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RESPONSE OF UNDERSTORY SPECIES TO TREE HARVESTING AND FIRE
IN PINYON-JUNIPER WOODLANDS
Richard L. Everett and Steven H. Sharrow
ABSTRACT: Fire and tree harvesting have been
used successfully to increase understory
production of pinyon-juniper woodlands when
applied at the proper seral stage. Natural
response is variable among sites and exhibits
multiple entrance points into the successional
model. Burning and tree harvesting stimulate
reestablishment of early-to-midsuccessional
species within the pinyon-juniper successional
process.
Where woodlands are managed for both forage and
wood products, forage may be enhanced in a
patchwork of small treated areas. Economic and
topographic constraints make small treatment
sites unamenable to mechanized seeding, thus
natural plant response must improve the forage
base. Understory establishment and growth can be
expected following release from tr~e competition
as indicated by pinyon-juniper successional
models (fig. 1). These models suggest basic
trends in plant succession, however, plant
response on a particular site is not totally
predictable. A site is a composite of subsites
(microsites), each with its own sere and rate of
successional advancement.
INTRODUCTION
Understory cover in pinyon-juniper woodlands
has declined dramatically over the last few
decades because of increasing tree dominance
(Blackburn and Tueller 1970; Tausch and others
1981). Unfortunately for both livestock and
wildlife, the forage base has declined even
faster. On some sites one-third of understory
production has been lost when trees are still
less than 6.6 feet (2 m) in height (West and
others 1979) and herbage yields may decline by
82 percent when tree canopy cover exceeds 50
percent (Arnold and others 1964).
CLIMAX
PINYON-JUNIPER
WOODLAND
FIRE
!
SKELETON FOREST & BARE SOIL
!
Removal of trees is imperative for forage
improvement, but indiscriminat~ destruction of
trees to releas.e the understory wastes the tree
resource. Tree products, such as cordwood,
charcoal, Christmas trees, and nuts have
economic value and a good case has been made
for their management (Johnson 1975). Jensen's
(1972) work in the House Canyon pinyon-juniper
management unit of central Nevada demonstrated
both tree and forage resources could be
improved through appropriate silvicultural
practices. Modern silvicultural (Meeuwig and
Bassett 1 ) and range management (Evans and
others 1975) practices recognize site quality
differences for tree and forage production and
recommend small (<5 acres [2 ha]) treatment
areas following an intensive site selection
process.
ANNUAL STAGE
J
ANNUAL-PERENNIAL FORB STAGE
1
PERENNIAL FORB-GRASS-HALF-SHRUB STAGE
"""
/
SHRUB S T A G E - - - -.... FIRE-----.... PERENNIAL
~
/GRASS STAGE
""
CLIMAX
PINYON-JUNIPER WOODLAND
Figure 1.--Pinyon-juniper succession following
fire (Arnold and others 1964).
Richard L. Everett is Range Scientist at the
Intermountain Forest and Range Experiment
Station, USDA Forest Service, Reno, Nev.
Steven H. Sharrow is Associate Professor of
Range, Department of Rangeland Resources,
Oregon State University, Corvallis.
.
1Meeuwig, R. 0.; Bassett, R. L. Pinyon-juniper.
Revision of silvicultural systems for the major
forest types of the United States. Agric.
Handb. 445. Washington, D.C.: U.S. Department
of Agriculture; In press.
62
,·
.···...
DISCUSSION
declined with increased tree cover, but the
transition zone adjacent the tree crown became
somewhat more "favorable" for understory growth
under these conditions. Plummer (1958)
reported opposite understory distribution
patterns associated with pygmy conifers in
Utah, thus no one scenario will fit all
understory-overstory species combinations.
Competition Between 'Dree and Understory Species
Plant response following tree release is
influenced by the tree-induced effects that
caused understory decline. Tree effects on
understory vary with tree species, associated
understory species, and site characteristics,
thus no one scenario fits all conditions. To
avoid confusion and the misinterpretation of
findings, both authorship and the predominant
tree species in cited studies are given.
bominant tree species discussed are: Pinus
monophyZZa (PIMO) singleleaf pinyon, Pinus
eduZis (PIED) pinon, Juniperus osteosperma
(JUOS) Utah juniper, Juniperus monosperma
(JUMO) one-seed juniper, Juniperus deppeana
(JUDE) alligator juniper, and Juniperus
occidentaZis (JUOC) western juniper. If more
than three tree species were covered in a given
study, the key (MULT) will be used.
