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Understory Production and Composition in
Pinon-Juniper Woodlands in New Mexico
Rex D. Pieper1
Abstract.-Herbaceous standing crop was determined on three locations
within pinon-juniper woodlands on the Fort Stanton Research Station in the
Sacramento Mountains near Capitan, NM for a 12 year period. End-ofseason standing crop varied from over 1000 kg/ha to a low of less than 200
kg/ha. These differences are comparable to those for other locations in the
Southwest. During the period from 1964 to 1975 variations in July-August
precipitation explained over 40% of the variation in herbaceous standing
crop. Across 25 pinon-juniper stands on Fort Stanton, overstory canopy'
nij
cover was negatively related to total understory herbage weight. A 2
polynomial equation explained over 93% of the variation in understory
herbage weight attributed to overstory cover while two C3 species Piptochaetium fimbriatum and Stipa neomexicana, were positively related to canopy cover. Two-way cabling resulted in increased herbaceous standing
crop of more than 185 kg/ha over a 6-year period after cabling on four location on the research station. Other studies indicated varying responses to
tree reduction. Manipulating overstory cover appears to be one way of altering understory production and composition. Pinon-juniper vegetation responds to topographic variation, but the response is quite variable across
these southwestern woodlands.
INTRODUCTION
more complete understanding of the ecology of
these woodlands.
Summaries of information concerning floristics of understory vegetation on pinon-juniper
woodlands have been presented by Pieper (1977)
and West and others (1975). While juniper and
pinon-juniper communities in the West are represented by only 9 or 10 tree species (Tausch, this
volume, Tueller and Clark 1975), the understory
component is much more varied. West and others (1975) listed over 30 species of shrubs, 8 succulent species, 30 grass species, and 14 fob
species on 20 locations in the western U.S. In
northern portions of the woodlands understory
grasses are dominated by C 3 cool-season species
while C4 warm-season grassers are more important in southern and southeastern areas (West
and others 1975). Pieper (1977) compared understory vegetation for areas in northern regions
with that from the southwestern and southeastern regions.
Pifion-juniper woodlands serve as important
habitat for wildlife and domestic livestock in the
Southwest. These woodlands occupy over 12.5
million hectares in Arizona and New Mexico alone
(Dortignac 1960 and West and others 1975). In
some cases these woodlands occur as extensive
stands while on other cases, they occur as scattered
and disjunct elements. Initially these woodlands
were valued mainly as habitat for animals, but recently other resource values have been recognized
within the pifi.on-juniper woodland (Bledsoe and
Fowler 1992 and Fowler and others 1985). These
other products include fence posts, fuelwood,
poles, wildings, pifi.on nuts, and Christmas trees.
Yet most of these woodlands continue to be used
for livestock grazing and wildlife habitat. Understory components are especially important for livestock, but an understanding of their relationship to
other environmental variables is necessary for a
1Professor of Range Science, Department of Animal and Range
Sciences, New Mexico State University, Las Cruces, NM.
120
Table 2. - Simple linear correlation coefficients between dependent variables of total herbage, blue grama, and forb biomass
and precipitation for certain periods of the year at Fort Stanton, NM (from Pieper and others 1971).
.
Ann. Ppt
1
Dependent variables Oct-Sept JJAS JAS
JA
July Aug
Total biomass
0.47*
0.68** 0.68** 0.69** 0.51 * 0.65**
The objective of this paper is to compare understory vegetation in pifton-juniper woodlands in
New Mexico and the Southwest and to discuss factors influencing development of understory vegetation.
Blue grama biomass
HERBAGE PRODUCTION
0.54*
0.44
0.46*
0.24
0.29
0.14
0.33
Several combinations of monthly precipitation
were related to total, blue grama, and forb biomass
at Fort Stanton in south-central New Mexico usm.g
a combination of location and yearly data sets
(Pieper an others 1971). Summer precipitation was
significantly related to total herbage biomass with
47% of the variation in understory herbaceous
biomass determined by differences in July and
August precipitation (Table 2). The regression
equa tion was
Y = 123.4 + 3.2X
where Y = herbage biomass in kWh-a and X = July
and August precipitation in milliliters. August
precipitation was the best single month predictor
of total herbaceous biomass. Correlation coefficients were lower for blue grama and forb biomass.
The only significant relationship for forb biomass
was with June, July, August, and September precipitation (Table 2). These data were obtained from
mature pifton-juniper woodland stands on Fort
Stanton and stand density and overstory variation
was minimal.
Clary (1971) reported that different plant
groups responded to precipitation distributions in
different manners. Perennial" grass biomass was
controlled largely by winter-summer precipitation
while annual grasses responded to summer precipitation. Shrubs were related more closely to
winter precipitation than that received during
other periods.
