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Implications of Snag Policies on Management of
Southwestern Ponderosa Pine Forests1
Peter F •. Ffolliott
2
Abstract.--The Southwestern Region of the USDA Forest
Servi~e is providing habitat for snag-dependent wildlife
species in ponderosa pine (Pinus ponderosa Laws.) forests.
To evaluate alternative snag retention policies, study areas
representing five silvicultural treatments and virgin conditions were evaluated to determine whether or not they met
the policies under present conditions, and if not, whether
or not policies would be met through natural mortality in the
future.
volumes would be lost and revenues foregone if
snags were artificially created.
INTRODUCTION
Southwestern ponderosa pine snags are an
important habitat component for cavity-nesting
wildlife species. At least 49 species of birds,
along with the 10 species of mammals and numerous
species of insects and herpetofauna, use tree
cavities in southwestern forests (Scott and Patton
1975, USDA Forest Service 1977). These cavitynesters depend on snags and dead portions of live
trees for denning, roosting, feeding, and nesting
cover. Sixty-three percent of the birds and
three-fourths of the mammals that are snag-dependent in southwestern forests are insectivorous
(USDA Forest SerVice 1977). These birds and
mammals are often credited with insect control
that helps to maintain a healthy ecosystem (Thomas
et al. 1975).
STUDY AREAS
Southwestern ponderosa pine forests exist in
uneven-aged stands of small, even-aged aggregates
ranging from a few trees to stands of several
acres (Schubert 1974). Cutting practices imposed
in these forests involve a variety of silvicultural
treatments undertaken to achieve a variety of multiple use objectives.
Data were obtained from study areas representing different silvicultural treatments, including
the Beaver Creek Watersheds, the Heber Watersheds,
and the Castle Creek Watersheds (fig. 1). Additionally, data were gathered on study areas
representing virgin conditions, including the
Castle Creek Watersheds, and the Fort Valley and
the Long Valley Experimental Forests.
Prior to recent years, snags were viewed as
potential fire hazards, harborers of disease, and
worthless space fillers (McClelland and Frissell
1975). However, in a change of policy, the Southwestern Region of the Forest Service is to provide
adequate habitat to maintain self-sustaining populations of snag-dependent wildlife species (USDA
Forest Service 1977).
Snag retention management alternatives were
examined on 10 study areas representing five silvicultural treatments; shelterwood-seed tree cut,
shelterwood-stripcut, group selection cut, thinningpatch cut, and uniform thinning (table 1). The
virgin forests studied provided a reference.
Clearcut treatments were not analyzed, since this
treatment precludes snag retention.
To provide snags to perpetuity to ensure
viable populations of snag-dependent wildlife
species, it could become necessary to sacrifice
timber production. Therefore, a study was undertaken to determine; (a) whether or not snag retention policies could be met solely through
natural mortality; and (b) whether or not timber
METHODS
Snag retention policies of one, two, or three
snags per acre were examined under present conditions on the study areas, and under simulated
conditions at the end of the 20-year period. Snags
were defined as standing dead trees at least
12 inches in diameter and 10 feet in height. No
differentiation was made between "hard" snags,
those composed of sound wood, and 11 soft" snags,
those characterized by advanced decay and
1
Paper presented at the Snag Habitat Management Symposium, Northern Arizona University,
Flagstaff, Arizona, June 7-9, 1983.
~eter F. Ffolliott is Professor, School of
Renewable Natural Resources, University of Arizona,
Tucson, AZ.
28
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Figure 1.
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~~~~Ponderosa Pine
Location of study areas.
deterioration. Twenty years was selected as the
simulation period because it coincides with a
"typical" cutting cycle in southwestern ponderosa
pine forests (Schubert 1974).
met through natural mortality, and if snags were
artificially created , timber volumes l os t and
revenues foregone were ascertained.
To quantify possible timber volumes lost due
to the creation of snags, it was necessary to
define a "target" stand toward which forest management should be aimed. A balanced uneven-aged stand
was selected as the "target" structure. To define
this structure , three parameters were quantified;
a "q" ratio3 of 1. 3, a maximum tree diameter of
24 inches, and a residual basal area of 60 square
feet per acre. Other quantifications of these
parameters could have been made, or another "target"
could have been selected . The "target" structure
presented herein is simply illustrative.
