Green Ash Woodlands
A Review
Peter Lesica
Affiliate Faculty, Division of Biological Sciences, University of Montana
Clayton Marlow
Research Bulletin No. 4601
Professor in Animal and Range Sciences, Montana State University
2
Introduction
Green ash woodlands or draws are widespread across eastern Montana and the Northern Great Plains. The majority occur on
private land. This review examines what is known about the physical environment, composition, values and management of these
important habitats and their component trees.
Historic Conditions
The Physical Environment
Woodland vegetation is uncommon amidst the grasslands of the northwestern Great Plains and eastern Montana in particular.
Native deciduous woodlands dominated by green ash (see Appendix for scientific names of plants), boxelder and chokecherry
occur on cool slopes or along ephemeral streams where flooding is more sporadic or of short duration. These woodlands are
often referred to as hardwood draws, woody draws, wooded draws or ash draws (Photo 1, 2, 3).
PHOTO 1. Green ash woodland south of Baker in Fallon County.
PHOTO 2. Green ash woodland south of Glasgow in McCone County. Fort Peck Reservoir is
in the background.
PHOTO 3. Green ash woodland along Burns Creek southwest of Sidney in Dawson County.
Green Ash Woodlands: A Review
Peter Lesica, Affiliate Faculty, Division of Biological Sciences, University of Montana
Clayton Marlow, Professor in Animal and Range Sciences , Montana State University
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3
Climate of the region is semi-arid and continental with
very cold winters and hot summers. Three climate recording
stations representative of the part of eastern Montana with
the most extensive green ash woodlands are at Baker, Sidney
and Terry. Mean January minimum and July maximum varied
from 0.8 to 6.8°F and 85.1 to 90.5°F respectively. Mean annual
precipitation varied between 11.6 and 14 inches with 77–80%
falling during the growing season (April-September)62. Green
ash woodlands are most abundant where topography is
broken, such as drainage divides or along incised stream beds.
Snow accumulates in sites sheltered from the wind, such as on
lee slopes and in ravines, resulting in improved early-spring
moisture conditions compared to the adjacent terrain
(Photo 4). Presumably it is these hydrologic conditions that
allow the recruitment and persistence of trees and tall shrubs
in this arid environment2.
Soils of green ash
woodlands in the
western Dakotas are
loamy to somewhat
fine-textured,
moderately deep and
well-drained with
near-neutral pH.
They are moderately
Photo 4. Snow accumulation in a green ash draw
fertile with 4–20%
following a spring storm in Prairie County.
(R. Reukauf photograph)
organic matter and
moderately high
levels of all nutrients
except nitrogen and phosphorus. The ability of the soil to
provide plants with nutrients, i.e. cation exchange capacity,
is high, but amounts of exchangeable sodium are low32,97.
Nonetheless, there is little correlation between soil properties
and understory vegetation in hardwood draws97.
More recently green ash woodlands have been formally
described for western North Dakota24,32, northwest South
Dakota and adjacent southeast Montana31 and eastern
Montana33. These woodlands have been classified into either
one or two community types. Hansen and colleagues consider
all ash woodlands to belong to the Green ash/chokecherry
habitat type31,32,33. Stands of this woodland vegetation with
a substantial presence of American elm occur in eastern
Montana counties as well as North Dakota and South Dakota.
Understory vegetation of these stands appears to be more
moisture-loving than stands without elm24,44, and they have
been classified as the American elm phase of the Green ash/
chokecherry habitat type24. Green ash woodlands along
floodplains of rivers and their major tributaries are similar to
the drier margins of upland ash woodlands. This vegetation has
been described as the Green ash/snowberry habitat type24,32, but
it is strictly riparian and will not be considered here.
Green ash woodlands are dominated by green ash in the
canopy (Photo 5). Basal area (sum of cross-sectional area of all
trees) of ash stands ranged from 24 to 195 square-feet/acre with
means of 90, 116 and 65 square-feet/acre for southeast Montana
and adjacent South Dakota31, southwest North Dakota32, and
east-central Montana45 respectively. Canopy cover of ash
averaged 40–45% in eastern Montana44,45. In eastern Montana,
American elm occurred in some stands and was occasionally
co-dominant with a mean canopy cover of 32% in stands in
which it occurred44,45. Boxelder also occurred in some stands
but never in any abundance.
Vegetation
Early explorers and settlers reported the
occurrence of green ash woodlands in eastern
Montana and the adjacent Dakotas71. Granville
Stuart85, an early cattle rancher, described the
country along Rosebud Creek and the Tongue
River in 1880 as having “plenty of big scrubby
ash trees along the dry creeks and bluffs” and
“small groves of ash and boxelder in ravines and
along little creeks.” Stephen Visher96 described
the vegetation and wildlife of ash woodlands
of northwest South Dakota. He reported that
bluegrass, wildrye, brome and thistle were
common in open stands, but closed-canopy
stands were dominated by more shade-loving
species. Such reports and early photographic
evidence indicated that ash woodlands were
common prior to European settlement71.
PHOTO 5. Green ash trees dominate in a small draw in the Slim Buttes of Harding County, South Dakota.
4
Understory vegetation of green ash woodlands is rich in shrub
and herbaceous species24, 31,33,44. Chokecherry and snowberry,
or buckbrush, are dominant shrubs, while American plum,
pin cherry, poison ivy, serviceberry, hawthorn and wild rose
are common in many stands
(Photo 6, 7, 8).
PHOTO 6. Understory of good-condition green ash
draw in Dawson County. The dominant shrub is
chokecherry.
PHOTO 7. Understory of good-condition
green ash draw in Harding County, South
Dakota. Chokecherry is the common shrub,
the ground layer is dominated by Sprengel’s
sedge.
The native Sprengel’s sedge
and introduced Kentucky
bluegrass, smooth brome,
PHOTO 8. Chokecherry flowers
and leaves.
Japanese brome and
quackgrass are common
grass-like plants. Common
native broad-leaved plants include strawberry, Canada violet,
northern bedstraw, black snakeroot, small-flowered buttercup,
cleavers, starry Solomon’s plume and meadowrue (Photo 9).
Common introduced forbs are dandelion and burdock.
Woodlands with more closed canopies have extensive leaf
litter on the ground with more native forbs and relatively less
cover of grass-like plants24.
