spotted knapweed with greater allocation of nitrogen and energy to the roots and less to shoots.
Sulphur cinquefoil (Potentilla recta) has high ecological amplitude, it is found in numerous forested and non-forested vegetation communities in Montana. Sulphur cinquefoil flourishes in Montana’s semiarid climate in areas similar to those inhabited by spotted knapweed. Sulphur cinquefoil is native to southeastern Europe and southwestern Asia. It is a perennial forb with a short caudex attached to a woody taproot. It is non-rhizomatous and does not form monospecific stands but it can be dense. Sulphur cinquefoil has no known mycorrhizal associations. It reproduces by seed and vegetatively by sprouting from a caudex. Cross fertilization (cross pollinated by wind or insects) is the most common means of fertilization but some seeds are produced by self-pollination. Sulphur cinquefoil is most common on disturbed areas but can invade relatively undisturbed sites. Generally more abundant on drier sites with less total grass cover. Sulphur cinquefoil is also intolerant of complete shade. Survival of plant parts depends on depth of burial and fire severity as perennating buds in the caudex can survive fire if not exposed to lethal temperatures. Sulphur cinquefoil likely resprouts following fire and establishes from on-site or off-site seed. Fall or spring fire did not have a long term impact on sulphur cinquefoil at Dancing Prairie in northwestern Montana. It did not result in mortality of large plants, but did reduce the density of small plants immediately after burning for one year and enhanced germination however the seedling survival depended on sufficient moisture. The Dancing Prairie is dominated by C3 plants and dormant season and late fall burns are more likely to harm nonnative, invasive plant populations without damaging native species, while late-spring and early-fall burns are more detrimental to native species.