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Restoration Studies in Degraded PinyonJuniper Woodlands of North-Central New
Mexico
Brian F. Jacobs
Richard G. Gatewood
Abstract-Small scale experiments were initiated in 1994, at two
degraded pinyon-juniper (Pinus edulis Engelm. and Juniperus
monosperma [Engelm.l Sarg.) woodland sites in north-central New
Mexico, to evaluate the efficacy of resto~ation methodologies for
reestablishment of native herbaceous cover. Results after three
years post-treatment were highly significant: a primary, overstory
reduction and slash mulching treatment produced two to sevenfold
increases in total herbaceous cover relative to both controls and pretreatment condition. Secondary, soil surface preparation and seeding treatments appeared to confer no significant benefits over the
primary treatment herbaceous response.
Degraded pinyon-juniper (Pinus edulis Engelm. and
Juniperus monosperma [Engelm.] Sarg.) communities occupy thousands of hectares within Bandelier National Monument and the adjacent Santa Fe National Forest in northcentral New Mexico. These woodland areas were inhabited
during prehistoric times by Puebloan Indians who left behind an abundance of cultural remains. Overgrazing, loss of
a fire regime and drought during the last 120 years are
thought to be causative factors of the overstocked and
degraded nature of many woodlands in this area. As increasing tree cover supplanted herbaceous cover, savanna like
communities were gradually transformed into closed woodlands. Older growth trees are common in many of these
woodland areas, but tree reproduction less than 120 years
old accounts for much of the observed density. Interspaces
between trees are characterized by scant herbaceous cover
and exposed, rapidly eroding soils. Rapid soil loss in these
degraded pinyon-juniper communities is unsustainable and
ultimately threatens the integrity of thousands of prehistoric archeological sites embedded in the cultural landscape
of north-central New Mexico.
The current study was initiated in 1994 to evaluate the
efficacy of potential methodologies for the restoration of
degraded pinyon-juniper (Pinus edulis Engelm. and
Juniperus monosperma [Engelm.] Sarg.) woodlands at
Bandelier National Monument and on the adjacent Santa Fe
National Forest. Specifically, we wanted to determine if a
primary, overstory reduction and slash mulching treatment
would yield a suitable herbaceous response and whether
In: Monsen, Stephen B.; Stevens, Richard, comps. 1999. Proceedings:
ecology and management of pinyon-juniper communities within the Interior
West; 1997 September 15-18; Provo, UT. Proc. RMRS-P-9. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Brian Jacobs and Richard Gatewood are natural resource specialists at
Bandelier National Monument, Los Alamos, NM 87544.
294
secondary, soil surface preparation and seeding treatments
would enhance reestablishment of an effective ground cover.
Previous work by Chong (1994) at Bandelier National
Monument and by land managers on the Santa Fe National
Forest (Elson, pers. comm.) set the stage for the current
study. Chong (1994) demonstrated the importance of favorable microsites in the establishment of herbaceous plants
from seed on degraded pinyon-juniper sites. She also noted
that without the addition of seed material, no new grass
plant establishment was observed (Chong, 1994) suggesting
a depleted soil seed bank. Santa Fe National Forest land
managers have repeatedly observed robust herbaceous responses following fuelwood harvests in pinyon-juni per woodlands, particularly when follow-up efforts were made to thin
out smaller diameter trees and scatter the resulting slash
(Elson, pers. comm.; Loftin, 1998). While numerous studies
have documented an herbaceous response to overs tory thinning in pinyon-juniper systems (Arnold and Schroeder,
1955; Bledsoe and Fowler, 1992), itis importantto assess the
efficacy of restoration methodologies relative to specific site
conditions and management objectives.
Intensive grazing pressures, beginning around 1880, are
thought to have reduced continuity of fine grass fuels,
effectively preventing fire from propagating in pinyon-juniper woodlands (Gottfried and others, 1995) as well as in
adjacent plant communities (Allen, 1989). Although pinyonjuniper communities are generally poor recorders of fire
occurrence, evidence of periodic fire in these systems is
provided by charcoal deposits in the soil and fire scars on
woody remains and living trees (Gottfried and others, 1995).
