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The Importance of Snags to Pine Marten Habitat In
the Northern Sierra Nevada1
Sandra
x.
Martin and Reginald H. Barrett2
Abstract. Snags were found to represent 2~ of all
dens located during a study of pine marten (Martes ueric&Da)
ecology conducted on the east slope of the Sierra Nevada.
Use of snags as den si tea by two geographically isolated
groups of pine marten occupying dissimUar habitats 1d.thin
the study area was significantly different. Relative levels
of snag utilization may be related to the availability of
other preferred den types within a specific marten home range.
INTRODUCTION
Heavy trapping pressure and major loss of
habitat through fires and logging have been blued
for the extreme reductions exhibited by many pine
marten (Martes americana) pol'\Jlations in North
America in the past century (Barbant 1922,
Yeager 1950). Conservationists in California,
alarmed by declines in local marten populations
which. began as early aa 1900, urged the passing of
a legislative ban on marten trapping in the state
(~lor 1913, GriDDell et al. 1937). Such a ban
was enacted in 1953 and remains effective to date.
By the 1970's, casual and intermittent information
suggested that marten populations had regained a
healthy status wherever semi-secluded tracts of
appropriate forest habitat were found in the state,
but few detailed studies of marten in California
existed (Schempf and White 1977). In 1979,
University of California personnel, in cooperation
with the u.s. Forest Service, undertook long-term
investigations of pine marten ecology. '!he data
presented below are part of this larger study.
METHODS
'!he study vas conducted in a 40 m2 area
defined by the upper watershed of Sagehen Creek,
located on the east slope of the Sierra Nevada in
the Truckee Ranger District, Tahoe National Forest,
Cal1.forn1a. Elevations in the study area range
from 1880 a in the east to 2620 m on the western
edge, S km from the Sierra Nevadan crest. Major
habitat types in the watershed include those
defined by meadow/fen/lodgepole pine (Pinus
contort&) complexes in riparian zones at lower
elevations. Drier forests dominated by Jeffrey
plne (Pinus 1effrex1) are found on south-facing
1
slopes. '!he Jeffre'f pine stands mix 1f1th wh1 te
fir ~Abies concolo~) at higher elevations. Red
fir Abies magnifica) becomes prevalent on even
higher mountain slopes.
Field work from which data below were extracted
began in June 1981 and continues to date (June
1983). Pine marten were li vetrapped in 20 x 20
x 60 em Toll8ha.vk traps ba1 ted 1d. th fish and
commercially prepared scent. 'Dle traps were placed
at stations on a 200 x 600 m grid. Trapped marten
were drugged with ketamine and acepromazine for
handling, and a radio collar was placed on each
marten.
Radio collared marten were located once daily
with radio receiving equipment and hand held
antennas. Marten were tracked to their precise
locations. Active marten could often be sighted
at a distance while being followed, and the dens
of inactive marten could be directly approached.
Habitat was characterized by collecting data
regarding the vegetation on 0.1 acre (0.25 ha)
circular plots. Plot centers were placed using
the marten trapping grid described above. Data
collected included tallies of all deadwood features
on the plots. Deadwood was interpreted as comprising all snags, stumps, and logs. Specificall;r,
a snag was tallied when found to be greater than
20 em in diameter at breast height (DBH) and taller
than 2 m. A stump was counted if it vas at least
20 em wide at its highest point. A stump was
defined as being less than 2 m tall. A log was
counted on the plot when it vas at least 40 em
1d.de at a point 1.5 m above its base, at least
2 m long, and at least half of the log lay on
the plot,
Data used below to estimate densities of
deadwood features in different subregions of the
study area were taken from habitat plots located
within the home ranges of radio collared pine
marten. Den sites were tabulated·for eight
1ndividual marten (four & 1 four t). Only dens
found at least 24 hours apart during tracking
were counted. For instance, five locations of the
.
Paper presented at the Snag Habitat Management Symposium (Northern Arizona University,
Flags~, Arimona, J~e 7-9, 1983).
