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Silvicliltural Research. in. COlliJ:erolls
SII.balpiIle Forests of tile Central Roc){y
l\1oI111tain.s
Frederick W. Smith 1
Abstract--Silvicultural research in the central Rocky Mountains has
been influellcf~d'by the high proportion of olel-growth forests, poor markl~ts, modest growth rates anel emphasis on resources other than wood
proeluction, Four areas of research have been stressed in the region
classiNcation, natural regeneration, reproduction methods and density
control. As a result, silvicultural prescriptions can be tailored for specific
site and stand conditions to achieve a variety of resource objectives .
_---------
.
Silvicultural problems and research in the subalpine zone
of the central Rockies are related to the unique dim ate and
forest uses in this region. The zone, whic.h extends from 9,000
to 11,00 feet is characterized by short growing seasons, low
temperatures and high radiation (Peet 1981). Tree species in
the zone are limited in number. Lodgepole pine (Pinus COlltorta Doug!. ex Loud.), Engelmann spruce (Picea englemannii
Parry ex. Engelm.) and subalpine fir (Abies lasiocarpa (Hook.)
Nutt.) dominate the coniferous subalpine forests, with aspen
(Populus tr(?nluloides Mic.h.~.) as a common deciduous associate. I'roductivity is low to moderate (20 to 120 ft 3a1 yl) (Green
and Van Hooser 1983) and regeneration can be difficult. Steep
environmental gradients, largely a function of elevation, topography and soils, result in large local variation among sites.
A high proportion of existing stands are unmanaged, are
relatively old and may be of low vigor. Markets for wood
products are poorly developed and stumpage prices are low
(Long et a1.1986). Production for consumptive use, especially
on public land, often emphasizes water, grazing or wildlife
over wood products. Non consumptive uses, whether recreation or preservation, are important determinants of management goals throughout the region.
SilviculturaJ. practices are commonly less intense than in
other regions and management to enhance wood production
is often subordinate to producing stands to meet other re··
source goals. Regenerating older stands is the major task
facing silviculturists in the region. With 1m" to moderate
productivity and poor markets, natural regeneration is the
dominant regeneration technique. Therefore, research has
emphasized reproducti.on methods and other techniques to
secure suitable natural regeneration. Intermediate operations,
mainly thi.nning, are used to control density, spacing and
c.omposition of naturally regenerated stands.
I will review four areas of research in subalpine forests of
the central Rocky l\lountains dominated by Engelmann
spruce and subalpine fir, or lodgepole pine classification,
natural regeneration, reproduction methods and density
control. These areas are of the most interest to silviculturists
and have been emphasized in research programs on the Fraser
Experimental Forest and throughout the region.
Classification
Classification schemes used in the Region meet two needs
indexing productivity and delineating vegetative associations.
Site index curves are available for Engelmann spruce (Alexander 1967) and lodgepole pine (Alexander et a1. 1967). In
conjunction with growth and yield simulators, they provide a
means to estimate site productivity under a variety of density
management regimes (Alexander and Edminster 1980, 1981).
Lodgepole pine is one of the few species where dominant
height is known to be reduced at high densiti.es. There.fore,
corrections factors have been developed to adjust site index
estimates for density related height reduction of site trees
(Alexander et a1. 1967).
Habitat typing is a vegetative classification system which
has recently come into \vide use in the central Rockies.
Methods for typing large geographic areas have been standardiz.ed (Pfister and Arno 1980), and habitat types have been
1Associate Professor, Department of Forest and Wood Sciences,
Colorado State Univl~rsity, Fort Collins, CO 8052:3.
15
developed for most National Forests in the region (e.g.
Hoffman and Ale.xander 1976, 1980, 1983, Steel(~ et al.. 1983,
Hess and Alexander 1986). Habitat types are defined as the
land area which fiU pports or will come to su pport a given climax
community or plant association, and are named for the dominant or indicator species of the overstory and understory
unions. \\'here climax vegetation does not occur, the habitat
type can be identified from keys based on occurrence and
abundance of seral species. A successional pattern for overstory and understory vegetation and response of differe.nt
successional stage.s to disturbance are usually described for
each habitat type.