To help determine whether or not snag retention policies can be met in the future, a computer
simulation model called SNAG was develop ed. This
model estimates natural tree mortality and snag
retention in southwestern ponderosa pine forests
through time, given inputs of current growing
stock and, if available, existing number of snags.
In this study, outputs from SNAG were used to
determine whether or not the study areas would
meet s uggested s nag r etention policies through
natural mortality.
Initially, it was determined whether or not
snag r etention policies could be met solely
through natural mortality. To this end, initial
forest inventories of the s tudy areas were
repeated to summarize current growing stock and
existing number of snags. These summaries provided the information to examine the alternative
policies under present conditions and to formulate
inputs to SNAG to simulate conditions at the end
of the 20-year period . If th e policies were not
After definine a "target" stand, any number
of scenarios that specify when during the 20- year
period snags ar e to be created, and what size of
trees will be sacrificed could be developed to
analyze timber volumes lost (Nowakowski 1980).
3de Liocount's "q" ratio r eflects a decreasing
geometric series of numbers of trees per acre for
successive diameter c l asses.
29
Table 1.--Silvicultural treatments and study areas
Silvicultural treatment
and study area
Size
(acres)
Shelterwood-seed tree cut
Beaver Creek Watershed 8
1,892
Residual densities of 60 sq ft of basal
area in size classes 10 in dbh and less,
70 sq ft in 12-22-in dbh classes; stands
averaging 24 in in dbh and larger cut in
shelterwood-seed tree system.
Shelterwood-stripcut
Beaver Creek Watershed 14
1,267
One-third of area cleared in irregular
strips averaging 60 ft wide; intervening
leave strips thinned to 60 sq ft of basal
area.
Group selection cut
Beaver Creek Watershed 13
867
Residual density of 99 sq ft of basal area,
in uneven-aged structure.
134
Residual density of 98 sq ft of basal area,
in uneven-aged structure.
Thinning-patch cut
Beaver Creek Watershed 10
571
Irregular openings (1 to 10 acres); timber
in leave areas cut in similar manner as
Beaver Creek Watershed 8.
West Fork Castle Creek
900
One-sixth area cleared in patches, with
remaining five-sixths cut to residual
density of 65 sq ft of basal area, in
unevened-aged structure.
Uniform thinning
Beaver Creek Watershed 17
299
Residual density of 30 sq ft of basal area,
in uneven-aged structure.
Heber Watersheds
Virgin forests
. East Fork Castle Creek
Description
1,163
Density of 134 sq ft of basal area, in
uneven-aged structure.
Fort Valley Experimental
Forest
76
Density of 175 sq ft of basal area, in
uneven-aged structure.
Long Valley Experimental
Forest
1,280
Density of 214 sq ft of basal area, in
uneven-aged structure.
The one presented below represents a situation
in which timber volumes lost are minimized.
the conditions specified by the "target," resulting
in additional, but unknown, costs. In this
scenario, it was assumed that, if growing stock had
to be sacrificed, snags would be created at the
present time, starting with trees 12 inches in
diameter. By starting with the smallest trees,
less timber volumes would be lost to subsequent
harvesting.
Trees in surplus to the "target" stand structure, if present, would be those initially used
to create snags. Of course, merchantable volumes
lost by creating snags from surplus! trees would
have to be reflected by losses in t;·imber volumes
and revenues foregone at the time of harvesting.
It was assumed that any timber harvesting operation would occur at the present time.
Calculations of revenues foregone were based
on stumpage value of $75 per thousand board feet
of lumber. Future values of stumpage at the end
of the 20-year period were calculated by the
general compound interest formula at 5 and 10
percent. Revenues foregone represent approximated
losses only, as these values will undoubtedly vary
If additional trees were required to meet a
policy, a sacrifice in growing stock prescribed
by the "target" stand would be necessary. Unfortunately, this latter action would delay meeting
30
with distribution of harvest volumes by size and
grade.
acre. Future values of this loss would be $18 at
5 percent and almost $45.50 per acre at 10 percent
interest rates. If managed for two snags per acre,
losses would be 165 board feet per acre. ' 'Present
value of this loss is approximately $12.50 per
acre, and future values would be in excess of $32.50
at 5 percent and nearly $83.50 at 10 percent interest rates. Finally, if managed for three snags per·
acre, losses would approach 280 board feet per acre,
presently valued at $21 per acre. Future values
of this loss would be about $56 at 5 percent and
$141 at 10 percent interest rates.
