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'
Chenzical Conzposition of the Sapwood
of Four Tree Species in Relation to
Feeding by the Black Bear
BY
M. A. RADWAN
Abstract, The contents of sugars, nitrogen, and mineral elements, and the kinds of sugars and
soluble nitrogenous compounds in the sapwood of 20- to 30-year-old Douglas-fir (Pseudotsuga
menziesii (Mirb.) Franco), western hemlock (Tsuga heteropl1ylla (Raf.) Sarg.), western red­
cedar (Tl111ja plica/a Donn), and red alder (AI11us mbra Bong.) were determined on two areas
in western Washington. One area was subject to considerable tree damage hy black bear
(Euarctos americanus Pall us) and the other showed very little damage. On both areas, there
were significant differences among species in contents and kinds of some chemical constituents,
but total sugars and ash were the only components which seemed to be related to bear prefer­
ence. There were only minor differences within species between the two areas, however. Chem­
ical analysis alone, therefore, was not sufficient to explain the problem of bear feeding on tree
sapwood.
Additional key words. Pseudotsuga menziesii, Tsuga heterop11ylla, Tlutja plicata, Alnus rubra,
Euarctos americanus, animal damage, carbohydrate content.
STRIPPING
OF BARK off trees is one of the
most important mammal-caused injuries,
resulting in complete or partial girdling.
In western W'ashington, a number of
wild mammals strip bark, but black
bear (Euarctos american us Pallus) perhaps
causes the most serious depredations.
Typically, bears remove bark and feed on
the exposed sapwood in the spring when
bark strips readily. The injury is usually
basal, but occasionally may extend up
the trunk to a height of 50 feet (Childs
and Worthington 1955). The most serious
damage occurs in lightly stocked stands
where rapidly growing trees of the 20- to
30-year-old class are selected (Levin 1954).
It is difficult to put an accurate
monetary value on losses caused by
black bear. The information available
in western 'Vashington, however, has
induced several public and private
organizations to support a control pro­
gram of hunting and trapping. The pro­
gram, which covers some 1,200 square
miles of forest land, has apparently
reduced bear damage, but better control
methods are needed.
Thus, the Cooperative Bear Research
Project1 was established in Olympia,
'Vashington, to investigate several phases
of the problem. Results of one study,
conducted on a low-elevation Douglas-fir
type forest north and south of the Che­
halis River, indicated a descending order
of preference for the principal species, as
1 A cooperative research program of bear research
organized in 1962 and sponsored by the Washington
Forest Protection Association. Cooperating agencies
arc: Washington State Department of Game,
Washington State Department of Natural Re­
sources, Weyerhaeuser Company, University of
Washington, U. S. Bureau of Sport Fisheries and
WildJif.,, and U.S.D.A. Forest Service.
The author is Principal Plant Physiologist,
.
Forestry Sciences Laboratory, Pacific Northwest
Forest and Range Expt. Sta., Forest Service, U. S.
Dept. of Agric., Olympia, Wash. He gratefully
acknowledges the help of the biologists of the
Cooperative Bear Research Project, Washington
Forest Protection Association, in collecting the
sapwood samples; and the assistance of Walter D.
Ellis, chemist on the staff, in conducting the
chemical analysis. Manuscript received Dec. 27,
1967.
Reprinted from FOREST SciENCE, Volume 15, Number 1, March, 1969
Purchased by the FOl'est Se1·vice, U.S. Department of Agriculture, for official use.
follows: Douglas-hr (Pseudotsuga men­
ziesii (Mirb.) Franco), western hemlock
(Tsuga heterophylla (Raf.) Sarg.), western
redcedar (Thuja plicata Donn), and red
alder (Alnus rubra Bong.).2 Of the many
tree characteristics that may have in­
fluenced this preference, chemical compo­
sition of the sapwood in the spring seemed
a major factor. This paper, therefore,
reports chemical analyses of sapwood
tissue collected in May from the above
four species gwwing on two areas, with
and without bear damage.