In addition, excellent fire history data documenting recurrent fire events are available from Ponderosa Pine communities immediately adjacent (Gottfried and others, 1995;
Touchan and others, 1994). We suggest fire was an important process in maintaining the former savanna like structure of pinyon-juniper communities in our area. The thinning effects of periodic fire prior to 1880 and the subsequent
loss of a fire regime would account for the pattern of widely
spaced, older growth trees within a now dense woodland
matrix of a much younger age.
The loss of fire disturbance, initially as a result of overgrazing, and subsequently through active suppression
(Gottfried and others, 1995), has had a profound influence on
the dynamics of pinyon-juniper systems. Age class information from pinyon-juniper study sites in our area suggest an
exponential increase in pinyon-juniper stem densities in
former savanna areas beginning around 1880 (Allen, pers.
comm.; Davenport and others, 1996; Gottfried and others,
1995;). Pinyon and juniper also expanded their ranges, with
USDA Forest Service Proceedings RMRS-P-9. 1999
both species invading upslope into Ponderosa Pine dominated forests and juniper invading downslope into former
shrub and grassland communities (Gottfried and others,
1995).
Active soil erosion on degraded pinyon-juniper sites during the last fifty years is clearly evidenced by exposed soils
and bedrock, soil pedestals, lobes of active sediment and
sediment accumulation behind fallen logs (Davenport, 1997).
While previous episodes of erosion are documented in soil
profiles from local pinyon-juniper sites (Davenport, 1997;
Reneau, pers. comm.), the timing of the current erosional
episode appears linked to historic land use practices. In
addition, an extended drought during the mid-1950's is
thought to have intensified competition for water in overstocked woodlands, perhaps reducing herbaceous cover below thresholds necessary to conta~n erosional processes
(Wilcox and Breshears, 1995; Wilcox and others, 1996a,b).
Occasional trunk remnants of Ponderosa Pine from individ uals killed during the 1950's drought can be found on both
sites, but no live Ponderosa Pine is currently present within
study site boundaries.
Intensive characterization of erosional processes in a one
hectare portion of a degraded pinyon-juniper woodland at
Bandelier (Wilcox and others, 1996a,b) suggest average
annual soil losses of 10,000 to 20,000 kg/ha, most of it
occurring during intense thunderstorm events typical ofthe
summer monsoons. On the basis of both soil erosion bridge
and sediment catchment data, soil loss within degraded
pinyon-juniper communities at Bandelier can be conservatively estimated at 2.5 cm per decade; an unsustainable rate
given shallow soils with average depths of 1.0 to 12.0 dm
(Davenport, 1997; Davenport and others, 1996; Wilcox and
Breshears, 1995).
Rapid soil loss in degraded pinyon-juniper communities
threatens the integrity of the thousands of prehistoric cultural sites located within Bandelier National Monument.
Well over three-quarters of the prehistoric sites at Bandelier
National Monument occur within pinyon-juniper communities; of these cultural sites, nearly 99 percent have sustained
erosional impacts (Mozzillo, in preparation).
Despite the cessation of intensive livestock grazing pressure around 1940, in Bandelier National Monument and on
portions of the Santa Fe National Forest, degraded pinyonjuniper communities appear to have little capacity to recover. Repeated measures of herbaceous cover in ungulate
exclosures established in 1975 on degraded pinyon-juniper
sites at Bandelier, suggest exclusion of grazing alone is
insufficient to promote recovery of these systems. (Chong,
1992).
Overstocked with young trees and lacking an effective
ground cover, degraded pinyon-juniper systems are poorly
equipped to manage limited soil and water resources. These
degraded communities can yield significant amounts of
runoff and sediment, at various scales, particularly from
bare ground interspaces during high intensity summer
thunderstorms (Wilcox and others, 1996a,b). Freeze-thaw
action on exposed soils is thought to facilitate erosional
processes, both by inhibiting new plant establishment
through root shear effects and by creating light textured
crusts vulnerable to the forces of wind and rain. Harsh
physical site conditions, characterized by exposed, nutrient
poor soils, impose severe restrictions on the successful
USDA Forest Service Proceedings RMRS-P-9. 1999
establishment of new herbaceous plants. Soil seed banks
appear to be depleted of perennial grass propagules and the
depauperate grass individuals are incapable of producing
viable seed in many years. With generally low herbaceous
productivity, seed predation by birds, rodents, and harvester ants can be very efficient. High levels of mortality are
common, in both germinating seed and young herbaceous
seedlings. Herbivory of cool season grasses by native ungulates may limit abundance and productivity of these species
relative to warm season grasses.
