SPECIAL FEATURE:
SAGEBRUSH STEPPE TREATMENT EVALUATION PROJECT
Long-term effects of tree expansion and reduction on soil climate in
a semiarid ecosystem
BRUCE A. ROUNDY,1,
R. F. MILLER,2 R. J. TAUSCH,3 J. C. CHAMBERS,3
AND
B. M. RAU4
1
Department of Plant and Wildlife Sciences, Brigham Young University, Provo, Utah 84602 USA
Eastern Oregon Agricultural Research Center, Oregon State University, Corvallis, Oregon 97331 USA
3
USDA Forest Service, Rocky Mountain Research Station, Reno, Nevada 89521 USA
4
USGS New England Water Science Center, Northborough, Massachusetts 01532 USA
2
Citation: Roundy, B. A., R. F. Miller, R. J. Tausch, J. C. Chambers, and B. M. Rau. 2020. Long-term effects of tree
expansion and reduction on soil climate in a semiarid ecosystem. Ecosphere 11(9):e03241. 10.1002/ecs2.3241
Abstract. In sagebrush ecosystems, pinyon and juniper tree expansion reduces water available to perennial shrubs and herbs. We measured soil water matric potential and temperatures at 13–30 and 50–65 cm
soil depths in untreated and treated plots across a range of environmental conditions. We sought to determine the effects of tree expansion, tree reduction treatments, and expansion phase at time of treatment over
12–13 yr post-treatment. Because the effects of tree reduction on vegetation can vary with the soil temperature/moisture regime, we also analyzed differences in soil climate variables between the mesic/aridic-xeric
and frigid/xeric regime classifications for our sites. Growing conditions during all seasons except spring
were greatly limited by lack of available water, low temperatures, or both. Advanced tree expansion
reduced wet days (total hours per 24 hr when hourly average soil water matric potential > 1.5 MPa),
especially in early spring. Fire and mechanical tree reduction increased wet days and wet degree days
(sum of hourly soil temperatures >0°C when soil is wet per 24 hr) compared with no treatment for most
seasons. Burning resulted in higher soil temperatures than untreated or mechanically treated woodlands.
Tree reduction at advanced expansion phases increased wet days in spring more than when implemented
at earlier phases of expansion. Added wet days from tree reduction were negatively associated with October through June precipitation and vegetation cover, rather than time since treatment, with more wet days
added on drier sites and years. The longer period of water availability in spring supports increased growth
and cover of not only shrubs and perennial herbs, but also invasive weeds on warmer and drier sites, for
many years after tree reduction. We found that sites classified as mesic/aridic-xeric had warmer soil temperatures all seasons and were drier in spring and winter than sites classified as frigid/xeric. Land managers should consider reducing trees at earlier phases of expansion or consider revegetation when treating
at advanced phases on these warmer and drier sites that lack perennial herb potential.
Key words: cheatgrass; fire; fuel; juniper; pinyon; sagebrush; soil temperature; soil water; Special Feature: Sagebrush
Steppe Treatment Evaluation Project.
Received 17 January 2020; revised 13 April 2020; accepted 20 April 2020; final version received 27 May 2020.
Corresponding Editor: James McIver.
Copyright: © 2020 The Authors. This is an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
E-mail : bruce_roundy@byu.edu
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INTRODUCTION
summer periods (Bates et al. 2000, Leffler and
Ryel 2012, Mollnau et al. 2014, Roundy et al.
2014b). Tree reduction could also increase soil
water availability by reducing tree cover and
interception of precipitation (Williams et al.
2018).
Understanding the effects of increasing tree
dominance and the associated shrub and perennial herb loss may be helpful in determining the
successional timing of tree reduction treatments
to reduce fuels and risk of severe wildfire.
Expansion has been classified into three phases
of increasing tree dominance (Miller et al. 2005):
Phase I, perennial shrubs and herbs dominate
with scattered trees; Phase II, understory perennials and trees share dominance; and Phase III,
trees dominate. Because the amount of understory cover remaining in relation to expanding
tree cover may vary greatly among sites (Roundy
et al. 2014a), it is useful to evaluate the effects of
tree expansion and reduction on resources across
a wide range of sites.
Trees and other woody fuels are usually
reduced by prescribed fire or mechanical means
such as cutting or shredding (Miller et al. 2019).
The Sagebrush Treatment and Evaluation Project
(SageSTEP) was initiated in 2005 to follow shortto long-term effects of woody fuel reduction
treatments across a wide range of sites (McIver
et al. 2010, McIver and Brunson 2014). SageSTEP
and other studies have reported shorter-term (1–
6 yr after treatment) effects of fire and mechanical tree reduction on soil moisture and temperature. Mechanical tree reduction increases the
time of soil water availability in the RGP (Bates
et al. 2000, Young et al. 2013, Roundy et al.
2014b, 2018) and MP (Mollnau et al. 2014), while
fire increases soil water availability and soil temperatures (Roundy et al. 2014b, 2018, Cline et al.
2018). Fire generally reduces both trees and
shrubs long term, and temporarily reduces cover
of perennial grass for about 2–3 yr (Roundy et al.
2014a, Miller et al. 2014b, Williams et al. 2017).
Mechanical treatments maintain shrub and grass
cover, and shade patches of the soil surface with
woody debris (Cline et al. 2010, Young et al.
2013). Previously, we found that soil water availability in spring was equally increased by both
fire and mechanical tree reduction up to 4 yr
after treatment (Roundy et al. 2014b). Tree reduction increased time of available water most when
For more than a century, woody plants have
been expanding into semiarid grasslands and
shrublands worldwide (Archer et al. 2017).
Because semiarid lands have short growing periods when soil water and nutrients are available
and temperatures are favorable, the expansion of
woody plants may reduce available resources,
alter ecosystem functioning, and fundamentally
change the ecological services provided by the
pre-expansion plant community (Archer and
Predick 2014, Archer et al. 2011, 2017, Williams
et al. 2018). In the western United States, pinyon
(Pinus spp.) and juniper (Juniperus spp.) trees are
expanding into sagebrush (Artemisia spp.)
ecosystems in the Great Basin and Colorado Plateau (Romme et al. 2009, Davies et al. 2011,
Miller et al. 2019). Environmental consequences
may include loss of important perennial shrubs
and herbs, increase in woody fuels and potential
severity of wildfire, and higher runoff and erosion associated with depleted ground cover
(Pierson et al. 2010, Young et al. 2013, Miller et al.
