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Influence of Soil Water Repellency on
Seedling Emergence and Plant Survival in
a Burned Semi-Arid Woodland
a
b
b
Matthew D. Madsen , Steven L. Petersen , Kaitlynn J. Fernelius ,
b
c
Bruce A. Roundy , Alan G. Taylor & Bryan G. Hopkins
b
a
USDA–Agricultural Research Service, Eastern Oregon Agricultural
Research Center, Burns, Oregon, USA
b
Department of Plant and Wildlife Sciences, Brigham Young
University, Provo, Utah, USA
c
Department of Horticulture, Cornell University, Geneva, New York,
USA
To cite this article: Matthew D. Madsen, Steven L. Petersen, Kaitlynn J. Fernelius, Bruce A. Roundy,
Alan G. Taylor & Bryan G. Hopkins (2012): Influence of Soil Water Repellency on Seedling Emergence
and Plant Survival in a Burned Semi-Arid Woodland, Arid Land Research and Management, 26:3,
236-249
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Arid Land Research and Management, 26:236–249, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 1532-4982 print=1532-4990 online
DOI: 10.1080/15324982.2012.680655
Influence of Soil Water Repellency on Seedling
Emergence and Plant Survival in a Burned
Semi-Arid Woodland
Matthew D. Madsen1, Steven L. Petersen2,
Kaitlynn J. Fernelius2, Bruce A. Roundy2, Alan G.
Taylor3, and Bryan G. Hopkins2
Downloaded by [Matt Madsen] at 14:25 17 September 2012
1
USDA–Agricultural Research Service, Eastern Oregon Agricultural
Research Center, Burns, Oregon, USA
2
Department of Plant and Wildlife Sciences, Brigham Young University,
Provo, Utah, USA
3
Department of Horticulture, Cornell University, Geneva, New York,
USA
High intensity wildfires in semiarid shrub and woodland plant communities can leave
ecosystems incapable of self-repair and susceptible to weed invasion. Subsequently,
land managers need effective restoration tools to reseed native vegetation back into
these degraded systems. In order to develop successful post-fire restoration
approaches in these communities, it is critical that we understand the mechanisms
that impair reseeding success. Our objective was to quantify the influence of soil
water repellency on seedling emergence and plant growth in a greenhouse study using
soil cores obtained from beneath burned Juniperus osteosperma trees. Soil cores
were seeded with either Elymus wawawaiensis or Agropyron cristatum, and
watered with either a high (watered daily) or a low water regime (watered every
5 days). During the first watering event, water repellency was ameliorated in half
the cores by adding a wetting-agent comprised of alkylpolyglycoside-ethylene oxide=propylene oxide block copolymers. Results showed that water repellency reduced
seedling emergence and seedling survival by decreasing soil moisture availability.
Wetting-agents improved ecohydrologic properties required for plant growth by
decreasing runoff and increasing the amount and duration of available water for
seedling emergence, survival, and plant growth. These results indicate that soil water
repellency can act as an ecological threshold by impairing establishment of reseeded
species after a fire. Where restoration efforts are limited by soil water repellency,
Received 18 April 2011; accepted 30 November 2011.
We are grateful to Daniel Zvirzdin and Alexander Zvirzdin for their aid in conducting
this research. Wetting agent and technical support was provided by Stan Kostka (Aquatrols,
Paulsboro, NJ, USA). Funding was provided by the USDA-Natural Resource Conservation
Service, Conservation Initiative Grant, USDA-National Institute of Food and Agriculture’s
Rangeland Research Program, and the USDA-Agricultural Research Service. Mention of a
proprietary product does not constitute a guarantee or warranty of the product by USDA
or the authors and does not imply its approval to the exclusion of the other products that also
may be suitable. USDA is an equal opportunity provider and employer.
Address correspondence to Matthew D. Madsen, USDA–Agricultural Research Service
67826-A Hwy 205, Eastern Oregon Agricultural Research Center, Burns, OR 97720, USA.
E-mail: matthew.madsen@oregonstate.edu
236
Wildfire Restoration in Water Repellent Soil
237
wetting agents have the potential to improve the success of post-fire reseeding
efforts. Future work is needed to validate these findings in the field.
