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ORGANIC MATTER FUNCTION IN
THE WESTERN-MONTANE FOREST
SOIL SYSTEM
Deborah Page-Durnroese
Alan Harvey
Martin Jurgensen
Russell Graham
was blanketed with a highly variable, sometimes extensive
ash-fall up to 60 em deep. Lesser eruptions from Mount
St. Helens and Glacier Peak left thin deposits of ash.
Ash cap soils support large tracts of forested land in the
Inland Northwest and have relatively high productivity
and water-holding capacity compared to non-andic soils
(Geist and others 1989). Below the ash cap in northern
Washington, Idaho, and Montana is glacial outwash material characterized by a high percentage of rock fragments
and low moisture and nutrient-holding capacity. Andic
soils, once the organic mantle has been removed, are very
susceptible to damage, particularly erosion, during harvest
and site preparation.
The Ii tter layer, also designated as Oi, consists of freshly
fallen needles, twigs, and other debris that have undergone
only slight decomposition. The fermentation layer (duff or
Oe), is the organic material beneath the litter layer. Decomposition in this layer is very active, and the duffis usually
permeated with fungal mycelia and root mats. Although
this horizon is undergoing decomposition, plant parts are
still distinguishable. Humus (Oa), is unrecognizable, dark
brown or black, amorphous organic material that has undergone complete decomposition.
Typical horizon development includes some form of an
organic horizon (0) underlain by an A and a Bs horizon
(Fosberg and others 1979). Relatively undisturbed surface
organic horizons typically consist of approximately 2-10 em
of litter, 0-5 cm of duff or humus (fig. 1) (collectively termed
the forest floor), and varying amounts of decayed soil wood
(the brown, crumbly mass left from decaying wood) (Harvey
and others 1987). Surface organic horizon depth is highly
variable, depending on climate, moisture, and topography.
Northern Idaho forests, which are cool and moist, generally
have substantial organic deposits, except in the driest habitat types. Central Idaho and northwestern Montana have
similar organic horizon depths. However, soils influenced
by lodgepole pine (Pinus contorta Dougl. ex Loud.) on wann,
dry sites in central Idaho have almost nonexistent organic
horizons and reflect a lower productivity potential (Steele
and others 1981).
Woody residue is a valuable component of montane-west
ecosystems and has an important role in carbon cycling,
nutrient storage, stream dynamics, erosion control, and
animal activity (Harmon and others 1986; Jurgensen and
others 1990; Maser and Trappe 1984). When the woody
residue becomes incorporated into the forest floor, it is then
termed soil wood. Organic horizons in combination with
woody residues and soil wood com prise most forest soil
ABSTRACT
Soil organic horizons are critical components of forest soil
productivity. Understanding their unique roles in moisture
retention and nutrient cycling before and after timber harvesting is key to managing postharvest productivity for
future stands. Diverse habitat types, variable volcanic
ash deposition, and temperature/moisture extremes make
organic horizons especially crucial to productivity and
management of western-montane forests.
INTRODUCTION
Productivity of western-montane forest soils is tightly
bound to the organic matter component. Additions, alterations, and reductions of forest litter, humus, and wood residues influence both biotic and abiotic properties of any given
site (Harvey and others 1987). Organic matter is especially
important for soil water retention, cation exchange, nutrient
cycling, and erosion control. Recent trends toward whole
tree harvesting, increased woody debris removal (including
slash burning and yarding unmerchantable material), and
shorter rotations (McColl and Powers 1984) have increased
awareness of effects of forest operations on soil processes
and overall site productivity (Jurgensen and others 1990).
FOREST SOILS
The western-montane area encompasses the area
from the eastern slope of the Cascade mountain range
in Washington and Oregon south to the Sierra crest in
California. It extends east to the Continental Divide in
Montana and Wyoming.