Habitat types provide useful information for prescribing
silvicultural treatments. Reproduction me.thods (.an be tailored to the pattern of ecological behavior (e.g. tolerance or
successional. status) of a spedes in specific habitat types.
Regeneration problems, such as release of aggressive competing understory spedes, can be p~edi.cted. From descriptions of
the understory union, forage p~oduction and palatability can
be estimated for domesti.c grazing or wildlife habitat requi.rements.
south-fadng slopes that natural regeneration of dearcut
openings is not a viable option.
Regeneration of lodgepole pine requires completely different si.lvicultural considerations. Lodgepole pine is an aggressive pioneer and an excess of regeneration may be. more
common than a regeneration failure. Where serotinous cones
are present, any harve.sting technique which leaves the cone
bearing slash distributed through the opening will. provide. an
adequate source of seed. Regeneration in excess of 100
thousand seedlings per acre can occur following natural
disturbance or harvest (Cleme.nts 1910). \\'here cones are
predominantly non~erotinous, seed dispersal distances must
be accounted for when designing openings. l\fechanical scarification or broadcast burning improve microsite conditions by
limiting temperature extremes and improving soil moisture
availability in relation to a duff surface (Hungerford 1980,
Lotan and Perry 1983). Reproduction methods and treatment
of the cone-bearing slash may be modified to reduce the
amount of seed available to produce lower numbers of seedlings.
Reproduction Methods
Regeneration
All of the high fore.st reproduction methods except the see.d
tree method are applicable to re.generate Engelmann spruce
stands. Choke of an appropriate method depends on the
pattern of ecological behavior of spruce in a particular stand
and the management objectives for the stand. Some form of
parti.al cutting may be required where spruce regeneration is
difficult to achieve in openings, or where management objectives dictate maintenance of a mature forest canopy for an
extended period.
Clearcutting in spruce is most commonly used in conjunction with natural rege.neration. In the open environment of a
dearcut, successful natural regeneration is more likely on a
north- or east-facing slope than a south- or west-facing slope
(Alexander 1984, Alexander 1986a). Size and shape of
dearcuts in spruce are limited to distances that will provide
adequate seed dissemination throughout the cut area. Provision of bare mineraJ. soil seedbeds and shade for regeneration
are important c.onsiderations for success in dearcuts (Alexander 1986a).
Size, shape and orientation of dearcuts are influenced by
a variety of forest management objectives, especially enhancement of water yield. Water yields from spruce-fir stands can
be increased by patch dear cutting. Optimum increases occur
where 30% to 4(Y~O of the area within a watershed is harvested
in small., 3- to 5-acre openings dispersed over the entire. area
(Leaf 1975, Leaf and Alexander 1975, Troendle and Leaf
1981, Troendle 1982, 1983). Differences in canopy height
causes snow to be accumulated in these openings, which, in
turn, increases stream flows at snowmelt in the spring. These
effects dedine with time as canopy height differences between
an opening and the surrounding stand disappear and as
Natural regeneration is the primary technique applied in
the region and artificial regeneration is used only under
extreme circumstances where natural regeneration has failed
or is likely to faiL Successful natural regeneration depends on
the provision of a sufficient supply of sound seed, and favorable conditions for germination and early survival. Successful
regeneration prescriptions, especially in the relatively harsh
environments common to the subalpine zone, depend on a
knowledge of these relations (Alexander and Shepperd 1984,
Alexander et al. 1984).
Natural regeneration of Engelmann spruce appears to be
dependent on high seed production and favorable mkrosites.
Spruce seed production may be. higher and more re.gular than
previ.ously thought. In a 15-year study of production rates,
good to excellent seed years (> 100,000 seeds per acre)
occurred in 7 out of 15 years, but production varied by location
and year (Alexander et al. 1986). On a stand basis, seed
production was correlated with the basal area of dominant and
codominant spruces. In dear cut openings, germination and
survival were related to aspect and microsite. On north-facing
slopes with scarified and shaded seedbeds, 25,600 sound seeds
per acre were required to produce 800 5-year-old seedlings
per acre (Alexander 1984). On south-facing slopes, germination and survival were poor even with the most favorable
seedbed conditions (Noble and Alexander 1977, Alexander
1984). Clearcut openi.ngs that can be successfully stocked on
north aspects are li.mited to 300 to 450 feet wide where
seedbeds are scarified and shaded and 200 to 350 feet where
seedbeds are scarified or shaded (Alexander 1986b). Survival
is so poor on north slopes with unfavorable seedbeds and on
16
transpiration increases with accumulation of canopy leaf area.