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Keywords erosion, hydrophobicity, pinyon-juniper, reseeding, restoration,
revegetation, surfactants, weeds, wetting-agents, wildfire
High intensity or catastrophic wildfires in semiarid shrub and woodland plant
communities can leave these ecosystems incapable of self-repair and susceptible to
weed invasion (Young & Evans, 1978; Brooks et al., 2004). Invasive species such as
Bromus tectorum L. (cheatgrass) have a negative impact on rangeland ecological
services, including decreased forage production, watershed stability, and water
quality (Young & Evans, 1978; Miller & Tausch, 2001). These flammable weeds also
alter the frequency, intensity, extent, and seasonality of natural fires, thus further
promoting weed dominance and loss of native species (D’Antonio & Vitousek, 1992).
One approach to preclude weed invasion is to reseed the site with desirable species
(Epanchin-Niell, Englin, & Nalle, 2009). Through this approach, recovery is aided by
replenishing depleted seed pools, which helps restore ecosystem processes while
reducing resource availability for invasive weeds (Cox & Anderson, 2004). In the past,
land managers typically selected introduced species for post-fire restoration, such as
Agropyron cristatum (L.) Gaertn. (crested wheatgrass), and Thinopyrum intermedium
(Host) Barkworth & D.R (intermediate wheatgrass). These species can increase establishment success, lower seeding costs, provide better weed competition, and improve
livestock forage quality (Steward, 1950). Currently, fire rehabilitation programs are
increasing the use of native plant materials in place of introduced species in an effort
to reinstate ecosystem processes and improve species diversity after a fire (Thompson
et al., 2006). However, these native species are costly and establishment success is
typically less than desirable (Roundy et al., 1997). Consequently, land managers
throughout the Intermountain West are requesting new techniques that will improve
establishment success of native plant materials in order to restore habitats lost to
wildfire, prevent subsequent weed dominance, and restore natural fire cycles.
In order to develop successful post-fire restoration approaches, it is critical to
understand the factors that impair native vegetation recovery. Soil water repellency
(or hydrophobicity), is one factor that may significantly limit post-fire recovery in
Pinus-Juniperus systems and promote subsequent weed domination. Soil water repellency is commonly found in arid and semi-arid ecosystems in unburned soils
(Krammes & DeBano, 1965; Doerr et al., 2000; Jaramillo et al., 2000). Sites with
coarse-textured soils that contain woody evergreen plants, which have high amounts
of resins, waxes, or aromatic oils, and associated thick litter layers, are most susceptible to soil water repellency development (Doerr et al., 2000; Jaramillo et al., 2000).
However, it cannot be assumed that all sites that have these soil and plant characteristics will be associated with soil water repellency, as there are multiple complex
processes associated with water repellency formation (Doerr et al., 2009).
In unburned conditions soil water repellency can have a strong control on
precipitation inputs. During a precipitation event, thick litter layers can help retain
precipitation from running off site, while underlying water repellent soil concentrates
and directs this moisture through preferential flow channels to greater depths than
could be achieved on wettable soil (Madsen et al., 2008; Robinson et al., 2010). In
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238
M. D. Madsen et al.
water limiting environments, soil water repellency may benefit woody plants by
minimizing near surface evaporation and increasing soil moisture within the root
zone (Madsen et al., 2008; Robinson et al., 2010). Burning can alter the severity
and depth of the water repellent layer as heat volatilizes organic substances in the
litter and upper water repellent soil layers (DeBano and Krammes, 1966). These
molecules penetrate into the cool underlying soil layers where they condense around
soil particles and either render them hydrophobic or further decrease their affinity
for water (Savage, 1974; Doerr et al., 2000).
Pinus spp. (piñon) and Juniperus spp. (juniper) are examples of woody vegetation
types which under both unburned and burned conditions can be strongly correlated
with the presence of soil water repellency (Jaramillo et al., 2000; Madsen et al., 2008;
Pierson et al., 2009). In the western United States, Pinus-Juniperus woodlands have
expanded since Eurasian settlement in the late nineteenth century, into primarily
sagebrush steppe and grassland communities (Burkhardt & Tisdale, 1976). These
species out-compete understory species, which decreases the potential for frequent,
moderate-severity fires carried through the understory vegetation, and increases
the potential for less-frequent large-scale catastrophic crown fires (Miller & Tausch,
2001). Madsen et al. (2011) demonstrated that within a Pinus-Juniperus woodland,
the spatial distribution of water repellency was strongly correlated with pre-fire tree
canopy cover. In this study critical water repellency (i.e., water repellency that can be
measured through the water drop penetration test (WDPT) (Krammes and DeBano,
1965) extended from the base of the tree to just beyond the canopy edge. Subcritical
water repellency (i.e., water repellency levels are insufficient to be detected through
the WDPT test but are at sufficient levels to influence soil infiltration; Tillman et al.,
1989) extended half a canopy radius past the edge of the critical water repellency
zone. At sites where critical water repellency was present, vegetation cover and density were dramatically reduced relative to non-water repellent sites; these variables
were also reduced in soils with subcritical water repellency, albeit to a lesser extent.