Understory species are not equally suppressed by
tree competition and this results in discernible
species zones around the tree stem (Arnold 1964
[JUMO]; Johnsen 1962 [JUMO]). Several cool
season grasses (i.e., prairie june grass KoeZaria
cristata; western wheatgrass Agropyron smithii;
bottlebrush Sitanion hystrix; mutton bluegrass
Poa fendZeriana) are more abundant under or
adjacent the tree crown of alligator juniper than
in the interspace between trees (Clary and
Morrison 1973). We found Sandberg bluegrass (Poa
sandbergii) only grudgingly displaced from
singleleaf pinyon duff while prostrate forbs were
readily expelled. These preexisting species
patterns at the time of either burning or
harvesting treatments are reflected in
post-treatment response as discussed later.
Adjacent trees outcompete most understory
species for soil moisture (Jeppesen 1977
[JUOC]), light (Jameson 1966 [JUMO]), and
nutrients (Barth 1980 [PIED]), thus understory
declines as succession proceeds. In dense
stands ubiquitous tree roots (Woodbury 1947
[MULT]; Plummer 1958 [PIED- JUOS]) cause rapid
surface soil moisture withdrawal from all soil
microsites--the duff (thick needle mat) under
the crown, transition zone adjacent to the tree
crown, and the interspace between trees. The
tree species competitive advantage is partly
due to the utilization of soil moisture prior
to the breaking of dormancy of many understory
species (Jeppesen 1977 [JUOC]) and the presence
of a tap root that withdraws moisture at a
depth unavailable to grass species (Emmerson
1932 [PIED - JUMO]).
Tree Harvesting and Prescribed Burning Treatment
The literature is replete with reports of
increased understory cover and production
following tree removal by hand chopping,
chaining, herbicides, and prescribed burning
(Arnold and Schroeder 1955 [MULT]; Aro 1971
[MULT]; Clary and others 1974 [JUDE- JUOS];
Evans and others 1975 [PIMO]). Mechanical
treatments are applicable when planted seed must
be covered to establish desirable species and
exclude invaders (i.e., cheatgrass Bromus
tectorum).
As trees increase in size and the stand
thickens, the duff under the tree crown
occupies much of the ground surface (~50
percent; Everett and Koniak 1981 [PIMO]). A
dense needle mat >10 m2 in area under a single
large singleleaf pinyon is not uncommon. Duff
visibly delineates an area of nutrient
enrichment (Barth 1980) at the expense of
interspace soils and associated understory.
Where seeding or seed covering is not required,
prescribed burning can be less expensive than
mechanical or chemical treatment (Blackburn and
Bruner 1975 [PIMO- JUOS]). Hand-harvested wood
can provide a cash crop in itself, and cleared
areas provide transitory range. Thus burning and
wood harvesting are two viable alternatives for
forage improvement. The choice between the two
treatments is decided by future use of the site,
economic value of the wood, and stand
physiognomy.
Pinyon litter physically reduced understory
plant establishment; shading or allelopthic
effects reduced understory persistence under
the tree crown. In time, the understory is
displaced from the tree stem of singleleaf
pinyon and one-seed juniper (Johnsen 1962). We
found understory cover declined in the
interspace between adjacent singleleaf pinyon
trees, increased at the crown edge, and became
negligible under the dense crown (Everett and
others 2 [PIMO]). Understory cover as a whole
Controlled burns require an understory adequate
to carry fire (Bruner and Klebenow 1979 [PIMO JUOS]; Richard Young, 3 this symposium [JUOC]),
thus early-to-midsuccessional stands are usually
burned. Prescribed burning has been used
successfully to increase understory production
and to eliminate tree species (Barney and
Frischknecht 1974 [JUOS]; Dwyer and Pieper 1967
[JUDE]) and was selected by Aro (1971 [MULT]) as
the optimum conversion method where practical.
Closed stands lack understory necessary for
2 Everett, R. L.; Sharrow, S. H.; Meeuwig, R. 0.
Pinyon-juniper woodland understory distribution
patterns and species associations. Submitted to
Torrey Botanical Club Bulletin.
3 Young, R. P. The use of fire to control and
improve wildland sites.