On north-facing slopes, shelterwood can be as effective as
clearcutting for increasing water yield. Interception a.nd consumptive water use are decreased while evaporation is less
than in a dearcut opening (Troendle and Meiman 1984).
Patch d.earcutting in successive entries is complicated by
the slow height growth of Engelmann spruce. Spruce may
require 20 to 40 years to reach breast height, and height of
dominant trees at a total age of 60 years (breast height age 30)
on SHOO 80 may only be 33 feet (Herring and l\fcMinn 1980,
Alexander 1986a). Long cutting cycles are required to restrict
the size of contiguous openings .and to obtain stand heights
necessary for adequate seed dispersal.
Partial cutting for even··aged management can be achieved
with uniform or group sheltenvood. A shelterwood can be
applied across a wide range of site conditions but may be
necessary to achieve even-aged natural regeneration on southor east-facing slopes (Alexander 1986a). Shelterwood is used
to promote natural regeneration by ameliorating site environmental conditi.ons and providi.ng a seed source. Also, shelterwood may be used to extend the period during which a stand
meets other resource objectives by maintaining a partial
mature forest canopy until regeneration is established.
Guidelines for uniform shelterwood cutting have been
developed for old-growth spruce.. fir stands based on stand
structure, wind throw hazard and advanced regenera.tion,
(Alexander 1986a). For single-storied stands with low
wind throw risk, a three step she.lterwood induding a light
preparatory cutting, an establishment cutting, and one or more
removal cuttings after adequate re.stocking is re.commended.
For stands with higher risk for windthrow, several preparatory
cuttings may be necessary to provide a windfirm stand for the
establishment cutting. For stands with two or more canopy
stories without a manageable unde.rstory on low wind throw
risk sites, the upper story may be wind firm, and the preparatory cuttings may be unnecessary. An overstory density of 40%
to 60% of the original stand basal area is recommended
following the establishment cutting.
The removal cutting of a uniform shelterwood (simulated
sheltenvood) may be applied where sufficient advanced reproduction is present as a product of natural stand development or inadvertent response to previous cutting (Alexander
1986a). "'here advanced reproduction is present, uncertainty
in achieving regeneration of harsh sites can be reduced, and
rotation age can be shortened. However, advanced regeneration must be of sufficient quantity, quality, species compositi.on and uniformity in spacing to constitute a well-stocked
stand. Estimates of damage. to reproduction during harvest
should be deducted from pre~harvest stocking to assess the
adequacy of the understory for management. Pre .. and postharvest stocking evaluations and design of logging operations
to control damage to reproduction are recommended.
Group shelterwood may be appropriate where mature
stand structures are dumpy (Alexander 1986a). Ope.nings less
than two tree heights in diameter are cut to take advantage of
the natural distribution of dumps in a stand. A stand can be
regenerated in three entries by removing about 30~t, of the
original basal area over about 1/3 of the stand area in each
entry. Second and third entries can be made only after
openi.ngs created in the previous entry are adequately restocked. Openings created in the final entry may require
alternative means of regeneration. Openings from the final
entry can be artificially regenerated, or if suffidentlywindfirm
to provide a reliable seed source, they can be cut as a uniform
shelterwood.