These results by Madsen et al. (2011) and other authors, such as the research of
Adams et al. (1970) on Larrea tridentata (creosote bush) and the studies of Salih
et al. (1971) with Artemisia tridentata (sagebrush), may indicate that soil water
repellency limits revegetation success immediately following fire. Consequently,
restoration approaches that focus on treating soil water repellency issues could
potentially improve the success of seeding projects while simultaneously decreasing
runoff and soil erosion, and preventing weed invasion.
Use of commercially available surface active agents (wetting-agents or surfactants) may provide an alternative post-fire restoration approach where water repellency inhibits site recovery. A wide variety of ionic and nonionic wetting-agents are
produced commercially, ranging from simple dish soaps to sophisticated polymers
chemically engineered to overcome soil water repellency. Wetting-agents are generally organic molecules that are amphiphilic (hydrophobic tails and hydrophilic
heads). While wetting-agents have different modes of action, water repellent
soils are rendered wettable by the hydrophobic tail of the wetting-agent chemically
bonding to the non-polar hydrophobic coating on the soil particle, while the
hydrophilic head of the molecule attracts water molecules into the soil (Cisar et al.
2000; Kostka 2000). Various small plot post-fire research projects located in the
mountains of southern California revealed that the application of wetting-agents
after a fire can reduce soil erosion and improve vegetation establishment (Osborn
et al., 1967; Krammes & Osborn, 1969; Debano & Conrad, 1974). These studies
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Wildfire Restoration in Water Repellent Soil
239
suggest that wetting-agent applications can be a successful post-fire treatment. While
wetting-agents have not been used in wildland systems since the 1970’s, they were
extensively used and further developed within various aspects of the agricultural
industry, with most applications in turfgrass systems (Cisar et al., 2000; Kostka &
Bially, 2005). These newly developed wetting-agents may provide an innovative
approach for alleviating the effects of water repellency on the establishment of
seeded species.
The primary objective of this research was to quantify the extent that soil water
repellency within burned Pinus-Juniperus woodland soils influences emergence and
growth of the non-native bunchgrass A.cristatum, and native bunchgrass, Elymus
wawawaiensis J. Carlson and Barkworth (Snake River wheatgrass), and in conjunction with this objective, determine if wetting-agent can be used to reduce soil water
repellency and subsequently improve seedling emergence and survival. We hypothesized that soil water repellency would decrease seedling survival by decreasing
soil moisture availability, and that the reduction of soil water repellency through
wetting-agent application would overcome these limitations.
Methods
Soil Collection
Research was conducted on soil obtained from the boundaries of Utah’s largest wildfire on record (145,000 ha), the 2007 Milford Flat wildfire. Soils were collected approximately 13.7 km NW of Milford, UT, USA (Lat: 38 26’ 12’’ N, Long: 112 51’ 46’’ W,
elevation 1847 m). A detailed description of the site is given by Madsen et al. (2011).
The site is predominantly on west-facing slopes, positioned at the base of the Mineral
Mountain Range. Parent material is characterized as alluvium derived from acid and
intermediate igneous rock, with soil classified as coarse sandy loam, mixed, mesic
Aridic Haploxerolls (3–10% slope). Volumetric soil water content at 1.5 MPa (permanent wilting point) and 0.33 MPa (field capacity) was equal to 12.8%, and 24.8%,
respectively (Soil Survey Staff, 2011). Mean annual precipitation is approximately
370 mm (PRISM Climate Group, 2009). Prior to the fire, the plant community was
a Phase III, Pinus-Juniperus woodland, with Juniperus osteosperma (Torr.) Little (Utah
juniper), and Pinus monophylla Torr. & Frém. (singleleaf piñon) trees acting as the
primary plant layer driving ecological processes (Miller et al., 2008). Soil was collected
one year after the fire, from the subcanopy of burned J. osteosperma trees. At the time
of collection, the soil remained almost completely bare of live vegetation, with trace
amounts of annual forbs and grasses (3.1 1.1 plants m2) (average and standard error
of the mean). The upper layer of the soil was wettable and appeared to be primarily ash
originating from burned litter and debris; this layer extended from the soil surface
down to 1.7 0.1 cm. Below this depth (1.7–4.5 cm), water repellent mineral soil
predominated. Average WDPT; within the water repellent layer was 5,256 258 sec.