Most forest soils in the montane-west are Inceptisols
(some Andosols, under the new taxonomy), developed
from volcanic ac;h deposits (see Meurisse and others and
Hironaka and others, these proceedings). The major ashfall affecting the area is from Mount Mazama (now known
as Crater L~ke, OR) ash, which was deposited during an
eruption 6,700 years B.P. The western-montane region
Paper presented at the Symposium on Management and Productivity
ofWestem-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Deborah Page-Dumroese, Alan Hanrey, and Russell Graham are Soil
Scientist, Project Leader, and Research Forester, respectively, Intermountain Research Station, Forest Service, U.S. Department of Agriculture,
Moscow, ID. Martin Jurgensen is Professor, Forest Soils, Michigan Technological University, Houghton, MI.
95
Habitat type series
Western redcedar
Grand fir
Douglas-fir (moist)
Western hemlock
Ponderosa pine
Douglas-fir (dry)
Subalpine fir
Lodge pole pine
Subalpine fir
Grand fir
Douglas-fir
Ponderosa pine
Lodgepole pine
Northern Idaho
Central Idaho
=
Subalpine fir
Lodgepole pine
Grand fir
Douglas-fir
Ponderosa pine
Northwestern Montana
o
2
I
I
I
I
I
4
6
8
10
12
I
I
14
16
Average forest floor depth (em)
Figure 1-Average depth of the forest floor
in selected habitat types in northern (Cooper
and others 1987) and central Idaho (Steele
and others 1981), and northwestern Montana
(Pfister and others 1977) forests.
Table 1-Volume of organic horizons and mineral soil in old-growth
stands in the montane-west (from Jurgensen and others
organic matter (table 1), and in many cases the woody
residue component may equal or surpass that of other
soil components.
Total soil organic matter contents generally mirror site
productivity. The most productive stands in our region
have the deepest organic matter deposits and are usually
in the cedarlhemlock (Thuja plicata Donn ex D. Don and
Tsuga heterophylla [Raf.] Sarg.) types. The least productive stands, with the shallowest organic matter deposits,
are ponderosa pine (Pinus ponderosa Dougl. ex Laws.)
stands. The exception to this rule is subalpine fir (Abies
lasiocarpa [Hook.] Nutt.) types; in these stands low temperatures limit organic matter turnover rates, leading
to deep organic matter deposits, but limited tree growth.
1990)
Residue
Location
Forest Soil
floor wood
Mineral
Yield
soiP capability
-----------Mglha ----------- m 3/ha/yr
Montana
Cedarlhemlock
Subalpine fir
Douglas-fir
Ponderosa pine
84
146
45
<20
50
36
26
7
51
36
26
2
145
153
133
160
7.7
7.7
4.9
2.9
Idaho
Cedarlhemlock
154
23
48
201
9.5
1Sampled
to a depth of 30 em.
Organic Matter and Nutrient Budgets
Table 2-Nitrogen content of organic horizons and mineral soil in
several old-growth stands in the montane-west
A primary portion of the nutrient capital, particularly
nitrogen (N), in the forest ecosystem is contained in the
Oi, Oe, Oa, and woody residue. Soils with the greatest N
content usually have the largest organic horizon accumulations (tables 1 and 2). Generally, as N in the organic horizons increases, stand productivity increases. The exception
to this is the warm, moist cedarlhemlock stands in Idaho
where there is a rapid turnover of forest floor (Jurgensen
and others 1990). In southeastern Wyoming, lodgepole pine
(Pinus contorta ssp.latifolia [Engelm. ex Wats.] Critchfield)
stands average 31 Mglha forest floor (Oe and Oi) volume and
have an average N content of 33 kglha (Fahey and others
1985). On these stands, woody residue contributed 13 kglha
N. Since lodgepole pine stands are usually N limited (Fahey
and others 1985), inputs from decaying wood and forest floor
can be very important for productivity.
Besides N, nutrients like calcium (Ca), magnesium (Mg),
potassium (K), and phosphorus (P) are also found in abundance within organic horizons (table 3). The availability
of all these nutrients is strongly influenced by the rate of
Residue
Location
Proportion
Forest Soli Minerai In mineral
floor wood soil 1
soil
- - - - - - - - - - - - kg/ha - - - - - - - - - - -
Montana2
Cedarlhemlock
Subalpine fir
Douglas-fir
Ponderosa pine
Idaho
Cedar/hemlock
Douglas-fir
Wyoming
Lodgepole pine4
125
219
68
<30
787
570
438
128
341
344
419
33
1,729
1,686
2,183
3,433
58
60
70
94
231
179
248
297
ns
3,045
3,160
81
ns
ns
400
86
5,270
ns = not sampled.
to a depth of 30 em.