Understory biomass is a function of several abiotic as well as biotic factors. Some of these relationships have been studied in detail and others only
superficially. Quantitative 'relationships have been
developed for the influence of precipitation and
overstory canopy on herbaceous biomass.
Table 1. - Herbage standing crop at three locations with piflonjuniper woodlands in the Sacramento Mountains in southcentral New Mexico (mean ± standard error).
Locations
Year
1
2
3
----------------------- kg/ha -------------------1964
486±35
435±44
336±30
1965
660±44
592±50
595±47
1966
570±65
400±22
630±52
1967
899±60
1048±47
725±52
1968
556±57
543±27
529±33
1969
462±37
391 ±44
527±33
1970
585±67
543±35
667±55
1971
375±35
335±25
250±22
1972
330±44
225±18
285±35
1973
410±50
440±40
385±39
1974
235±28
300±32
185±15
1975
425±38
330±40
385±35
452
0.55*
Preci pitati on
FACTORS INFLUENCING UNDERSTORY
BIOMASS
457
0.51 *
Forb biomass
0.36
0.46*
'June, July, August, and September.
*P < 0.05
**P < 0.01
Clary (1978) summarized results of understory
herbaceous biomass for 6 different studies in different locations. These data ranged from less than
50 kglha before removal of overstory trees to more
than 2200 kglha following tree removal. In addition
to large spatial variations in herbage biomass, there
is considerable variation from year to year. At Fort
Stanton in the foothills of the Sacramento Mountains in south-central New Mexico, end-of-season
biomass varied from less than 200 kWh-a to over
1000 kWh-a at three locations over a 12 year period
(Table 1). There were two severe short-term
drought periods during this period in 1970-71 and
in 1974 (Pieper an Donart 1973, Pieper and others
1992). In general growing conditions were more favorable during the 1960s than during the 1970s and
herbage production shows a general decline from
the peak in 1967 to 1975 (Table 1).
Average
0.37
Tree Overstory
Several studies have evaluated the relationship
between overs tory cover or tree basal area and understory production or cover (Pieper .1983). These
relationships are similar although the functional
form of the relationship varies among locations and
514
(
121
Table 3. - Equations depicting relationships between herbage production and overstory cover In southwestern plflon Juniper
ecosystems.
Independent
Dependent
Author(s)
Equation
Variable
Varlable{s)
Location
Northern
Arizona
Northern (Utah
Arizona Juniper)
Northern (Alligator
Arizona Juniper)
Great Basin
New Mexico
Herbage
Biomass
Herbage
Biomass
Herbage
Biomass
Understory
cover
Herbage
Percent
Canopy cover
Basal Area (X1)
June-Aug. PPT (X3)
Basal Area (X1)
June-Aug. PPT (X~
Overstory
cover
Overstory
years. Herbage biomass is reduced by small iricrements of canopy in dense stands but at lower canopy cover, the curve is much flatter. Several studies
report equations for prediction of understory biomass from overstory cover (Table 3) (Clary 1974,
Jameson 1967, Pieper 1990, and Tausch and others
1981) while others showed the curves without
equations (Arnold and others 1964, Dalen and
Snyder 1987, and Short and others 1977). Reduction of canopy cover from 60 to 70% increases herbaceous biomass by small amounts, but reduction
from 20 to 10% increases herbage biomass considerably. The only linear relationship was that reported by Tausch and others (1981) for several
areas throughout the Great Basin but the dependent variable in their study was herbaceous cover
and not biomass. Large variations in understory
response to tree reductions is to be expected since
the studies represent different areas with different
soils, climatic patterns, and time since treatment.
Arnold and others (1964) reported that maximum
response in understory production occurred 10 to
12 years following treatment. Bledsoe and Fowler
(1992) found that herbage response was highest 4
to 7 years following cutting on two sites and sooner
on another site in New Mexico. Degree of disturbance and precipitation patterns following control
undoubtedly influence the response of understory
species.
While the general pattern is for herbaceous
biomass to decrease as overstory cover increases,
some species actually increase as canopy cover increases. Clary and Morrison (1973) reported that
cool-season grasses such as Poa fendleriana, Sitanion
hystrix, Koeleria cristata, and Agropyron smithii were
more abundant under the canopy of large alligator
junipers Uuniperus deppeana) than in the open
spaces in northern New Mexico. In New Mexico,
several cool season grasses were positively correlated with overs tory tree canopy cover
(Armentrout and Pieper 1988, Pieper, 1991, and
Schott and Pieper 1985).