At Bandelier, the selection of restoration methodologies is
constrained by self-imposed restrictions designed to protect
cultural, natural, and wilderness resource values (Sydoriak,
1995). Overstory reduction must be accomplished using
minimum tools and techniques that are sensitive to multiple
park resources. Rough terrain, high cultural site density,
and the presence of designated wilderness essentially preclude mechanized ground disturbing activities (such as
chaining, drilling and other agronomic techniques utilizing
heavy equipment) typically associated with large scale restoration efforts. Planting methodologies for seed material
are generally limited to hand methods. All plant materials
must be locally native, requiring custom production at
considerable expense.
Methodology
Two degraded pinyon-juniper sites were included in the
current study; one located on Frijoles Mesa within Bandelier
National Monument and a second on Garcia Mesa in the
adjacent Santa Fe National Forest. Study sites are located
at the upper end ofthe pinyon-juniper zone on gently sloped
mesas between 1980 m (6600 ft) at Frijoles Mesa and 2160
m (7200 ft) at Garcia Mesa. Canopy closure ranged from 23.0
to 60.0 percent, with herbaceous cover ranging from 5.0 to
15.0 percent, litter ranging from 38.0 to 84.0 percent and
bare soil ranging from 7.0 to 56.0 percent. Soils are derived
from volcanic ash deposits and are generally shallow and
poorly developed (Davenport, 1997; Davenport and others,
1996; Wilcox and Breshears, 1995). Precipitation increases
with elevation and ranges from around 40.0 cm. (16 in.) at
the Frijoles Mesa site to nearly 50.0 cm. (20 in.) at the Garcia
Mesa site (Wilcox and Breshears, 1995). Summer thunderstorms account for nearly half ofthe annual rainfall; winter
snows are variable in depth and persistence (Wilcox and
Breshears, 1995).
Experimental Design
The experiment consisted of 18, 15m 2 plots at each of two
sites. Because of constraints in implementing this study, the
primary, overstory reduction and slash mulching treatment
was assigned to 15 contiguous plots, in a non-random fashion, with the remaining three plots serving as controls.
Evaluation of the primary treatment relative to controls, is
considered separately from the secondary treatment.
For the purposes of evaluating secondary treatments, the
15 plots within the bounds of the primary, overstory reduction and slash mulching treatment, were divided into three
blocks of five plots each. Within each block, plots were
randomly assigned one of five secondary treatment
295
combinations: slash mulch only; slash mulch + imprinting;
slash mulch + seeding; slash mulch + imprinting + seeding;
or slash mulch + raking + seeding.
Data Collection Protocols
Herbaceous cover was measured as an indicator of system
response to treatment, since these data can be reliably
collected by a seasonal workforce and are indicators of
available soil moisture and rates of soil erosion. Changes in
soil surface cover (that is vegetation, litter, bare soil) were
measured using a modified University of New Mexico, Long
Term Ecological Research program design; vegetation data
was collected by species and growth form and included basal
intercept and canopy cover components. Two 21.21 meter
vegetation transects were permanently established in each
plot for a total of 42.42 m sampled per plot. Pre-treatment
data was collected from both study sites during the fall of
1994. Year 2 and 3 post-treatment data was subsequently
collected at each study site during the fall of 1996 and 1997.
These data were compiled and summarized using the Statistical Package for Social Sciences. Large and small scale
repeat photos were taken along the vegetation transects to
provide additional visual documentation of treatment
response.
Restoration Treatments
The primary, overstory reduction and slash mulching
treatment was applied to both sites in the spring of 1995.