RESULTS AND DISCUSSION
Analysis of snag retention policies met
solely through natural mortality indicated that,
under present conditions, none of the study areas
subjected to silvicultural treatments met any of
the suggested policies. In fact, only one of the
virgin forests examined, the Long Valley Experimental Forest, met snag retention policies under
present conditions, and this area met all of the
policy standards.
It must be emphasized that the study areas
evaluated are illustrative only, since implementation of a given silvicultural treatment usually
varies in its prescription. Therefore, the magnitudes of timber volumes lost and revenues foregone
discussed are only general guidelines.
Under simulated conditions at the end of the
20-year period, the areas subjected to the shelterwood-seed tree cut, the shelterwood-stripcut, and
the group selection cut would only meet a snag
retention policy of one snag per acre through
natural mortality. Areas representing the other
silvicultural treatments would meet none of the
policies. All of the virgin forests would meet
snag retention policies of one or two snags per
acre and, of course, the Long Valley Experimental
Forest would support three snags per acre.
SUMMARY
Depending upon the snag retention policy followed, some southwestern ponderosa pine forests
will meet the policy requirements at the end of a
20-year period through natural mortality. Other
forests subjected to particular silvicultural
treatments will require artificial creation of
snags. As a result~ timber production may be
sacrificed.
From this analysis, virgin forests would meet
a snag retention policy of one or two snags per
acre through natural mortality at the end of the
2~year period. If three snags per acre were
required, ·additional snags must be artificially
created. Losses in timber volumes in virgin
forests being managed for three snags per acre
(and if these forests are to be harvested) would
approximate 60 board feet per acre, mostly in
trees 12 inches in diameter. Present value of
this loss is $4.50 per acre. Future values of
this loss at the end of the 20-year period would
be nearly $12 per acre at 5 percent and approximately $30.50 per acre at 10 percent interest
rates.
To determine timber volumes lost and revenues
foregone through artifici~l creation of snags, a
"target" stand toward which forest management
should be aimed must be defined. Also, the point
in time when snags are to be created must be determined. Given this information, and assuming a
specific interest rate, it is possible to estimate
timber volumes lost and revenues foregone.
While the results of this study pertain to
southwestern ponderosa pine forests, a similar
approach could be helpful in evaluating snag policies with respect to timber volumes lost and
revenues foregone in other forest types
The study areas subjected to the shelterwoodseed tree cut, the shelterwood-stripcut, and the
group selection cut would generally meet a policy
of one snag per acre through natural mortality
over the 20-year period. However, if managed for
two snags per ·acre, losses of ~pproximately 115
board beet per acre would occur, primarily in
trees 12 to 14 inches in diameter. Present value
of this loss is almost $8.75 per acre, and future
values are about $23 per acre at 5 percent and
nearly $58 per acre at 10 percent interest rates.
If managed for three snags per acre, losses over
the 20-year period would be 220 board feet per
acre. Present value of this loss is $16.50 per
acre, while future values would be nearly $44 per
acre at 5 percent and $111 per·acre at 10 percent
interest rates.
LITERATURE CITED
McClelland, B. R., and S. S. Frissell. 1975.
Identifying forest snags useful for hole-nesting
birds. Journal of Forestry 73:414-417.
Nowakowski, N. A. 1980. Assessment of snag policies and their effects on timber harvests.
Master's Thesis, University of Arizona, Tucson,
85 p.
Schubert, G. H. 1974. Silviculture of southwestern
ponderosa pine: The status of our knowledge.
USDA Forest Service, Research Paper RM-123,
71 p.
Study areas representing the thinning-patch
cut and the uniform thinning silvicultural treatments would not meet any of the snag retention
policies through natural mortality at the end of
the 20-year period. Therefore, if managed for one
snag per acre, losses would approximate 90 board
feet per acre, presently valued at about $7 per
Scott, V. E., and D. R. Patton. 1975. Cavitynesting birds of Arizona and New Mexico forests.
USDA Forest Service, General Technical Report
RM-10, 52 p.
31
Thomas, J. W., G. L. Crouch , R. S. Bumstead, and
L. D. Bryant. 1975. Sil vicultural options
and habita t va lues in coniferous fores t s .
In Proceedings of Symposium on Management
of Forest and Range Habitats for Nongame
Birds. USDA Forest Service, General Technical
Report W0-1, pp. 272-287.
32
USDA Forest Service . 197 7. Fo r est Service Manual,
Title 5151.13c (Policy), Fire Management, np.