The Study Areas
The two areas studied had been selected
earlier by the Cooperative Bear Research
Project. Both are located in Grays Harbor
County, Washington; one north of the
Chehalis River valley where considerable
bear damage occurs, and the other south
of the valley where very little damage has
been reported, although bears are present.
Together, the two areas encompass ap­
proximately 180 square miles.
Materials and Methods
Sapwood of Douglas-fir, western hemlock,
western redcedar, and red alder trees
from each of the two areas served as
test material. Only healthy trees of the
20- to 30-year-old class and approximately
10 to 12 inches dbh were used.
Tissue Sampling. Two composite sapwood
samples, of approximately 400 g each,
were taken during the morning hours in
�May 1965 from each of the four species on
both areas. Each sample was taken from
10 trees selected from approximately 5
acres of similar elevation, aspect, soil
series, and vegetation composition. In
each case, the bark was removed and
0.2- to 0.4-cm-deep samples were cut
out with a stainless steel knife. These
depths approximated those found on
2 Pierson, Douglas J. Black bear study; black
bear-forest rei ationships. Washington State Game
Dept. Proj. W71-R-2, job completion rep., 13 pp.,
mimeo., 1965.
12 /
Forest Science
bear-damaged trees on the study areas.
Sample Preparation. Each sample was
cut into .7\!-inch-square pieces with stain­
less steel scissors and homogenized in a
blender with stainless steel blades. Sub­
samples were taken for moisture deter­
mination and fot· analyses of sugars
and soluble nitrogenous compounds.
The remaining tissue was dried to
constant weight, ground to 40 mesh
in a Wiley mill, and stored in closed
containers at -15° C until analyzed.
Sugars and soluble nitrogenous com­
pounds were extracted from the fresh
tissue by adding hot alcohol at a final
concentration of 80 percent and extracting
in Soxhlet for 12 hours. The alcoholic
solution was then made to volume with
the extracting solvent.
For sugar determinations, portions of
the extracts were concentrated on a water
bath and clarified with lead acetate.
Additional portions of the extracts
were partitioned between water and
chloroform to remove the lipids and
chlorophyll. Aqueous phases were then
evaporated to dryness with warm air
jets and residues dissolved in 1-ml
amounts of distilled water. Resulting
extracts were used for separation of
sugars and soluble nitrogenous com­
pounds by paper chromatography.
Analytical Methods. Moisture was de­
termined in a forced-air oven at 65° C,
and total nitrogen (N) was measured in a
Coleman Model 29 analyzer.
Ground tissue was ashed in platinum
crucibles at 500-550° C, and the ash was
analyzed for phosphorus (P) by the colori­
metric method of Fiske and Subbarow
(1925), calcium (Ca) by a titrimetric
technique (Horwitz 1960), and mag­
nesium (Mg), iron (Fe), and manganese
(Mn) by the magnesium ammonium
phosphate titrimetric method, 0-phen­
anthroline colorimetric method, and the
colorimetric periodate procedure, re
spectively (Chapman and Pratt 1961).
Results
Sugar was determined in the clarified
tissue extracts before and after hydroly­
sis by Hassid's eerie sulfate method
(1937). Reducing and total sugars were
calculated as glucose and nonreducing
sugars as sucrose.
Concentrations of the chemical constitu­
ents of sapwood, summarized in Table 1,
are expressed on a fresh-weight basis to
allow comparison of levels as the animal
encounters them in feeding.
Sugars and soluble nitrogenous com­
pounds were separated by two-dimen­
sional chromatography on 'Vhatman No.
1 paper. The paper was developed in
the short direction with liquid phenol:
water (4: 1 V/V) with 0.04-percent 8­
hydroxyquinoline, and then in the long
direction with the upper phase of n­
butanol: acetic acid: water (25:6:25 V/
V/V) (Block et a/. 1958). Sugars and
soluble nitrogenous compounds, respec­
tively, were located on separate chro­
matograms with aniline-diphenylamine­
phosphate (Block et a/. 1958) and 1.0
percent ninhydrin (Thompson et a/. 1951)
color reagents. Identity of resultant spots
was then made from their RF values and
by co-chromatography with authentic
compounds.
