Effects of thinning,
burning, seeding, and slash arrangements on understory communities in pinyon-juniper woodlands of northern
Maria R. Irwin1, Alex J. Finkral 2*, and John D. Bailey3=
Authors are 1 Forest Ecologist, Ecosphere
Environmental Services, Durango, CO 81301, USA; 2Assistant Professor, Schoo=
l of
Forestry, Northern Arizona University, Flagstaff, AZ 86001 USA; 3Associate
Professor, Department of Forest Resources, Oregon State University, Corvall=
is,
OR 97331, USA. *Correspondence: Alex J. Finkral=
,
Grafical Abstract
�Abstract:�
Pinyon-juniper woodlands are a domin=
ant
ecosystem in the American Southwest that have been increasing in density ov=
er
the last century, generating concerns about the effects on wildlife habitat,
livestock forage, and wildfire risk. We tested 16 treatment combinations
designed to restore stands to historic conditions by examining the impact o=
n understory
plant richness and abundance.� We t=
hinned
three sites comprised of different parent soil materials: limestone, sandst=
one,
and basalt.� Each site had four sla=
sh
arrangements: piled, broadcast, clustered, or no thinning.� Each of these arrangements received a
different burning/seeding treatment: prescribed fire, seeding, prescribed f=
ire
and seeding, or none.� This study
corresponded with the driest period in the last 55 years, and plant species
richness decreased by an average of 40% from the previous year in the contr=
ol
plots.� Richness was significantly
different due to slash arrangement at the basalt site only.� Burning or seeding did not affect richn=
ess at
any of the sites. Plant species abundance was generally low and not influen=
ced
by treatment or site.� This study
demonstrates that extensive ecosystem manipulation in the pinyon-juniper
woodlands of northern
Keywords: pi�on, restoration, prescribed fire,
understory response, silviculture
Introduc=
tion
Pinyon=
span>-juniper woodlands occupy almost 30 million hectares in the western Uni=
ted
States and are one of the largest ecosystems in the American Southwest.1
Pinyon-juniper woodlands are usually found at
elevations of 1,370 to pinyon-juniper areas have expanded their geographic e=
xtent
and/or increased in density.6,7,8,9,10 This period coincides with
Euro-American settlement in many areas, when livestock grazing, climatic ch=
anges,
and fire suppression were introduced to pinyon-=
juniper
forests.11
The current expansion and
densification of pinyon-juniper woodlands is
generally considered an undesirable trend for land managers, who thin or re=
move
pinyon-juniper woodlands for wildlife habitat
improvements, increased forage for livestock, and fuels mitigation.� Many managers remove all pinyon
and juniper trees by chaining (dragging a heavy chain between two bulldozer=
s to
knock over the trees) or mastication.12 Few<=
/span>
studies have used historic reference conditions to guide thinning in pinyon-juniper woodlands.13, 14��
The inverse relationship
between overstory and understory cover in pinyon-juniper woodlands has been documented.15,=
16,17,18,19
Increasing understory diversity and abundance has become a goal for many la=
nd
managers.� Techniques for increasing
understory health have included thinning, slash additions to bare soil, pre=
scribed
burning, and seeding.13,20,21,22
Leaving the slash created =
by
thinning on the ground may create favorable microsites
for understory establishment.� Slash
amendments to the soil significantly increased residual woody and li=
tter
debris, reduced soil movement, and increas=
ed arbuscular mycorrhizal fu=
ngi and
microbial carbon levels.22� Brockway
et al. found that plant species
richness and diversity increased most on sites where slash was either
completely removed or scattered to serve as mulch and that understory bioma=
ss
increased for all harvest treatments.21� Jacobs and Gatewoo=
d
determined that overstory reduction and slash
mulching treatments produced two to sevenfold increases in herbaceous cover
relative to controls.13 It remains an open question whether slash
additions to bare soil alone, without the confounding factors of thinning, =
increase
understory diversity and cover.�
The use of prescribed fire=
has
had limited applications in pinyon-juniper wood=
lands
because of the difficulty of burning 23 and uncertainty about the
historic fire regimes.24 ��In
many areas, only extremely dry and windy conditions will carry a fire throu=
gh
the canopy, resulting in a high severity, stand replacing fire.25,26=
sup>
Prescribed fire success depends on stand structure, weather conditions, fuel
availability, and fuel conditions.27 Some land managers have use=
d prescribed
fire followed by seeding to convert woodlands to grasslands, thus improving=
their
rangeland for livestock. Jacobs et =
al.
