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Effects of Tebuthiuron and
Fire on Pinyon-Juniper
Woodlands in Southcentral
New Mexico 1
Roger D. WiHie and Kirk C. McDaniel 2
Pinyon-juniper (P /J) woodlands
are one of the most extensive vegetation types in the southwestern
United States. Springfield (1976) estimates 3.6 million ha of P /J in New
Mexico with about 160 million ha in
the southwestern states (Aro 1971).
Historical evidence suggests P /J
woodlands have increased in range
and tree density since the turn of the
century. Migration into perennial
grasslands along the woodlands'
lower elevational range has decreased forage production for livestock, increased poisonous plant densities, created soil erosion problems
and increased the costs of livestock
management (Parker 1948, Johnsen
1962 and Srneins 1983).
Herbicide treatments generally
have not been used widely to reduce
P /J dominance (McDaniel and
WhiteTrifaro 1987). Control of
pinyon-oneseed juniper by herbicides
is highly dependent upon both range
site and tree growth characteristics
(McDaniel et al. 1989). Prescribed
burning has also not been widely
used in P /J woodlands because results are often inconsistent. Integrated (complementary) brush man1Poster paper presented at the conference. Effects of Fire in Management of
Southwestern Natural Resources (Tucson,
AZ. November 14-17, 1988).
2 Witfie is graduate research assistant.
and McDaniel is Professor, Department of
Animal and Range Sciences, New Mexico
State University, Las Cruces, NM 88003.
agernent systems using two or more
control methods may be needed in
order to meet multiple use objectives.
Scifres (1987) has shown that application of a herbicide followed in later
years by a controlled burning program can offer an efficient way to
rnal).age brush canopies when longterm economics are considered. The
primary objective of this research
was to examine the response of trees
and shrubs following application of
the herbicide tebuthiuron (N-[5-{1,1dirnethylethyl)-1,3,4-thiadiazol-2-yl]N,N-dirnethylurea) and/ or fire. A
secondary objective was to monitor
change in herbaceous composition
and yield following treatment.
Methods
This study was conducted at the
New Mexico State University Ft.
Stanton Experimental Ranch in Lincoln County near Capitan, New Mexico. Soils and vegetation have been
previously classified (Groce and
Pieper 1967, Pieper et al. 1971). Average annual precipitation is 390 rnrn
with about 250 rnrn falling from May
to September. Within the study area,
three range sites (hill, shallow and
loamy) were identified. Each range
- site has a different slope, soil, tree
density and mixture of herbaceous
vegetation. All sampling was stratified by range sites within treatment
blocks.
174
Hill range sites were characterized
by 10% to 45% slope, a maximum soil
depth of 30 ern and tree densities of
about 1900 individuals/ha. Soils
were predominantly lithic haplustolls
classified as Tortugas-rock outcrop.
Herbaceous vegetation was dominated by sideoats grarna (Bouteloua
curtipendula), New Mexico rnuhly
(Muhlenbergia pauciflora) and plains
lovegrass (Eragrostis erosa). Shallow
range sites were characterized by
slopes of 3% to 10%, maximum soil
depths of 100 ern and tree densites of
about 700 individuals/ha. Soils are
predominantly petrocalciustolls classified as Plack variant gravelly
loarns. Herbaceous vegetation is
dominated by sideoats grarna, blue
grarna (Bouteloua gracilis), wolftail
(Lycurus pheloides) and various
threeawns (Aristida spp.). Loamy
range sites are characterized by
slopes of 1% to 10%, maximum soil
depths over 150 ern and tree densities up to 235 individuals/ha. Soils
were predominantly aridic haplustolls classified as Deacon loarns. Tree
cover was almost solely afforded by
oneseed juniper. Herbaceous vegetation was dominated by blue grarna,
galleta (Hilaria jamesii), creeping
rnuhly (Muhlenbergia repens) and sand
dropseed (Sporobolus cryptandrus).
Tebuthiuron pellets (40% active
ingredient) were applied to P /J
woodlands within the study area in
August 1983. Rates of 0.84, 1.26and
1.40 kg/ha active ingredient were
flown onto 6 ha (1250 X 120m)
blocks placed side by side, each separated by a 35m buffer area. Onethird of each herbicide block, the
buffers and a small control area were
burned in March 1987 using headfire/backfire techniques (Wright 1974
and Scifres 1980). Fire containment
was accomplished by burning out
fuels around the perimeter of treatment areas. Fuel moisture and fire
weather conditions were monitored
for 2 weeks prior to actual ignition.
One-tenth ha sampling clusters
were established on each range site
within treatment blocks. Five trees of
each species were randomly selected
from each cluster for evaluation.
Evaluations included tree height,
canopy cover, tree position (overstory /understory), percentage canopy removed by herbicide and percentage canopy damaged by fire.
Herbaceous sampling was conducted
using 0.186 m 2 rectangular frames.