PHOTO 9. Ground layer of a good-condition green ash draw in Dawson
County. Sprengels sedge, Oregon grape and sweet cicely are common.
Fire
The role of fire in destroying or maintaining green ash
woodlands on the northwestern Great Plains is unknown. Fire
frequency is thought to have declined following European
settlement88, and it has been hypothesized that woodlands
were less common in pre-settlement times partly as a result
of higher fire frequency43. Green ash woodlands burned by
wildfire had fewer tree seedlings and more dead trees than
adjacent unburned stands46.
However, there are several reasons to think that fire plays only
a minor role in green ash woodland dynamics. Woodlands
maintain a higher humidity than adjacent grasslands, and
fuels are expected to be less combustible. Many woodlands
occupy deep ravines or steep slopes with cool aspects that act
as natural fire breaks. Furthermore, the important tree and
shrub species (e.g., ash, elm, chokecherry) of ash woodlands
produce sprouts from the root crown when the main trunk is
damaged25,45,74,94,105.
The ability of dominant
species to sprout
prolifically (Photo 10)
suggests that these
communities are
adapted to fire and able
to recover quickly. Green
ash trees in southeast
Montana woodlands
had three times as many
crown sprouts and were
twice as large as those
in adjacent unburned
stands46, and in western
PHOTO 10. Green ash trees sprouted after being top- North Dakota the
killed by wildfire in Prairie County.
density of understory
shrubs was higher in ash
woodlands exposed to wildfire compared to unburned stands105.
However, very intense fire can destroy trees and convert
woodlands to shrublands, at least temporarily 46 (Photo 11).
PHOTO 11. Dead green ash and boxelder trees following wildfire in Custer County.
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Grazing
Bison were common in eastern Montana prior to European
settlement. Some have speculated that bison had a strong
impact on Great Plains woodlands74. However, evidence
from modern herds suggests that bison did not severely
affect woodlands in most cases. 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 margin40,63. The observation
that bison avoid timbered habitats is corroborated by early
reports of explorers in southwest Montana48. Past effects of
deer browsing on green ash woodlands are not known.
Life History of Common Woody Species
Green ash
Green ash is the dominant and often only tree of deciduous
woodlands in eastern Montana. It is a small tree with male
and female flowers on separate plants. In Montana green
ash can become up to 65 feet tall and 20 inches in diameter.
It is common along streams and in floodplains and other
bottomland habitats from Nova Scotia to Alberta, and
southward from Texas to Florida20,28. Although natural stands
are usually confined to bottomlands along streams, green
ash will grow well on moist upland soils, and it is commonly
planted in Great Plains shelterbelts20. Flowers are produced as
the leaves expand, commonly in late May in eastern Montana.
However, female flowers and young fruits are very sensitive
to late spring frosts104. Fruits are winged and mature in late
summer but may remain on the tree through the winter
(Photo 12).
Cold stratification is required for germination of seed14,83.
Green ash is reported to have a short-lived seed bank18, but
harvested seeds may be stored at room temperature for at least
one year without loss of viability20.
Seedlings grow equally well in sun or shade9 (Photo 13),
but are intolerant of saline soil conditions65. Seedlings can
survive for 1–2 years in competition with dense herbaceous
cover, but growth is greatly curtailed20. Grasses deplete soil
moisture, lowering seedling survival2. Recruitment of green
ash from seed is reported to be curtailed by competition
with grass in eastern Montana woodlands44,45,47. Droughtstressed seedlings become dormant earlier than those that are
well-watered75.
Green ash readily sprouts from the root crown, allowing
it to rejuvenate if mature trunks are lost20,25,31,74,105 (Photo 14).
More than 90% of green ash trees had basal sprouts after
being cut in an experimental study in western North Dakota94.
On average, 33% of green ash trees in east-central Montana
woodlands had basal sprouts, and 30% of the ash trees had
more than one trunk, suggesting a minimum of one-third of
the trees in these stands arose as basal sprouts instead of from
seed45 (Photo 15). Crown sprouts are capable of regenerating
a canopy-size tree in approximately 20 years47,93. Root systems
are horizontally extensive but shallow, extending only 3­–5 feet
deep into the soil at maturity20.
PHOTO 14. Stump sprouts from a cut green
ash tree.
PHOTO 15. Multiple-stem green ash tree
that probably arose from stump sprouts.
PHOTO 12. Green ash with maturing seeds.
PHOTO 13. A first-year green ash
seedling. The lowest “leaves” are
actually the cotyledons (seed coat).
6
Mean lifetime girth growth rate (radius/age) for green
ash in east-central Montana ranged from 0.02 inch/year to
0.17 inch/year with a mean of 0.07 inch/year45. Growth rate
between 1980 and 2000 averaged 0.12 inches/year across the
entire Northern Great Plains region100. A maximum age of
approximately 100 years has been reported for green ash in
the region10,18,23,31,32,42,45,76. These estimates are for above-ground
trunks and do not take recruitment from stump sprouts into
account. Life expectancy for individual root systems is not
known. Green ash trees from more arid parts of the species’
range in western North Dakota are more resistant to drought
stress than those from eastern Nebraska and Kansas59.
Crown dieback in green ash has been variously attributed
to disease, drought and freeze injury50,100 (Photo 16).
Green ash is very susceptible to white stringy heartrot
fungus (Perenniporia fraxinophila =Fomes fraxinophilus) in the
northwestern portion of its range69,104. Heartrot is common in
native and planted stands of green ash on the Great Plains67,68,99,
and is common in eastern Montana’s ash woodlands. This
native fungus enters through wounds or broken branches and
causes decay of the wood of the trunk and major branches.
Sporocarps (fruiting bodies or conks, Photo 18) develop in
areas where decay is extensive35,69. Heartrot rarely kills its
host tree, but infected
trunks and branches
are weakened and more
susceptible to breakage by
wind or ice (Photo 19).
PHOTO 18. Sporocarps (conks) of white
stringy heartrot on green ash.
PHOTO 16. Crown dieback of green ash trees along Cherry Creek in
Prairie County.