Authors • address J Dept. of Forestry and
Resource Management, University of California,
Berkeley, Ca. 94720
114
same marten in the same d~n in a single 24 hour
(midnight to midnight) period would be tabulated
as one den site location.
Table 1. Den types used by pine mart~n in the
upper and lower subregions of Sagehen
Creek Watershed.
Relationships between marten sex and den type,
and between geographic location of marten dens and
den type, were examined with the 2 x 2 test of
independence using the G-statistic (Sokal and Rohlf
1969• 591-921. A.2Student's t test, modified to
account for d': = a:, was used to compare average
densities of ~abitlt components in two different
subregions of the study area (Cochran 1964).
Total number of den
sites located
Number of snag dens
Percent of total
N\UIIber of deadwood dens
Percent of total
RESULTS
Pine marten investigated in this study typically occupied a den for periods ranging from one
to seven days. They remained inactive within the
den for several hours at a time, often leaving to
hunt and returning later. Marten are usually
solitary except during brief mating liasons in
summer. Another social grouping forms when kits
stay with their mother from birth in early spring
until late summer. r.,embers of these social groups
often occupy the same den simultaneously.
Upper
Basin
Lower
Basin
165
29
17.6
121
85
28
)2.9
49
7).)
57.6
the designated areas. Mean snag densities were
10.74/acre (26.54/ha) in the lower basin (below
2250 m elevation) and 8.78/acre (21.70/ha) in the
upper basin (table 2). Using a modified t test,
no significant difference between these two means
was found (test statistic=0.699 1 t'=1.674).
Deadwood dens (snags, stumps, and logs) pooled
together represented 6~ of all marten den sites.
As with snag dens alone, the hypothesis of independence between geograyhic location (upper and lower
basin) and den type ~deadwood and other dens) was
rejected (G=6.224, X:
=3.841 ). Marten
(l::S0.05, d.f.=1
in the upper basin were found to be using proportionally more deadwood dens than those in the
lower basin (table 1).
·
A common movement pattern identified was for
a pine marten to occupy a den for one or two days,
then shift to another den w1 thin a few hundred
meters of the first. Another shift would occur
a day or tKo later. Dens within one specific
area of the home range were often used for a few
weeks, with the marten then completely shifting
activity and den occupation to another area of its
home range. Specific den sites were often used
again after the initial occupation. Home ranges
of the pine marten overlapped to varying degrees,
and some sites were used by two or even three
individuals at different times.
Densities of stumps and logs in the two geographic areas were obtained in the same manner that
snag densities were calculated. Modified t tests
indicated that stump density in the upper basin
was significantly higher than that in the lower
basin (test statistic=2.)2, t'::s1.680)(table 2).
A similar result was found when log densities in·
the upper and lower basin areas were compared
(test statistic=).o., t'=1.671)(table 2).
Types of den sites included the following
categories; snags, stumps, logs, the canopy of
trees, willow (~ spp.) clumps, other brush,
rock slides, and subni vea.."l dens of unknown structure. Snags represented 23% of all den si. tea
found (n=250). When only those dens used more
than once were considered, snags were found to
incorporate 27% of the total (n=90) •
DISCUSSION
Dens were divided into the categories •snags"
and "other dens". Den type was then found to be
independent of the sex of the occupying marten
2
(G=0.)01, 'X: Cl=0.05 , d.f .at =).841 ). Marten den
locations were also divided into two geographic
categoriesa those of marten located in the western half of the study area, above 2250 m elevation
(upper basin marten), and those found in the
eastern end of the watershed (lower basin marten).
The independence of snag and non-snag dens was
tested with these two geographic categories. The
null hypoth~sis of independence was rejected
(G=7.268, X Cl:Oe05 , d.f 1=).841 ). Specifically,
marten in the lower basin area used proportionally
more sn~s as dens than did upper basin marten
(table 1).