Frames were placed beneath the dripline of sample trees and 1 m from
the drip line into the interspace area
on the tree's east/west axis producing four quadrats per tree and 40 per
cluster. Annual production values by
species were generated using weightestimate techniques (Cook and Stubbendieck 1986). Each species encountered within four quadrats was
clipped and weighed to form a green
weight regression between estimated
and actual values. Clipped samples
were oven-dried at 60° C for 72 hours
and weights were adjusted to dry
forage values. Fire damage and tree/
shrub seedling densities were estimated using ten 7.18-m diameter
subplots within each cluster. All trees
greater than 0.3 min height within a
subplot were counted and evaluated
prior to, and at 6 and 12 months after
burning. Seedling densities were
taken from within subplots at the
same intervals. All data were analyzed in a randomized complete
block design and subjected to mean
separation using protected LSD,
Duncan's multiple range test or Chisquared analyses (Steele and Torrie
1980). Because treatments were applied to only one block, pseudo-replication (over time) was used to estimate experimental error for this
an~lysis. Therefore, extrapolation of
ciifferences to other areas must be
done with caution and values reported here are preliminary results.
Results
Tree and Herbage Response to
Tebuthiuron
Pinyon was highly susceptible to
tebuthiuron regardless of rate or
175
range site (table 1). Mortality averaged 60%, 58%, and 78% from the
low to high rate, respectively.
Oneseed juniper was less susceptible
to the herbicide at low and moderate
rates (mortality was 10% and 43%),
compared to the high application
rate (74%). Generally, moderately
large pinyon (those 2.5 to 4 min
height) and smaller juniper (those
less than 2.5 m in height) were most
susceptible to the herbicide
(McDaniel et al. 1989).
Grass yield did not increase in the
herbicide treated areas compared to
the untreated areas until the second
or third growing season after treatment (table 2). After the third year,
average grass yield increased with
increasing rate of application, being
615, 775 and 1000 kg/ha compared to
428 kg/ha for the untreated area. Initial depression and subsequent release of grass yields after 2 to 3 years
is common for soil applied herbicides
(Duncan and Scifres 1983 and James
and Pettit 1984). Forb production decreased uniformly across herbicide
rate the first and second growing
season after treatment. By the third
season, forb production began to increase in areas treated with the low
and medium rate of herbicide but
remained depressed in areas treated
at the high rate. Several studies
found forbs to recover between 18
and 36 months after treatment with
tebuthiuron depending upon applied
rates (Scifres and Mutz 1978 and
Jacoby et al. 1982). Perennial grasses
comprised about 70% of the composition by weight in untreated areas
but nearly 95% in areas treated at the
high tebuthiuron rate after three
years. Total herbage yield also varied
by range site within a treatment area.
Overall, herbage yields averaged 810,
915 and 1334 kg/ha on shallow, hill
and loamy sites, respectively. Peak
yields from tebuthiuron treatment
may not yet be realized. Arnold
(1964) reported peak herbage yields
following tree removal in pinyonjuniper woodlands of Arizona occurred 12 years after treatment. Pro-
duction values given in table 2 for
1986 represent the quantity of fine
fuels present in the study area prior
to burning.
vived burning except where oneseed
juniper canopies enclosed the
pinyon's and crown fires developed.
The relationship between tree height
and percentage of canopy removed
by fire has been reported from other
Tree and Herbage Response to
Burning
. ·. ·.,
Table 3 indicates canopy reductions achieved from burning treatments applied 4 years after tebuthiuron. Sixty percent of the remaining
juniper in tebuthiuron treated areas
were damaged. Fire was effective in
reducing oneseed juniper canopies
within areas previously treated with
tebuthiuron at the low and medium
rate but did not further reduce canopies in areas treated at the high
tebuthiuron rate. Oneseed junipers
were damaged slightly (5% to 20%
crown scorch) on hill and shallow
sites where fuel beds were light or
inconsistent but heavily damaged
where adequate fuels were present.
Oneseed juniper on loamy range sites
received moderate to heavy damage
(25% to 75% crown scorch) at the low
and medium herbicide rate. Those
trees less than 2 m in height were
heavy damaged or removed by fire.
Larger juniper received light to moderate crown scorch and generally
where crowns were in close proximity, complete crown fires developed.
Similar results were reported after
burning herbicide-treated eastern
redcedar (Juniperus virginiana) in
Oklahoma (Engle et al. 1988).
Few pinyon were damaged by fire
where burning was not preceded
with tebuthiuron application. Where
tebuthiuron had been applied, fuel
loading and distribution were improved. Twenty percent of the remaining pinyon were damaged by
fire. Most exhibited scorching
around the bole and 15% of the canopies were slightly scorched (table 3).
Smaller pinyon (those less than 1.2 m
in height) received the heaviest damage to the canopy or were burned out
completely. Larger pinyon (greater
than 1.3 m in height) generally sur176
studies (Dwyer and Pieper 1967 and
Engle et al. 1988).