Overstory reduction treatment was applied using chainsaws.
Bledsoe and Fowler (1992) suggested overstory reduction of
pinyon-juniper woodlands through selective thinning, as
compared with agronomic techniques such as chaining, can
meet multiple management objectives. Main limbs were
lopped off and the trunk was flush cut at the base. The
resulting slash, lopped branches and trunk sections, were
then scattered preferentially.into the bare interspaces to
serve as a rough mulch. On the basis of land manager
recommendations (Elson, pers. comm.), previous experimental restoration work (Bledsoe and Fowler, 1992), spatial
patterns of older growth trees, and water relation studies in
pinyon-juniper systems (Breshears and others, 1997), a
spacing between mature tree individuals offrom 15 to 20 m
was considered optimal for restoration of former pinyonjuniper savanna types in the study area. Following these
recommendations, we elected to remove all of the tree
canopy within and immediately adjacent to individual plots
for the purposes of this small scale experiment.
Secondary soil surface preparation and seeding treatments were applied to bare soil areas of both sites during the
summer of 1995. Soil surface preparation techniques applied prior to seeding included: no soil surface preparation,
light raking, and imprinting. Raking was accomplished
using a council fire rake to cut shallow (1-2 cm deep) furrows
perpendicular to the slope. Imprinting was accomplished
using custom made, hand implements to create a pattern of
sloped depressions (5 cm deep) on moist soil surfaces. Imprints provide temporary catchments for seed, litter, soil
and water and thus may serve as favorable microsites for
germinating plant materials. Seed material was applied at
296
a rate of 1291 seeds/m2 and consisted of a mix of blue
gramma (50 percent), little bluestem (30 percent), and sand
dropseed (20 percent). The relatively high seeding rate was
used in an attempt to compensate for anticipated losses due
to seed predation, seed mortality and marginal seedbed
preparation. Seeding was accomplished by broadcasting the
seed mix onto designated treatment plots subsequent to soil
surface preparation. Loose dirt was lightly brushed back
into the furrows cut on raked plots.
Results _ _ _ _ _ _ _ _ _ __
Primary Treatment
A comparison of the primary, overstory reduction and
slash mulching treatment to controls suggests that exposed
soil coverage decreased by a mean of 222.0 percent at the
Frijoles Mesa site and 200.0 percent at the Garcia Mesa site
(fig. 1a,b). Total herbaceous cover had a mean increase of
773.0 percent at Frijoles Mesa and 241.0 percent at Garcia
Mesa, by the third year post-treatment (fig. 1a,b). Grass
cover increased 446.0 percent at Frijoles Mesa and 179.0
percent at Garcia Mesa (fig. 1c,d), while forbs increased
1267.0 percent at Frijoles Mesa and 705.0 percent at Garcia
Mesa (fig. 1c,d). The difference in relative contributions of
forbs and grasses at the two sites may be due to the initially
high grass cover at Garcia Mesa (25.0 percent) as compared
with Frijoles Mesa (6.0 percent). The low initial grass cover
at the Frijoles Mesa site may have provided more opportunities for annual and biennial forbs to establish.
Secondary Treatment
Analysis of the secondary, soil surface preparation and
seeding treatments suggest that there was no significant
increase in total grass cover over the primary treatment
response (fig. 2). While Blue Grama (Bouteloua gracilis
[H.B.K.] Lag.) showed a mean increase of 252.0 percent
cover on seeded versus 119.0 percent cover on unseeded
plots, this increase was apparently offset by contributions
to total grass cover on unseeded plots by other non-seeded
species (fig. 2). The other two seeded species, Little Blue
Stem (Schizachyrium scoparium [Michx] Nash) and Sand
Dropseed (Sporobolus cryptandrus [Torr.] Gray), had mean
increases ofless than 1.0 percent across both sites.
