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Paciric

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Wrest

FOREST AND RANGE
EXPERIMENT STATIO

USDA FOREST SERVICE RESEARCH NOTE

PNW-306

December 1977

ROOT STRENGTH CHANGES AFTER LOGGING
IN SOUTHEAST ALASKA

ee
by

R. R. Ziemer and D. N. Swanston!

gle 3
J

a
ABSTRACT

the role of plant roots in maintaining the shear strength of soil
mantles.

Roots add strength to the soil by vertically anchoring
through the soil mass into failures in the bedrock and by laterally
tying the slope together across zones of weakness or instability.

Once the covering vegetation is removed, these roots deteriorate
and much of the soil strength is lost.

A crucial factor in the stability of steep forested slopes is

Measurements of change in strength of roots remaining in the
soil after logging at Staney Creek on Prince of Wales Island, south-

east Alaska, indicate that loss of strength in smaller roots occurs
rapidly for all species the first 2 years. Western hemlock (Tsuga
heterophylla (Raf.) Sarg.) roots are more resistant to loss of
strength than are Sitka spruce (Picea sitchensis (Bong.) Carr.)
roots.

By 10 years, even the largest roots have lost appreciable
strength.

KEYWORDS:

Root morphology, root damage, soil stability, logging
(-forest damage, Alaska (southeast).

i/ Research Hydrologist, USDA Forest Service, Pacific Southwest
Forest and Range Exp. Station; and Research Geologist, USDA Forest
Service, Pacific Northwest Forest and Range Exp. Station.

REST SERVICE - U.S. DEPARTMENT OF AGRICULTURE - PORTLAND, OREGON |

INTRODUCTION

Averucial factor ani the
Stability vor steep aoresued
sllopess 1s) the, role of sp lLanit
roots in maintaining the shear
Strength of Soil mantlesa) jRogts
add strencth, to the ysozieipy
vertically anchoring through
the soil mass anto frac tumesaum
the bedrock and by laterally
tying the slope together wenross
zones Of weaknesis o2) inSitalbauleiieye

In Japan, Endo and Tsuruta
(1969) reported that soil shear
strength increases in proportion
to the amount of roots imnvene
soil. Kitamura and Namba (1966,
1968) noted that the resistance
of tree stumps to uprooting
decreases rapidly as the root
systems decay following timber
harvest. They concluded, when
considering root growth of
planted trees, thatathe score sit
$Q0il would reach a minimum
strength between 5 and 10 years
after cutting and replanting.

Bishop and Stevens (1964)
and Swanston (1967, 1969) dem-
onstrated the probablewetrcer
Of roots on the “stabi lanayro
slopes in southeast Alaska and
correlated increased landslide
activity with time after logging.
Wu (1976) measured the contri-
bution of root tensile strength
to shear strength of Karta Soils
at Hollis, Prince of Wales Island,
and found it equivalent to at
least a cohesion of 120 pounds
per square inch (8.44 kg/cm2)
or about 25 percent om the total
soil strength available to resist
failure. Preliminary smeasurements
of loss in root Strength ot born
Sika spruce (Picea sttchensis
(Bong.) Carr.) and western hemlock
(Tsuga heterophylla (Raf.) Sarg.)
by simple penetration tests
further indicate a maximum loss

of root strength is attained 3
fo. Syyears after cutting
(Swanston 1970).

Similar rates of strength
loss are reported: for Douglas — iam
(Pseudotsuga menztestt (Mirb.)
Franco) FoOts in coastal Brews
Columbia (O'Louglin 1974) and in
the Oregon Coast Ranges (Burroughs
and Thomas 1977). The senior
author measured the influence
of roots on in-situ shear
strength in the Oregon-northern
California coastal province and
found that the reinforcing effect
of the root mass significantly
strengthened the soil.

The present study was
initiated to quantify these
relationships for southeast
Alaska and to extend the data
base of a much larger inves-
tigation to evaluate the inter-
actions of root Strength, soot
biomass, and soil strength in
natural forest stands and the
influence of logging on these
factors. The larger investigation
is a portion of a west-wide
cooperative inter-Station
research program to study mass
wasting process and the influence
of forest operations on slope
Sica bad aey..