Moisture. There were only slight vari­
ations in moisture content among species
and none within species from the two
study areas (Table 1). The average
moisture content for all samples was
approximately 90 percent, indicating a
low dry matter content of about 10
percent.
dslz. The ash content was lowest in
Douglas-fir, intermediate in hemlock,
and highest in redcedar and red alder
(Table 1). Here again, there were no
differences between the species from
the two areas.
Mineral elements in the sapwood, as
shown by the ash content, were low. They
averaged less than 1 percent on a fresh­
weight basis, and about 7 percent wh;;"n
TABLE 1. Concentration of selected chemical constituents in the fresh sapwood of four tree
species in tl1e spring.'
Species and
collection area2
Moisture
Ash
Reduc- Nonreing clueing Total
sugars sugars sugars
N
p
Mn
Ca
Mg
. 013a
. 015a
. 010ab .00027a ,00065a
. Ollah . 00031a .00060a
Fe
Percmt offresh weight
Douglas-fir
Damage
.43a
89.2a3
90.2ah
.43a
!\Tondamage
Western hemlock
.65b
89.2a
Damage
.64b
89.1a
Nondamage
Western redcedar
91.4b
.83c
Damage
91.4b
.SSe
Nondamage
Red alder
91.3b 1 .02c
Damage
90.6ab
.91c
Nondamage
3.63a
3. 75a
.64a 4.30a
.58a 4.36a
. 13a
.12a
1.97c 1.62c 3.67a .13a
1. 76c 1.96c 3.83a .13a
.034a
. 031a
.033a .021h . 013cb
.035a .029h . 015c
.00042a .0021 b
. 00044a .0019 b
. 00027c
. 00018c
.SOh 1.63c 2 . 69h
. 65b 1.93c 2. 69b
. 13a .032a
. 13a .028a
. 025h .009a
. 022h .009a
. 00042a
.00045a
.50a 2.10c
.43a 1.80c
.17h .035a
.22c .030a
.025b .040b
. 025b .032e
.0012 b . 00079a
.0012 b . 00074a
1.56c
1 . 45c
1 Values are averages of two composite samples from 10 trees each.
2 Damage and nondamage areas are, respectively, north and south of the Chehalis River.
3 Means in each column followed by the same letter(s) do not differ significantly at the 5-percent level, with a
modification of Tukey's test (Snedecor 1961).
volume 15, number 1, 1969
/ 13.
expressed on a dry-weight basis This
indicates that an average of approxi­
mately 93 percent of the dry n)atter
was organic in nature.
Sugars. Concentrations of reducing, non­
reducing, and total sugars differed sig­
nificantly among species. Within species,
however, sapwood from the two areas
contained similar amounts of the sugars.
Total sugar content was highest in
Douglas-fir and to a lesser extent in
hemlock, medium in redcedar, and lowest
in red alder. The concentration range of
the sugars, calculated on a fresh-weight
basis, was 1.80 to 4.36 percent (Table 1).
However, calculation to an organic­
matter basis shows that the average
concentrations of total sugars were 43,
37, 33, and 24 percent in Douglas-fir,
hemlock, redcedar, and red alder, re­
spectively. Sugars, therefor , were one
of the major organic constituents of the
sapwood.
Contributions of reducing and nonre­
ducing sugars to the total sugar content
varied among species. The simpler re­
ducing sugars predominated in Douglas-fir
and, to a lesser extent, in red alder. In
contrast, the more complex nonreducing
sugars contributed approximately 50 per­
cent of the total sugars in hemlock and
prevailed over reducing sugars in red­
cedar.
Three sugars were detected chromato­
graphically: glucose, fructose, and sucrose.
In addition, visual examination of the
chromatograms showed that in all ex­
tracts, concentrations of glucose and
fructose were approximately equal. All
sapwood samples, therefore, contained
the same kinds of sugar and glucose/
fructose ratio.