63
Plant response often varies as much within an
array of burn or tree harvest sites as between
the two treatments. Precise response is
unpredictable for either treatment because of
potentially unknown species immigration, soil
seed reserves, and post-treatment environmental
variables. General plant response may be
estimated from soil characteristics such as caco 2
levels (O'Rourke and Ogden 1969 [JUDE]), soil
depth, or stoniness (Stevens and others 1974
[JUOS -PIED]). Elevation, aspect,
precipitation, and indicator plant species may
also provide an estimate of potential
post-harvest response (Stevens and others 1974
[JUOS- PIED]; Winward 6 ). The qualitative
character of succession is predetermined at an
early stage on many sites because of the presence
of numerous plant forms, including many
midsuccesssional species (Everett and Ward 4 ).
controlled burns, but are more easily harvested
for cordwood. ~2nd-cutting has been used
effectively to red~ce tree competition in Arizona
(Arnold and Sch:coc.der 1955 [MULT]) and Oregon
(Jeppesen 19/7 [JUUC]).
Plant Response F,_,]_:t:l\·,_._.'"
Prescribed Burni '~~
'_rr,;-O:
r;::itv~st:ing
or
The character of plant response following burning
or tree harvesting is dependent upon the
predisturbance level of understory suppression,
which varies with tree cover, the tree species,
and the soil type (Clary and other 1974 [JUOS JUDE]; Springfield 1976 [JUMO- JUDE]). The
selected conversion technique and resultant site
disturbance further alters plant response (Clary
and others 1974) such that quantitative response
values can rarely be extrapolated from one
project to another. Nevertheless, general trends
of plant response and their probable causes can
provide insight for land managers in the
estimation of potential response on future
treatment sites.
Following burning, number of understory species
increases because of the rapid return of
pretreatment species together with the
establishment of numerous fire-induced species
(Everett and Ward 4 ). This result could be
expected from Nabi's (1978) report that species
numbers decline as pinyon-juniper succession
proceeds. Tree harvest of fully stocked stands
did not dramatically increase species numbers
because soil seed reserves were low and few
understory plants remained when succession had
proceeded this far (Everett 1978; Koniak and
Everetts).
Plant succession following burning or tree
harvesting may tend toward the standard
successional model, but post-treatment plant
assemblages may depict any number of the early
successional stages (fig. 2). Even though
pretreatment stands may be similar, initial plant
composition of post-treatment communities can
vary considerably (Everett and Ward 4 ). This is
due to the irregular germination of soil seed
reserves (Koniak and Everetts) and the random
immigration of off-site species.
Understory species provided approximately 30
percent ground cover 5 years following prescribed
burning (Everett and Ward 4 [PIMO]), but less than
9 percent cover following tree harvesting on
fully stocked singleleaf pinyon stands. Greater
understory response following burning was
expected because predisturbance understory cover
on burns (38 percent) was 10 times greater than
predisturbed understory on tree-harvested (3
percent) plots.
TREE
~
SHRUB
Increases in forage production following fire of
400 to 1100 lbs/acre (~48 - 1232 kg/ha) have been
reported by Aro (1971 [MULT]). Similar increases
in herbage production following tree harvesting
have been reported by Arnold and Schroeder (1955
[MULT]), Clary (1974 [JUDE]), and Springfield
(1976 [MULT]) .· Herbage production can continue
to increase for 13 years following tree harvest
(Arnold and others 1964 [MULT]), but response is
site and year specific (Clary 1974 [JUDE]). No
increase in herbage production following tree
removal has been noted on some sites (O'Rourke
and Ogden 1969 [JUDE]; Clary and others 1974
[MULT]). Unfavorable moisture regimes and the
absence of understory species capable of
utilizing released resources were suggested
causes for the lack of response.
PERENNIAL FORB/GRASS
ANNUAL FORB
/
FIRE
or
TREE HARVEST
MULTIPLE PATHWAYS OF EARLY SUCCESSION
Figure 2.--Natural response following fire or
tree harvesting in pinyon-juniper woodlands.
4 Everett, R. L.; Ward, K. Early plant succession
on pinyon-juniper controlled burns. Northwest
Sci. In press.
5Koniak, S.; Everett, R. L. Soil seed reserves
in successional stages of pinyon woodlands. Am.
Midl. Nat. In press.
6 Winward, A. H. Using sagebrush ecology in
management of wildlands. In: Proceedings, Utah
Shrub Ecology Workshop; September 1981. In press.
64
Plant response is not uniform within treated
sites partly because of the various soil
microsites present and the pretreatment plant
distribution patterns. Burned duff zones have
remained devoid of vegetation much like the
burned slash piles following mechanical
treatments (Arnold and others 1964 [MULT]).
Thus plant response has been most rapid in the
microsites between the trees on harvest plots.
Arnold, J. F.; Jameson, D. A.; Reid, E. H. The
pinyon-juniper type of Arizona: effects of
grazing, fire and tree control. Prod. Res. Rep.