2014a, b, 2019, Roundy et al. 2014a, Williams
et al. 2017, 2018). Cascading effects can be loss of
wildlife habitat (Miller et al. 2011, 2017, Wilson
at al. 2011), exotic grass dominance after wildfire
(D’Antonio and Vitousek 1992, Chambers et al.
2016), and loss of ecosystem productivity
(Williams et al. 2018).
The climate of sagebrush ecosystems in the
Great Basin of the western United States consists
of cool wet winters and warm dry summers (Leffler and Ryel 2012, Williams et al. 2018, Miller
et al. 2019). Shrubs, herbs, and trees all rely on
available water in spring for growth. Ryel et al.
(2008) and Leffler and Ryel (2012) have referred
to the upper 30 cm of soil as the resource growth
pool (RGP) and soil depths below 30 cm as the
maintenance pool (MP). Soil nutrients, especially
N, are most available in the upper 30 cm and can
move to roots at soil water potentials > 1.5 MPa.
Available water below 30 cm helps sustain plants
through the dry summers but supports limited
growth because of limited nutrient availability.
Thus, reduction in trees as major water users
should result in longer periods of water availability for nutrient transport in the RGP to support
growth of shrubs and herbs, and more water for
maintenance of shrubs in the MP during dry
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typically more resistant to invasive annual
grasses and forbs (Chambers et al. 2014, Roundy
et al. 2014, Bybee et al. 2016, Williams et al.
2017). However, both resilience to treatments
and resistance to invasive annual grasses are
strongly influenced by residual vegetation and
resource availability. Reducing trees when and
where enough residual and desirable vegetation
is available to use the resources that the trees previously used may increase the capacity for recovery and help maintain ecosystem resilience.
For the current study, we measured seasonal
soil water availability and temperature continuously on 11–12 expansion sites within plots that
were either untreated or received tree reduction
treatments up to 13 yr earlier. Our purpose was
to evaluate the responses of seasonal soil water
availability and temperature to (1) tree expansion
phase, (2) prescribed fire compared with
mechanical tree reduction, (3) phase of expansion
at the time of tree reduction, and (4) time since
tree reduction. We also compared seasonal soil
water availability and temperature between the
mesic/aridic-xeric (expected warmer and drier)
and frigid/xeric (expected cooler and wetter)
NRCS soil temperature/moisture regimes into
which our sites were classified. This study is
unique in the years of measurement after treatment, geographical scope, range of environmental conditions, and the intensity of measurement
at each site.
implemented at Phase III expansion where less
understory plant cover probably resulted in
lower transpiration compared with treatments
applied at Phases I and II. Williams et al. (2017)
considered that this extra soil water availability
in spring supported major increases in perennial
grass cover 6 yr after tree reduction at high
pretreatment tree dominance, although perennial
grass cover still remained lower than when
trees were reduced at less pretreatment tree
dominance.
Resilience to disturbance and resistance to exotic annual grasses have been associated with soil
temperature and moisture in sagebrush ecosystems (Chambers et al. 2007, 2014, Condon et al.
2011, Davies et al. 2012). More favorable environmental conditions for native plant establishment
and growth and greater productivity of perennial herbaceous species due to higher precipitation and cooler temperatures typically equate to
greater resilience at higher than lower elevations
(Condon et al. 2011, Davies et al. 2012, Knutson
et al. 2014, Chambers et al. 2014). Also, climate
suitability to exotic annual grasses decreases as
soil temperatures become colder resulting in
greater resistance to these grasses at higher than
lower elevations (Brooks et al. 2004, Chambers
et al. 2007, 2014, Condon et al. 2011). Chambers
et al. (2017, 2019), Miller et al. (2014a, 2019), and
Pyke et al. (2017) have based management recommendations on soil temperature/moisture
regimes as classified by the US Department of
Agriculture Natural Resources Conservation Service (USDA NRCS 1999). These regimes are
defined by estimated conditions at the 50 cm soil
depth. Freund et al. (2020) found differences in
vegetation response to fuel control treatments
between mesic/aridic-xeric and frigid/xeric soil
temperature/moisture regimes. However, soil
temperature and moisture conditions in the RGP
have not been quantified by on-site measurements of these NRCS regimes.
Understanding the longer-term effects of treatment, expansion phase at time of treatment, and
longevity of these effects on resource availability
after treatment can help land managers decide
when, where, and how to conduct fuel treatments. Sites in Phases I to II are often more resilient to treatments (Chambers et al. 2014, Roundy
et al. 2014a, Bybee et al. 2016, Williams et al.
2017). Similarly, sites in earlier phases are
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ROUNDY ET AL.
METHODS
Study sites
This study was implemented as part of the
Sagebrush Steppe Treatment Evaluation Project
(SageSTEP), a Great Basin regional research project described by McIver et al. (2010) and McIver
and Brunson (2014). Study sites included four
different cover types; four western juniper
(Juniperus occidentalis) sites in California and Oregon (Blue Mountain, Walker Butte, Bridge Creek,
and Devine Ridge); four single-leaf pinyon (Pinus
monophylla)–Utah juniper (Juniperus osteosperma)
sites in central Nevada (pinyon–juniper; Seven
Mile, South Ruby, Marking Corral, Spruce Mountain); and two Utah juniper (Stansbury and Onaqui) and two Utah juniper–Colorado pinyon
(Pinus edulis) sites (juniper–pinyon; Scipio,
Greenville) in Utah (McIver et al. 2010, Miller
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et al. 2014b, Freund et al. 2020). Sites were
selected as wooded shrublands (Romme et al.