Green ash is on the western and most arid margin of its
range in eastern Montana and is likely at the limit of its
environmental tolerances. Hence, extended periods of drought
may have an adverse effect on regeneration and probably
enhance other problems74 (see below). The great drought of
the 1930s caused dieback of many green ash and boxelder
trees in shelterbelt plantings and native stands throughout the
Northern Great Plains2. Ranchers in eastern Montana report
that a severe late spring freeze caused extensive dieback of
many native ash trees in the mid-1980s49 (Photo 17). However,
crown dieback increased with tree age but was not related to
mean annual
precipitation
among
stands in
east-central
Montana49.
PHOTO 17. Leaves and flowers of green ash killed by a late spring
freeze. New leaves will sprout, but some dieback may occur as a
result of the freezing.
In east-central Montana,
an average of 38% of ash
trees displayed heartrot with
a mean of five sporocarps
per tree50. Older, larger
PHOTO 19. Trunk of green ash weakened by
trees are more likely to
heartrot and broken by wind. Note the white
fungal mycelium in the center of the trunk.
demonstrate symptoms of
heartrot, and growth rate
declined with increasing
numbers of sporocarps50. Dead branches are common in the
crowns of green ash trees throughout the Northern Great
Plains. Green ash with heartrot sporocarps had nearly twice
as much crown dieback as those without50. Heartrot is more
common in arid climates, suggesting that it may contribute
to the decline of ash woodlands where drought stress is
common50.
Ash yellows is a native disease of ash trees caused by a
bacteria (mycoplasma) that infects the tree’s vascular system.
In eastern North America, ash yellows is reported to cause
reduced growth and crown dieback in mature trees and
witches’ brooms in sprouts80, but these symptoms have not
been confirmed for green ash on the Great Plains100. Seedlings
and saplings are most severely affected, suffering root dieback
and suppressed shoot growth80. Young trees with ash yellows
may fail to develop to maturity98. Ash yellows is difficult to
detect in mature trees without laboratory analysis, but surveys
throughout the Northern Great Plains indicate the mycoplasma
is found in most native stands100. There are few reports from
7
Montana, but incidence is high in native woodlands of adjacent
North Dakota100. Ash yellows was weakly associated with
higher levels of dieback; however, it is not clear how much or in
what way the disease contributes to this dieback100.
Little is known about the insect pests and their long-term
effects on green ash in Great Plains woodlands. Ash bark
beetles (Hylesinus californicus) were present in a small number
of stands in east-central Montana, and were thought to be at
least partly responsible for killing
a number of trees in these stands49
(Photo 20). The adults and larvae
create galleries in the sapwood
underneath the bark, essentially
girdling the trunk15. Larvae of
the black-headed sawfly (Tethida
cordigera) skeletonize ash leaves by
consuming the mesophyll tissue
between the epidermal layers.
Evidence of black-headed sawfly
was observed in four stands in
east-central Montana with 18% to
86% of the sampled twigs affected49.
Evidence of forest tent caterpillars
(Malacosoma disstria) was common
in ash woodlands of east-central
Montana in 2000. Typical injury
PHOTO 20. Dead green ash
tree with bark removed to show
was one to several leaflets with large
horizontal galleries excavated by
holes. Occasionally leaflets were
ash bark beetles.
completely consumed49.
The emerald ash borer (Agrilus planipennis), a metallic
wood-boring beetle native to China, was introduced into
Michigan around 2000 and has since spread to at least 13 states
and two Canadian provinces89. The emerald ash borer attacks
green ash and is capable of killing tress in one to three years. It
has potential to cause economic and ecological damage to ash
on a scale similar to the impacts of invasive pests on American
chestnut and American elm13. It is not known whether emerald
ash borer will spread into Montana. It has been documented to
occur as far west as southeastern Minnesota and southeastern
Missouri89. The native range in China as well as the current
invaded range in North America encompasses regions having
a frost-free period of at least 150 days and receiving annual
precipitation of 20–40 inches102. It may not be able to tolerate
the harsher climate of the semi-arid west. Furthermore,
experimental evidence suggests that emerald ash borers cannot
tolerate winter climates where warming events (45°–50°F) of
several days are followed by cold spells (10°–15°F)81. Emerald
ash borers can be transported long
distances in firewood collected from
infested forests. Beetles thought to be
emerald ash borers (Photo 21) should
be collected and sent to a county
PHOTO 21. Emerald ash borer
(J. Hahn photograph).
Extension agent for identification.
American elm
American elm occurs in many green ash woodlands in eastern
Montana counties and may occasionally be co-dominant with
green ash (Photo 22). Mean canopy cover of American elm
was 32% in stands in which it occurred in eastern Montana44,49
and 29% in southwestern North Dakota24. American elm is a
large deciduous tree reaching 100 feet in height and 70 inches
in diameter in Montana. It is shallow-rooted and common on
floodplains from Newfoundland to Saskatchewan and south
to Florida through Texas28,66. Flowers are produced before
the leaves (late May in Montana), and winged seeds are shed
in June. American elm is capable of sprouting from the base
if the trunk is lost to fire or woodcutting25,31. American elm
has hard, strong wood and was commonly planted as an
ornamental. Maximum age is thought to be about 200 years18.
PHOTO 22. Leaves and fruits of American elm.
American elm displays significant dieback in green ash
woodlands49. Trees often have large-diameter branches with
numerous small twigs but few intermediate branches. The
cause of the dieback is not known but is likely related to
drought or late spring freezes. Dutch elm disease (Ceratocystis
ulmi) occurs in native and planted elms throughout the Great
Plains82. Although Dutch elm disease is reported in street trees
in many towns in eastern Montana79, there are no reports from
native stands49.
Boxelder
Boxelder occurs
sporadically but is never
dominant in green ash
woodlands in eastern
Montana (Photo 23). It is a
small deciduous tree with
separate sexes growing to 65
feet in height and 40 inches
in diameter. It is intolerant
of shade. In northeastern
North America it is fastgrowing but lives for
PHOTO 23. Leaves and fruit of boxelder.
8
only about 60 years18. Boxelder is common on floodplains
and stream banks and in other moist, disturbed sites from
New England to eastern Alberta and south to California,
Arizona, Texas, Mexico and Florida66. Flowers are produced
with expanding leaves; in late May or early June in Montana.
Winged fruits mature in late summer. Boxelder is capable
of sprouting from the root crown if the trunk is destroyed.