2From Jurgensen and others 1990.
3From Clayton and Kennedy 1985.
4From Fahey and others 1985.
1 Sampled
96
Percent
Table 3-Soil nutrient budgets of organic horizons and mineral soil
from selected undisturbed stands in the montane-west
Horizon
Ca
Mg
K
Table 4-Moisture content of woody residue and
mineral soil in the montane-west
P
Woody residue
Location
- - - - - - - - - kg/halyr - - - - - - - - Ponderosa pine/Douglas-fir
mixed forest-Silver Creek, IDI
Litter (Oi)
Mineral (0-1 0 em)
347
319
340
111
340
184
190
175
Lodgepole pine-Lolo Pass, MI'!
Forest floor (Oi,Oe)
Mineral (0-10 em)
349
278
48
40
120
177
100
100
Mineral soli
- - - - Percent dry weight - - - Southwestern Oregon
Ponderosa pine
Western Montana
Douglas-fir
Subalpine fir
Hemlock
1
157
6
98
163
161
17
34
27
2
lFrom Amaranthus and others 1989.
2From Harvey and others 1979.
lFrom Clayton and Kennedy 1985.
2From Entry and others 1987.
organic matter decomposition. Again, nutrient concentrations vary depending on overstory species and stand locations, but 0 horizons provide a large proportion ofnutrients critical for seedling establishment and growth.
Table 5-Seasonal moisture content fluctuations in soil substrates from an Abies/asiocarpalC/intonia uniflora
habitat type in western Montana (from Harvey and
others 1978)
Month
Organic Matter and Moisture-Holding
Capacity
Humus
Woody residue
Mineral soli
- - - - - - - - - - -Percent dry weight - - - - - - - - - May
July
September
As we have seen, forest floor material and decayed logs
are a reservoir for nutrients. They also act as a storehouse
for moisture. Fallen, decaying logs can contain especially
large amounts of moisture (table 4). Amaranthus and
others (1989) noted, in southwestern Oregon, that during
the winter months decayed wood acts like a sponge to absorb water and retains much of that water throughout the
following growing season. This water supply can be particularly important for seedling establishment, especially
where available soil water would otherwise be insufficient
for surviving summer drought or for maximizing growth
in highly competitive situations.
Comparisons of moisture contents on a dry weight basis
do not provide a ready measure of how much is available
for plant uptake. However, field capacity and permanent
wilting point moisture data for a Douglas-fir stand in northern Idaho show soil wood has 5.5 times more available moisture than mineral soil per gram of substrate. On a weightl
weight basis soil wood has an average available moisture
of 84.5 percent, litter 18.7 percent, and mineral soil 15.4
percent (Page-Dumroese 1990). Although soil moisture
levels fluctuate seasonally, decayed wood maintains higher
water contents throughout the growing season (table 5)
than the forest floor or underlying mineral soil. This makes
decayed wood of particular importance to drier ecosystems
where moisture is limited throughout the year. The forest
floor, by acting as a mulch, may also be helpful for maintaining moisture levels in the mineral soil throughout the
growing season.
130
74
141
204
118
244
37
27
40
times greater CEC than surface mineral soil. Mter harvesting, an eightfold difference occurred in CEC between
the forest floor and the mineral soil.
In northern Idaho, site preparation treatments that
mound the soil organic matter and mineral top soil together
(Page-Dumroese and others 1986, 1989) had significantly
greater CEC's than a scalp treatment that removed the
forest floor (table 6). The undisturbed treatment, with the
forest floor left relatively intact, had a similar CEC to the
mounded treatment. While knowledge about a soil's CEC
is important, very little work has been done to link the
effects of timber harvesting/site preparation to changes
in CEC and resulting site productivity.
Organic Matter and Disturbance
Stand disturbances, either natural or artificial, have a
dramatic impact on the depth of organic horizons (table 7).