2009) or expansion woodlands (Miller et al. 2008,
McIver et al. 2010) where trees have expanded
into sagebrush (Artemisia spp.) communities on
loam soils with native species still present in the
understory across a range of tree cover (Roundy
et al. 2014a). Sites represent a wide range in elevation, soil, and climatic conditions, but some
regional characteristics are evident. Across the
Great Basin from west to east, western juniper
sites represent the lowest elevation, pinyon–
juniper sites in central Nevada have the highest
elevation, and Utah juniper sites in Utah are
intermediate (Roundy et al. 2014b). On the northwestern Great Basin sites, soils are derived from
basalt lava flows and the climate is Pacific maritime, with most precipitation falling between
November and June (McIver et al. 2010, Rau
et al. 2011, Miller et al. 2014b). The central and
eastern sites include igneous, metamorphic, and
sedimentary-based soils, which are carbonatic.
The climate is more continental, with lower precipitation than Pacific maritime between November and June, and highly variable summer
precipitation in July and August (McIver et al.
2010, Rau et al. 2011, Miller et al. 2014b).
ROUNDY ET AL.
in 2006 and 2007 for all but the South Ruby site,
where treatments were applied in 2009. This
stagger-start design avoids the potential
restricted inferences associated with implementing all treatments under the same set of climatic
conditions (Loughlin 2006). Plots were burned
between August and October, and trees were cut
or shredded from September through November.
The fire treatment was a broadcast burn ranging
from low to moderate severity across all sites
(Miller et al. 2014b). The reduction in tree canopies in the fire treatment and mechanical treatments averaged 86%, across the 11 study sites
(Roundy et al. 2014a, Miller et al. 2014b), indicating that treatments were effective in accomplishing targeted tree removal goals. The burn
treatment resulted in 90% reduction in shrub
cover and <5% remaining tree canopy cover
(Miller et al. 2014b). For the mechanical treatment, all trees >2 m tall were cut or shredded
and debris left in-place on the ground. Tree cutting was done by chain saw and mastication by
rotation of a large, toothed drum or Fecon Bullhog attachment (Fecon, Lebanon, Ohio, USA)
mounted on a large rubber-tired vehicle as
described by Cline et al. (2010). Tree canopies
were reduced to <1% in the mechanically treated
plots.
Treatments
Measurements
Treatments were applied across the network as
a randomized complete block, with each site considered a block (Roundy et al. 2014a, Miller et al.
2014b). We attempted to place treatment plots at
each site within the same ecological site (Miller
et al. 2014b). Plots were fenced where necessary
to exclude cattle grazing. Throughout the network at each site, three 8- to 20-ha treatment
plots were left as an untreated control plot or
received a broadcast burn, or cut-and-drop treatment. In addition, the four Utah sites received
tree mastication (shredding) treatment. The Blue
Mountain, Walker Butte, and Bridge Creek western juniper sites and the Seven Mile, South Ruby,
and Marking Corral pinyon–juniper sites all had
an untreated control, prescribed fire, and cutand-drop treatment plots. The Utah juniper or
juniper–pinyon sites of Stansbury, Onaqui, Scipio, and Greenville all had these same treatments
plus an additional mastication treatment plot.
Because plots could not all be burned in the same
year (Miller et al. 2014b), treatments were applied
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Soil water and temperature measurement stations were located in three tree expansion phases
within treatments at each site by observing relative tree, shrub, and perennial herb cover to
determine dominance of life forms. Eight sites,
Blue Mountain, California; Devine Ridge and
Bridge Creek, Oregon; Marking Corral and South
Ruby, Nevada; and Stansbury, Onaqui, and
Greenville, Utah, were fully instrumented by the
year after treatment (2007–2008). One of these
sites, Stansbury, was measured through spring
2009 until a wildfire burned the treatment plots.
Another site in Utah, Scipio, was fully instrumented starting in 2011, while two other sites,
Walker Butte, Oregon, and Seven Mile, Nevada,
were fully instrumented in summer 2014. The
Spruce Mountain, Nevada, untreated plot was
instrumented in 2006. Since tree reduction treatments were never applied on this site, data were
only used to determine expansion effects. When
fully instrumented, stations were installed on
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hourly average soil temperatures for each hour
that average soil temperature was >0°C per 24 hr
and when soil water matric potential > 1.5
MPa), and hourly average soil temperatures
(Rawlins et al. 2012, Roundy et al. 2014b, 2018,
Cline et al. 2018).
untreated, burned, cut, and shred plots at expansion Phases I, II, and III. Each of these sites had 9
stations (3 phases 9 3 treatments; untreated,
burned, cut) or 12 stations (Utah sites only; 3
phases 9 4 treatments; untreated, burned, cut,
shred).
Each of the soil water and temperature stations
installed across the 12 study sites was equipped
with a Campbell Scientific CR10X or CR1000
micrologger and multiplexer that measured 16
soil temperature and soil water matric potential
sensors. At each station, thermocouples to measure temperature and gypsum blocks (Delmhorst) to measure soil water matric potential
were buried at 1–3, 13–15, 18–20, and 28–30 cm
deep in tree and shrub microsites at the east-side
dripline and on two interspaces between shrubs
or trees (4 depths 9 4 microsites = 16 thermocouples and 16 gypsum blocks at each station).
Starting in 2011, an additional thermocouple was
installed at 50 cm deep to measure soil temperature and gypsum blocks were installed at 50 and
65 cm depths where possible to measure matric
potential. These sensors were installed in one
interspace for each station. Microloggers were
programmed to read sensors every 60 s and to
store hourly averages. We converted gypsum
block resistance data to water potential using
standard calibration curves (Campbell Scientific
1983). Although some error may be introduced
by not individually calibrating each gypsum
block, blocks calibrated with standard equations
were relatively consistent and sensitive to soil
drying in a growth chamber study (Taylor et al.
2007). We also measured air temperature and
precipitation (1–1.5 m height) on one station at
each site (untreated Phase III). Precipitation was
measured with an electronic tipping bucket rain
gage (Texas Electronics) and removable precipitation adapter for snowfall (Campbell Scientific).
Air temperature was measured in a gill shield
using a Campbell Scientific Model 107 temperature probe.