Soil cores were obtained by pressing polyvinyl chloride (PVC) tubes (20.3 cm
diameter by 36-cm deep) into the soil with the bucket of a front-end loader. To minimize soil compaction, the end of each tube was sharpened before being pressed into
the soil. Cores were modified for use as growing pots by fastening a woven porous
landscape fabric to the bottom of the PVC pipe. To allow for drainage at the soil
surface, pots were fitted with a runoff spout 19.1 mm in diameter (Figure 1). Pebbles
were placed in front of the spout to minimize seed and soil loss.
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M. D. Madsen et al.
Figure 1. A. Diagram of rainfall simulator; B. soil core modified for use as a growing pot,
attached with a runoff catchment tube.
Study Design
This experiment was conducted in a greenhouse located on the campus of Brigham
Young University (BYU), Provo UT, USA. Treatments included soil cores treated
with and without wetting-agent, two watering regimes, and two seeded species, with
all combinations arranged in a randomized block design, with three soil core replicates per block, and five blocks in the study (2 soil treatments 2 watering
regimes 2 species 3 core-replicates 5 blocks ¼ 120 soil cores in the study). Each
pot was seeded with 35 seeds of either A. cristatum or E. wawawaiensis. Total germination of A. cristatum and E. wawawaiensis, was 90% and 85%, respectively. During
the first watering event, we applied the non-ionic wetting-agent IrrigAid Gold
(Aquatrols, Paulsboro, NJ, USA), which is composed of a blend of alkylpolyglycoside and ethylene oxide=propylene oxide block copolymers, at 0.012 ml cm2. In preliminary tests, we found that this wetting-agent concentration was required to allow
complete wetting of the soil column. After the pots were treated with wetting-agent,
we verified that the soil had been made wettable using the WDPT test.
As Pinus-Juniperus woodlands typically occur on sloping terrain and the influence of soil water repellency can be more pronounced on a slope, pots were placed
on a 15 angled bench, similar to methods described by Osborn et al. (1967). We
watered the pots by running a rainfall simulator with the bottom of the simulator
40 cm above the soil surface (Figure 1). Individual rainfall simulators were designed
to water one pot at a time, which were constructed from seven liter buckets that had
a 25.4 cm radius bottom, and 50 drip tubes (10 mm long, with an inside diameter of
0.864 mm and wall thickness of 0.305 mm), to distribute the water. The rate of flow
Wildfire Restoration in Water Repellent Soil
241
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from the bucket was controlled by adjusting the head pressure using a Mariotte tube
(4.8 mm diameter) that slid within a bored rubber stopper attached to the top of the
bucket. To start and stop the flow of water from the rainfall simulator, surgical
tubing and a clamp were placed at the top of the Mariotte tube.
Pots were watered under either a low or a high watering regime. The low watering regime was designed to replicate spring-like conditions, with periods of adequate
soil moisture, followed by intermittent periods of no precipitation. This watering
treatment consisted of watering daily for the first 4 days, to insure seed germination,
then once every 5 days. We also watered pots daily on days 20–24 to see if seeds not
yet germinated would emerge. The high water regime was designed so the seedlings
would not be water stressed. Under this watering regime, we watered the pots daily
throughout the project. At each watering (low and high) we delivered a total of
0.62 cm (400 mL) of water at a rate of 3.0 cm hr1.
Measurements
The experiment was conducted for 48 days from the time of seeding (14 September
2008–2 November 2008). During this period, runoff was collected from three randomly-chosen blocks. Runoff was measured from each pot within a catchment tube
made of 4.5-cm-diameter thin-wall PVC cylinders, with the cap hung just below the
runoff spout of each pot (Figure 1). For each different watering regime and soil
treatment, soil water content was measured on four randomly-selected pots. Soil
water content was continuously recorded, at the 1–3 cm soil depth using EC-5 Soil
Moisture Sensors in conjunction with Em5b data loggers (Decagon Devices, Pullman, WA, USA). For all blocks, the number of live seedlings was recorded every
2 days throughout the experiment. Upon harvest, below-ground and above-ground
biomass was measured separately after drying at 65 C for 72 h.
