Values and Management of Montana’s Green Ash Draws
Green ash woodlands provide shelter for livestock and important habitat for a wide range of wildlife even though they occupy only a small proportion of the eastern Montana landscape. Conservation of green ash woodlands rests on understanding the history and biology of these important habitats.Last Updated: 12/11
by Peter Lesica and Clayton Marlow
GREEN ASH WOODLANDS (HARDWOOD DRAWS,
ash draws) are important habitats on the Northern Great Plains. Although they occupy only one to four percent of the landscape, green ash woodlands are habitat for many wildlife species. In addition, these habitats provide economic benefits to livestock growers and rural communities. Understanding the history and ecology of these relatively uncommon habitats can provide information useful for successful rangeland management and enhancement of livestock and wildlife habitats.
The Ecological Setting
Woodland vegetation is uncommon amidst the grasslands of the northwestern Great Plains including eastern Montana. In this continental, semi-arid climate, native deciduous woodlands dominated by green ash, box elder and chokecherry are most abundant where topography is broken, such as drainage divides or along incised stream beds. The unique hydrologic conditions created by cool slopes and short duration flooding within these landforms make possible the establishment and persistence of trees and tall shrubs in this arid environment. Equally important to tree establishment and survival is snow accumulation on lee slopes and in ravines resulting in higher early-spring moisture conditions than adjacent uplands. Even though soils of green ash woodlands in the western Dakotas, and presumably eastern Montana, are loamy to somewhat fine-textured, moderately deep and well-drained, there is little correlation between soil properties and understory vegetation.
Early explorers and settlers reported the occurrence of green ash woodlands in eastern Montana and the adjacent Dakotas. More recently green ash woodlands have been recognized as distinct habitats within an otherwise grass-dominated landscape and formally described for western North Dakota, northwest South Dakota and adjacent eastern Montana. Canopy cover of green ash woodlands averaged 45 percent in east-central Montana and 42 percent in eastern Montana. American elm occurred in some stands and was occasionally codominant in the eastern tier of Montana counties. Boxelder occurs in some stands but never in any abundance. The understory vegetation often is comprised of up to eight shrub species, five or more grass species and numerous forbs. Woodlands with more closed canopies have extensive leaf litter on the ground with more native forbs and relatively less cover of grass-like plants.
Basic Biology of Common Tree Species
Green ash: Eastern Montana appears to be the most western (arid) margin of green ash distribution. Seedlings grow equally well in sun or shade under non-saline conditions; however, recruitment from seed is reported to be curtailed by competition with grass. Green ash readily sprouts from the root crown, allowing it to rejuvenate if mature trunks are lost; these sprouts are capable of regenerating a canopy- sized tree in about 20 years. Extended drought may have an adverse effect on regeneration and probably enhances other problems such as crown die-back.
American elm: American elm occurs in many green ash woodlands in the eastern tier of Montana counties and may occasionally be codominant. Elm made up 32 percent of the canopy in these woodlands in eastern Montana and 29 percent in southwestern North Dakota. Seedlings have low salinity tolerance and grow best in more open stands. This species is capable of sprouting from the base following trunk loss to fire or woodcutting.
Boxelder: Occuring sporadically in green ash woodlands in eastern Montana, boxelder is intolerant of shade and may be out-competed by more shade-tolerant American elm and green ash. Boxelder is capable of sprouting from the root crown if the bole is destroyed by fire.
Role of Wildfire and Grazing
The role of fire in destroying or maintaining green ash woodlands on the northwestern Great Plains is unknown. Even though it has been hypothesized that woodlands were less common in presettlement times partly as a result of the higher fire frequency, the ability of dominant species (e.g., ash, elm, chokecherry) to sprout prolifically suggests that these woodland communities were adapted to fire. In southeast Montana burned-over green ash woodlands had three times as many crown sprouts and stems were twice as large as those in adjacent unburned stands. Previously burned stands in western North Dakota had a higher density of understory shrubs than unburned stands.
Prolific stump sprouting has led some ecologists to speculate that bison had a strong impact on Great Plains woodlands. However, bison in Theodore Roosevelt National Park in western North Dakota did not preferentially use woodlands, did little browsing, and their impacts occurred mainly at the grassland-woodland margin. This suggests that negative impacts may result from animals seeking shelter rather than forage.