Discussion ___________
Additional work is clearly needed to more completely
understand the mechanisms responsible for the observed
herbaceous response. Breshears and others (1997) provide
evidence to support shallow water harvest by one-seed
juniper from intercanopy spaces. Bledsoe and Fowler (1992)
document an increasing herbaceous response to decreasing
densities of overstory trees and report no significant increases in grass production without a minimum two-thirds
overstory thinning. Preliminary greenhouse studies conducted at Bandelier support the benefits oflitter and slash
as moderators of soil moisture and temperature; growth
performance of blue gramma seedlings was significantly
USDA Forest Service Proceedings RMRS-P-9. 1999
90
Figure 1-Comparisons of the primary, overstory
reduction and slash mulching treatment (n =15) to
control (n =3) using four cover components measured at Frijoles Mesa (Graphs A and C) and
Garcia Mesa (Graphs B and D). The clustered bars
represent three years of measurements: 1994
(pre-treatment), 1996 (two years post-treatment),
and 1997 (three years post-treatment). Graphs A
and B compare treatment effects on exposed soil
and total herbaceous cover across both sites.
Graphs C and D compare the forb and grass
response between control and primary treatment
plots for each site. The standard error bars are
times one standard error.
80
70
60
~
...
50
>
40
CII
0
0
30
20
10
0
80
70
~
...
CII
>
60
50
0
0
:,
40
30
20
10
~.------------,-------------------------------~
0
Control
Treatment
Control
Treatment
9r-----------L-------~------~--------+_----~
Total Herbaceous
Cover
Exposed Soil
~r-H!H------_4~------i~M------~tH-----~miH----~
~
70
Frijoles Mesa
~ 30
o
C
(J
60
20
50
10
C
...
40
CII
>
0
0
30
20
0
60
50
~
...
CII
>
0
Grass Grama
Sl
SIS
Figure 2-Comparisons of total grass cover
among secondary, soil surface preparation and
seeding treatment combinations, for each year
of measurement, and relative to one of the
seeded species, blue grama (Bogr). Treatment
codes: S = Slash only; SI = Slash Mulch +
Imprinting; SIS = Slash Mulch + Imprinting +
Seeding; SRS =Slash Mulch + Raking + Seeding. Standard error bars are times one standard
error.
10
Garcia Mesa
Grass Grama
40
30
0
20
10
0
Control
Treatment
Forbs
Control
Treatment
Grasses
USDA Forest Service Proceedings RMRS-P-9. 1999
297
enhanced by litter and! or slash mulching treatments
(Snyderman and Jacobs, 1995). Watershed level restoration
studies underway at Bandelier National Monument are
attempting to correlate the herbaceous response to overstory reduction and slash mulching treatment with soil
moisture and soil erosion.
We suggest that tree overstory removal reduces competition for limited water and nutrient resources while the
scattered slash provides benefits to exposed soils: reducing
runoff and sediment transport, increasing infiltration and
soil moisture, moderating soil temperature, freeze-thaw and
evaporation, redistributing nutrients, and mitigating grazing impacts. Combined, these effects create favorable
microsites for increased productivity of remnant herbaceous
plants as well as for germination, establishment and growth
of new individuals from seed.
High seed loss in secondary treatments, from some combination of predation and mortality, was evident at both sites
based on seedbank analysis at one, three, and twelve weeks
post-seeding (J acobs and Snyderman, 1995). Marginal planting techniques, which did not effectively provide seed with
good soil contact, combined with uneven precipitation patterns may have been responsible. Slash mulch was observed
to be effective in keeping wind and rain from transporting
broadcast seed off ofindividual plots. Intense seed predation
by harvester ants was observed at the Frijoles Mesa site,
beginning soon after application and continuing until effective rain either concealed seeds or stimulated germination
several weeks later. Surprisingly, loss of seed was also high
at the Garcia Mesa site, despite the apparent absence of
harvester ants and occurrence of effective precipitation
within several days of planting.
Acknowledgments _ _ _ _ _ __
We would like to thank the eighteen Student Conservation Assistants who supported this study through their
cumulative seventy-six months pffield work over a four year
period. David Snyderman, Craig Allen and Sam Loftin
provided valuable technical assistance. This study received
financial support from the Challenge Cost Share Program of
the National Park Service, the Friends of Bandelier, the
Student Conservation Association, and Bandelier National
Monument.
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USDA Forest Service Proceedings RMRS-P-9. 1999