A field guide for predicting snow damage to ponderosa pine plantations

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United States
Department of
Agriculture

Forest Service

Intermountain
Research Station

Research Note
INT-385

October 1988

Robert Steele’

ABSTRACT

Describes a procedure for predicting potential damage to
ponderosa pine plantings due to weight and movement of
snowpack. Provides an example of the procedure for field
use and discusses management implications of planting
ponderosa pine in areas with high potential for snow dam-
age. Current area of application covers the Weiser and
Payette River drainages in central Idaho.

KEYWORDS: reforestation, silviculture, forest manage-
ment, tree damage, snow pressure

INTRODUCTION

For more than five decades, ponderosa pine (Pinus
ponderosa Laws.) has been the preferred species for refor-
esting burned and cut-over areas in many of the warmer
and drier portions of the Northern Rockies. Because of its
high timber value, ease of establishment, dependable
growth rates, and lower susceptibility to insects and dis-
ease, this tree is usually preferred to other species. Al-
though researchers have reported on susceptibility to snow
damage of various conifers in the Western United States
(Kangur 1973; Leaphart and others 1972; Schmidt and
Schmidt 1979; Watt 1951, 1960; Williams 1966), we could
find no reports of damage by snowpack to ponderosa pine
in the Northern Rockies. Snow damage to ponderosz pine
has been reported in California (Powers and Oliver 1970)

and Arizona (Ffolliott and Thompson 1976; Schubert 1971).

Most damage studies have been concerned with wet
snowfalls that overload tree crowns and cause bending and
deformation. Our study deals with damage caused mainly
by lateral snow movement and pressure against the stem,
although some damage from crown overloading may have
also occurred. Recent reconnaissance of ponderosa pine
plantations in west-central Idano revealed widespread
snowpack damage to pine saplings under certain site con-
ditions. Some damage to Douglas-fir (Pseudotsuga
menziesii [Mirb.] Franco) was also noted but was less wide-
spread. Type and degree of damage varied from bent
(probably temporarily) terminal stems to permanent 90

1Principal research hydrologist and research forester, respectively, lo-
cated at Intermountain Station’s Forestry Sciences Laboratory, Boise, ID.

A Field Guide for
Predicting Snow |
Damage to Ponderosa
Pine Plantations |

Walter F. Megahan

degree bends in the main stem and to entire saplings
pushed into permanent, critical departures from vertical
(fig. 1). Other causes of deformed trees included rodents,
soil creep, and rolling rocks or debris, but these were of
minor importance compared to the effects of snow.

Once deformed, the pine’s height growth is reduced
(Rehfeldt 1987; Williams 1966), compression wood forms
on the downhill side of the stem (Panshin and others
1964), and the tree becomes increasingly vulnerable to
shrub competition. In some cases, severely deformed trees
are killed by the brown-felt snow mold (Neopeckia coulteri
[PK.] Sacc.) during years with prolonged snow cover. Thus
snow damage may reduce timber yield, wood quality, and
plantation survival.

A recent study involved the evaluation of 45 ponderosa
pine plantations in the Douglas-fir/ninebark and the grand
fir/mountain maple habitat types. Prior to logging, all of
these sites appear to have supported naturally established
ponderosa pine in varying amounts. These two habitat
types represent some of the more productive timber sites
in southwestern Idaho, and a common practice was to
clearcut and plant ponderosa pine. The high potential for
shrub competition usually required that contour stripping
or pile-and-burn site preparation be used on these sites.
Slopes too steep for these treatments were often broadcast
burned.

Many of the pine plantations studied exhibited snow
damage. Plantations were considered as damaged if more
than 10 percent of the trees were obviously deformed by
snow. Snow damage occurred to 65 percent (22) of the 34
grand fir/mountain maple sites sampled, but to only 9
percent (1) of the 11 Douglas-fir/ninebark sites. Actual
percentage of damaged trees ranged from close to 10 per-
cent to virtually 100 percent.

Analyses of snow-damaged versus undamaged pine
plantations revealed that certain site features were re-
lated to snow damage. These findings led to development
of a procedure for predicting snow damage potential from
site features easily obtained by forest managers (Megahan
and Steele 1987). The purpose of this paper is to adapt the
snow damage assessment procedure for field use.

The present area of application includes the Weiser and
the Payette River drainages in west-central Idaho (fig. 2).

Figure 1a, b—Snow-damaged ponderosa pine.