�used prescribed fire to maintain t=
he
mechanically created savanna structure by killing tree seedlings, but warn =
that
excessive fuel loadings or less than optimal burning conditions can damage
grass and forb communities.13 Prescribed fire has also been used=
to
consume the slash created by thinning.�
Understory abundance can increase when a site is burned several years
after thinning.13,20
The success or failure of
seeding in pinyon-juniper woodlands is highly
dependent on precipitation.� Water
availability is critical for seedling establishment in arid ecosystems.28,29 Seeding is also affected by animal predation=
30
and the availability of favorable microsites.31 Slash additions =
and
minor soil disturbances can create favorable microsites for seed establishm=
ent.22
Seeding after wildfires is a common practice for the US Forest Service, and=
has
been shown to effectively increase graminoid co=
ver in
degraded pinyon-juniper woodlands in northern
Arizona.22
The objective of this study
was to determine the effect of different silvicultural=
treatments in a pinyon-juniper
woodland on understory richness and abundance.�
The treatments consisted of overstory
thinning, different arrangements of slash, and burning and seeding in diffe=
rent
combinations.� Our specific research
questions were: (1) Does burning and/or seeding =
after
thinning influence resulting understory richness and abundance?� (2) Does slash arrangement influence
resulting understory richness and abundance?�
To answer these questions, we measured post-treatment changes in for=
est
structure, fuel creation and consumption, maximum soil temperature reached
during the prescribed burn, and understory vegetation responses.� We hypothesized that broadcasting slash
followed by seeding would lead to the greatest understory abundance and
richness and that burning would decrease both abundance and richness.� The results from this study will assist=
land
managers designing thinning prescriptions and in understanding the interact=
ions
of slash arrangements, burning, and seeding on resulting understory richness
and abundance.� Methods Study Site This study was conducted in
2005 and 2006 on Anderson Mesa, located Because of the effect of s=
oil
parent material on the developmental dynamics of vegetation in this region =
we
selected three sites with different parental substrates.10 These sites also have well documented historic forest
structures (see 10 for detailed site descriptions).� These sites were named after their soil
parent material: limestone, sandstone, and basalt.� All three sites were in the middle of t=
he
local pinyon-juniper eleva=
tional
gradient.� The limestone and sandstone
sites have had limited fall and spring livestock grazing since the 1950�s.<=
span
style=3D'mso-spacerun:yes'>� The basalt site has not been grazed fro=
m 1920
to the present (Jack Metzger, Flying M Ranch, personal communication).� Other important grazers in the area inc=
lude
elk (Cervus elaphus=
i>),
mule deer (Odo=
coileus hemionus<=
/i>),
and pronghorn antelope (Antilocarpa
Experimental Design
We created a split-plot de=
sign
with one of four slash arrangements applied to the subplots and one of four
seed/burn methods applied to the whole-plots (Figure 1).� At each site we created three 160 x 80 m whole plots, and =
the
whole plots were divided into four 40 x �
Figure 1.� Split-plot experimental design replicated at each of the three sites.