Derived variables were calculated for six seasons: early spring (March–April), late spring
(May–June), all of spring (March–June), summer
(July–August), fall (September–November), and
winter (December–February). Derived variables
included total number of wet days (total hours
per 24 hr when hourly average soil water matric
potential > 1.5 MPa), wet degree days (sum of
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ROUNDY ET AL.
Analysis
Mixed model analysis (Proc Glimmix, SAS v9.3;
SAS Institute, Cary, North Carolina, USA) was
used to test fixed effects of year of measurement
and expansion phase on untreated plots for 10–12
sites, depending on the year (2008 through spring
2018). Site, site 9 phase, and site 9 phase 9 year
were considered random variables in these analyses. Similarly, mixed model analysis was also used
to test fixed effects of year of measurement, treatment, and expansion phase (subplot within treatment) for 7–11 sites, depending on the year. Site,
site 9 treatment, site 9 treatment 9 phase, and
site 9 treatment 9 phase 9 year were considered random variables. Using the same fixed and
random variables, we also conducted analyses on
the difference between untreated and treated
responses for each expansion phase on these sites
during the same year to best adjust for differences
in annual weather among sites. This allowed us to
determine additional wet days, wet degree days,
and soil temperature degrees associated with tree
reduction. Tukey’s tests were used to determine
significant differences among years, treatments, or
phases when significant. All analyses were conducted separately for each season (Roundy et al.
2014b, 2018). Significance was at P < 0.05 unless
stated otherwise. Data were not transformed
because examination of residual plots indicated
that assumptions for analysis of variance were
generally met as well without as with transformation. Data from tree shredding and cutting were
pooled for Utah sites because preliminary analysis
showed similar responses. To best represent the
RGP, we analyzed across the depth intervals of
13–15, 18–20, and 28–30 cm and across the four
microsites (Roundy et al. 2014b). To represent the
MP, we analyzed wet degree days and average
soil temperatures at 50 cm deep and wet days
across the 50 and 65 cm depths. Regression analysis was used to determine correlations among
spring wet days, added spring wet days, October
through June precipitation, and total and herbaceous vegetation cover. Data from all available
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years and sites from spring 2008 through 2018
were used with spring wet days and additional
wet days averaged across burn and mechanical
treatments, including averages across phases,
microsites, and depths to produce one data point
per site per year. Foliar vegetation cover was
quantified as described in Freund et al. (2020). To
quantify differences in soil temperature/moisture
regimes in the RGP, we analyzed data from fall
2014 through spring 2018 when the maximum
number of sites was fully instrumented. Six sites
were classified as mesic/aridic-xeric (Bridge Creek,
Greenville Bench, Marking Corral, Onaqui, Scipio,
and South Ruby) and four sites were classified as
frigid/xeric (Blue Mountain, Devine Ridge, Seven
Mile, and Walker Butte) according to Freund et al.
(2020). Mixed model analysis of seasonal wet days
and soil temperature was used to test significance
of the fixed factors of soil temperature/moisture
regime, tree reduction treatment, phase at time of
tree reduction, and their interactions. Site,
site 9 regime, site 9 regime 9 treatment, and
site 9 regime 9 treatment 9 phase were considered random variables.
ROUNDY ET AL.
Appendix S1: Table S1). Soil temperatures in the
RGP were also lower for Phase III than Phase I in
fall, but the difference was small (0.9°C).
Although year was significant for all three variables and for all seasons, the year-by-phase interaction was not. Wet days and wet degree days
were greater in some years than others, while soil
temperatures were cooler in the wetter years.
Response to phase of expansion for these variables paralleled each other over the different
years, with the numerical order of Phase
I > Phase II > Phase III, but differences among
phases on a given year were small.
For the MP, none of the response variables
varied by phase, although there was a trend
toward decreasing response with increasing
phase for early spring, as occurred with the
RGP (Fig. 1; Appendix S1: Table S1). Wet days
differed significantly by year for early spring,
late spring, and winter, with some years wetter
than others. For example, wet days in spring
ranged from a low of 29.4 8.19 in 2018 to a
high of 98.8 8.35 in 2016. Wet degree days
and average soil temperature varied among
years for all seasons, but the year-by-phase
interaction was not significant, indicating that
whether years were wetter or drier, variables
responded similarly to phase.
RESULTS
Seasonal effects
As expected, wet days were highest in early
spring, wet degree days were highest in early
spring and late spring, and soil temperatures
were highest in summer (Fig. 1). Although winter had a high number of wet days, it had limited
wet degree days due to low soil temperatures.
While fall had warm soil temperatures, wet
degree days were limited due to few wet days.
The RGP and MP reflected similar seasonal patterns with the RGP usually having more wet
days, wet degree days, and higher soil temperatures than the MP (Fig. 1). An exception was that
the MP had 1.9°C higher average soil temperature than the RGP in winter. In late spring, soil
temperatures averaged 16.3°C higher in the RGP
than MP (Fig. 1).
Effects of treatments
Mechanical tree reduction had more wet days
than no treatment for the RGP in all seasons and
for the MP in early spring and late spring (Fig. 2;
Appendix S1: Table S2). The burn treatment had
more wet degree days than no treatment for all
seasons in the RGP, and for early spring in the
MP. Wet days and wet degree days were generally similar for burn and mechanical treatments,
except that for the RGP, wet days were greater
for the mechanical than burn treatment in late
spring. The burn treatment had the highest soil
temperatures for all seasons in the RGP, and a
trend toward highest temperatures in the MP
(Fig. 2).
Effects of expansion
Effects of phase of expansion when treated
For the RGP on untreated plots, wet days, wet
degree days, and soil temperatures had a trend
toward decreasing with increasing woodland
expansion phase, but the trend was only significant for all three variables in early spring (Fig. 1;
For the RGP, tree reduction added more wet
days at Phase II and Phase III than were added
at Phase I in early spring and late spring, and
more wet days were added at Phase III than
Phases I and II in summer, fall, and winter
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Fig. 1. Effects of tree expansion (phase) for untreated plots on wet days (top), wet degree days (middle), and
soil temperature (bottom) for the resource growth pool (RGP, left, 13–30 cm soil depth) and maintenance pool
(right, 50–65 cm soil depth for wet days and 50 cm soil depth for others) for different seasons (ES, early spring;
LS, late spring; S, all of spring; SU, summer; F, fall; W, winter). Different letters above bars for a season indicate a
significant difference (P < 0.05). Lines above bars indicate 1 standard error.