Recent wildfires and intense, long-duration prescribed
burns seem particularly devastating to organic matter
depth (Harvey and others 1986). Destruction of soil organic
horizons by repeated wildfires over the past 75 years may
be a contributing factor to the development of aggressive
shrubfields in northern Idaho (Harvey and others 1987).
Harvesting and different site preparation methods and
their effect on stand nutrient balances can be seen in table 8.
Clearcut and burn operations maintain more total N, P, and
cations in the organic horizons than does a mechanical residue (bulldozer piling) removal system. Acceleration of nutrient loss and increased erosion occur after removing the
protective organic mantle (Megahan and Kidd 1972). Soil
organic matter promotes the formation of water-stable aggregates as long as substantial levels are maintained. Once
Organic Matter and Cation Exchange
Capacity
.
Organic matter, because ofits many negatively charged
sites, is a major source of a soil's cation exchange capacity
(CEC) (Tate 1987). In a northern hardwood forest, Brooks
(1987) found that in uncut stands the forest floor had six
97
1.8
Table 6-Cation exchange capacity (cmollkg) and soil organic
matter content (percent) as affected by site preparation technique in two northern Idaho stands
Site
treatment
Low elevation1
O.M.
CEC
Percent
Mounded
Scalped
Undisturbed
15
9
8
14
11
-
High elevation 2
O.M.
CEC
cmol/kg
15
1.
Percent
emoVkg
28
15
29
18
11
20
S
1.
CD
j
tJ)
~
.2
lAbies grandislSymphoricarpos a/bus h.t., elevation 715 m.
2Tsuga heterophyllwClintonia uniflora h.t., elevation 1,456 m.
o
0.0
depth in northwestern Montana (from Harvey and others
1986)
Stand age
Forest floor depth
Years
em
>250
80
15-20
15
3.5
1.8
1.5
.5
>250
80-1(>0
60-120
50
2.3
.6
1.2
1.9
Subalpine fir
Undisturbed
Wildfire
Clearcut and burn
Wildfire
Douglas-fir
Undisturbed
Partial cuVunderburn
Selective cut
Wildfire
Figure 2-Ponderosa pine seedling weight response
to increasing forest floor depth in the montane-west.
This increase is usually short-lived (Cromack and others
1979) and decreases with new stand development (Kraemer
and Hermann 1979). Therefore, planted seedlings, with
reduced access to soil organics, may have, or will likely
soon experience, growth declines (Graham and others
1989).
Organic horizon depth can directly infl uence seedling
biomass production (fig. 2). Seedling weight of naturally
regenerated ponderosa pine in a Douglas-fir (Pseudotsuga
menziesii Beissn. [Franco]) habitat type is positively correlated with depth of the organic horizon. The correlation
shown in figure 2 is particularly striking because organic
matter depth did not exceed 1 cm. Although this study
had a relatively small sample size, organic matter depth
explained 51 percent of the variation in weight of these
seedlings (Harvey and others 1988).
In the past, mineral seedbeds for natural regeneration
have been the "norm" (Haig and others 1941). However,
soil organic components can also act as valuable seedbeds
for natural regeneration (table 9). Organic substrates in
the Canadian Rockies occupy a large portion of the stand
and are used extensively as a seedbed.
Harvey and others (1987) noted that, in terms of a competitive advantage, conifers seem to be the only species using woody debris as a substratum for regeneration. There
is also species differentiation in the use of organic horizons
for regeneration (Day and Duffy 1963). Lodgepole pine
favors a mineral seedbed, but Engelmann spruce (Picea
engelmannii Parry ex Engelm.) and Douglas-fir prefer organic seedbeds. Organic horizons and the upper 30 cm of
mineral soil then become the primary rooting substrate as
seedlings mature (Harvey and others 1986; Kimmins and
Hawkes 1978).
Growth of planted seedlings after intensive site preparation on two soil types in northern Idaho was influenced
by soil organic matter content (table 10). Western white
pine (Pinus monticola Dougl. ex D. Don) and Douglas-fir
growth was greater after 3 years in treatments with high
organic matter content compared to scalped treatments.