Wood is soft and branches are weak. Boxelder is susceptible to
heartrot. It is fed on by boxelder bugs (Leptochoris spp.), but
rarely suffers permanent damage.
Values of green ash woodlands
Wildlife
Game mammals
Green ash woodlands are important elements of mule
deer summer and winter range and critical winter habitat
for white-tailed deer55,73,86. Swenson86 found that they are
excellent wildlife habitat because of the multi-level canopy
structure, high edge-to-area ratio and prevalence of succulent
foliage, fruits, and buds. His 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. Thompson87 reported that
although they comprised approximately one percent of
available habitat in McCone County, five percent of all mule
and white-tailed deer observations occurred in green ash
woodlands. Aerial surveys indicated that mule deer and whitetailed deer use green ash woodlands more than any other
habitat in southeast Montana53.
observed in green ash woodlands in northwest South Dakota,
22 of which were observed breeding36. The highest total
biomass and species diversity of breeding birds in McCone
County, Montana was found in green ash draws87. Although
northwest North Dakota woodlands had lower species
diversity than bottomland forests, green ash woodlands
supported a much higher density of breeding pairs than any
other habitat38,39. Breeding bird diversity of ash woodlands
was as high as riparian forest and much higher than any other
vegetation type in northeast Montana86.
Several species of raptors used green ash woodlands
for nesting in the western Dakotas. These include Greathorned Owl, Long-eared Owl, Swainson’s Hawk, Red-tailed
Hawk, Ferruginous Hawk, Sharp-shinned Hawk, and
American Kestrel21,36.
At least 10 bird species of conservation concern
commonly occur in green ash woodlands64. Sharp-shinned
Hawk, Swanson’s Hawk and Eastern Screech Owl require
the tree canopy for nesting as do the Yellow-billed Cuckoo,
Downy Woodpecker and Red-eyed Vireo. Black-billed
Cuckoo, Gray Catbird, and Warbling Vireo require tall shrubs,
and the Ovenbird requires dense ground cover and leaf litter.
Plants
Several species of herbaceous plants commonly found in
coniferous or deciduous forests of more humid regions of
North America survive in the benign conditions of green ash
woodlands of the Great Plains (Photo 24, 25, 26).
Game birds
During the fall and winter months green ash woodlands
provide critical habitat for Sharp-tailed Grouse, particularly
when snow makes grain fields inaccessible86, and nearly ten
percent of Sharp-tailed Grouse observations were made in
these communities. Wild Turkeys commonly occur in green
ash woodlands16,38,36,71.
PHOTO 25. Canada violets (Viola
canadensis)
PHOTO 26. Carrion flower
(Smilax herbacea)
Non-game mammals
Green ash woodlands provide important habitat for coyotes,
weasel, red fox, and bobcat78,86. Meadow voles were more
common in ash woodlands, while deer mice and thirteenlined ground squirrels preferred grasslands and shrublands91.
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, including many songbirds typical of eastern deciduous
forests71. Forty-seven species of breeding birds were recorded
in western North Dakota woodlands; of these, 22 species were
neotropical migrants16,17. Eighty-two species of birds were
PHOTO 24. Sprengel’s sedge (Carex sprengelii).
9
These include agrimony, bristle-leaved sedge, Sprengel’s sedge,
striped coralroot, fairybells, frog orchis, fringed loosestrife,
pin cherry, small buttercup, and carrion flower44 (see
Appendix for scientific names). Pregnant sedge, although not
globally rare, is a species of conservation concern60 and known
from only a handful of woodland sites in eastern Montana.
Economic values
There has been only one analysis of the economic value of
green ash woodlands7. Woodlands provide critical habitat
that supports deer populations (see above), and there was
approximately $9 million dollars spent on deer hunting
associated with Montana green ash woodlands in 1982. The
same study estimated that $264,000 was derived from turkey
hunting that same year. Fur harvest associated with green ash
woodlands in Montana in the 1975–1976 season was valued at
$492,000, although trapping in eastern Montana has declined
by 36% the past 30 years22. After adjusting for inflation90, this
study suggests green ash woodlands account for approximately
$19.5 million dollars of hunting and trapping revenue each
year in Montana. It has been suggested that firewood could be
harvested on a sustained yield basis by cutting only declining
ash trees on a 20-year rotation7. The value of firewood that
could be produced each year from eastern Montana green ash
woodlands is not known. Systematic and sustained firewood
cutting would also likely reduce the value of woodlands for
some cavity nesting bird species and other organisms that
depend on coarse woody material. Green ash woodlands
provide undetermined economic benefits to farmers and
ranchers by providing habitat for livestock to escape heat during
summer and shelter from wind during winter. Wild fruit such as
chokecherries and plums harvested from green ash woodlands
are used by local residents to make jams and preserves.
Most green ash trees in woodlands of eastern Montana
and the adjacent Dakotas are approaching the putative
maximum age of 100 years. Mean age of green ash in native
stands in western North Dakota was reported to be greater
than 50 years in 1984 and 60–70 years in 198410,32. Mean age
of ash trees in eastern Montana was reported to be 70–80
years in 198831 and 60 years in 200145. Greater ages reported
by Hansen and collaborators31,32 is likely the result of
subjectively choosing stands presumed to be “undisturbed”
and “mature.” Stand-age distributions indicate that recent
recruitment has been greatly reduced compared to the first
half of the 20th Century45.
PHOTO 27. Declining green ash woodland in the Chalk Buttes, Carter County. The ground
layer is dominated by smooth brome.
Current condition
Description of declining woodlands
Evidence from studies throughout the Northern Great Plains
between 1978 and the present suggests that the majority of
green ash woodlands have declined8,10,25,31,37,44,45,54,72,97. 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
rhizomatous, usually exotic grasses (Photo 27, 28). In some
cases, what were once woodlands have become meadows8
(Photo 29).
It is believed that these more open stands are degraded
examples of rich, “good-condition” woodlands with 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
(Photo 6, 7). Most ash woodlands are intermediate in
composition between these two extremes.
PHOTO 28. Declining green ash draw in Cave Hills, Harding County, South Dakota.
Buckbrush and smooth brome are dominant species.
PHOTO 29. Green ash draw in the final stages of decline, Richland County.