METHODS
Sampling

A prime objective of this
study was to measure the change
in strength of roots remaining
in the soil after lopeamanauho
keep the influence of other
variables at a minimum, we
selected sites having a wide
range of cutting ages concen-
trated within a small geographic
area. Staney Creek is one of
the very few locations in south-
east Alaska having such a wide
diversity of cutting ages with

136° 134° 132° 130°

-N-

a aiiee aie
ee
‘ BY

SOUTHEAST ALASKA

oO 15 30 45
——l
SCALE: MILES

137° 135° 133° 131°

1 ©

‘Figure 1.--Planimetric map of southeast Alaska
_ showing location of the Staney Creek study
meee on the west coast of Prince of Wales

i

eae

an
>? =>

Island.
] number of miles by 1.609.)

(For kilometers, multiply the

Similar environmental character-
mets (tap. 1).

Within the Staney Creek
area, five comparable sites,
on which the trees had been cut
en 1966, 1970, 1972, and 1974,
and an uncut stand were found.
Within each of the five sites,
a western hemlock and a Sitka
Spruce tree or stump was
selected for study. Live
huckleberry (Vaccinium parvifolium
Sm.) roots were also sampled
in the clearcut units. Root
samples were collected from
the subsurface mineral soil
rather than from within the
extensive surface organic layer.

We planned to collect 15
separate root segments approx-

imately 12 centimeters long for
each of the six diameter classes
for each species. " These were
oe Se) eles ao, ‘and
50-millimeters inside the

bark. We were unable to obtain
the desired number of roots for
some of the large size classes;
and, in the areas cut in 1966
and 1970, root decay was so
advanced that dead roots in the
smaller size classes could not
be found.

The selected roots were
immediately packed in wet sphagnum,
Seauedan plastuc. bags, and air
mailed to the laboratory in
Arcata, California. At the
laboratory, the roots were
inventoried and stored at 2°C.
Within 2 weeks, the roots were
tested in the shear apparatus.

The results reported here
represent data collected from a
small geographic area. The
slopes were limited to gentle
terrain that was characteristic
of the Staney Creek area but not
generally characteristie of
southeastern Alaska. The data
are further limited to one tree
per species an; each of the five
cutting age classes selected.

Shearing

Ten of the roots, | 1£
available, were prepared for
shearing by removing the bark
and marking each root into five
equally spaced segments. The
separation between segments was
at least twice the root diameter
to prevent one break from in-
fluencing subsequent tests.

For large soots, only one. or

two breaks per root sample was
possible. The minimum and
maximum diameters of each seg-
ment were measured. Each root
was then sheared at each segment
mark.

The shear apparatus (fig.
2) consists of a stationary
steel block machined to allow
the insertion of hardened
steel dies.£/ A hole was
drilled in the die to hold a
root for each diameter class.
A root having a diameter ap-
proximately equal to that of
the hole was inserted through
the ‘dies it protruded iat sborn
sides. A movable steel block
with a hardened steel V-shaped
blade was machined to slide
along the surface of the sta-
tionary block. The movable
block was pushed by a mechanical

a Ziemer, R. R. An apparatus
to measure the strength of roots.
Unpublished report on file at Pacific
Southwest Forest and Range Experiment
Station, Arcata, Calif.

Figure 2.--Root shearing apparatus.

jack until the protruding root
segment failed.

The moving friction of the
shear block and the maximum
stress applied toledch root
segment at failure was measured
by means of a proving ring and
dial gage. Stress was applied
to. the To0teat a Tateroteors
centimeters per minute. Both
the moving friction and the
maximum stress at failure were /
converted to kilograms of force.=
The net maximum stress (shear
strength) was obtained by
subtraction of the moving
friction component from the
maximum shearing force component.

In an earlier study, ssuch
direct shear strength measure-
ments were found to give an
excellent prediction of root
tensile strength measured: aa
dependently with an apparatus
designed by Burroughs and Thomas
(see footnote 2). A linear
regression on paired breaks of
the same root yielded the equation:

(Shear

hensastc alan
(Ryo (AN ays 7) strength aml

strength —

where, the root strengths were
expressed in kilograms and the
root diameters ranged from 1 to
10 millimeters. The explained
variance (r2) was 0.972. The
direct shear strength measure-
ments were preferred in our
study because many more and
larger roots could be tested

in the limited time available.