Values of Green Ash Woodlands
Green ash woodlands provide excellent wildlife habitat because of the multi-level canopy structure, high edge-to- area ratio and prevalence of succulent foliage, fruits, and buds. Additionally, several species of herbaceous plants commonly found in coniferous or deciduous forests of more humid regions survive in green ash woodlands of the Great Plains.
Big Game: Research indicates that green ash woodlands provide important escape cover, travel corridors, late summer and winter forage, and fawning grounds for white-tailed and mule deer. Although these woodlands comprised approximately one percent of available habitat in McCone County, five percent of all mule and white-tailed deer observations occurred in them.
Game birds: During the fall and winter months green ash woodlands provide critical habitat for sharp-tailed grouse, particularly when snow makes grain fields inaccessible, and nearly ten percent of sharp-tailed grouse observations were made in these communities. Wild turkeys commonly occur in green ash woodlands.
Non-game mammals: Green ash woodlands provide important habitat for coyotes, weasel, red fox, and bobcat. Meadow voles were more common in ash woodlands, while deer mice and thirteen-lined ground squirrels preferred grasslands and shrublands.
Non-game birds: Green ash woodlands provide critical habitat for many species of birds that would otherwise not occur in the semi-arid Great Plains. For example, the highest total biomass and species diversity of breeding birds in McCone County, Montana was found in green ash draws. Seven raptor species rely on green ash woodlands for nesting cover in the western Dakotas. Additionally, ten song bird species that commonly occur in green ash woodlands are currently listed by the Montana Natural Heritage Program as Species of Concern because of declining populations and loss of habitat.
While assessment of the economic value of green ash woodlands is limited, it is clear that this habitat supports sufficient mule deer, whitetail deer, grouse, and turkey hunting opportunities to generate approximately $9 million dollars annually for eastern Montana communities. When adjusted for inflation (www.bls.gov/data/inflation_calculator.htm) the limited economic measures suggest that green ash woodlands account for approximately $19.5 million dollars of hunting and trapping revenue each year in Montana.
Historically green ash woodlands have been valued for abundant, late season forages and water but many producers have come to value green ash as critical cover for range livestock during adverse weather conditions. High condition woodlands provide a thermal blanket that protects livestock from the full force of cold winds and overhead cover for calves and cows during late spring snow storms. Even small increases in survival rates during blizzards and spring snow storms would repay producers for the time and effort to improve the condition of green ash woodlands.
Many green ash woodlands in eastern Montana and the adjacent Dakotas are relatively open with few young trees and understories dominated by snowberry, grassland forbs and introduced, rhizomatous grasses. It is believed that these more open stands are degraded examples of rich, “good- condition” woodlands characterized by a relatively dense tree canopy, ash trees of all ages, and understories dominated by chokecherry, wild plum, hawthorn, serviceberry, Sprengel’s sedge and shade-loving forbs. Importantly, stand-age distributions indicate that recent tree recruitment has been greatly reduced compared to the first half of the 20th Century. There are several potential causes for the decline of green ash woodlands in Montana.
Woodcutting: While harvesting may have been substantial from 1900 to1918, homestead abandonment from 1920 to 1936 released the pressure on green ash stands. Consequently, peak recruitment of green ash stems in eastern Montana occurred in the 15-year period of 1926 to1940.
Grazing: The decline of green ash woodlands has most often been attributed to overgrazing by livestock. Woodlands provide shade, succulent forage and sometimes water. Consequently, grazing animals tend to congregate in these communities during hot, dry summer and early fall months. Since ash woodlands occupy only a small portion of most pastures, they will usually be overgrazed even at moderate stocking rates. It is possible that deer browsing may also reduce tree regeneration in ash woodlands. Substantial white-tailed deer use of ash woodlands during winter months may have led to the heavily browsed stump sprouts in two of three livestock exclosures in southeast Montana. This level of use can probably prevent them from growing into mature trees. In many areas of the upper Midwest and the Appalachian Mountains dramatic increases in white-tailed deer populations have curtailed preferred tree regeneration and altered the structure, composition and productivity of surviving stands.