Data Analysis
Mixed model analysis was used to analyze runoff, soil water content, final seedling
density, above-ground biomass, and below-ground biomass (SYSTAT 12, Systat Software Inc. Chicago, IL, USA; P < 0.05). Blocks were considered random, while species,
watering regimes, and soil treatments were considered fixed factors. Runoff and soil
water content were analyzed as repeated measures. For treatments found to affect the
response variables, mean values were separated using Tukey’s Honestly-SignificantDifference test. Pairwise comparison among the treatments for runoff and soil water
content response variables were averaged by treatment over the period of the study.
Analysis of soil water content was performed on values at the time of watering (i.e.,
peak SWC), and 4 days after watering (i.e., trough SWC). Final seed density was analyzed by the number of live plants at the end of the study and after normalizing the
density data, by species, with the percent of germinable seeds planted.
Results
Runoff
Runoff was not influenced by species, or watering regime, but was influenced by
wetting-agent treatment (Table 1A). The control had, on average, 24%, and 19%
of the added water runoff, while the wetting-agent treatment decreased runoff with
242
M. D. Madsen et al.
Table 1. A. Mixed-model analysis results indicating significance of water regime and
wetting-agent treatment on runoff and soil water content just after watering (SWC:
peak) and four days after watering (SWC: trough). B. Pair-wise comparisons showing mean and associated standard error for the response variables that were found to
be significant
Hydrologic Response
A.
Response variables
Runoff
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Effect variables
Water regime
Wetting-agent
Species
SWC: trough
F
P
F
P
F
P
2.3
178.3
0.002
0.144
<0.001
0.966
54.5
8.9
–
<0.001
0.004
–
139.1
17.1
–
<0.001
0.001
–
Pair-wise comparisons
B.
Runoff (mL)
low water þ control
low water þ WA
high water þ control
high water þ WA
SWCa: peak
94.7 5.7
18.0 3.4
76.1 5.8
22.1 3.6
a
b
a
b
SWC: peak (%)
17 2
25 2
31 1
33 1
a
b
c
c
SWC: trough (%)
62
15 2
27 1
30 1
a
b
c
c
Means without a common letter differ at P < 0.05, Tukey HSD test.
a
SWC ¼ soil water content.
only 4%, and 5% of the water runoff, for the low and high watering regimes, respectively (Table 1).
At harvest, it was observed that the majority of the control pots still had a band
of air-dry water-repellent soil just below the soil surface (Figure 2). It was speculated
that for the control pots, water that did not run off from the soil cores infiltrated
around the water repellent layer along the sides of the pot, into the wettable soil
below. In contrast, wetting-agent treated soils appeared to allow for an even wetting
of the entire soil core (Figure 2).
Soil Water Content
Soil water content was affected by watering regime and wetting-agent treatment
(Table 1A). Under the low watering regime, soil water content was higher in the
wetting-agent treated soil for both the peak and trough periods in relationship to
the control (Table 1B). Under the high watering regime, soil water content was
statistically similar, although the peaks and troughs on average were higher for
wetting-agent treated soil (Table 1B).
Density
Seedling density was influenced by watering regime, wetting-agent application, and
species (Table 2). However, relative seedling density, normalized by percent of
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Wildfire Restoration in Water Repellent Soil
243
Figure 2. Aerial view and cross section photo of E. wawawaiensis seedlings grown under a low
water regime on: A. water repellent soil; B. water repellent soil that was treated with
wetting-agent (WA).
germinable seeds, did not differ by species (Table 2). Under the low water regime,
within the control pots, seedling density first peaked at about 10 days after seeding
(Figure 3). Under the same watering regime, wetting-agent treated pots peaked at
day 14, with seedling densities 268% higher than the control peak densities. Between
days 10–20, 72% of the seedlings growing in the control desiccated (Figure 3). Unlike
the control pots, wetting-agent treated pots under the low water regime had
relatively few plants desiccate before the next daily watering period, with a drop
in seedling density of only 9%.