3/

=’ Here we follow the common
engineering practice of expressing
force in units of mass. Force is
correctly expressed in units of mass-
length/time2 or in newtons. To
convert: Newtons = mass (kg) X
9.807 m/sec2 (gravitational ac-
celeration). We further define the
shear strength as the maximum shearing
force applied at failure.

SO aw

ate ie meee

Tensile strength measurements
are generally limited to roots
less than 10 millimeters in
diameter.

Physical and strength
characteristics of each root
segment were coded for subse-
quent analysis. The data were

initially screened for anomalies

such as cracked or dried roots,
instrument malfunction, and
observer errors. Such data
were rejected from subsequent
compilation.

The mean shear strength,
standard deviation, and number
of breaks were calculated for
each species, age, and size
class, (table 1)... Further
statistical analysis of these
data was not justified because
the samples were, in no sense,
collected at random.

RESULTS

During excavation, tree
roots were found to range from
resinous roots with a high re-
Sistance to decay to nonresinous
roots that decayed rapidly. In
companion studies, the senior
author has observed similar decay
resistant roots in coastal Oregon,
the Oregon Cascades, and in coast
redwood (Sequota sempervirens
(D. Don.) Endl.) stands and
interior Douglas-fir stands of
the northern California Coast
Ranges. In the Sierra Nevada
and southern California there
is such heavy resin in pine roots
infected with Fomes annosus that
larger roots (100- to 200-mm
diameter) have not deteriorated,
even after 50 years.4

4/ Robert Bega, Plant Pathologist,
Forest Disease Research, Pacific South-
west Forest and Range Experiment Station,
Berkeley, California, personal corre-
spondence, August 2, 1976. On file at
Redwood Sciences Laboratory, Arcata,
California.

Table 1--Root strength of western hemlock, Sitka spruce, and huckleberry, by size class and time since Logging

Root |Statisticgl Live roots

diameter| measureL

Dead roots

Years since logging

6 10
Sitka Western| Sitka Western| Sitka
hemlock| spruce} hemlock| spruce} hemlock] spruce
mm
2 t's 3.58 9.72 Sas 5.68 Phat) 4.00 4.85 Zieh 3.44 3.26 =-
0 1.80 3.06 - 60 Lead .83 Asia SaL9 (Apa) .90 2.02 ==
n 50 50 45 50 50. 45 50 50. 20 20 --
5 Xx 30.38 42.73 PATE 28.09 20.44 21.87 38.42 21.48 13.09 17.96 13.10
o 8.28 11.63 7.54 7.02 7.79 9.53 16.23 11.58 7 pe ky 5.18 6.72
n 50 50. 50 50 50. 55 50 50. 50 50 14
10 D4 146.91 128.98 106.11 101.41 70.54 78.16 99.09 157.28 87.90 68.29 50.50
0 S07 40.63 45.41 32.47 25.03 Bio, 51.80 50.41 39.02 37.56 30.31
n 44 50 49. 50 50. 50 49 iis 49 50 47
17 p 442.5 454.4 501.4 318.6 149.4 255.6 319.0 551.2 303.1 409.6 153.9
o 194.8 120.5 164.9 193.8 23.0 74.6 162.5 130.2 141.3 181.5 82.6
n 23 36. Ze 50. 30. 30 46 48. 49 45 35
25 4 -- 757.8 811.2 518.0 420.6 fA | 937.8 800.9 530.4 507.9 438.7
o of ~ 194.1 337.4 190.8 104.4 290.5 294.6 207.8 145.6 170.8 176.8
n -- 2a 23 32 30. 37 19 49. 39 46 24.
50 sd -- 2884. 2427 2806 EAE fe 2581. 2705S. 2984. 2608. 1391. 314.
o == 124. 588 265 556. 679. 174. 80. Zio. 387. a Arle
n == 10. 10 14 Si 8. Tie 6. : I 8. 6.
if X = mean shear strength in kilograms; o = standard deviation in kilograms; n = number of breaks.