Introduced species: Several species of invasive rhizomatous sod grasses have been introduced into the Great Plains with European settlement and have increased in rangelands under the influence of heavy livestock grazing. These include smooth brome, quackgrass and Kentucky bluegrass. A decline in tree seedling density was associated with a change from the native Sprengel’s sedge to introduced rhizomatous grass and this change was associated with heavier livestock grazing of adjacent grasslands and presumably the woodlands themselves. Treating grass sod with herbicide in degraded hardwood draws resulted in increased green ash and chokecherry seedling recruitment and survival. Other experiments conducted in declining woodlands showed that densities of ash and elm seedlings were higher on grazed plots compared to those protected from grazing, but seedling survival was low in the grazed plots either because seedlings were grazed or because grass vigor recovered too quickly.
Climate: An increase of more drought-tolerant, grazing- adapted species and a decline in tree seedling recruitment might be expected with a decrease in precipitation even in the absence of grazing. Therefore, it is likely that recruitment of green ash from seed will become a rare occurrence in many stands if the future climate of the northwestern Great Plains becomes warmer and drier.
Hardwood draws should be maintained in good condition because restoration of eastern Montana woodlands will be difficult. While livestock grazing has been implicated as the primary cause of woodland decline, current woodland condition may be more reflective of the 1880 to 1930 grazing levels. In order to maintain woodlands in good condition, grazing systems need to be tailored to the local landscape and operator skills. Hence, one or a combination of these management strategies may fit your needs.
- Multiple-pasture, rotational grazing systems and winter- only pastures had more green ash seedlings and saplings than summer pastures; the difference is most apparent in stands furthest from water developments.
- Most studies indicate that summer-long grazing is detrimental, probably because cattle spend much of their time in the shade, trampling and overgrazing the understory. However, summer grazing can be compatible with good condition green ash woodlands if it is followed by at least two years of rest. Good-condition woodlands have been maintained in Wibaux County by grazing season-long then implementing two years of rest from grazing.
- Fall and winter use may also maintain good-condition ash woodlands when cattle are fed hay within 1/8 to ¼ mile from the stands.
- Unacceptably high levels of livestock browsing can occur in late summer and fall. This can be avoided by moving cattle to a new pasture when browse utilization approaches 50 percent of annual twig growth.
Discontinuing livestock grazing by itself is not likely to result in enhanced tree recruitment because exotic grasses curtail seedling survival, and browsing by large deer populations will prevent growth of seedlings into full sized trees. For example, unbrowsed green ash sprouts grew 16 inches per year; those exposed to heavy browsing grew only five inches per year, and sprout height was strongly correlated with the number of times they had been browsed.
Fire has been suggested for regenerating Great Plains woodlands, but there is little evidence to support this idea. Even though wildfire increased sprouting at seven sites in eastern Montana, uncontrolled, introduced sod grasses depressed green ash seedling regeneration.
Restoration of green ash from seed in eastern Montana woodlands will be difficult, requiring an unlikely combination of biological and environmental conditions or large expenditures of time and money. One study found that applying herbicide (glyphosate) to the grass sod greatly increased green ash seedling recruitment and survival, but seedlings grew poorly and probably did not persist as herbaceous plants began to reoccupy treated areas after only a few years.
Effective recruitment of green ash seedlings suggested by the frequency of green ash seedling-, sapling- and pole- size classes is positively associated with the canopy cover of chokecherry in many woodlands in the northwestern Great Plains. Recruitment of shade-tolerant green ash from seed may benefit from a tall shrub overstory that would reduce the vigor of sod grasses through shading and create more safe sites for tree seedlings.
Herbicide-treated plots seeded to chokecherry produced more than six times as many seedlings compared to seeding into undisturbed sod. Most seedlings grew at an average of three cm/year, but five percent of them grew at more than 10 cm/year, suggesting that it may be possible to establish stands of chokecherry by using herbicide to lower perennial grass cover followed by supplemental seeding. Herbicide treatment did induce a flush of annual (not noxious) weeds that eventually declined to pretreatment levels. Chokecherry establishment would then be followed by an “under- seeding” of green ash. This method of hardwood draw restoration is plausible but should be tested first before it is widely applied. Livestock would have to be controlled to minimize browsing.
Coppicing, or the downing of large diseased trees, will not produce more trees but it can increase tree canopy cover with new and more vigorous boles and branches. In southeast Montana, green ash trees cut to the base sprouted vigorously and grew at a rate of 16 inches per year even if the old trunks had been diseased. However, successful coppicing will require careful control of livestock to minimize browsing and may not be successful in areas with high densities of whitetail deer.
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