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Pacific Northwest
Forest and Range
Experiment Station
Research Note

PNW-425
June 1985

Abstract

Introduction

Tree Basal Area as an Index of
Thermal Cover for Elk
J. Edward Dealy

LIBRARY U.S. FOREST SERVICE BERKELEY

Po is Se
PSW FOREST AND RANGE
EXPERIMENT STATION

AUG 7 1985

STATION LIBRARY COPY ,

The relationship of basal area to crown closure was studied in five major forest
types of the Blue Mountains of Oregon and Washington. The regressions
developed give wildlife and forest managers a tool for estimating the amount of
crown closure if data are not available from stand examinations. Information is used
in determining quantity and quality of elk thermal cover.

Keywords: Basal area, crown closure, thermal cover, wildlife habitat, Cervidae.

Thermal cover for elk is a habitat feature, such as a stand of conifer trees, that
provides protection against changes in an animal’s body temperature above and
below critical tolerances (thermoregulation). Generally, as crown closure increases,
the effectiveness of a stand to provide thermal cover improves. Optimum thermal
cover is determined by the animal as well as by the habitat. Changes in season,
temperature, wind, and radiation create different thermoregulation demands on the
animal. Crown closures that provide optimum thermal cover under one set of
weather conditions may not be optimum under others. Variability among and within
conifer stands often provides a wide range of crown closure and thermal cover
characteristics that correspond to animals’ changing needs.

For summer ranges in the Blue Mountains region of Oregon and Washington, the
optimum level of elk thermal cover for management purposes is defined as a stand
of coniferous trees at least 12 m (40 ft) tall and exceeding an average of 70 per-
cent crown closure (Thomas and others 1979). Use varies with the animals’ needs
and the amount of cover; for example, during summer high temperature periods,
elk need clumps of trees that provide dense shade (often 90 to 100 percent crown
closure and multitiered crowns) for maximum cooling (Leckenby and Adams 1981),
whereas on a winter range the best available thermal cover may be less than op-
timum for protection.

A better understanding of optimum thermal cover under a variety of weather, site,
and seasonal conditions will help managers to appropriately manipulate forest
stands to provide this component of elk habitat.

~~ $ TATION LIBRARY COPY

J. EDWARD DEALY is principal research ecologist
at the Pacific Northwest Forest and Range Experi-
ment Station, Forestry Sciences Laboratory, PO.
Box 909, Juneau, Alaska 99802.

Methods

Results and
Discussion

The objective of this study was to determine if crown closure of conifers in un-
managed natural stands in the Blue Mountains of Oregon and Washington could
be predicted from tree basal area measurements. If so, stand basal area, which is
usually available, could be used to estimate crown closure and in turn to help
managers manipulate stands to achieve optimum thermal cover.

Plots were located in six geographical areas of the Blue Mountains region of
Oregon and Washington: (1) the north portion of the Umatilla National Forest (Walla
Walla District); (2) the north portion of the Wallowa-Whitman National Forest
(Wallowa Valley District and Hells Canyon National Recreation Area); (3) the
southeastern portion of the Umatilla National Forest (Ukiah and Dale Districts plus
part of the Baker District of the Wallowa-Whitman National Forest); (4) the 4
southwest portion of the Umatilla National Forest (Heppner District); (5) the Malheur —
National Forest (Prairie City and John Day Districts); and (6) the Ochoco National
Forest (Paulina District).

Plots were established by Oregon State Department of Fish and Wildlife research
scientists at specific locations used by elk during all seasons (Leckenby and |
Adams 1981); 609 plots were stratified into five Society of American Foresters cover —
types (Eyre 1980) and were identified by symbols (Hall 1976) (referred to as
“formation-associations ’):

Cover type (SAF) No. Symbol (Hall 1976)
Interior Ponderosa Pine 237 CP
Interior Douglas-Fir 210 CD
Grand Fir 213 CW
Lodgepole Pine 218 CL
Engelmann Spruce-

Subalpine Fir 206 CE

All plots sampled were in unmanaged stands (there was no evidence of tree
removal). A single sample of basal area, using a basal area factor of 10 (Dilworth
and Bell 1967) was taken at each plot with a prism. Four readings of crown closure ~
were taken at cardinal directions and 15 feet from the plot center with a type A
densiometer (Lemmon 1956) and were averaged.

For each cover type, crown closure and basal area (BA) were related by use of
regression analysis. An empirical fit of the data was made by using the models of
Y as a function of Logio (X + 1), and Y as a function of various powers of x (x1,
x2, and x3).

From this analysis the equation model percent crown cover = a + b[LOG;> (BA+1)]
was found to be the best fit.

For 70 percent crown closure in CP, CD, CW, SL, and CE formation-associations,

the corresponding basal area values are 190, 80, 64, 145, and 70 square feet per

acre, respectively (figs. 1-5). The percentage of variation in crown closure '
associated with basal area (r2) ranged from 0.21 for CE to 0.49 for CW (table 1). :
These regressions are best used in cases where crown closure data are not 3
available. It should be noted that, whatever the basal area, when 70 percent crown ;
closure is approached on most curves (figs. 2-4), reliability is relatively high (con- :

fidence limits are narrow). F tests for all the regressions presented for formation-
associations were significant (p < 0.01).

These regressions apply only to unthinned stands. Use of these regressions to
judge the level of thermal cover that will remain after a stand is thinned is not ap-
propriate. Crown characteristics of remnant trees after cutting in a dense stand are
different from crown characteristics of an open stand that developed naturally to
the same level of closure. Use of the crown closure-basal area relationships
described here would overestimate crown closure—sometimes dramatically.