(kg)

SHEAR STRENGTH

LIVE ROOTS

There is a linear relation-
ship between the logarithm of
root strength and the logarithm
of root diameter for live roots
of the three southeast Alaska
Species Lesized | (ile aes)

100

O——O HEMLOCK
@—-@ SITKA SPRUCE

2 @-----@ HUCKLEBERRY

ns. Points not significantly different

| 2 5 10 20 50 ~=100
ROOT DIAMETER (mm)

Figure 3.--Root shear strength vs.
root diameter for live western
hemlock, Sitka spruce, and
huckleberry.

Sitka /spruce: roots mane
weaker than hemlock roots (fig.
3. table wl). pant 1c Ullargleymtane
smaller noots. “For ‘example,
Sitka’ Spruuce Moots . enmeOm on annl
in diameter, are about 40 per-
cent weaker than similar size

hemlock roots. This difference

is less’ in 10-mmeSsitkas spruce
roots, which a@revabout lem pem.
cent weaker than 10-mm hemlock
roots. There is little difference
in) Streneth of Mave noose ancien
than 17 mm.

After logeange.. wedehuckiic—
berry commonly invades the site.
Very small) huckilebernysnoers
have substantially less strength
than either hemlock or Sitka
Spruce roots of comparable msaaer
For example, 2Z-mm roots of
huckleberry are about 63 percent
weaker than hemlock roots and
about 40 percent weaker than
Savtkay spruce Toots. Wlbersmichat
ference vanishes in roots with
larger diameters, and sthemcmmes
little difference betweenmunte
Cimees Specvesmart. leanne

DEAD ROOTS

IMME eommiereal WIE O WM Cut IPSS
LOOES tO Che Strep th Ode enc
soudl-as, a function of ene sane
dividual root strengths and the
number of roots per unit volume
Ose S@mLIES = Vilsbieloiatin jel seis Sy
years after lopeoing., themenussed
decrease in the average strengun
Ob (the Toocs, 1n) the "somal Gea
ay. Then; 4 to © years aipten
logging, the strength of resmude
wal roots 1S at least as manent
as the average strene thvot edema
roots. By this time, however:
most of the nonresinous roots
have completely decayed and only
GFEesinous, TOOES an! the nesaidiues
root biomass are left. Resimneus
roots are only a small fraction
of the total root mass in the
Onlginal, live ores f “stands

Several important differences
between hemlock and Sitka spruce
roots are related to size and :
change in strength with time
dee oe Gere Ut tlm or

chee
[2mm | Smm | 10mm |

461
oe 6 246 wets 10

YEARS AFTER CUTTING

Figure 4.--Change in root shear
strength, in years after cutting,
for different root diameters of
western hemlock and Sitka spruce.

Hemlock Roots

Theré is a continual loss
in average strength of residual
hemlock roots less than 25 mm in
diameter with time after logging
with about 32 percent of the
Suheopin wast. during the first
Zowedrs. {Lapie a... fic, 4). “For
example, for 2-mm diameter roots,
there is a 42-percent loss in
strength the first 2 years after
logging. Within 4 years, there
is a 59-percent loss. By 6 years,

70 percent of the strength of
the residual roots is lost.
Residual roots having a diameter

of 5 mm show a 34-percent loss

in: strength. the first 2 years.
Within 4 years, half the strength
1S LOSE A eoVe Lee years-atter cut-
ting, se percent 1s Lost.

A reduction in residual
strength of intermediate size
hemlock roots (10-25 mm) during
the first 4 years after logging
is followed by an apparent in-
erease in Strength vas. a result
of dominance of resinous roots
im “the residual’ biomass. +This
increase reaches a peak about
oO) years after cutting, then
declines as the resinous roots
begin to decay. The root strength
Ato years 1S) ‘ati ieast as igneat
aS that of the original live woots.
This does not imply that the
strength of the soil-root matrix
i inencasine, because the total
number of roots continually
becomes smaller and the roots
remaining in the soil are the
decay resistant resinous roots
which represent a small fraction
of the original root biemass.