10
Stand condition is important to wildlife, and several
researchers have found that wildlife diversity is reduced in
declining (open, grass-dominated) stands compared to goodcondition (closed-canopy/shrub-dominated understory)
woodlands. Bird density and diversity was higher in goodcondition stands in northwestern South Dakota; Rufous-sided
Towhee, Black-capped Chickadee, Field Sparrow, American
Goldfinch, Dark-eyed Junco, small flycatchers, Orangecrowned Warbler, Wilson’s Warbler and Swainson’s Thrush
were all more common in closed-canopy woodlands37,78.
Deer mice, white-footed mice and woodrats occurred more
commonly in closed-canopy stands, while no mammalian
species was more common in the open stands37. Of the 81
species of birds observed in ash woodlands, 65 species
require woodland habitat. Closed-canopy stands had
greater numbers of tree-nesting and shrub-nesting birds but
fewer ground-nesting species. Mourning Doves, American
Goldfinches, Bell’s Vireos, Yellow Warblers, Rufous-sided
Towhees and Brown-headed Cowbirds were more abundant in
stands with a more closed canopy and denser, tall shrub layer.
No bird species were significantly more abundant in the more
open stands72.
Causes of woodland decline
There are several potential causes for the decline of green
ash woodlands in Montana25. None are mutually exclusive,
and more than one may be acting together synergistically.
Moreover, different factors may have more or less importance
in different parts of the state and in different environments.
Woodcutting
A large influx of homesteaders arrived in eastern Montana
during the years of 1900–191856. Wood was needed
for building houses, barns and fences. Pine was used
preferentially, but where pine was scarce, ash could be used
for fence posts and some building construction. Homesteads
occurred wherever there was land level enough to plow, and
in areas without pine it is likely that ash was cut extensively96.
Starting around 1920 a decline in farm prices and a series
of severe droughts led to a rapid reduction in the rural
population56 and a concomitant lessening of woodcutting.
Presently woodcutting is minimal49. Peak recruitment of green
ash stems in eastern Montana occurred in the 15-year period
of 1926–194045. This surge in ash stem recruitment was likely
the result of sprouting after trees were cut and then the land
abandoned in response to drought45. Presently woodcutting is
probably sustainable in most areas.
Grazing
The decline of green ash woodlands has most often been
attributed to overgrazing by livestock10,31,44,97. Livestock grazing
in eastern Montana began in earnest in the 1870s and 1880s56.
Cattle and sheep were common up until approximately the
1930s when the number of sheep began to decline, and the
number of cattle to increase. There are few sheep operations
in eastern Montana at the present time. Woodlands provide
shade, succulent forage, relief from biting insects and
sometimes water. Consequently, grazing animals tend to
congregate in these habitats during the 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 rates74.
Overgrazing has a number of purported adverse effects on
the structure of woodlands. Heavy browsing of tree seedlings
and saplings leads eventually to opening of the canopy10,11,94.
Rubbing and trampling also reduce the cover of tree
species10,11. Browsing by livestock can stimulate production of
more basal sprouting by green ash trees; however, ash saplings
subject to grazing grow more slowly, are shorter, and have
less chance of survival than those protected from grazing93,94.
It is thought that constant herbage removal associated with
overgrazing allows soil temperatures to rise and increases
evaporation from surface horizons which, in turn, impedes
tree seedling recruitment74. Another effect of overgrazing is
soil compaction which reduces infiltration rates and causes
increased runoff and erosion74. Erosion of the drainage results
in gully formation and a concomitant drop in the water table86.
Decreased infiltration and a lower water table would likely
result in decreased establishment of deciduous trees.
It is possible that deer browsing may also reduce tree
regeneration in ash woodlands. In many parts of eastern
Montana the development of land for agriculture has resulted
in an increase in white-tailed deer populations compared to
what was present prior to European settlement. Prior to 1941,
white-tailed deer in eastern Montana primarily inhabited
the Yellowstone and Missouri river bottoms; however, in
recent times, populations have grown and extended their
range into the uplands61. Equally important is that Montana
deer populations, like those nationwide, were low during
the homesteading and dust bowl eras52, which coincides
with the apparent
peak recruitment of
green ash during the
period 1926 to 1940.
White-tailed deer now
use ash woodlands
heavily during winter
months86. Green ash
stump sprouts were
heavily browsed by
deer in two of three
livestock exclosures in
southeast Montana47
(Photo 30), likely
preventing them from PHOTO 30. Green ash sapling browsed
repeatedly by deer.
growing into mature
11
trees. Similar effects could be expected with ash saplings
arising from seed. Studies in deciduous or mixed forests in
central and eastern North America also suggest that overbrowsing by white-tailed deer can impact forest canopy
composition. Deciduous forests in southeastern Nebraska
with high white-tailed deer densities had fewer ash saplings
compared to forests with low deer densities29. Deer browsing
is a significant threat to green ash growth and survival in
Georgia19. In many areas of the upper Midwest and the
Appalachian Mountains, white-tailed deer populations have
increased dramatically since the introduction of agriculture
and timber harvest, curtailing regeneration of preferred
tree species, altering structure, composition and productivity
of stands3,4,5,51,57,84.
Exotic species
Several species of invasive, rhizomatous sod grasses were
introduced into the Great Plains with European settlement
and have increased in rangelands under the influence of heavy
livestock grazing70. Reduced canopy cover of tall shrubs was
associated with heavier livestock grazing in North Dakota
woodlands10,31. Herbivory of the tall shrub layer and young
trees would be expected to result in more sunlight reaching
the ground layer and an increase of light-loving, grazingadapted species, especially crested wheatgrass and invasive
exotic sod grasses such as smooth brome, Kentucky bluegrass
and quackgrass (Photo 27). A decline in tree seedling density
was associated with a change from a ground layer of the native
Sprengel’s sedge to exotic rhizomatous grass45, and this change
was associated with heavier livestock grazing of adjacent
grasslands and presumably the woodlands themselves44. Green
ash seedlings were common in good-condition woodlands
in Theodore Roosevelt National Park where livestock and
elk grazing was minimal32. Treating exotic-grass sod with
herbicide in degraded hardwood draws resulted in increased
green ash and chokecherry seedling recruitment and
survival47. These studies indicate that tree recruitment from
seed is curtailed as the tall shrub layer declines and the ground
layer becomes dominated by a sod of exotic grasses. Tree
recruitment is reported to be reduced by competition with
grass in many systems because grasses deplete soil moisture
thereby lowering seedling survival2,27,95.