Uneven-aged management can be appHed i.n Engelmann
spruce and subalpine fir stands by individual tree or group
selection (Alexander'1986a). "'here spruce and fir occur, they
are generally climax species but mayor may not be major
components of seral communities (Alexander and She.pperd
1984, Alexander et al. 1984). Mature stands frequently have
more than one canopy story, which can decrease the time
required to achieve a balanced diameter distribution. The
major difference between selection and some modifications to
dear cutting and shelterwood is the way growing stock is
regulated. Selection is distinguished by controlling stand
structure throughout a unit on the relation of numbers of trees
in successively smaller diameter classes (q), the basal area of
the stand, and the maximum diameter. A q between 1.3 a.nd 1.5
is recommended as a reasonable initial goal for regulating
previously unmanaged stands (Alexander and Edminster
1977). Selection is an intensive management practice that can
be difficult and expensi.ve to apply. Alexander and Edminster
(1977) have. suggested marking procedures for conditions
c·ommon to the region.
Normally, even-aged reproduction methods are best
suited to maintain vigorous, productive lodgepole pine stands
because of the intolerance of the species and the prevalence
of dwarf mistletoe (Arceuthobium amcricanum Nutt. ex
Engelm.) in mature stands (Alexander 1986c). However, it is
possible to apply selection reproduction methods in mature
old ..growth pure or mixed lodgepole pine stands where management goals require minima.l disturbance. to the forest.
Clearcutting is the most common reproduction method
applied in lodgepole pine stands because of the advanced age
of many stands in the region and their susceptibility to insects,
disease and wind throw. "'here natural regeneration is dependent on a nonserotinous seed source, size of cutting units,
loggi.ng plans, slash disposa.l and seedbed preparation should
be designed to provide for seed dispersal, promote seedling
establishment and create conditions favorable for growth.
Effective wind di.spersal of lodgepole seed limits dearcut size
to a diameter of 300 to 400 feet, or about 5 to 6 times tree height
on favorable sites (Lotan and Perry 1977). A bare mineral soil
seedbed is the most favorable for regeneration, and can be
created by broadcast burning or mechanical treatment in
conjunction with 10ggjJlg or slash disposal, since seed is not
contained in cone bearing slash.
Clearcut unit size is not li.mited by seed dispersal where
cones are serotinous, but there is only one opportunity to
'1'1
achieve successful natural regeneration. However, Alexander
(1986c) sees no advantage to openings larger than 30 to 40
acre.s, even for dwarf mistletoe control, and smal.ler openings
may be more consistent with non-timber resource objectives.
Slash treatment alternatives are limited since the cone bearing
slash must be. preserved and distributed throughout the cutting
unit (Tackle 1964, Alexander 1966). Slash treatnlent can be
achieved by lopping and scattering or rolli.ng and chopping.
Bare mineral soil should be present on at least 40% of the area
for suitable regeneration.
Partial cutting to regenerate an even-aged stand can be
accomplished with uniform or group shelterwood where
shade is required because of hi;rsh site conditions or where
management goals dictate extension of the time a mature
canopy is present on the site. "In group shelterwood small
openings are created and regenerated in a sequence so that the
regeneration in the entire stand can be treated as an even-aged
management unit. Recommendations for uni.form and group
shelterwood are based on stand structure, windthrow risk, and
insect and disease problems (Alexander 1986c). Repeated
entries are recommended at five to ten year intervals or when
current openings are adequately stocked. In uniform, singlestoried, low windfall risk situations, a stand can be complete.l.y
regenerated in 2 cutting cycles with uniform shelt:erwood and
3 to 4 cycles with group shelterwood. As windfall risk increases, the basal area or unit area cut i.n one entry decreases,
and the number of cutting cycles required to complete.l.y
regenerate. a stand increases.
stands (Edminster 1978), and PROGNOSIS, an individual
tree model, for even and uneven-aged stands (Stage 1973).
Treatment effects on stand volume. tree size, and disease or
insect infestations can be simulated with these models. Density
management diagrams (McCarter and Long 1986) and stocking charts are useful tools for devising sound density mana.gement prescriptions to achieve specific resource goals. Increasingly, growth projection systems are being linked with produc··
tion models for non timber resources. For example, stand
structures meeting ungulate thermal and hiding cover guidelines are displayed on density management diagrams (Smith
and Long 1987) and a hiding cover simulator is directly linked
to PROGNOSIS.
Conclusions
Silviculturists have the knowle.dge base to prescribe stand
level treatments to meet management goals specific to regional conditions as the result of the accumulated research on
the Fraser Experimental Forest and throughout the region.