By 0b years after) cutting. seven
these resinous roots have begun
coOmose their strengicn.

there is no Loss in -streneth
of large hemlock roots (50 mm)
during the first 6 years after
cuttings. Within the next 4
years, however, 50 percent of
the=strength is lest.

Sitka Spruce Roots

There is a decrease of about
Spercent Im Strength otysitka
spruce roots less, than 25 mm in
diameter within the first 2 years
after logging, followed by an
mcrease In the 4th ‘year toca
strength approximating that of
tuve roots,” This. apparent in-
crease 15 also due to the
dominance of resinous roots in
the biomass. After this 4-year
peak, there is a continual

reduction in root strength as the
resinous roots decay. By the 10th
year; no Gootse less» than Ze mmean
diameter remain in the soil and
few roots smaller than 5 mn.
Therefore, essentially all strength
from these smaller roots is gone.
The remaining resinous roots,

from 10) to) 25 mm ined taneter.

lose about 50 percent vor steiesa,
strength between the 4th and 10th
yee

There 1S no Loss ingest remot h
of 50-mm roots for the first 6
years, but between 6 and 10) years,
these, large) roots losers percent
of their Strene the

Comparison of Roots of Hemlock
and Sitka Spruce

In general, there were fewer
residual Sitka spruce roots per
age class, which suggested that
they ane esse nresistante so sdecay.
Because the nonresinous fraction
of the Sitka spruce roots decays
more rapidly than the resinous
fraction, the resinous roots
dominate the residual biomass
Z years earlier thane domehe
EESIMOUS LTOOES sot swemlomk= a aNs
the proportion of nondecayed
resinous roots rises, the average
GLesidual Ss treniptinalSOm Gases.
Therefore, this observed) ancreasie
in strength ofvthevortukasspruce
roots 4 years after logging and
of the hemlock roots 6 years after
logging reflects the rate of
total decay of the nonresinous
Groot fraction. | hha sidoesmmnoit:
imply that the strength of the
soil through the binding action
of roots increases after logging
since the total number of roots
or biomass of roots continues to
decline.

There appears tombe momres—
inosis of hemlock roots less than
5 mm diameter, whereas resinosis
is apparent in small Sitka spruce
roots.

The average strength of
residual Sitka spruce roots is
consistently less than thatward
hemlock roots until 4 years after
logging when the impact of res-
inosis is observed in Sitka spruce.

SUMMARY AND CONCLUSIONS

Il. There 1s: ac lanear rede
tionship between the logarithm
of root strength and the logarithm
of root diameter for the live
roots of Sitka spruce, western
hemlock, and red huckleberry.

Zz. in uncut Stands,” hemlock
roots are stronger than! sxtkal
SPEUGE LOCOS.

3. Very small huckleberry
roots have substantially less
strength than either hemlock or
Sitka spruce, but strength rapidly @
increases with diameter and closely ©
approximates the strength of the
tree roots above 10 mm diameter.

4. Sitka spruce roots decay
more rapidly than do hemlock roots.

5. As tree roots decay masem
time after logeing, residuat
hemlock roots continue to be
stronger than residual Sitka
spruce foots until 4 yearsvanven
cutting.

6. Hemlock loses about
one-third of the average strength
of its roots smaller than 25 mm
in diameter within 2 years after
logging.

7. Sitka spruce losessabome
one-half the average strength of
its roots smaller than 25 mm in
diameter within 2 years after
logging.

8. Within 2 years sheen
logging, most of the original
roots larger than 1 mm of the
three Species can still ‘beviound

a ee ae

in the soil. By 4 years, many
of the roots have totally
decayed, and a proportionately
larger number of decay resistant,
resinous roots are left. By

10 years after cutting, even
these resinous roots have lost
appreciable strength.

9. Since this study was
exploratory and designed to
provide data for a more regional
evaluation of root strength,
several limitations should be
mentioned:

a. We studied a specific
geographic area, namely
Staney Creek, Prince of
Wales Island.

b. We selected only one
Site for each age class.

c. We selected only one
tree per species for each
age class.

d. We located sites on
relatively gentle terrain.

Subsequent studies and papers

wil evaluate the significance of
these factors on root strength
and its rate of reduction after
logging. A more definitive
measure of the rate of root
biomass loss after logging in
southeast Alaska is also needed.