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Figure 2—Area covered by the field guide to predicting snow damage.
Annual precipitation, mostly snowfall, ranges from 25 deeper, better developed, and more basic than the granitic
inches at the lowest elevation to 60 inches at the highest soils.

elevation. Topography is typical of that found in the
Northern Rocky Mountains, with steep, dissected slopes PROCEDURE FOR PREDICTING

ranging in gradient from 10 to 100 percent. Geology in-
cludes the intrusive, acid, igneous rocks of the Idaho SNOW DAMAGE
ee ie ide - ie are oe HUGE EKNC as pe Snow that accumulates on the ground undergoes a

Si : ae ee ey es a cer sane ae car change in its crystalline structure that causes a plastic
cine Coe ee era eumee cata sin teu te oe aus. x y deformation of the snowpack and exerts pressure on young
acidic. In contrast, basaltic soils are finer textured, trees. Three types of snowpack movement can occur,

{ie RESULTANT CREEP PATH
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152 CREEP PATH PARALLEL
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SNOW COVER HEIGHT WHEN COLUMN INSTALLED

ORIGINAL SNOW COVER HEIGHT
AT EXCAVATION OF COLUMN

GLIDE PATH PARALLEL
TO SLOPE

—_ HORIZONTAL GLIDE PATH

Figure 3—Example of snow glide
and creep (after Frutiger and
Kuster 1967).

namely vertical settlement at all sites plus creep and glide
on steeper slopes. Snow creep refers to differential motion
throughout the pack with more movement in upper layers
than in lower layers. Glide involves the slow downslope
movement of the entire snowpack along the soil-snow
interface (fig. 3). Glide tends to be greater on south as-
pects, is directly proportional to snow depth, and is in-
versely proportional to slope roughness. Frutiger and
Kuster (1967) documented glide movement of up to 3 feet
or more on study slopes in Switzerland. Creep varies
directly with snow depth, snow density, and slope gradient.
Martinelli (1960) measured snow creep averaging more
than 7 inches per 70 inches of snow depth on snow fields in
Colorado. Frutiger and Martinelli (1966) adapted the snow
pressure concept, originally presented by Haefeli (1951), to
quantify the static forces caused by creep and glide ina
snowpack. We used a multiple, discriminant analysis to
adapt the snow pressure approach to predict snow damage
hazards on ponderosa pine plantations (Megahan and
Steele 1987).

In order to calculate snow pressure for each plantation,
the following site data are needed:

1. Elevation in feet

2. Slope gradient in percent

3. Slope azimuth in degrees

4. Roughness (a rating based on site characteristics).

Measurement precision for the various factors should be:
elevation — 100 feet; slope gradient — 5 percent; slope
azimuth — 10 degrees; roughness — 0.1.

Table 1—Roughness as defined by surface features (derived from
Frutiger 1962)

Surface feature Roughness
Class |
Big boulders (d' >30 cm, 12 in)
Terrain with more or less big
outcroppings of rock U2
Class Il
Surface covered with shrubs at least
1 m (39.4 in) tall
Well-expressed mounds covered by
grass and low shrubs; mounds must
be at least 50 cm (20 in) high
Well-pronounced livestock or game
trails
Boulders (d' about 10-30 cm, 4-12 in) 1.6
Class Ill
Short grass (such as pinegrass) with
shrubs less than 1 m (39.4 in) in
height
Small boulders (d' <10 cm, 4 in)
intermingled with grass and shrubs
Only a few mounds up to 50 cm
(20 in) tall covered by grass and
shrubs
Grass with indistinct livestock or
game trails 2.0
Class IV
Long-bladed grass (such as bromes)
Smooth rock plates with stratification
planes parallel to slope
Smooth scree or scree-soil mixtures
Swampy depressions 2.6

‘dis diameter of the blocks that determine roughness of the surface.

The calculation assumes uniform site conditions within
the plantation. If there are large variations in any of the
site factors, the plantation should be divided into subunits
and calculations made accordingly. Roughness is deter-
mined with the use of table 1 and the photographs illus-
trating various levels of roughness (figs. 4-7). Interpola-
tions can be made between roughness levels if necessary.

Snow pressure (P) is calculated as the product of three
variables as follows:

PD ACG
where

P = snow pressure in pounds per foot of tree diameter

D = depth factor in pounds per foot of tree diameter

C = creep factor

G = glide factor.

The depth factor (D) is obtained from figure 8. Enter
figure 8 at the appropriate elevation in feet and project a
vertical line to the curve. At the intersection of the curve,
project a horizontal line to the left to read the depth factor
(see example on fig. 8). The creep factor (C) is obtained
from figure 9 in a similar manner as for the depth factor
on figure 8 except that the figure consists of a family of
curves representing various slope gradients. In this case,
the appropriate slope gradient for the site is used as the
point of intersection. Interpolate between the curves if

Figure 4—An example of class | roughness (1.2) due to the many downed logs and tall stumps;
boulders and rock outcroppings can create the same effect. This site occurs at 7,700 feet in
elevation with a 20-degree azimuth and a 55 percent slope. These site conditions can produce
snow pressures of 1,210 pounds/foot.