In spite of broad negative effects, the disturbance
caused by occasional heavy grazing can benefit tree seedling
recruitment under some circumstances. Experiments
conducted in declining woodlands showed that densities of
ash and elm seedlings were higher on grazed plots compared
to those protected from grazing47,94, although seedling
survival was low in the grazed plots either because seedlings
were grazed or because grass vigor recovered too quickly.
Apparently loss of shrub layer and increased sod grasses cause
a decline in tree recruitment. However, once a woodland
is in an advanced state of decline and the ground layer is
dominated by grass sod, seedling survival may be enhanced
by herbicide or grazing that breaks up the sod and weakens its
competitive advantage47,94.
Climate
Drought and grazing often have similar effects on rangeland101.
An increase of more drought-tolerant, grazing-adapted
species and a decline in tree seedling recruitment might
also be expected with a decrease in precipitation even in
the absence of grazing. More open stands are associated
with drier sites or regions24,37,45. It is likely that the future
climate of the northwestern Great Plains in particular will be
characterized by decreases in precipitation and increases in
temperature and the frequency of extreme climatic events30,34.
Such changes could make recruitment of green ash from seed
a rare occurrence in many stands at the arid edge of the tree’s
geographic range.
Management
Conservation
Hardwood draws in good condition should be maintained as
such because restoration of eastern Montana woodlands will
be difficult due to the presence of non-native sod-forming
grasses and recurring drought. Even though recent livestock
grazing and wildlife use have been implicated as the primary
causes of woodland decline, the current condition of green ash
draws may be more a reflection of past (1880–1930) grazing
pressure. Regardless, it is essential to manage livestock in
ways that are compatible with good-condition woodlands.
Few studies have focused on determining grazing strategies
most compatible with maintaining woodlands in good
condition. There was a greater density of ash saplings and
higher canopy cover of tall shrubs in a stand protected from
livestock grazing for 40 years compared to grazed stands in
western North Dakota10. Similarly there was a higher density
of green ash seedlings and saplings in stands that were in a
multiple-pasture, rotational grazing system, and higher tree
recruitment in winter pastures compared to summer pastures,
and in stands farthest from water developments41. Green ash
and chokecherry grew 2–5 times faster when protected from
livestock grazing in western North Dakota93. Good-condition
woodlands have been maintained in Wibaux County,
Montana by a system of season-long grazing followed by two
years of rest6. However, these woodlands on the YellowstoneLittle Missouri Divide receive mean annual precipitation
of 16 inches compared to 12–14 inches for most of eastern
Montana. Girard et al.25 also suggest electric fencing, winter
grazing and moving water developments for managing woody
draws, but admit that their suggestions are largely untested
and based on other rangeland systems.
The few available studies indicate that season-long (more
than 30 days) summer grazing is detrimental, probably
12
because cattle spend much of their time in the shade and
overgraze the understory. It appears that summer grazing can
be compatible with maintaining good condition woodlands
if livestock are removed as soon as browsing becomes
noticeable (25–30% of shrubs and seedling exhibiting use)
and the grazing period is followed with at least two years
of rest. Under light stocking cattle will sometimes not
go into the draws92. The grazing reduces grass cover and
creates disturbed sites that allow tree seedlings to establish;
subsequent protection from grazing promotes survival.
While complete protection from grazing using permanent or
temporary fencing is also likely to conserve good condition
stands, regular cooperation with state wildlife managers
will be necessary to gain further protection from deer
browsing. Some fall and winter use may also be compatible
with maintaining good-condition ash woodlands as long as
livestock are not fed within one-eighth to one-quarter mile of
wooded draws.
Restoration
Overgrazing by livestock may have been the primary
agent initiating woodland decline; however, discontinuing
livestock grazing alone is not likely to result in enhanced tree
recruitment because sod of exotic grasses curtails seedling
survival, and browsing by large deer populations will prevent
growth of tree seedlings. For example, excluding livestock
access to degraded woody draws has not resulted in new
tree recruitment at Cherry Creek on the Reukauf Ranch in
Prairie County.
Fire has been suggested as a process that could facilitate
regeneration of woodlands on the Great Plains1,25,44, but there is
limited evidence to support this idea. Fire creates open sites in
grass sod for tree seedling germination and growth in Pacific
Slope woodlands58, and low-severity, controlled spring burns
increased the frequency of green ash sprouting in western
South Dakota77. Wildfire caused increased sprouting at seven
sites in eastern Montana; however, cover of introduced sod
grasses was not reduced, green ash seedlings were fewer, and
many mature trees were killed in burned stands46 (Photo 11).
Restoration of green ash recruitment from seed in
eastern Montana woodlands will be difficult at best, requiring
either a coincidence of increasingly unlikely biological and
environmental conditions or large expenditures of time and
money. Applying herbicide (glyphosate) to the grass sod
greatly increased green ash seedling recruitment and survival
compared to untreated sod, probably by reducing competition
for water and nutrients in a study in southeast Montana.
Unfortunately, seedlings grew very poorly, and would probably
not persist in competition with herbaceous plants that began to
reoccupy treated areas after only a few years47 (Photo 31).
The reason for poor tree seedling growth is unknown,
but other conditions required for successful regeneration of
green ash from seed may occur at long intervals or may no
longer occur at all. The northwestern Great Plains semi-arid
environment is marginal for tree growth2, and green ash is at
the western, most arid margin of its range in eastern Montana.
Green ash is primarily a tree of humid to sub-humid climates,
occurring mainly in bottom lands19,20, so it is reasonable to
assume that hydrology is an important limiting factor for the
growth of green ash in eastern Montana. In the first decade
of the 21st century, winter (December-February) precipitation
was approximately 25% lower than the 20th century average
in southeast Montana. Perhaps more importantly, the winters
averaged more than 3°F warmer than in the last century62.
These conditions have probably reduced snow accumulations,
early spring flows and the deep water penetration into the
soil compared to the past. Hydrologic conditions conducive
to recruitment and growth of green ash seedlings in
eastern Montana may have been sporadic, even prior to
the introduction of Eurasian sod grasses into the woodland
understory25. These conditions may be even less common now
in a warmer, drier climate30.