LITERATURE CITED

Bishops De Ma) ands MER mi ome mens.
1964.: Landslides on slogped
areas in southeast Alaska.
USDA. Fome* Sei veus heSmese ior
NORS ae Sape

Bibbmeowwudoic, Ia Ie = die. . unl
B. Re thomas.
1977. Declining Tooke strengeh

im Douglas Fim sateen ae laine
aS a factor anesloperstabaklaty:.
WSN Miche sw Siow. IES. Pea.
INT =2905 27 ope sinwermin weor.
and Range Exp. Stn., Ogden,
Utah.

Endo, ia, and i 4eisunmtisar

1969) GAY repome ine re gard ib0
the reinforcement action of
the vegetational roots upon
the tensile strength of the
Na tucads soul eLOc Amu:
Rep. Hokkaido Branch, For.
qe Sem 5 ws LSS Ise).
[Translated from the Japanese
by J. Ma Anataieand Ree Re
Ziemer, USDA For. Serv.
ArGaltasm: Galleisees 9) 5) eulcd ean]

Kittamuna, Yo, ands. Namba
IS OCRe AG sereldexpeasiment
on the uprooting resistance

Of Eree LOO GS ee roc. 7 eh
Meet) Japs, Home Soc... pr
568-570. [Translated from

the Japanese by J. M. Arata

and R. Re) Zener USDA GE Oa:
Serica ) Niceaticel, Gailimiec IO:
NS joe |

Kitamura, Yo5) ands.) Namba

1963. A’ field experiment von
the uprooting resistance of
tree nootsen eroca/Jthe Meera.
Japs .FOm. SOC as spi 100 ao le
[Translated from the Japanese
by Ji: Me Araitcar and eRe Re
ZemMeT USAGE On OC tare
Areata ss Calbict., S967 lUnap rn

10

OM Lough loins Geet

1974, whe eLtece jos sermmbex
removal on the stability of
forest Somlis.. (d25 hydrous
(NZ) 4 1S: C29) eae ese

Swanston, Douglas N.
1967. Geology and slope
failure in the Maybeso Valley,
Prince of Wales Island, Alaska.
Ph.D). “thesis. Machssisaitse
Univ. , East Lansing Z0Gnspe

Swanston, Douglas N.

1969. Mass wasting in coastal
Alaska. USDA® Fors SeiViaakese
Paps -PNW= 83), WS) jp less
Pac. Northwest For. and

Range Exp. Stn.;, Por tiland:
Onece

Swanston, Douglas N.
1970. Mechanics of debris
avalanching in shallow till
soils of southeast Alaska.

USDA’ For. Sexuyv. Rese bape
PNWo103 ye pe asllase
Pac. Northwest For. and

Range Exp. Stn., Portland)
Oreg.
Wilts leq ide
1976. Investigation, of) Wand—

slides on Prince of Wales
Island, Alaska. Dep." \Cangigl
Eng. , Geotech. Eng? {RepeaNer
5, 95 p. ..Ohao States Unger
Columbus.

GPO 999-908

The mission of the PACIFIC NORTHWEST FOREST
AND RANGE EXPERIMENT STATION is to provide the
knowledge, technology, and alternatives for present and
future protection, management, and use of forest, range, and
related environments.

Within this overall mission, the Station conducts and
stimulates research to facilitate and to accelerate progress
toward the following goals:

1. Providing safe and efficient technology for inventory,
protection, and use of resources.

2. Developing and evaluating alternative methods and
levels of resource management.

3. Achieving optimum sustained resource productivity
consistent with maintaining a high quality forest
environment.

The area of research encompasses Oregon, Washington,
Alaska, and, in some cases, California, Hawaii, the Western
States, and the Nation. Results of the research are made
available promptly. Project headquarters are at:

Fairbanks, Alaska Portland, Oregon
Juneau, Alaska Olympia, Washington
Bend, Oregon Seattle, Washington
Corvallis, Oregon Wenatchee, Washington

La Grande, Oregon

Mailing address: Pacific Northwest Forest and Range
Experiment Station
P.O. Box 3141
Portland, Oregon 97208