Vegetation Surveys
A pre-treatment vegetation
survey was conducted using a modification of the Modified-Whittaker plot in
June of 2005. 33 We drew out a
Thinning and Slash Arrangements
Thinning was conducted in =
the
summer of 2005, following a BDQ prescription for each site that was based on
the 1860 stand structure at each site.10 B stands for basal area=
in
m2/ha, D stands for maximum diameter measured at root collar (DR=
C)
in cm, and Q stands for the q-factor, a fixed ratio <=
/a>of trees in one d=
iameter
class to the next largest diameter class.34 BDQ thinn=
ing
prescriptions are a silvicultural approach for
controlling uneven-aged forest structure by setting targets of desired numb=
ers
of trees in each diameter class. 34 This method seeks to balance
standing tree density with expectations for growth and mortality up to some
maximum diameter.35 These prescriptio=
ns did
not consider P. edulis,
which composed 1-10% of the woodland and much of which suffered from recent
drought-induced mortality.�
In applying the prescripti=
on,
we attempted to retain trees in a clumpy arrangement (3 trees or more toget=
her)
when possible to mimic 1860 spacing patterns.10 All of the thinn=
ing
was done by hand with chainsaws.� A=
fter
each subplot was cut, we tallied the root collar diameter of all the
stumps.� These data, coupled with
pre-treatment inventory data, allowed us to calculate forest density and
diameter distribution at each plot before and after thinning.�
We arranged the slash as we
were thinning the subplots.� There =
were
four possible slash arrangements: pile, cluster, broadcast, and no thinning
(Figure 2).� We piled the slash for=
the
pile arrangement.� We felled the tr=
ees at
the base and then left the limbs intact for the cluster arrangement.� For the broadcast arrangement, we cut t=
he
slash into approximately one meter sections and then scattered it uniformly
around the subplot. �We left unthinned=
plots as controls.�
Figure 2.
Fire Measurements and the Prescribed Burn
We measured surface fuel
loading on each pile, cluster, broadcast, and no thinning sup-plot using pl=
anar
intercept transects after the thinning.36 We=
estimated the volume of slash piles according to Hardy et al.37
We used prescribed fire in=
the
designated burn units in early November of 2005.� Even under windy conditions (gusts >=
24
km/hour) we had a difficult time getting the fire to carry because of a lac=
k of
continuous surface fuels.� We place=
d 3
pyrometers at each subplot into areas of high, medium, and low slash
accumulations.� The pyrometers were
composed of an �L� shaped strip of thin sheet metal, painted with 11
temperature-sensitive paints that detect temperatures ranging from
Seeding
We hand-seeded a custom na=
tive
seed mix after the prescribed burn in November of 2005.� We applied the seed mix directly to the
ground in the whole-plots designated to be seeded.� Our seed mixture was composed of three
shrubs, one forb and six grasses (Artemisia tridentate, Krasche=
ninnikovia
lanata, Purshia tri=
dentate,
Linum lewisii, Achnatherum hymenoides, <=
span
class=3DSpellE>Aristida purpurea, Muhlenbergia wrightii, Pleuraphis jamesii, Elymus elymoides=
i>) and was applied at a rate of 62.9 kg/ha.� All species in the seed mix
were found on the sites in the 2005 vegetation survey.� The seed and seeding rates were provide=
d by
Granite Seed in
To measure seed predation,=
we
measured seedling emergence of pairs of protected and unprotected seeds.
�
Data Analysis
Since each of the three si=
tes
had different thinning prescriptions, they were treated as independent
experiments and were analyzed separately.�
We used a split-plot design analysis of variance to test the influen=
ce
of thinning, slash arrangements, seed/burn methods, and their interactions =
on
understory richness and abundance.� We
used Tukey-Kramer honestly significant differen=
ce
tests (HSD) to test for differences among means.� We compared the differences in abundanc=
e and
richness between years in the control plots at each site using paired
t-tests.� Analyses were conducted u=
sing
the statistical package JMP version 6 (SAS Institute, Inc. 2004).� All significances were found at the α=3D0.05
level.���
Results
Thinning
The prescriptions based on
reference conditions resulted in basal area reductions across the three sit=
es
ranging from 28% to 61% (Table 1).�
Table 1.� Summary o=
f the
changes in forest structure after implementing the BDQ thinning prescriptio=
n at
each of three sites.