(Fig. 3; Appendix S1: Table S3). The MP had a
similar trend, but phases were not significantly
different. Additional wet degree days followed a
similar response to phase as did additional wet
days. Treating at Phase III warmed soils more
than treating at Phase I for early spring in the
RGP and for all seasons except summer for the
MP (Fig. 3). Warming differences among phases
were small (<1.6°C).
much more related to annual weather than years
since treatment (Fig. 4). All sites except South
Ruby were treated in either 2006 or 2007 so that
2008 was 2–3 yr after treatment and most recent
data were 12–13 yr after treatment. Instead of
following a pattern of decreasing wet days with
time after tree reduction, treated and untreated
wet days in spring tracked October through June
precipitation. The additional wet days from tree
reduction in the RGP were highest on drier years
such as 2012, 2015, and 2018, and least on wetter
years such as 2010, 2011, and 2016 (Fig. 4). Additional wet day differences after tree reduction
between wet and dry years were greatest in early
spring, but also occurred in late spring (Fig. 4).
Annual effects and time since treatment
For the RGP, wet days for all treatments and
additional wet days after treatment varied significantly by year for all seasons (Appendix S1:
Tables S2, S3). However, annual variation was
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Fig. 2. Effects of tree reduction on wet days (top), wet degree days (middle), and soil temperature (bottom) for
the resource growth pool (RGP, left, 13–30 cm soil depth), and maintenance pool (MP, right, 50–65 cm soil depth
for wet days and 50 cm for others) for different seasons (ES, early spring; LS, late spring; S, all of spring; SU,
summer; F, fall; W, winter). Different letters above bars for a season indicate a significant difference (P < 0.05).
Lines above bars indicate 1 standard error.
followed the same pattern as the RGP, with more
wet days added by tree reduction on drier years,
rather than responding to time since treatment.
However, data for the MP were not available
until 2012, which was already 5–6 yr after treatment for all sites except South Ruby.
For both the RGP and MP, soil temperatures
varied by year for every season (Appendix S1:
Table S2). Treatments paralleled each other over
the years with the burn treatment always highest
(Fig. 5). As with wet days, soil temperatures
were associated with October through June
Even 12–13 yr after treatment, tree reduction at
Phase III still added 20.8 3.63 wet days and
281 46.67 wet degree days in the RGP in
spring of 2018, a relatively dry year. Wet degree
days generally followed the same response as
wet days, being driven more by wet days than
soil temperature.
For the MP, wet days varied significantly
among years for all seasons except summer, and
additional wet days varied significantly among
years for all but summer and winter
(Appendix S1: Tables S2, S3). Wet days in the MP
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Fig. 3. Additional wet days (top), wet degree days (middle), and soil temperature (bottom) for the resource
growth pool (RGP, left, 13–30 cm soil depth), and maintenance pool (MP, right, 50–65 cm soil depth for wet days
and 50 cm for others) for different seasons (ES, early spring; LS, late spring; S, all of spring; SU, summer; F, fall;
W, winter), after tree reduction by fire and mechanical methods. Different letters above bars for a season indicate
a significant difference (P < 0.05). Lines above bars indicate 1 standard error.
precipitation. However, soil temperatures were
cooler on years when October through June precipitation was higher (e.g., in 2010 and 2011).
There was no strong warming trend seen over
the 9–10 yr shown for the RGP, or for the 6–7 yr
shown for the MP in Figure 5. The RGP generally
had warmer temperatures than the MP, except in
fall and winter (Fig. 5).
were greater for the mechanical than burn treatment on most years, but there was little difference between treatments on wetter years. The
interaction of year and phase was significant for
additional wet days in the MP in late spring.
Additional wet days were greater for Phase III
compared with Phase I in drier than wetter years.
The interactions of treatment by phase and treatment by year were both significant for the RGP
for late spring for additional soil temperature
degrees. Burning added more soil temperature
degrees than the mechanical treatment at Phase
III than at Phase I or Phase II. Burning also
Interactions of year, treatment, and phase
The year-by-treatment interaction was significant for additional wet days for the RGP in early
spring (Appendix S1: Table S3). Added wet days
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Correlations of total and herbaceous cover with
added spring wet days were negative and significant (P < 0.0002), also with low r2 values (0.19
for total and 0.17 for herbaceous cover).
Comparison of soil temperature/moisture regimes
The frigid/xeric regime had 19 7.9 more wet
days in spring than the mesic/aridic-xeric regime
across the time interval of fall 2014 through
spring of 2018 (Table 1; Appendix S1: Table S4).
In contrast, soil temperatures were higher for the
mesic/aridic-xeric than frigid/xeric regime for all
seasons except winter. Some regime x treatment
interactions were significant for some seasons
(Appendix S1: Table S4). Wet days in spring were
greater for the frigid/xeric than mesic/aridic-xeric
regime for all treatments, but less so for the burn
than mechanical treatment (Table 1). On the
other hand, soil temperatures for all seasons
were more similar among treatments in the
mesic/aridic-xeric regime, while in the frigid/
xeric regimes, warmest temperatures were in the
burn treatment and coolest temperatures were in
the mechanical treatment (Table 1).
Fig. 4. Additional wet days in spring after tree
reduction for the RGP (resource growth pool) and MP
(maintenance pool) in relation to October through June
precipitation (A). Additional wet days for early spring
and late spring on wetter and drier years for the RGP
(B). Lines above bars are 1 standard error.
DISCUSSION
Resource growth pool
added many more soil temperature degrees than
the mechanical treatment on some years than
others. Burning had a small, but long-term effect
on increasing soil temperatures in both the RGP
and MP, especially when implemented at Phase
III. Both the RGP and MP dried to < 1.5 MPa by
late spring every year of measurement.