The tebuthiuron-fire treatment increased herbage production above
that of either treatment alone. By further reducing tree canopy cover at
the low and moderate tebuthiuron
rate, burning allowed for an additional release of herbaceous species
(table 2). Grass production increased
an average of 13%, 19% and 38%
across range sites from the low to
high herbicide rate, respectively, after burning. Burning stimulated a
release of forbs at the low and medium herbicide rate, increasing
yields 53% and 140%, but was not
effective at the high tebuthiuron rate.
Percentage composition by weight
for the forb component equalled that
of untreated areas after burning the
low and medium tebuthiuron rate. In
untreated areas annual forbs were a
major component of the coppice
zones associated with tree canopies
whereas perennial forbs were predominant in interspaces zones. The
complement of annual forbs increased in both zones after treatment
(Arnold 1964 and Arnold et al. 1964).
tual tree densities being greatest on
hill range sites and lowest in loamy
range sites. Generally, there was little
difference in pinyon or oneseed juniper seedling numbers between burning or herbicide with burning treatments. Burning alone appeared to
favor oneseed juniper and pinyon
establishment on shallow sites, but
this may be in part due to a poorly
distributed fuel bed across these areas which resulted in a cool or erratic
fire front. Seedling numbers for
pinyon increased following burning
on loamy sites where it only occurs
rarely. This suggests the presence of
a viable seed source but an inability
to become established against a competitive perennial grass component.
Research from other P /J woodlands
in the Southwest indicates recolonization may take as little as 15 years
on some sites after overstory removal (Tausch et al. 1971 and Rippel
et al. 1983).
Regeneration Sampling
Response of Associated Shrubs
As expected, seedling densities of
both pinyon and oneseed juniper
were decreased immediately after
burning (table 4). However, new
seedlings began to appear within the
first year following fire. Seedling
numbers corresponded closely to ac-
Associated shrubs in these woodlands were susceptible to tebuthiuron rates applied (McDaniel et al.
1989). Wavyleaf oak (Quercus undulata) and algerita (Berberis haematocarpa) densities were reduced by
80% or more whereas skunkbush
sumac (Rhus trilobata) was more tolerant (50%) at the high tebuthiuron
rate.
Only algerita occurred in high
enough numbers on loamy sites to
achieve an adequate measure of response in these areas and was heavily damaged or completely removed
by fire. Algerita' s close association
with canopies of oneseed juniper
provided it some protection from
herbicide but increased its susceptibility to fire. Burning opened microsites for the reestablishment of
algerita across all sites. Since this
species prefers microsites beneath
tree canopies for establishment, once
trees are removed, the seed source
present is probably released. Oak
was more tolerant to burning than
was sumac on hill and shallow sites.
Wright (1972) found oak sprouts to
increase immediately after fire then
decrease to preburn levels over 18
years. The combination of tebuthiuron and burning severely depleted
cover and browse afforded by both
oak and algerita, but stimulated resprouting in stagnant sumac thereby
increasing its value as a browse species. Competition from perennial
grasses may have been a factor for
suppression of these shrubs on
loamy sites.
Management Implications
Because of a low understory fuel
source, the application of controlled
burning under stable, nonhazardous
conditions in early spring is generally
ineffective in reducing P /J canopies
such as those in our study. However,
following herbicide application and
subsequent herbage release, the application of spring burning holds
some promise for further reducing
remaining live tree canopies. This
may be especially beneficial where
herbicide treatments only produced
marginal results such as the low and
medium rates used at Ft. Stanton.
Results from the application of the
medium rate of tebuthiuron achieved
a partial canopy reduction and forage release which when later burned
produced a mosaic (savanna-like)
cover pattern offering both quantity
and quality in the forage supplied.
Application of a high rate of tebuthiuron alone achieved essentially the
same results as the combination of a
moderate rate of tebuthiuron followed by burning.
Neither tebuthiuron or burning
alone or in combination produced
consistent results when seedling
numbers of both species were considered. AI though treatment can be considered beneficial with respect to
some criteria, it also opens microsites
for the release of an unquantified
seed source. Caution and a complete
resource assessment must be utilized, otherwise treatment life may be
severely shortened through enhancement of seedling germination. Additionally, above normal precipitation
during the 1987 growing season may
have been a factor in both herbage
and seedling release following treatment. Results may vary following
spring burns if soil moisture conditions are unfavorable to plant
growth.
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36:568-571.
Dwyer, D. D. and R. D. Pieper. 1967.
Fire effects on blue grama-pinyonjuniper rangeland in New Mexico.
J. Range Manage. 20:359-362.
Engle, D. M., J. F. Stritzke and P. L.
Claypool. 1987. Herbage standing
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Engle, D. M. J. F. Stritzke and P. L.
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