Recruitment of green ash seedlings might be possible by
first establishing a chokecherry understory to act as “nurse
plants.” The frequency of seedling-, sapling- and pole-size
green ash was positively associated with the canopy cover of
chokecherry in many woodlands in the northwestern Great
Plains25,31,44,45,72, and this association suggests that recruitment
of green ash from seed may be enhanced by a tall shrub
understory. Reduced vigor of sod grasses associated with
shading by a healthy shrub layer would likely mean more
safe sites for tree seedlings2,95. Green ash seedlings are shadetolerant9, so interference from the chokecherry canopy would
be minimal.
Herbicide-treated plots seeded to chokecherry produced
more than six times as many seedlings compared to seeding
into undisturbed sod47. Most seedlings grew at an average of
one inch/year, but 5% of them grew at more than four inches/
year, suggesting that
it may be possible
to establish stands
of chokecherry by
using herbicide to
lower perennial grass
cover followed by
supplemental seeding.
Once the chokecherry
thicket is established
and sod grass cover
reduced, green ash
could establish.
Establishment of
tree seedlings in
PHOTO 31. A three-year old green ash seedling
semi-arid climates is
growing in an area of sod previously treated with
commonly facilitated
herbicide.
13
by shrubs that provide a more humid environment with less
herbivory12,26,103. This method of hardwood draw restoration is
plausible but should be tested first before it is widely applied.
Livestock and deer access would have to be controlled to
minimize browsing.
Coppicing, or pruning to ground level, cannot produce
more trees but it can increase tree canopy cover by replacing
diseased or weakened trees with new and more vigorous
trunks 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 diseased47
(Photo 32).
Similar results
were obtained in
western North
Dakota93,94,
suggesting that
average mature
tree height (22
feet) could be
obtained in less
than 20 years.
Ash trees are
generally capable
of sprouting
vigorously, but
sprouts regularly
browsed by deer
grow very little. While unbrowsed
sprouts grew 16
inches per year,
PHOTO 32. Three-year old stump sprouts from a green
those exposed to
ash are five feet tall.
heavy browsing
grew only 5 inches
per year and sprout height was strongly correlated with the
number of times they had been browsed47. Stump sprouting
may have been the dominant form of reproduction for green
ash in the northwestern Great Plains even in the absence of
livestock grazing, woodcutting or exotic grasses.
The last major recruitment event for green ash across
eastern Montana occurred 70–80 years ago as a result of
stump sprouting during a time when deer populations were
low45. Successful coppicing would require controlling livestock
to minimize browsing and may not be possible in areas with
high densities of white-tailed deer.
Acknowledgements
Helen Atthowe, Adam Carr, Dan Brunkhorst, and Carla
Lawrence helped in the field, and Louise DeMontingy, Dale
Tribby, Kristi DuBois and Ann Fisher provided logistical
support. Rob and Judy Reukauf, Monte Herzog, Fred Hoff,
Jerry and Kathy Sikorski, Ray Bannister and many others
allowed me to conduct research on their land. Daniel Uresk,
Jane Mangold, Rob Reukauf and Jon Siddoway provided
helpful comments on the manuscript. James Knight was
instrumental in bringing this project to completion. I am
grateful to Sara Adlington and Millie Veltkamp for careful
editing. Funding for P. Lesica’s research was funded by the
Bureau of Land Management, Conservation Biology Research,
The Nature Conservancy, Montana Department of Fish
Wildlife and Parks, and the Grazing Lands Conservation
Initiative.
Appendix: Species List
14
Common Name
Scientific Name
Family
Cystopteris fragilis
Polypodiaceae
Agrimonia striata
Heuchera richardsonii
Vicia americana
Artemisia biennis
Rosaceae
Saxifragaceae
Fabaceae
Asteraceae
Polygonum
convolvulus*
Medicago lupulina*
Lactuca pulchella
Sisyrinchium
montanum
Cirsium vulgare*
Arctium minus*
Sanguisorba annua
Solidago canadensis
Polygonaceae
Cirsium arvense*
Viola canadensis
Smilax herbacea
Nepeta cataria
Galium aparine
Plantago major*
Heracleum
sphondylium
Rumex crispus
Grindelia squarrosa
Asteraceae
Violaceae
Liliaceae
Lamiaceae
Rubiaceae
Plantaginaceae
Apiaceae
Hesperis matronalis*
Taraxacum officinale*
Zigadenus venenosus
Apocynum
androsaemifolium
Silene douglasii
Elissia nyctelea
Disporum
trachycarpum
Camelina microcarpa*
Thlaspi arvense*
Cerastium arvense
Astragalus agrestis
Mentha arvensis
Descurainia sophia*
Cirsium flodmanii
Brassicaceae
Asteraceae
Liliaceae
Apocynaceae
* indicates exotic species
FERNS
Fragile fern
FORBS
Agrimony
Alum root
American vetch
Biennial
wormwood
Bindweed
Black medic
Blue lettuce
Blue-eyed grass
Bull thistle
Burdock
Burnet
Canada
goldenrod
Canada thistle
Canada violet
Carrion flower
Catnip
Cleavers
Common plantain
Cow parsnip
Curly dock
Curlycup
gumweed
Dame’s rocket
Dandelion