Effects of expansion.—Advanced tree expansion
decreased wet days (time of available water),
especially in early spring. Wet days on untreated
plots had a decreasing trend with increasing tree
dominance. Reasons for this are likely greater
interception and lower net precipitation as tree
cover increased, as well as greater transpiration
by trees than from shrubs and herbs. Interception
by individual trees ranges from 30% to 70% (Williams et al. 2018). Individual tree and stand-level
interception vary depending on tree cover and
amount of precipitation falling as snow compared with rainfall, as well as variations in intensity, and duration. Interception is substantial
enough that it affects soil water inputs and is
implicated in understory decline with expanding
woodlands (Mollnau et al. 2014, Williams et al.
2018). Pretreatment tree cover for untreated plots
ranged from about 9% at Phase I to about 42% at
Phase III (Williams et al. 2017). This expanding
tree leaf surface area increases both tree interception of precipitation and transpiration. Ivans
et al. (2006) considered that the greater stand-
Spring wet day and added wet day associations
with precipitation and vegetation cover
For the RGP, spring wet days and added
spring wet days from tree reduction had a quadratic response to October through June precipitation (Fig. 6, P < 0.0001, R2 = 0.46, for spring wet
days, P < 0.0001, R2 = 0.37, for added spring wet
days). However, spring wet days were positively
correlated, while added spring wet days were
negatively correlated with October through June
precipitation (Fig. 6). Correlations of total and
herbaceous vegetation cover with October
through June precipitation were positive and significant (P < 0.0003), but had low r2 values (0.14
for total and 0.15 for herbaceous cover).
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Fig. 5. Seasonal soil temperatures for untreated, burned, and mechanically treated plots for the resource
growth pool (RGP, 13–30 cm soil depth) and maintenance pool (MP, 50 cm soil depth). Note that RGP data range
from 2008 to 2018 and MP data from 2012 to 2018. Average standard errors (°C) are spring, 0.57; summer, 0.64;
fall, 0.48; and winter, 0.36.
associated runoff, which varies with many different site conditions such as soil texture, slope, and
incidence of high-intensity rainfall (Williams
et al. 2018).
Effects of treatments.—For most seasons, burned
and mechanical plots had similar wet days
level leaf area index for juniper compared with
sagebrush and bunchgrass-dominated communities was largely responsible for its greater seasonal CO2 uptake and water flux. Another way
that tree expansion may decrease water availability is through increased bare ground and
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woodlands decreased soil temperatures in spring,
but that shredded woodlands still had warmer
soils and more wet days than intact woodlands
when averaged or summed across all seasons. Our
burned plots also had greater cover of cheatgrass
and annual forbs (Williams et al. 2017, Freund
et al. 2020). Fewer wet days in spring on burned
compared with mechanically treated plots may
have been associated with higher transpirational
water loss from cheatgrass and annual herbs. In
addition, soil water repellency, indicated by
unvegetated patches on some burned tree
mounds, may have reduced soil water availability
on some burned plots (Zvirzdin et al. 2017).
Effects of phase when treated.—Tree reduction by
fire or mechanical means added more wet days
and wet degree days in most seasons when treated at Phase III than at Phase I and sometimes
Phase II (Fig. 3). One reason for this is that on
untreated plots, wet days and wet degree days
were less in early spring at Phase III than at
Phases I and II (Fig. 1). This difference only
accounts for part of the added wet days and wet
degree days from tree reduction at Phase III. For
example, on untreated plots in spring, Phase III
had 4.6 fewer wet days than Phase I woodlands
over the years of measurement. Yet reducing
trees at Phase III added 13.8 wet days compared
with 3.8 wet days added when reducing trees at
Phase I (Figs. 1, 3). After tree reduction at Phase
III, plant cover was dominated by perennial and
annual herbs, whereas trees provided the dominant plant cover for untreated Phase III. Shrub
cover is limited at Phase III expansion both
before and after treatment (Williams et al. 2017,
Freund et al. 2020). Transpiration would be
expected to be reduced most when trees are controlled at Phase III because understory herb and
shrub residuals are fewer and cover of these
components takes more time to recover from
effects of expansion at Phase III than at the other
phases (Williams et al. 2017, Freund et al. 2020).
Treatment effects over time.—In this longer-term
study, we found that October through June precipitation had a greater effect on added wet days
from tree reduction than time since treatment
(Figs. 4, 5). Measurement of water potential in
the RGP up to 4 yr after tree reduction showed a
declining trend in added spring wet days with
time since treatment (Roundy et al. 2014b). This
was considered a result of increasing herb and
Fig. 6. Additional spring wet days (A) and spring
wet days after tree reduction (B) at 13–30 cm in spring
in relation to October through June precipitation. Each
data point represents one site and year between 2008
and 2018, N = 78. Additional wet day and wet day
data were averaged across prescribed fire and mechanical tree reduction treatments. All data were averaged
across three tree expansion phases, four microsites,
and three soil depths.
(Fig. 2). Even though they had less shrub cover
than mechanically treated plots, burned plots
had fewer wet days than mechanical plots in late
spring (Fig. 2). Burned plots had consistently but
only slightly warmer soil temperatures than
mechanical and untreated plots (Fig. 2). Slightly
warmer soils could make a difference in growth
and transpiration over a long period, especially
from annual grasses. Much warmer temperatures
on burned plots in surface soils (1–3 cm, Cline
et al. 2018) and slightly warmer temperatures in
burned plots at 13–30 cm may have resulted in
greater evaporation from soil and greater transpiration from perennial and annual herbs than
occurred on mechanically treated plots. Decreased
canopy shading and increased absorption of solar
radiation would be expected to increase soil temperatures after fire. Young et al. (2013) found that
woody debris from shredding trees in Phase III
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Table 1. Wet days and soil temperature standard error for tree reduction treatments in two soil temperature/
moisture regimes for different seasons from fall 2014 through spring 2018.