This may be due to several interacting factors including:
Table a-Forest floor and woody residue nutrient content left on an
Abies lasiocarpa' Xerophyllum tenax site after harvesting
and site preparation in northwestern Montana (from Entry
and others 1987)
Harvest method
Ca
Mg
K
p
N
- - - - - - - - - - - - - kg/ha - - - - - - - - - - - - - CC 1/residue left
CC/residue removed
CC/residue burned
331
188
215
46.1
26.8
27.3
79.7
40.8
75.4
145
60
10
634
392
476
lClearcut.
".'
'.-
1.0
Depth of organic horizons (cm)
Table 7-Disturbance effects on soil forest floor (Oi, Ce, and Oa)
Site and disturbance
0.5
.
the forest floor is destroyed, these aggregates break down
and erosion increases. Clayton and Kennedy (1985) indicated it may take more than 50 years to restore a heavily
disturbed ecosystem to its former nutrient status and perhaps centuries to restore soil lost through erosion.
Organic Matter and Regeneration
Postharvest natural and artificial regeneration success
depends, in many cases, on soil organic matter content.
Increases after harvesting and site preparation in organic
matter percentage in the surface mineral soil are most
likely the result of forest floor and a considerable amount
oflogging slash being mixed into the surface mineral soil.
98
REFERENCES
Table 9-Seedbed composition and natural seedling distribution in logged-over stands in the Canadian
Rockies (from Day and Duffy 1963)
Seedbed
Area
Amaranthus, M. P.; Parrish, D. S.; Perry, D. A. 1989.
Decaying logs as moisture reservoirs after drought and
wildfire. In: Alexander, E. B., ed. Proceedings of Watershed '89: a conference on the stewardship of soil, air, and
water resources; 1989 March 21-23; Juneau, AK. Juneau,
AK: U.S. Department of Agriculture, Forest Service,
Alaska Region: 191-194.
Brooks, R. H. 1987. Effects of whole-tree harvest on organic
matter, cation exchange capacity, and available water
holding capacity. Houghton, MI: Michigan Technological
University. 29 p. Thesis.
Clayton, J. L.; Kennedy, D. A. 1985. Nutrient losses from
timber harvest in the Idaho batholith. Soil Science Society of America Journal. 49: 1041-1049.
Cooper, S. T.; Neiman, K. E.; Steele, R.; Roberts, D. W.
1987. Forest habitat types of north em Idaho: a second
approximation. Gen. Tech. Rep. INT-236. Ogden, UT:
U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 135 p.
Cromak, K., Jr.; Swanson, F. J.; Grier, C. C. 1979. A comparison of harvesting methods and their impact on soils
and environment in the Pacific Northwest. In: Youngberg,
C. T., ed. Forest soils and land use: Proceedings, fifth
North American forest soils conference; 1978 August; Fort
Collins, CO. Fort Collins, CO: Colorado State University:
445-476.
Day, R. J.; Duffy, P. J. 1963. Regeneration after logging in
the Crowsnest forest. Publ. 1007. Calgary, AB: Canadian
Department of Forestry. 31 p.
Entry, J. A.; Stark, N. M.; Loewenstein, H. 1987. Effect of
timber harvesting on extractable nutrients in a Northern
Rocky Mountain forest soil. Canadian Journal of Forest
Research. 17: 735-739.
Fahey, T. J.; Yavitt, J. B.; Pearson, J. A.; Knight, D. H.
1985. The nitrogen cycle oflodgepole pine forests, southeastern Wyoming. Biogeochemistry. 1: 257-275.
Fosberg, M. A.; Falen, A. L.; Jones, J. P.; Singh, B. B. 1979.
Physical, chemical, and mineralogical chamcteristics of
soils from volcanic ash in northern Idaho: I. Morphology
and genesis. Soil Science Society of America Journal. 43:
541-547.
Geist, J. M.; Hazard, J. W.; Seidel, K. W. 1989. Assessing
physical conditions of some Pacific Northwest volcanic
ash soils after forest harvest. Soil Science Society of
America Journal. 53: 946-950.