100

Number of plots 89
Formula Percent crown closure = -28 50 + 43.20 [LOG « (BA+1)])
r 45

& 3 i)

Crown closure (percent)

Le)
(=)

0
10 20 50 100 200
Basal area (square feet)
Figure 1—Regression curve showing crown closure as a
function of basal area for the Ponderosa Pine (CP)
formation-association.

Number of plots) 110
Formula: Percent crown closure = -1.27 + 37.33 [LOG (BA+1)]}

i 32

80
~~
apse
8
i“
®
& 60
o 70 percent crown closure for
5 optimum thermal cover for elk
”
oO
B 40
iS
s
i“
oO

20

i)
10 20 50 100 200 250
Basal area (square feet)

Figure 2.—Regression curve showing crown closure as a
function of basal area for the Douglas-Fir (CD)
formation-association.

100

Number of plots: 318
Formula: Percent crown closure = -5.55 + 41.59 [LOG 1. (BA+1)]
re 49

80
~~
=
=
o
o a
2 60 70 percent crown closure for
5 optimum thermal cover for elk
te
=)
n
&
© 40
=
F3
=
O

20

0

10 20 50 100 200 250

Basal area (square feet)

Figure 3.—Regression curve showing crown closure as a
function of basal area for the White Fir (CW) formation-
association.

100
Number of plots: 55
Formula: Percent crown closure = -20.29 + 41.80 (LOG 1. (BA+1)]

rm 41
80
=
¢
@
©
o
& 60
@
i=
=}
a)
&
oO 40 70 percent crown closure for
§ optimum thermal cover for elk
(2)
&
oO
20
(0)
10 20 50 100 200 250

Basal area (square feet)

Figure 4.—Regression curve showing crown closure as a
function of basal area for the Lodgepole Pine (CL)
formation-association.

Number of plots: 37
Formula: Percent crown closure = -5.01 + 40.46 [LOG 1» (BA+1)]

m2)

80
-
—
—
°
a
2 60 70 percent crown closure for
-— F
o optimum thermal cover for elk
=
=]
n
AJ
© 40
‘=
=
°
=
O

20

0

10 20 50 100 200 250

Basal area (square feet)
Figure 5.—Regression curve showing crown closure as a
function of basal area for the Engelmann Spruce-
Subalpine Fir (CE) formation-association.

Table 1—Values by formation-association for regression coefficients a and b in
the formula percent crown cover = a + b[LOG;0(BA+1)], plus information for
evaluating reliability of the crown closure-basal area correlations!

ee

Sampled basal area Significance of

Number Fille ia ee
Formation- of
association plots Maximum Minimum a b F r2
eee
CP 89 220 10 —28.50 43.20 0.01 0.45
CD 110 280 10 = 1027. Sio3 01 acy
CW 318 320 10 — 5.55 41.59 .01 .49
GL 55 270 10 —20.29 41.80 .01 .41
CE 37 270 20 = 5.01 40.46 .01 “2

ee ee SS

1/ Symbols are from Hall (1976), cover types from Eyre (1980): “C” designates a
stand of conifers; P, Ponderosa Pine; D, Douglas-Fir; W, White (or Grand) fir; L,
Lodgepole Pine; and E, Engelmann Spruce-Subalpine Fir; r?, coefficient of
determination.

Literature Cited

Dilworth, J.R.; Bell, J.F. Variable plot cruising. Corvallis, OR: Oregon State Unive
sity Book Stores, Inc.; 1967. 117 p.

Eyre, F.H., ed. Forest cover types of the United States and Canada. Washington, ©
DC: Society of American Foresters; 1980. 148 p. + map.

Hall, Frederick C. Pacific Northwest ecoclass identification: concept and codes.
R-6 Regional Guide 1-3. Portland, OR: U.S. Department of Agriculture, Forest
Service, Pacific Northwest Forest and Range Experiment Station; 1976. 44 p.

Leckenby, Donavin A.; Adams, A.W. “Bud.” Eastern Oregon cover study interim —
report. Inf. Rep. Ser. Wildl. 81-1. La Grande, OR: Oregon Department of Fish and
Wildlife, Research and Development Section; 1981. 68 p.

Lemmon, Paul E. A spherical densiometer for estimating forage overstory density.
Forest Science. 2: 314-320; 1956.

Thomas, Jack Ward; Black, Hugh, Jr.; Scherzinger, Richard J.; Pedersen,
Richard J. Deer and elk. In: Thomas, Jack Ward, ed. Wildlife habitats in
managed forests: the Blue Mountains of Oregon and Washington. Agric. Handb.
553. Washington DC: U.S. Department of Agriculture, Forest Service; 1979:
104-127.

The Forest Service of the U.S. Department of
Agriculture is dedicated to the principle of multiple
use management of the Nation’s forest resources
for sustained yields of wood, water, forage, wildlife,
and recreation. Through forestry research,
cooperation with the States and private forest
owners, and management of the National Forests
and National Grasslands, it strives — as directed by
Congress — to provide increasingly greater service
to a growing Nation.

The U.S. Department of Agriculture is an Equal
Opportunity Employer. Applicants for all Department
programs will be given equal consideration without
regard to age, race, color, sex, religion, or national
Origin.

Pacific Northwest Forest and Range
Experiment Station

319 S.W. Pine St.

PO. Box 3890

Portland, Oregon 97208