Wet days
Soil temperature (°C)
Season
Regime
Untreated
Burn
Mechanical
Untreated
Burn
Mechanical
Early spring
Mesic/aridic
Frigid/xeric
Mesic/aridic
Frigid/xeric
Mesic/aridic
Frigid/xeric
Mesic/aridic
Frigid/xeric
Mesic/aridic
Frigid/xeric
Mesic/aridic
Frigid/xeric
42.5 1.89
55.7 2.3
16.9 4.21
27.4 5.15
59.4 5.43
82.6 6.65
1.3 0.84
1.5 1.02
8.8 2.52
14.2 3.08
38.9 6.97
65.9 8.52
50.6 1.89
52.8 2.36
18.4 4.21
25.7 5.18
68.8 5.44
78.5 6.71
2.7 0.84
3.3 1.05
14.8 2.52
21.2 3.14
53.1 6.97
70.4 8.58
51 1.87
58.4 2.36
22.2 4.2
39.3 5.18
73.4 5.42
97.7 6.7
1.7 0.83
3.5 1.04
13.4 2.5
21.1 3.13
50.7 6.95
64.4 8.57
7.5 0.43
5.4 0.52
18.6 0.69
15.4 0.84
13.2 0.58
10.4 0.71
24.7 0.68
21.5 0.84
12.4 0.44
10.5 0.53
1.5 0.46
0.9 0.56
7.9 0.43
6.8 0.53
18.8 0.69
17.3 0.85
13.5 0.58
12 0.72
25.3 0.69
24.2 0.85
13 0.44
11.8 0.54
2 0.46
1.2 0.56
8 0.43
4.7 0.53
18.2 0.69
14.5 0.85
13.2 0.58
9.6 0.72
24.5 0.68
20.5 0.85
12.8 0.44
9.3 0.54
2.1 0.46
0.3 0.56
Late spring
Spring
Summer
Fall
Winter
drier years or on drier sites for a given year,
plant transpiration is reduced due to low growth
and productivity of shrubs and perennial herbs.
Also, the establishment and growth of both
native annuals and nonnative invasive annuals
are significantly less, further reducing plant
community productivity and water use. In contrast, a longer period of available water in the
RGP on wetter years and on wetter sites supports more establishment, growth, and production of plants.
Water availability in sagebrush ecosystems is a
zero-sum game because available water for
growth is transient, even in wet years. Predominately, winter precipitation generally creates a
block of time when water is stored and then
available in the RGP as spring begins in March.
On wet years, this time of continuously available
water ended the first week of May for both
untreated and treated plots, while on drier years
it ended by the third week of April on untreated
plots and by the first or second week of May on
treated plots. This suggests another factor that
may be especially affecting water availability in
Phase III woodlands during dry years. Because
the time of water availability is truncated on
drier years, removal of trees as the major interceptor of precipitation and major water user in
the RGP, especially at Phase III, may be contributing to a longer period of available water
compared with wetter years. This is especially
probable during years when establishment and
shrub cover with time since treatment (Roundy
et al. 2014, Miller et al. 2014b, Williams et al.
2017). With a longer measurement period, we
found that on drier years, such as 2012, 2015, and
2018, tree reduction added many more wet days
in spring than on wetter years such as 2010, 2011,
and 2016 (Fig. 4). For example, increased spring
wet days from tree reduction averaged
13.8 2.34 for Phase III over a 10-yr period of
measurement (2008 through spring of 2018), but
were 20.8 3.63 in spring of 2018, when the
October 2017–June 2018 precipitation was relatively low (Fig. 4). This could be due to several
interacting factors. Wet days are limited in summer and fall while wet days are high in winter,
but wet degree days are limited by cool temperatures. Therefore, spring is the season when water
availability and warm temperatures best coincide
to support plant growth. This is further evidenced by the much higher wet degree days in
spring than any other season (Figs. 1, 2). Regression analysis indicated the positive correlations
between spring wet days and vegetation cover
with October through June precipitation, but
negative correlation between spring wet days
added by tree reduction and October through
June precipitation (Fig. 6). There was also a negative, but weaker correlation of added spring wet
days and total or herbaceous vegetation cover.
This negative correlation may indicate that transpiration by vegetation extant after tree reduction is affecting spring soil water availability. In
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to respond to increased resources with tree
reduction, a perennial grass-dominated community on burned treatments and on Phase III-treated mechanical treatments should resist invasive
grass and forb dominance as shrubs slowly
recover (Roundy et al. 2018). However, on drier
and warmer sites, especially with warm, wet
falls and warmer late springs where cheatgrass
is more favored than perennial grasses (Roundy
et al. 2018), invasive weed dominance is a continued risk. Reducing trees at earlier phases of
tree dominance or revegetating sagebrush and
perennial herbs in association with tree reduction at high pretreatment tree dominance may
best support ecosystem resilience and resistance
to nonnative invasive plants on warmer and
drier sites. Deciding not to reduce woody fuels
could result in severe wildfire, and the subsequent increased soil water availability in spring
and higher seedbed and RGP temperatures
could support dominance of cheatgrass and
recurrent high-frequency fire. High fire severity
can result in over 85% mortality of perennial
grasses occupying the site, reducing both resilience and resistance to invasive annual grasses
(Bates et al. 2011).
The cooler temperatures of the mechanical
treatment compared with the warmer temperatures of the burn for the frigid-xeric regime suggest that mechanical treatment is better suited to
resist cheatgrass for this regime. Burning
increases seedbed temperatures and cheatgrass
germination potential (Cline et al. 2018).
Increased soil temperatures in the RGP after fire
could also support increased growth and seed
production of cheatgrass (Chambers et al. 2007).