�
Site |
BDQ1 |
Pre-thinning density (trees/ha) |
Post-thinning density (trees/ha) |
Density |
Basal area reduction=
(%) |
Limestone=
|
30-100-1.4 |
531 |
284 |
53 |
28 |
Sandstone=
|
20-100-1.25 |
212 |
156 |
26 |
42 |
Basalt |
10-100-1.5 |
441 |
138 |
69 |
61 |
1 B =3D basal area (m2 ha-1<=
/sup>),
D =3D maximum diameter at root collar (cm), Q =3D ratio of trees in one dia=
meter
class to the next largest diameter class.
Prescribed Fire and Fuels
Each of the four slash
arrangements created a different fuel structure on the ground before and af=
ter
the prescribed burn.� The most cons=
umption
was seen in the pile arrangement, then the broadcast, then the cluster
arrangement, and lastly in the no thinning subplots.� The pyrometer readings showed that the =
pile
slash arrangement burned hotter than all of the other slash arrangements,
between 680 and 750 �C.� There was little difference between the
maximum temperatures reached in the broadcast and the cluster slash
arrangements; both ranged between 450 and 550 =
�C.� The plots that were not thi=
nned
reported the lowest maximum temperature readings, between 50 and 200 =
�C.
Understory Vegetation
In 2005, we identified 115
species in the understory over all 3 sites.�
The basalt site had the greatest richness and abundance of the three
sites.� In 2006, we found 80 specie=
s over
all 3 sites and few understory responses to treatments.� Understory species richness was not
influenced by thinning, slash arrangement, or burn/seed method at the limes=
tone
site (Table 2).� At the basalt site,
understory richness was influenced by thinning and slash arrangement, with =
the
thinned plots and broadcast arrangement plots yielding the greatest richness
(Figure 3).� At the sandstone site,=
we
found a significant difference in richness only due to the slash arrangemen=
t by
seed/burn method interaction, but the three treatments with the greatest
species richness included the control (no thinning and no burn/seed method
combination).� Understory abundance=
did
not significantly differ by thinning, slash arrangements, or burn/seed meth=
od at
any of the three sites (Table 2).
Table 2.� P-values=
for
split-plot ANOVA testing understory species richness differences due to the
influences of thinning (thin vs no thin), slash
arrangement (pile, cluster, broadcast, or no thinning), seed/burn method
(burning, seeding, burning and seeding, or none), and the slash arrangement=
and
seed/burn method interaction.� All
understory plant abundance results for the same variables were not signific=
ant
(ns).
Variable |
Limestone<=
span
lang=3DES style=3D'font-size:8.0pt;font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";mso-ansi-language:ES;mso-fareas=
t-language:
ES'> |
Sandstone<=
span
lang=3DES style=3D'font-size:8.0pt;font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";mso-ansi-language:ES;mso-fareas=
t-language:
ES'> |
Basalt |
Thinning<=
/span> |
ns=
|
ns=
|
0.02 |
Slash arrangement |
ns=
|
ns=
|
0.0004 |
Seed/burn method<= o:p> |
ns=
|
ns=
|
ns=
|
Slash arrangement x
thin/burn method |
ns=
|
0.003 |
ns=
|
Figure 3.
We compared understory pla=
nt
richness and abundance for 2005 vs.
Seeding
In June of 2006 we surveyed
the seed cages and exposed seed plots and found no seedling emergence in ei=
ther
of the plots, at either of the sites.� We
found bare seeds lying on top of the soil inside of the cages.� In the exposed plots, the seeds were no=
longer
present, either consumed by herbivores or blown away by wind.� There was no germination and therefore,=
no
analysis was performed on the seed cage experiment.�
Discussion
Although our experiment was
not designed to test for the effect of moisture, we believe plant responses=
to
our thinning, slash arrangements, and burning and seeding treatments were m=
uted
by the severe drought of the preceding winter and spring. Pre-treatment
vegetation measurements were conducted in a relatively wet period and post-=
treatment
vegetation measurements were conducted in a very dry period.� The seasonality of precipitation is ver=
y important
in semiarid ecosystems.40 Our vegetat=
ion
surveys were conducted in June, which is traditionally the peak of the
understory plant abundance and richness at our research sites.41=
The
growing season of 2005-2006 was the 3rd driest growing season ev=
er
recorded.� January to May of 2006 w=
as the
driest winter and spring in the last 55 years (
Figure 4.� Differences in understory plant (A) ric=
hness
and (B) abundance between 2005 and � An asterisk after site names indicate
significant differences in the understory at the α=3D0.05 level.=
� Data are expressed as means (n =3D 16) =
+/-
SE.� Species richness decreased 40%=
at
the limestone site, 33% at the sandstone site, and 45% at the basalt site.