Graham, R. T.; Harvey, A. E.; Jurgensen, M. F. 1989.
Effect of site preparation on survival and growth of
Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco)
seedlings. New Forests. 3: 89-98.
Haig, I. T.; Harris, D. P.; Weidman, R. H. 1941. Natural
regeneration in the western white pine type. Tech. Bull.
767. Washington, DC: U.S. Department of Agriculture,
Forest Service. 99 p.
Harmon, M. E.; Franklin, J. F.; Swanson, F. J.; Sollins, P.;
Gregory, S. V.; Lattin, J. D.; Anderson, N. H.; Cline, S. P.;
Aumen, N. G.; Sedell, J. R.; Lienkaemper, G. W.; Cromack,
K., Jr.; Cummins, K. W. 1986. Ecology of coarse wood
debris in temperate ecosystems. Advances in Ecological
Research. 15: 133-302.
Seedling distribution
- - - - - - - - - Percent - - - - - - - - - -
Muck
Litter
Moss
Decayed wood
Humus
Mineral
9
9
11.8
6.3
16.5
44.9
20.4
24
24
12
22
10ata for area not available.
Table 10-Soil organic matter content and 3-year-old western
white pine and Douglas-fir seedling biomass after
three site preparation techniques in northern Idaho
(from Page-Dumroese and others 1986; Graham and
others 1989)
Treatment
Organic matter
P,ercent
Mounded
Scalped
Undisturbed
27
14
23
WWP biomass
DF biomass
- - - - - - - - - Grams - - - - - - - - -
19
8
8
16
7
9
(1) organic matter on the surface of the mounded treatments may have acted as a mulch to enhance water retention, (2) organic matter incorporated into the mounded
treatments significantly lowered soil bulk density, and
(3) organic matter left on the surface or incorporated into
the mounded treatments improved the nutrient status of
the soil (Page-Dumroese and others 1986, 1990). Scalping,
which is commonly used in the montane-west, can in some
instances, benefit seedling establishment and survival by
reducing competition (Sloan and Ryker 1986). However,
removal of the surface organic and mineral horizons can
also severely limit growth and impair long-term survival.
MANAGEMENT IMPLICATIONS
Soil organic matter affects the cation exchange capacity,
water-holding capacity, bulk density, nutrient budgets,
and erosion potential. Removal of organic horizons during
harvesting and site preparation may seriously reduce overall site productivity, stability, and regeneration potential.
Postharvest treatments should be planned to limit damage to fragile organic horizons. There may be occasional
instances of extreme competition or heavy fuel loading that
warrant intensive site treatments and forest floor removal
to achieve adequate regeneration. Although maintenance
of the organic mantle may limit some initial site preparation options, in the long run productivity will be maintained
or improved. Economic investments made to conserve organic matter or reduce bulk density in many stands in the
montane-west can provide substantial returns in the form
of improved long-term soil productivity.
99
Harvey, A E.; Jurgensen, M. F.; Graham, R. T. 1988.
The role of woody residue in soils of the ponderosa pine
forests. In: Baumgartner, D. M.; Lotan, J. E., eds. Ponderosa pine-the species and its management; 1987
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Harvey, A. E.; Jurgensen, M. F.; Larsen, M. J. 1978.
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Harvey, A. E.; Jurgensen, M. F.; Larsen, M. J.; Graham,
R. T. 1987. Decaying organic materials and soil quality
in the Inland Northwest: a management opportunity.
Gen. Tech. Rep. INT-225. Ogden, UT: U.S. Department
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Station. 15 p.
Harvey, A E.; Jurgensen, M. F.; Larsen, M. J.; Schlieter,
J. A 1986. Distribution of active ectomycorrhizal short
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Harvey, A. E.; Larsen, M. J.; Jurgensen, M. F. 1979. Comparative distribution of ectomycorrhizae in soils of three
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Jurgensen, M. F.; Harvey, A E.; Graham, R. T.; Larsen,
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in the Inland Northwest. In: Gessel, S. P.; Lacate, D. S.;
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of the fallen tree. Gen. Tech. Rep. PNW-164. Portland,
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Harvey, A E. 1986. Soil physical properties of raised
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