Figure S—An example of class Il roughness (1.6) due to the nearly complete cover of tall shrubs.
This site occurs at 5,930 feet in elevation with a 240-degree azimuth and a 38 percent slope.

These site conditions can produce snow pressures of about 365 pounds/foot and result in damaged
pine plantations as shown.

Figure 6—An example of class IIl roughness (2.0) due to the scattered low shrubs and cover
of short grass, sedges, and forbs. This site occurs at 5,500 feet in elevation with a 40-degree
azimuth and 34 percent slope. These conditions can produce snow pressures of about 205
pounds/foot, resulting in some snow damage to pine saplings.

Figure 7—An example of class IV roughness (2.6) due to the smooth surface and extensive

cover of tall grass. This site occurs at 5,050 feet in elevation with a 260-degree azimuth and
a 36 percent slope. In spite of the smooth surface, the combination of these conditions can

only produce snow pressures of about 160 pounds/foot, resulting in pine saplings with virtu-
ally no snow damage.

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DEPTH FACTOR

4488 4889 9288 3688 6800 6480 6888

BEE VATION: [PEE]

Figure 8—Depth factor as a function of elevation.

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ELEVATION — FEET

Figure 9—Creep factor as a function of elevation and slope aradient.

SLOPE GRABIENT —- PERCENT

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Figure 10—Glide factor as a function of roughness
and azimuth.

necessary for intermediate slope gradients (see example).
The final component, glide factor (G), is obtained from
figure 10 from the slope roughness and the azimuth for the
site. Enter the figure with slope roughness, project a verti-
cal line to the correct azimuth class, then read horizontally
to the left to obtain the glide factor (see example).

The product of the depth, creep, and glide factors is the
snow pressure for the plantation. A hypothetical example
to illustrate the calculation procedure is as follows:

Slope gradient = 53 percent
Slope azimuth = 170 degrees
Elevation = 5,950 feet
Roughness = 1.8

Entering figure 8 at an elevation of 5,950 feet, find a depth
factor of 390. Enter figure 9 with an elevation of 5,950 feet
and obtain a creep factor of 0.67 for the slope gradient of
53 percent. For the roughness of 1.8, obtain a glide factor
of 2.10 from figure 10, using the south azimuth curve
(based on the plantation azimuth of 170 degrees). Note
that the glide factor is greater on south aspects than on
north aspects. The snow pressure for the site is the prod-
uct of the depth, creep, and glide factors of 390, 0.67, and
2.10, respectively, and equals 549 pounds per foot.

USE OF THE SNOW DAMAGE
PREDICTION PROCEDURE

Megahan and Steele (1987) show that plantations are
subject to damage if snow pressures are equal to or greater
than 188 pounds per foot of tree diameter. The overall
prediction success for this procedure averages 80 percent

at a level of confidence of 95 percent (74 percent correct
for plantations predicted as damaged that are actually
damaged and 91 percent correct for plantations predicted
as undamaged that are actually undamaged). The hypo-
thetical plantation site given in the example above had a
predicted snow pressure of 549 pounds per foot andisa
candidate for serious snow damage!

The snow damage prediction procedure presented here
was developed for the study area shown in figure 2. An
important component of the procedure is a relationship
between elevation and the 20-year average (1961-80)
annual snow depth at the time of annual maximum snow
water content at the site. Such a relationship was devel-
oped from 24 snow courses operated within the study area
as a part of the USDA Cooperative Snow Survey network.
The resulting elevation-snow depth relationship may not
apply outside the Weiser and Payette River drainages
(fig. 1). Thus, the prediction results obtained from fig-
ures 8, 9, and 10 should not be used for areas outside
these areas without validation. Megahan and Steele
(1987) discuss the approach for development of the snow
damage prediction procedure for other locations.

At current development, the prediction procedure al-
lows us to define the threshold for damage. Common
sense and our observations suggest that damage is di-
rectly proportional to the amount that predicted snow
pressures exceed the threshold. Additional research is
needed to define the nature of this relationship as well as
recovery capabilities of damaged trees in relation to seed
sources. In the meantime, the snow pressure prediction
procedure provides a means to “red flag” probable damage
potential.

MANAGEMENT IMPLICATIONS

Where ponderosa pine has been chosen for reforesta-
tion, selecting seedlings from the proper genetic seed
source is critical. Seedlings from improper seed sources
may be less likely to recover from snow bending. But it
should not be assumed that pine seed from appropriate
elevations will result in successful plantations on sites
where high snow pressure is predicted and ponderosa
pine was never a predominant species. In high-snow-
hazard areas, forest managers should consider silvicultu-
ral alternatives other than clearcutting and planting
ponderosa pine. If there are no alternatives, then special
care should be taken to protect the planted ponderosa
pines. The site should be carefully inspected, including
during the period of maximum snow accumulation. This
will enable silviculturalists to identify and avoid planting
of localized deep snow sites such as the lee side of adja-
cent uncut stands, the lee side of ridges, and the toe slope
of cut banks or road beds. Additional protection can be
provided by planting trees downhill from local obstruc-
tions that reduce downslope creep and glide, such as
stumps, rocks, and logs. Intense broadcast burning
should be avoided on these sites because this treatment
removes logging debris and stimulates shrub develop-
ment. The shrubs can then outcompete the planted pines
more easily because snow damage has reduced growth
rates of the young trees and the trees, in turn, spend more
years within the snow damage window. Obviously, the

best time to make these assessments is during prepara-
tion of the initial site prescription so that necessary miti-
gating measures can be included.