Death camas
Dogbane
Douglas silene
Ellisia
Fairybells
False flax
Fanweed
Field chickweed
Field milkvetch
Field mint
Fixweed
Flodman’s thistle
Fabaceae
Asteraceae
Iridaceae
Asteraceae
Asteraceae
Rosaceae
Asteraceae
Polygonaceae
Asteraceae
Caryophyllaceae
Hydrophyllaceae
Liliaceae
Brassicaceae
Brassicaceae
Caryophyllaceae
Fabaceae
Lamiaceae
Brassicaceae
Asteraceae
Fringed
loosestrife
Frog orchis
Fumitory
Harebell
Hook-spur violet
Horse mint
Hyssop
Lamb’s quarters
Lysimachia ciliata
Primulaceae
Habenaria viridis
Fumaria vaillantii*
Campanula rotundifolia
Viola adunca
Monarda fistulosa
Agastache foeniculum
Chenopodium album*
Lance-leaved
bluebells
Large-leaved
avens
Leafy spurge
Macoun’s
buttercup
Meadow rue
Mugwort
Northern
bedstraw
Northern bog
violet
Pellitory
Poison hemlock
Prickly wild
lettuce
Pussytoes
Sanicle
Seaside
buttercup
Side-flowering
sandwort
Small buttercup
Smooth blue
aster
Smooth daisy
Starry Solomon’s
plume
Stickseed
Stickseed
Stiff goldenrod
Stinging nettle
Striped corallroot
Mertensia lanceolata
Orchidaceae
Fumariaceae
Campanulaceae
Violaceae
Lamiaceae
Lamiaceae
Chenopodiaceae
Boraginaceae
Geum macrophyllum
Rosaceae
Euphorbia esula*
Ranunculus macounii
Euphorbiaceae
Ranunculaceae
Thalictrum dasycarpum Ranunculaceae
Artemisia ludoviciana
Asteraceae
Galium boreale
Rubiaceae
Viola pratinicola
Violaceae
Parietaria pensylvanica
Conium maculatum*
Lactuca serriola*
Urticaceae
Apiaceae
Asteraceae
Antennaria neglecta
Asteraceae
Sanicula marilandica
Apiaceae
Ranunculus cymbalaria Ranunculaceae
Arenaria lateriflora
Caryophyllaceae
Ranunculus abortivus
Aster laevis
Ranunculaceae
Asteraceae
Erigeron glabellus
Smilacina stellata
Asteraceae
Liliaceae
Hackelia deflexa
Lappula redowskii
Solidago rigida
Urtica dioica
Corallorhiza striata
Boraginaceae
Boraginaceae
Asteraceae
Urticaceae
Orchidaceae
Appendix: Species List
FORBS (continued)
Summer cypress
Kochia scoparia*
Sweet cicely
Sweet cicely
Tall goldenrod
Thimbleweed
Tumble mustard
Osmorhiza chilensis
Osmorhiza longistylis
Solidago gigantea
Anemone cylindrica
Sisymbrium
altissimum*
Lychnis alba*
Trifolium repens*
Glycerrhiza lepidota
Fragaria virginiana
White campion
White clover
Wild licorice
Wild strawberry
GRAMINOIDS
Woodland
strawberry
Yarrow
Yellow avens
Yellow prairie
violet
Yellow
sweetclover
Bristle-leaved
sedge
Canada
bluegrass
Cheatgrass
Clustered field
sedge
Common
spikerush
Crested
wheatgrass
Golden sedge
Green
needlegrass
Indian ricegrass
Inland bluegrass
Japanese brome
Kentucky
bluegrass
Liddon’s sedge
Little ricegrass
Little-seed
ricegrass
Low northern
sedge
Meadow barley
15
SHRUBS
Chenopodiaceae
Apiaceae
Apiaceae
Asteraceae
Ranunculaceae
Brassicaceae
Caryophyllaceae
Fabaceae
Fabaceae
Rosaceae
(grass-like plants)
Fragaria vesca
Rosaceae
Achillea millefolium
Geum allepicum
Viola nuttallii
Asteraceae
Rosaceae
Violaceae
Melilotus officinalis*
Fabaceae
Carex eburnea
Cyperaceae
Poa compressa*
Poaceae
Bromus tectorum*
Carex praegracils
Poaceae
Cyperaceae
Eleocharis palustris
Cyperaceae
Agropyron cristatum*
Poaceae
Carex aurea
Stipa viridula
Cyperaceae
Poaceae
Oryzopsis hymenoides
Poa interior
Bromus japonicus*
Poa pratense*
Poaceae
Poaceae
Poaceae
Poaceae
Carex petasata
Oryzopsis micrantha
Oryzopsis micrantha
Cyperaceae
Poaceae
Poaceae
Carex concinna
Cyperaceae
Hordeum
bracyantherum
Poaceae
Pregnant sedge
Quackgrass
Slender
wheatgrass
Smooth brome
Sprengel’s sedge
Timothy
Virginia wildrye
Black wild currant
Bristly
gooseberry
Buckbrush
Buffaloberry
Chokecherry
Common juniper
Golden currant
Oregon grape
Peach-leaf willow
Pin cherry
Poison ivy
Red osier
dogwood
Red raspberry
Serviceberry
Silver sagebrush
Carex gravida
Agropyron repens*
Agropyron caninum
Cyperaceae
Poaceae
Poaceae
Bromus inermis*
Carex sprengelii
Phleum pratense*
Elymus virginicus
Ribes americanum
Ribes setosum
Poaceae
Cyperaceae
Poaceae
Poaceae
Grossulariaceae
Grossulariaceae
Symphoricarpos
occidentalis
Shepherdia argentea
Prunus virginiana
Juniperus communis
Ribes aureum
Berberis repens
Salix amygdaloides
Prunus pensylvanica
Toxicodendron rydbergii
Cornus stolonifera
Caprifoliaceae
Rubus idaeus
Amelanchier alnifolia
Artemisia cana
Rosaceae
Rosaceae
Asteraceae
Rhus aromatica
Anacardiaceae
Crataegus macrantha
Rosaceae
Parthenocissus vitacea
Clematis ligusticifolia
Prunus americana
Elaeagnus commutata
Rosa woodsii
Ulmus americana
Acer negundo
Fraxinus pennsylvanica
Populus deltoides
Vitaceae
Ranunculaceae
Rosaceae
Elaeagnaceae
Rosaceae
Ulmaceae
Aceraceae
Oleaceae
Salicaceae
Populus tremuloides
Betula occidentalis
Juniperus scopulorum
Salicaceae
Betulaceae
Cupressaceae
Elaeagnaceae
Rosaceae
Cupressaceae
Grossulariaceae
Berberidaceae
Salicaceae
Rosaceae
Anacardiaceae
Cornaceae
TREES
Skunkbush
sumac
Succulent
hawthorn
Virginia creeper
Virgin’s bower
Wild plum
Wolf willow
Wood’s rose
American elm
Boxelder
Green ash
Plains
cottonwood
Quaking aspen
River birch
Rocky Mountain
juniper
Russian olive
Elaeagnus angustifolia* Elaeagnaceae
16
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