Classific.ation systems provide the means to characteriz.e site
specific problems and opportunities for production of various
resources. Relations describing seed production and dissemination, and seedling survival with respect to tnic.rosite and
topographic condi.tions have been generally de.scribed for
natural regeneration. Guidelines are available to prescribe
reproduction methods for old-growth stands of lodgepole
pine, and Engelmann spruce and subalpine fir in relation to
stand structure, wind throw risk and management goals. An
array of quantitative tools, including simulati.on models and
applied density management guides, are available for use in
evaluating stand specific density management alternatives.
Further research in the central Rockies should proceed in
two directions····developing cost effective, site-specific. techniques for devising stand level prescriptions, and developing
stand and landscape scale relations between stand structures
and nonti.mber resources. J\.lore precise techniques to characterize site environmental variation are needed. Incorporation
of topographic. and soil influences on site may improve the
resolution of dassification systems (e.g. Peet 1981). Clarification of site-specific. relations for seedling establishment will
allow si.lviculturists to more closely tailor regeneration prescriptions to stand conditions. Development of techniques for
less intensive applications of selection would allow more
uneven-aged management practic.e.s in the region. Quantification of early growth rates and stocking relations of managed
stands will. lead to clearer choices for early density control to
achieve desired stand structures.
Design of prescriptions to meet multiple use objectives will
become increasingly important in the central Rocky Mountains. ~Tays to integrate individual stand prescriptions to
produce desi.red conditions on a variety of landscape scales
are needed. This will include of better understanding of the
relation between specific stand structures a.nd other compo-
Thinning
Thinning represents the most powerful silvicultural tool
available to shape the structure of naturally regenerated
stands of lodgepole pine, or Enge.l.mann spruce and subalpine
fi.r. Precommercial thinning can be used to shorten rotations,
to create economically viable manageme.nt regimes, to inlprove stand composition, and to improve stand vigor and
value, and resistance to insect attack (Long et a1.1986). Recent
evidence suggests that susceptibility to mountain pine beetle
attack (Berryman 1982) and defoliators such as the spruce
budworm (Cates et a1.1983) is related to reduced stand vigor.
Precommerdal and commercial thinning can maintain high
individual growth rates to maintain high stand vigor. Thinning
to reduce canopy leaf area and transpirati.on may be effective
in increasing water yield from subalpine forests (Knight et al.
1985, Troendle 1987). Habitat for various wildlife species may
be improved by thinning (Crouch 1986). Ungulate hiding
cover guidelines are most effectively met by sapling stands
precommerdally thinned to forestall the age of self-pruning of
tree crowns (Smith and Long 1987).
Ouantitative models are available to test the results of
thinning regi.mes for volume producti.on and stand structural
development. Growth and yield simulation models include
RI\IYLD, a whole stand model for even-aged and two-storied
18
nents of subalpine forests such as wildlife habitat requirements, water yield, sediment production, sce,nie values and
recreational use.
Alexander, R.R., R.C. Shearer, and ~T.[). Shepperd. 1984.
Silvical characteristics of subalpine fir. USDA For. Servo
Gen. Tech. RJ\.f-115, 29 p.
Acknowled,~ement
Alexander, R.R. and W.O. Shepperd. 1984. Sil.vical characteristics of Engelmann spruce. USDA For. Servo Gen. Tech.
Rep. RJ\f-114, 38 p.
Alexander, R.R., D. Tackle, and ~T.G. Dahms. 1967. Site
indexes for lodgepole pine with corrections for stand
density: Methodology. USDA For. Servo Res. Pap. RJ\f29,24 p.
Berryman, A.A. 19812. Mountain pine beetle outbreaks in
Rocky Mountain lodgepole pine forests. J. Forest. 80:410413.
Cates, R.O., R.A. Redak, and C.B. Henderson. 1983. Stress
physiology of Douglas fir, patterns in defensive chemistry,
and western spruc.e budworm success. Bull. Ecol. Soc..
Amer. 64:72.
Development of this manuscript was supported by J\fcIn··
tire-Stennis Project No. COL00540.
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20