Snags are an important habitat component for
pine marten ln the Sagehen Creek watershed. One
quarter of all den sites located for eight radio
collared marten were in snags. 'lbe sex of marten
appears irrelevant to discussions of den type
selection. Geographic location, i.e. separating
marten found in the upper or lower elevations of
the study area, had a significant effect' on analysis
of den type selection. This geographic separation·
is justifiable on two counts J the categories
represent two completely divided groups of marten
occup,ying dissimilar habitat. Home ranges of the
four marten found in the lower basin overlap
extensively, but none of these overlap with home
ranges of any upper basin marten. A similar
situation exists among the four upper basin marten.
~arten in the lower basin occupy lodgepole pine/
white fir forests. Those in the upper basin are
found in red fir/white fir forests.
Snag densities for each of these two geographic areas were taken from habitat plots within
Marten in the lower basin used snags proportionally more than did upper basin marten. However,
.=
115
Table 2. Average densities of deadwood features in the upper and lower subregions
of Sagehen Creek Watershed.
Snags per acre
s2
i
n
Upper
Basin 41
Lower
Basin 2?
8.?8 1?0.98
10.?4
99.43
Snags
per
hectare
Stumps per acre
s2
i
n
Stumps
per
hectare
Logs per acre
-2
n
i
8
Logs
per
hectare
21.?0
41 16.10
169.39 39.?8
41 1).1?
19?.20 32.62
26.54
2?
8.89
148.?2 21.9?
2?
4.44
94.8? 10.8?
snag densities appear to be similar in these two
areas. Marten in the upper basin used more deadwood dens overall than did marten in the lower
basin. These facts appear somewhat contradictory,
but when stump and log densities in the two subregions are compared, a probable explanation
emerges. The upper basin habitat provides a greater
number of stumps and logs as possible den sites
for marten than can be found in the lower basin,
Marten are opportunistic, to a degree, in den
selection. Deadwood sites provide preferred den
locations, but where densities of snags, stumps,
and logs are not high, other habitat features
are utilized.
Snags probably gain importance as marten den
sites in areas where logging has not recently
occurred, or where stump and log densities are
naturally low. A history of timber harvest activities in the Sagehen Creek basin reveals that
extensive logging occurred at lower elevations
between 1900-1920, and that a timber sale in 196?
resulted in str11>-cut harvests in the upper basin
(West 1982). The logging operation of 16 years
ago is the most likely reason for the current high
log and stump densities found in the upper basin.
Further analyses of den characteristics are
being undertaken, and may disclose the importance
of features beyond simple availability in marten
den selection. Characteristics of suspected
significance include size (DBH, height, and length),
state of deterioration, proximity and amount of
overhead canopy cover, and clustering of acceptable
dens.
Snags should be respected as potential den
sites in all areas where the marten occurs. However,
snags are particularly important where logging has
not been common. The natural decay of snags would
116
also be a primary factor in providing stUlllps and
logs for marten dens in such undisturbed habitat.
LITERATURE CITED
Brabant, A. 1922. The fire fiend's threat to the
fur trade. Illustrated Can. For. l'.ag.
18(12)• 12o4-120S.
Cochran, W. G. 1964. Approxitna.te significance
levels of the Behrens-Fisher teat. Biometrics
201 191-195.
Grinnell, J •• J. ·s. Dixon, and J. M. Linsdale. 193?.
Fur-bearing mammals of California. 3?5 P•
Univ. of Calif. Press, Berkeley, Calif.
Schempf, P. F. and fe!. White. 19??. Status of six
furbearer populations in the mountains of
northern California. 51 p. USDA Forest
Service, San Francisco, Calif. (unnumbered
publication)
Sokal, R. R. and F. J. Rohlf. 1969. Biometry.
776 p. w. H. Freeman and Co., San Francisco,
Calif.
Taylor, W. P. 1913. Fur-bearing mammalsJ an.
unappreciated natural resource. Science
3?• 485-487.
West,
c. I. 1982. Management options for the
Sagehen Creek basin. 110 P• M. s. F~ Prof.
Pap., Univ. of Calif., Berkeley.
Yeager, L. E. 1950. Implications of some harvest
and habitat factors on pine marten management. Trans. N. Am. Wildl. Conf. 1St :319-
334.