Y = 597-527(-e~.36x) 'h
Jameson 1967
Y=345.4=129.4X1
+ 28.3X3
Y=593.3=357.0X1
+ 75.4X2
Y =20.36-0. 752X
Clary and
others 1974
Clary and
others
Tausch and
others 1981
Pieper 1990
2
Y-1 062-170X +0.82X
Table 4. - Percent Increase In understory herbaceous biomass
following plflon-Junlper control In the Southwest
Location
% Increase
Authors
Arizona
Arizona
Arizona
Nevada
New Mexico
Taos
Chama
Cloudcroft
New Mexico
1227
339
105
441
316
1617
2907
33
Clary and Jameson 1981
Clary 1971
O'Rourke and Ogden 1969
Everett and Sharrow 1985
Bledsoe and Fowler 1992
Rippell and Others 1983
The strong relationship between overstory
canopy cover and herbaceous biomass suggests
that reduction of trees in the pinon-juniper
woodlands would increase forage for livestock
and possibly game. Consequently, during the
1950s and 1960s, large scale woodland control
projects were undertaken on public lands (Aro
1971). Many of these projects were not evaluated, but several studies have been conducted to
determine the response of herbaceous vegetation
to pinon-juniper reductions. The results of these
studies have been varied (Table 4) with understory biomass increasing form less than 35% to
nearly 30 times the untreated control. In Arizona
O'Rourke and Ogden (1969) reported understory
vegetation did not respond to pinon-juniper
control on some sites because of high calcium
carbonate content of the soil and probable low
phosphorus availability. The overall response in
herbaceous biomass was higher following control on three locations and nearly double on the
four locations studied (Table 4). Table 5 shows
that herbaceous biomass increased over 185
kglha during a six-year period following two
way cabling at Fort Stanton. The increase was
greater than 200 kglha for most years, but the
average was reduced by the lack of response
during the second year following the cabling.
122
Aro, Richard S. 1971. Elevation on pinyon-juniper conversion to grassland. Journal of Range Management.
24:188-197.
Bledsoe, F. Neal; Fowler, John M. 1992. Economic
evaluation of the forage-fiber response to pinyonjuniper thinning. New Mexico State University Agricultural Experiment Station Bulletin 753.
Clary, Warren P. 1971. Effects of Utah juniper removal on
herbage yields from Springerville soils. Journal of
Range Management. 24:373-378.
Clary, Warren P.; Baker, Malchus B. Jr; O'Connell, Paul F.;
Johnsen, Thomas N.; Campbell, Ralph N. 1974., Effects 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.
Clary, Warren P.; Morrison, Douglas C. 1973. Large alligator junipers benefit early-spring forage. Journal of
Range Management. 26:70-71.
Clary, Warren P.; Jameson, Donald A. 1981. Herbage
production following tree and shrub removal in the
pinyon-juniper type of Arizona. Journal of Range
Management: 34-109-113.
Clary, Warren P. 1987. Herbage production and livestock grazing on pinyon-juniper woodlands. In: Gen.
Tech. Rep. INT-215. U.S. Department of Agriculture,
forest Service, Intermountain Forest and Range Experiment Station. Ogden, UT.
Dalen, Raymond S.; Snyder, William R. 1987. Economic
and social aspects of pinyon-juniper treatment-then
and now. Gen. Tech. Rep. INT-215. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station.
Dortignac, E. J. 1960. Water yield from pinyon-juniper
woodland. In: Water yield in relation to environment
in the southwestern United States. Southwestern
and Rocky Mountain Division of the American Association for the Advancement of Science. Alpine, TX.
Everett, Richard L.; Sharrow, Steven H. 1985. Response
of grass species to tree harvesting in singleleaf pinyon-Utah juniper stands. Res. Rap. INT-334. U.S.
Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station.
Everett, Richard L.; Sharrow, Steven H.; Meeuwig, Richard O. 1983. Pinyon-juniper woodland understory
distribution patterns and species associations. Bulletin of the Torrey Botanical club. 110:454-463.
Fowler, John M.; Peacock, Bruce E.; Schaber, Michael J.
1985. Pinyon-juniper woodland type in New Mexico:
asses or liability. New Mexico State University Agricultural Experiment Station Bulletin 718.
Hill, Alison. 1990. Ecology and classification of the pinyon-juniper woodlands in western New Mexico.
Ph.D. Dissertation, New Mexico State University, Las
Cruces, NM.
Jameson, Donald A. 1967. The relationship of tree overstory and herbaceous understory vegetation. Journal
of Range Management. 20:247-249.