84. U.S. Department of Agriculture; 1964. 28 p.
Arnold, J. F.; Schroeder, W. L. Juniper control
increases forage production on the Fort Apache
Indian Reservation. Station Paper 18. Fort
Collins, CO: U.S. Department of Agriculture,
Forest Service, Rocky Mountain Forest and Range
Experiment Station; 1955. 35 p.
On tree harvest sites with a contiguous grass
understory, response has been greatest at the
crown edge. Increased soil moisture storage
and nutrient reserves under the duff promote
the vigor of established plants in accord with
the preharvest plant distribution patterns
discussed earlier. Where understory is more
sparse, response is less orderly because of the
random location of remnant plants, soil seed
reserves, and plant establishment from off-site
immigration.
Aro, R. S. Evaluation of pinyon-juniper
conversion to grassland. J. Range Manage.
24(3): 188-197; 1971.
Barney, M. A.; Frischknecht, N. C.. Vegetation
changes following fire in the pinyon-juniper
type of west central Utah. J. Range Manage. 27:
91-96; 1974.
Barth, R. C. Influence of pinyon pine trees on
soil chemical and physical properties. Soil
Sci. Soc. Am. J. 44: 112-114; 1980.
CONCLUSIONS
In pinyon-juniper woodlands, understory
declines dramatically under intense tree
competition for soil rr,c:fstu:re, light, and
nutrients. Thus, atte~pts to improve forage by
grazing management alone are unsuccessful.
Prescribed burning and tree harvesting at
different seral stages have generally increased
herbaceous growth but response varies
considerably. Attempts to predict
post-treatment response based on standard
successional models have failed because of the
multiple entr3. 11J~P points int:c• the successional
model and the ~tJS&.:",;CS
.:;::::-::fie seral stages
as succession proceeds. Response Lqs been most
fruitful when desirable species al~cqdy p~esent
were capable of utili zing released ""'esources
and occurred in sufficient quantities to do so.
Blackburn, W. H.; Tueller, P. T. Pinyon and
juniper invasion in black sagebrush communities
in east-central Nevada. Ecology 51: 841-848;
1970.
Bla:::kburn, W. H.; Bruner, A, D. Use of fire in
manipulation of the pinyon-juniper ecosystem.
In: Proceedings, the pinyon-juniper ecosyster;i:
a symposium; 1975 May; Logan, UT: Utah State
University; College of Natural Resources. Utah
Agricultural Experiment Station; 1975: 91-96.
Bruner, A. D.; Klebenow, D. A. Predicting success
of prescribed fires in pinyon-juniper woodland
in Nevada. Res. Pap. INT-219. Ogden, UT: U.S.
Dep~rtment of Agriculture, Forest Service,
Int~rmountain Forest and Range Experiment
Station; 1979. 11 p.
Cover and productivity of understory are not
uniform across tre:1tment sites follo'.ving tree
removal, but reflect pretreatment plant
distribution patterns, and an array of soil
surface microsites that vary in available soil
nutrients. Response on grossly similar sites,
that vary in the proportion of each soil
microsite, may well be qualitatively but not
quantitatively similar.
Cl2ry,
P. Response of herbaceous vegetation to
of alligator juniper. J. Range Manage.
27: ~S7-3g9; 1974.
~.
f~lling
Clary, W. P.; Baker, M. B. Jr.; O'Connell, P. F.;
]".]hnsen, T. N.; Campbell, R. E. Effect of
pinyon-juniper removal on natural resource
products and uses in Arizona. Res. Pap. RM-128.
Fort Collins, CO: U.S. Department of
Agriculture, Forest Service, Rocky Mountain
Forest and Range Experiment Station; 1974. 28
p.
Understory response to burning in
midsuccessional stands can be more rapid than
to tree harvesting of fully stocked stands due
to the negligible understory of the latter
sites. Where understory is more vigorous,
post-harvest understory response provides
substantial increases in forage.
Clary, W. P.; Morrison, D. C. Large alligator
junipers benefit early-spring forage. J. Range
Manage. 26: 70-71; 1973.
Dwyer, D. D.; Pieper, R. D. Fire effects on blue
grama-pinyon-juniper rangeland in New Mexico.
J. Range Manage. 20: 359-362; 1967.
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around a juniper tree. J. Range Manage.
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Emmerson, F. W. The tension zone between the
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1932.
65
-Evans, R. A.; Eckert, R. E.; Young, J. A. The
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Springfield, H. W. Characteristics and
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1981.
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~· M. S. Thesis.
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66