However, for this regime, cheatgrass cover
responded differently to burn compared with
mechanical treatments in relation to pretreatment tree dominance (Freund et al. 2020). By
10 yr after treatment, cheatgrass cover on the
burn treatment was highest at low pretreatment
tree dominance and then decreased with increasing pretreatment tree dominance. On the
mechanical treatment, cheatgrass cover was
slightly lower than that on the burn at low pretreatment tree dominance and then increased
slightly with increasing pretreatment tree dominance. At the same time, the dominant perennial
grass, Festuca idahoensis, cover was higher on the
mechanical than burn treatment at low-to-
productivity of the residual native and nonnative
invasive species are low on areas where trees
have been reduced. During wetter years and on
untreated areas, net precipitation might increase
for some storms if the canopy storage is saturated, interception capacity is exceeded, and
more through-fall occurs. Thus on wetter years,
spring wet days on untreated and treated areas
could be more similar due to reduced effects of
interception. On drier years, interception may
reduce the net precipitation more than on wetter
years because canopy saturation may be less
likely. Normally, interception and evapotranspiration water losses would be expected to increase
with increased tree cover (Williams et al. 2018),
but specific quantitative effects on cooler and
wetter vs. warmer and drier years have not been
determined. More complete measurement of
hydrologic inputs and outputs is needed to better understand differential effects of tree reduction on wetter compared with drier years.
Tree reduction, especially when implemented
at Phase III, can add weeks to the limited spring
growth period in sagebrush ecosystems and
favor plants that are available to use the additional resource. Perennial grasses recover much
more quickly than shrubs when trees are reduced
at Phase III (Williams et al. 2017, Freund et al.
2020). By both 6 and 10 yr after mechanical tree
reduction, cover of both sagebrush and perennial
grasses had increased relative to pretreatment,
but perennial grass cover was much higher than
sagebrush cover (Williams et al. 2017, Freund
et al. 2020). Additionally, cover of both was
higher on the mechanical than on the burn treatment. Because invasive weeds may also use the
added resource, increased wet days in the RGP
for drier and warmer sites and drier years after
tree reduction also present a risk.
Soil temperature/moisture regimes.—Field measurements of the RGP confirmed that the mesic/
aridic-xeric regime was warmer most seasons
and drier in spring than the frigid/xeric regime.
Sites with warmer and drier springs are associated with more cheatgrass and less perennial
grass cover, and especially on burn compared
with mechanical treatments (Roundy et al. 2018,
Freund et al. 2020). Where climate supports
perennial grass recovery (cooler, wetter, especially with wetter winters and wetter early
springs) and residual populations are sufficient
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untreated plots from Phase I to Phase II and Phase
III (Fig. 1), and also a significant increase in spring
wet days with tree reduction (Fig. 2). Our data
suggest that expansion limits and tree reduction
increases MP soil water availability needed to support shrub survival.
moderate pretreatment tree dominance, but
decreased greatly and was replaced by the lower
successional Elymus elymoides on both the
mechanical and burn treatments at high pretreatment tree dominance (Freund et al. 2020). Even
though the lower cover of cheatgrass on the
mechanical than burn treatment for this regime
at low pretreatment tree dominance was associated with lower soil temperatures, it was also
associated with greater perennial grass cover.
The cooler soil temperatures for the mechanical
treatment may have disfavored cheatgrass, while
the greater number of wet days in spring may
have favored perennial grass for this regime
(Table 1). Thus, the soil climate response to
mechanical tree reduction compared with burning for this regime may have increased resistance
to cheatgrass.
CONCLUSIONS
Managers remove trees to reduce fuel loads
and increase cover and density of desirable
understory species. Tree expansion decreased,
while tree removal by prescribed fire or mechanical means increased the time of available water
in springtime, which is associated with a longer
period of nutrient diffusion to roots and growth
of whichever plants are present (Leffler and Ryel
2012). We found that the time of soil water availability for understory plant growth was
increased even 12–13 yr after tree reduction. This
increase was greatest when trees were reduced at
an advanced phase of expansion and when measured on drier years. Tree expansion had the
most severe negative effects on soil water availability for understory vegetation on drier years.
On the other hand, increased water availability
from tree reduction may support not only establishment and growth of perennial shrubs and
herbs, but also invasive annual grasses and forbs
for many years. Reducing trees at earlier phases
of expansion to maintain the community, or
revegetation with shrubs and perennial herbs in
association with tree reduction at advanced
phases of expansion are recommended to
increase resistance to invasive plants. This is
especially important after prescribed fire and on
warmer and drier sites that have limited perennial grass cover and where resistance to invasive
annual grasses is low. On cooler and wetter sites
with limited perennial grass cover, tree reduction
by mechanical methods rather than by fire may
best resist cheatgrass by avoiding associated
increased soil temperatures.
Maintenance pool
The consequences of tree expansion and reduction on the MP relative to plant succession are less
clear than for the RGP. On untreated plots in
spring, the maximum dry period or maximum
period when soil water potential < 1.5 MPa
began as late as the third week in May. As with the
RGP, wet days were limited in summer and fall
(Fig. 1). Cavitation resistance of trees (for juniper
shoots = 8.2 MPa, for pinyon shoots = 2.7MPa,
Koepke and Kolb 2013), shrubs ( 5 MPa, Leffler
and Ryel 2012), and perennial grass ( 2.5 MPa,
Leffler and Ryel 2012) occurs at water potentials
lower than 1.5 MPa, the lower limit that we
were able to measure with gypsum blocks. Leffler
and Ryel (2012) considered that available water
(soil water potential > cavitation water potential)
at lower soil depths was necessary to maintain
shrub survival through the dry summers of the
Great Basin. Bates et al. (2000) found much higher
summer water potentials in a few small western
juniper trees left uncut in tree-cut plots than in
uncut plots, indicating a definite effect of summer
water use by trees. Decreased water availability for
maintenance as trees infill from Phase I to Phase III
may contribute to shrub mortality and declining
shrub cover. On drier years on some sites, soils at
50–65 cm did not wet up from winter and spring
precipitation. This could result in lack of maintenance water in summer and contribute to shrub
mortality. For the MP, we found a trend of decreasing wet days in spring with tree expansion on
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ROUNDY ET AL.
ACKNOWLEDGMENTS
This is contribution number 140 of the Sagebrush
Steppe Evaluation Project (SageSTEP) funded by the
U.S. Joint Fire Science Program, Bureau of Land Management, National Interagency Fire Center, Great
Basin Landscape Conservation Cooperative, and
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SPECIAL FEATURE: SAGEBRUSH STEPPE TREATMENT EVALUATION PROJECT
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