Figure 5.
We documented decreases in
plant richness from 2005 to
Despite the dry growing
conditions, we did see a significant plant richness response to the slash
arrangement at one of the three sites.�
At the basalt site, broadcasting the slash resulted in the highest
richness, followed by the cluster, then the pile and lastly the no thinning
(Figure 3).� In other words, the mo=
re
dispersed the slash was, the greater the resulting richness in the plant
community.� Our findings on the bas=
alt
site support the idea that slash additions to bare soil can create favorable
microsites for understory establishment.10, 22
Slash arrangement did not
significantly influence resulting understory richness at the limestone and
sandstone sites (Table 2).� The bas=
alt
site may have had a greater response because of increased moisture, a great
reduction in overstory, soil type, or a combina=
tion
of factors. The basalt site was
Burning did not influence
understory richness or abundance at any of the sites after one year.� Other studies have burned slash created=
by
thinning in pinyon-juniper woodlands and record=
ed an
immediate decrease in plant abundance, followed by an increase in years
following the burn.20,43 Burning heavy
loads of juniper slash creates very hot soil temperatures and may have nega=
tive
impacts on future understory regeneration.43 Our study showed th=
at
the maximum surface temperature exceeded
Since seedling emergence o=
ften
depends on soil water availability,46,47 we
attribute the total lack of germination in seeding cages to the dry growing
season of 2006.� Seeding success in=
other
studies had been mixed.� Stoddard (=
2006)
found seeding increased biodiversity in degraded pinyo=
n-juniper
woodlands in northern
A longer monitoring period=
is
needed to determine the effects of treatments on understory response in
Management Recommendations
Using an 1860 thinning
prescription, as opposed to total tree removal, assures that structure of t=
he pinyon-juniper woodland is maintained within the hist=
orical
range of variability.10 Thinning represents a compromise between
total tree removal which would maximize forage production and no management
action.48
Broadcasting the slash cre=
ated
by thinning increased initial understory diversity on the basalt derived so=
il
site, despite the dry year.� Burning
slash did not affect initial grass and forb abundance and diversity, althou=
gh
it did produce exceedingly hot soil temperatures.� Land managers must weigh the tradeoffs =
of
burning slash for wildlife and livestock mobility benefits, with the potent=
ial
negative effects mentioned above.� =
Hand
seeding was not found to be effective.� =
span>
Variation in precipitation=
is
the norm in the Southwest.� Therefo=
re,
understanding temporal and spatial variability in the =
pinyon-juniper
woodland understory plant community is vital to interpreting the influence =
of
management actions.� Global climate
change is expected to affect ecosystems worldwide49 by raising
temperatures and changing precipitation patterns.50 Given the central role that precipitation plays in sem=
iarid
ecosystems, changing precipitation regimes and inter-annual variability may
have a stronger effect on pinyon-juniper unders=
tory
biodiversity and abundance than land management decisions.�
=
Acknowledgements
This research was supporte=
d by
USDA Forest Service, Rocky Mountain Research Station under Research Joint
Venture RMRS-02-JV-11221615-135.� We are
extremely grateful to the Diablo Trust, particularly the Flying M Ranch, for
allowing us to conduct research on private property.� Special thanks to Bryan Zebrowski,
Stephanie Powers, and other members of the NAU School of Forestry Silviculture Lab, for making this project possible an=
d to
Elizabeth Kalies for thoughtful review comments=
.
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