REFERENCES

Ffolliott, P. F.; Thompson, J. R. 1976. Snow damage in
Arizona ponderosa pine stands. Res. Note RM-332. Fort
Collins, CO: U.S. Department of Agriculture, Forest
Service, Rocky Mountain Forest and Range Experiment
Station. 2 p.

Frutiger, H. 1962. Avalanche control in the starting zone.
[Translation of Swiss guidelines]. Paper 71. Fort
Collins, CO: U.S. Department of Agriculture, Forest
Service, Rocky Mountain Forest and Range Experiment
Station. 60 p.

Frutiger, H.; Kuster, J. 1967. Veber das gleiten und
kriechen der schneedecke in lawinenverbauugen. On
slide and creep of the snow cover among avalanche
defenses. Schweizerischen Zeitschift fuer Forstwesen.
10: 633-643. [Chapelle, E. Translation No. 9. Salt Lake
City, UT: U.S. Department of Agriculture, Forest Serv-
ice, Wasatch National Forest, Alta Avalanche Study
Center.]

Frutiger, H.; Martinelli, M. Jr. 1966. A manual for plan-
ning structural control of avalanches. Res. Pap. RM-19.
Fort Collins, CO: U.S. Department of Agriculture, For-
est Service, Rocky Mountain Forest and Range Experi-
ment Station. 68 p.

Haefeli, R. 1951. Nevere entwicklungstendenzen und
probleme des lawinenverbaus in autruchgebiet. Mit-
teilungen des Eidg Institutes fur Schnee-und Lawinen
forschung. 9: 28-56.

Kangur, R. 1973. Snow damage to young western hemlock
and Douglas-fir. Res. Pap. 21. Corvallis, OR: Oregon
State University, School of Forestry. 11 p.

Leaphart, C. D.; Hungerford, R. D.; Johnson, H. E. 1972.
Stem deformities in young trees caused by snowpack
and its movement. Res. Note INT-158. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Intermoun-
tain Research Station. 10 p.

Martinelli, M., Jr. 1960. Creep and settlement in an al-
pine snowpack. Res. Note 43. Fort Collins, CO: U.S.
Department of Agriculture, Forest Service, Rocky
Mountain Forest and Range Experiment Station. 4 p.

Megahan, W. F.; Steele, R. 1987. An approach for predict-
ing snow damage to ponderosa pine plantations. Forest
Science. 33(2): 485-503.

Panshin, A. J.; DeZeeuw, C.; Brown, H. P. 1964. Textbook
of wood technology. Vol. 1. New York: McGraw-Hill.
643 p.

Powers, R. F.; Oliver, W. W. 1970. Snow breakage in a
pole-sized ponderosa pine plantation...more damage at
high stand-densities. Res. Note PSW-218. Berkeley, CA:
U.S. Department of Agriculture, Forest Service, Pacific
Southwest Forest and Range Experiment Station. 3 p.

Rehfeldt, G. E. 1987. Components of adaptive variation in
Pinus contorta from the Inland Northwest. Res. Pap.
INT-375. Ogden, UT: U.S. Department of Agriculture,
Forest Service, Intermountain Research Station. 11 p.

Schmidt, W. C.; Schmidt, J. A. 1979. Recovery of snow-
bent young western larch. Gen. Tech. Rep. INT-54.
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station. 13 p.

Schubert, G. H. 1971. Growth response of over-aged pon-
derosa pine stands related to stand density level. Jour-
nal of Forestry. 69: 857-860.

Watt, R. F. 1951. Snow damage in a pole stand of western
white pine. Res. Note 92. Missoula, MT: U.S. Depart-
ment of Agriculture, Forest Service, Northern Rocky
Mountain Forest and Range Experiment Station. 4 p.

Watt, R. F. 1960. Second-growth western white pine
stands. Tech. Bull. 1226. Washington, DC: U.S. Depart-
ment of Agriculture, Forest Service. 60 p.

Williams, E.B., Jr. 1966. Snow damage to coniferous
seedlings and saplings. Res. Note PNW-49. Portland,
OR: U.S. Department of Agriculture, Forest Service,
Pacific Northwest Forest and Range Experiment
Station. 10 p.

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