Nitrogen. Total N content in sapwood
ranged from 0.12 to 0.22 percent, indi­
cating a low level of this element in the
tissue. Regardless of the area, Douglas­
fir, hemlock, and redcedar contained
similar amounts of the element. In con­
14 / FQT'est Science
trast, red alder from the nondamage
area was higher in N than that from the
damage area. Also, the average N con­
tent for red alder sapwood of both
areas at 0.20 percent was significantly
higher than that in sapwood of the other
three species.
vVithin species, nitrogenous compounds
detected by chromatography did not
differ with area of collection. However,
amounts and, to a lesser extent, kinds
of nitrogenous compounds varied among
the species. Alanine, serine, aspartic
acid, and 'Y-aminobutyric acid were
found in all extracts. Additional nitro­
genous compounds were also found in the
extracts as follows: traces of glycine in
Douglas-fir and hemlock, traces of gluta­
mine in redcedar, and traces of glutamic
acid and large amounts of citrulline in
red alder. 'Vith the exception of citrul­
line, which has been reported only in a
few plants including alder, the nitrogenous
compounds found in sapwood occur
widely in plants (Bollard 1957).
Jvfineral Elements. Concentrations of some
of the mineral elements known to be
important in animal nutrition (Maynard
1951) are shown in Table 1. For all
samples, the average concentrations of
P, Ca, Mg, Fe, and Mn were .032, .022,
.017, .00059, and .00090 percent, re­
spectively.
With the exception of Mg in red alder,
where sapwood from the damage area
contained higher concentrations of the
element than that from the nondamage
area, contents of the individual elements
within species did not differ with the
area. Among species, there were no
differences in the P contents and only
a limited number of major differences in
contents of the other elements. Thus,
Douglas-fir was lowest in Ca, hemlock
highest in Mn, redcedar lowest in Mn, and
red alder highest in Mg and Fe.
Discussion and Conclusions
The analysis of sapwood shows that sugar
was a major nutrient component of the
tissue and mineral elements and nitro­
gen occurred in minor amounts. Also,
sapwood tissue w s lower in mineral
elements and nitrogen, but higher in
sugar than vegetation which serves as
bear food on the study areas (unpublished
chemical analyses of red whortleberry
[Vaccinium parvifolium J. E. Sm.] and
salal [Gaultheria shallon Pursh] shoots
collected from the same areas and at the
same time as tree sapwood). Thus, sap­
wood appeared to be a good source of
sugar for bear nutrition.
On both areas, there were significant
differences among species in contents
and kinds of some chemical constituents,
but total sugars and ash were the only
components which seemed to be related
to bear preference. High preference was
generally associated with high sugar
and low ash. Obviously, it is impossible
to state conclusively that one, or a
combination of these two components,
was actually responsible for the prefer­
ence. However, one may speculate that
sugar alone was the important factor.
That sugar was apparently a major
animal nutrient in sapwood tends to
support this speculation. Furthermore,
sugar has long been recognized as ex­
tremely important in the nutrition and
metabolism of animals (Fruton and
Simmonds 1958), and has been suspected
to be a factor in food preference of some
animals (Plice 1952, Radwan and Camp­
bell 1968). However, the fact that bears
do not feed on tree sapwood south of the
Chehalis River, despite the similar sugar
content and the general chemical compo­
sition of sapwoods on the two study areas,
is inconsistent with the sugar hypothesis
or any other explanation of preference
based on the data from this study. In
addition, bears' preference for trees is
known to vary by State (Lutz 1951,
Glover 1955, Zeedyk 1957) and even by
area within western Washington.
In this study, therefore, chemical
analysis alone was not sufficient to
explain the problem of bear feeding on
tree sapwood. Attempts by other workers
(Fritz 1951, Lauckhart 1956) were also
unsuccessful, mainly because of con­
jecture in absence of factual information.
Clearly, the problem is influenced by
many factors involving both the animal
and its habitat. Only by concurrent
studies of these factors can bear behavior
toward the trees be adequately under­
stood.
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