Table 5. - Herbaceous biomass (kg/ha) on four locations on Fort
Stanton following two way cabling for tree control.
Control
Cabled
Difference
Year
1975
1976
1977
1978
1979
1980
600
642
556
455
512
530
797
375
859
685
825
717
+197
-267
+303
+230
+313
+187
Topography
Considerable research has been conducted on
topographic influences on tree species in the pinon-juniper woodland, but little on understory responses. Lymbery and Pieper (1983) reported that
several species were found on most locations sampled in south-central New Mexico, and were not
restricted to a particular topographic position or
aspect. In western New Mexico, cool season grasses
tended to be more abundant at higher, mesic habitats while warm season grasses were more abundant at lower elevations (Hill 1990). In Nevada,
Everett and Others (1983) reported that understory
species responded differently to the presence of
one-seed juniper Ouniperus monosperma) depending
on the aspect (north, west, or south).
Shrubby components appear more responsive
to topographic variables than herbaceous species.
Density of cholla cactus was greater on relatively
dry south facing slopes compared to north slopes
in pmon-juniper woodlands in the foothills of the
Sacramento mountains in south-central New Mexico. Other shrubs were not greatly influenced by
topographic position.
Mountain mahogany
(Cercocarpus moujntanus) density was greater at
higher and intermediate elevations in western
New Mexico while grey oak (Quercus grisea) was
more abundant at intermediate elevations (2000 to
2200 m). Other species occurred on different aspects and elevations.
LITERATURE CITED
Armentrout, Susan M.; Rex D. Pieper. 1988. Plant distribution surrounding Rocky Mountain pinyon pine
and oneseed juniper in south-central New Mexico.
Journal of Range Management. 41:139-143.
Arnold, Joseph F.; Jameson, Donald A.; Reid, Elbert H.
1964. The pinyon-juniper type of Arizona: Effects of
grazing, fire, and tree controL U.S. Department of
Agriculture Production Research Report 84.
123
Lymbery, Gordon A.; Pieper, Rex D. 1983. Ecology of
pinyon-juniper vegetation in the northern Sacramento Mountains, New Mexico State University Agricultural Experiment Station Bulletin 698.
O'Rourke, James T.; Ogden, Phil R. 1969. Vegetative response following pinyon-juniper control in Arizona.
Journal of Range Management. 26:302-303.
Pieper, Rex D.; Montoya, James R.; Groce, V. Lynn. 1971.
Site characteristics on pinyon-juniper and blue grama
ranges in south-central New Mexico. New Mexico
State University Agricultural Experiment Station
Bulletin 573.
Pieper, Rex D. 1977. The southwestern pinyon-juniper
ecosystem. In: Gen. Tech. Rep. RM-39. U.S. Department of Agriculture, Forest Service, Rocky Mountain
Forest and Range Experiment Station. Fort Collins,
CO.
Pieper, Rex D. 1983. Overstory-understory relationships;
pinyon-juniper woodlands. In: Overs tory-understory
relationships in western forests. Western Regional
Publications 1. Colorado Agricultural Experiment
Station, Fort Collins, CO.
Pieper, Rex D. 1990. Overstory-understory relations in
pinyon-juniper woodlands in New Mexico. Journal
of Range Management. 43:413-415.
Pieper, Rex D.; Donart, Gary B. 1973. Drought effects on
blue grama rangelands. New Mexico State University
Livestock Feeders Report. Las Cruces, NM.
Pieper, Rex D.; Parker, E. E; Donart, Gary B.; Wallace, Joe
D.; Wright, Jimmie D. 1992. Cattle and vegetational
response to 4-pasture and continuous grazing systems. New Mexico State University Agricultural Experiment Station Bulletin 756.
Schott, Martin R.; Pieper, Rex D. 1985. Influence of canopy characteristics of one-seed juniper on understory
grasses. Journal of Range Management. 38:328-331.
Short, Henry L.; Evans, Wain; Boeker, Erwin L. 1977. The
use of natural and modified pinyon-juniper woodlands by deer and elk. Journal of Wildlife Management. 41:543-559.
Tausch, Robin J.; West, Neil E.; Nabi, A.A. 1981. Tree age
and dominance patterns in Great Basin pinyonjuniper woodlands. Journal of Range Management.
34:259-264.
Tueller, Paul T.; Clark, James. 1975. Autecology of pinyonjuniper species of the Great Basin. In: The pinyonjuniper ecosystem: A symposium. Utah State University, Logan, UT.
West, Neil E.; Rea, Kenneth L.; Tausch, Robin J. 1975. Basic synecological relationships in juniper-pinyon
woodlands. In: The pinyon-juniper ecosystem: A
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124