Daily watering on days 20–24 produced a minor increase in seedling densities for
control and wetting-agent treatments, as a result of seeds emerging that had not
Table 2. Mixed-model analysis of Agropyron cristatum and Elymus wawawaiensis
seedling responses to water regime, and wetting-agent treatment
Vegetation response
Effect variables
Water regime
Response variables
Seedling survival
Survival of germinable seeds
Above-ground biomass
Below-ground biomass
Wetting agent
Species
F
P
F
P
F
P
129.1
130.2
100.1
82.6
<0.001
<0.001
<0.001
<0.001
102.1
102.0
50.1
59.7
<0.001
<0.001
<0.001
<0.001
4.9
1.8
11.3
2.4
0.034
0.191
0.002
0.128
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244
M. D. Madsen et al.
Figure 3. Combined results for A. cristatum and E. wawawaiensis seedling densities over the
course of the experiment, with results normalized by total germination for pots watered under
a low or high watering regime, treated with or without wetting-agent (WA). Significant differences (P < 0.05) at the end of the experiment are shown by unique letters.
emerged previously under the low water regime (Figure 3). At the end of the experiment under the low watering regime, wetting-agent treated pots had significantly
higher final seedling densities, with 292% more seedlings in the wetting agent treatment in comparison to the control (i.e., 12.2 and 46.3% of the germinable seeds
planted produced a live plant at the end of the study for control and wetting-agent
treatment, respectively). Under the high water regime, control pot seedling densities
steadily increased over the course of the experiment, with seedling densities similar to
wetting-agent treated soils under the low watering regime (Figure 3). Wetting-agent
treated pots under the high regime also had higher seedling densities than the control
pots, with 49.6% higher seedling survival (i.e., 50.2 and 74.9% of the germinable
seeds planted produced a live plant at the end of the study for control and wettingagent treated pots, respectively).
Biomass
Total above-ground biomass was influenced by watering regime, wetting-agent
application, and species (Table 2). Pots treated with wetting-agent under the low
water regime had above-ground biomass 690% and 722% greater than the control,
for A. cristatum and E. wawawaiensis respectively (Figure 4). Above-ground biomass
of wetting-agent treated pots for both species, under the low watering regime was
statistically similar to the control under the high watering regime. Under the high
watering regime, biomass of seedlings in the wetting-agent treated pots was statistically higher than seedlings in the control, with above-ground biomass 50% and 66%
greater than the control for A. cristatum and E. wawawaiensis, respectively (Figure 4).
However, under the high watering regime biomass of A. cristatum was similar to that
of E. wawawaiensis grown in wetting-agent treated soil.
Below-ground biomass was influenced by watering regime and wetting-agent
application, but unlike above-ground biomass it did not differ by species (Table 2).
Of all the response variables measured in this study, below-ground biomass showed
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Wildfire Restoration in Water Repellent Soil
245
Figure 4. Above and below ground biomass of A. cristatum and E. wawawaiensis grown on
water-repellent soil treated with or without wetting-agent (WA), under high and low watering
regimes. Significant differences (P < 0.05) at the end of the experiment are shown by unique
letters.
the greatest increase with wetting-agent treatment. Pots treated with wetting-agent
under the low water regime had below-ground biomass 1056% and 1196%, greater
than the control, for A. cristatum and E. wawawaiensis, respectively.
Similar to above-ground biomass, wetting-agent treated pots under the low watering regime had similar above-ground biomass as control treatments under the high
watering regime, but less biomass than wetting-agent treated pots under the higher
watering regime. Under the high water regime, above-ground biomass in the wettingagent pots was greater than the control by 119%, and 127%, for A. cristatum and
E. wawawaiensis, respectively. While harvesting the control pots, we observed that
seedling roots generally did not grow through the water repellent layer, but grew along
the sides of the soil pot to avoid the water repellent layer. In contrast, roots were evenly
distributed throughout the wetting-agent treated soil (Figure 2).
Discussion
These results are consistent with previous wildland fire studies that have documented
decreased germination and seedling survival as a result of soil water repellency
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M. D. Madsen et al.
(DeBano, 1969; Krammes & Osborn, 1969; DeBano & Conrad, 1974), and this is the
first study to demonstrate that water repellency limits reseeding success within
Pinus-Juniperus woodland soils. Madsen et al. (2011) examined the spatial distribution and severity of soil water repellency within the area the soil was collected
for this study, and determined that revegetation success was correlated to these parameters. However, the methods implied by Madsen et al. (2011) do not imply causation. In this study causation was shown by using wetting-agent to cancel out (or
ameliorate) the effect of soil water repellency and allow us to compare revegetation
on water repellent and non-water repellent soils. Specifically the results of this greenhouse experiment suggest that within burned Pinus-Juniperus woodlands, soil water
repellency can promote runoff, decrease soil water content, and impair seedling
emergence, growth, and plant survival. In this study decreased soil water content
near the soil surface is most likely associated with soil water repellency promoting
runoff (Table 1) and directing infiltrating water through preferential flow channels
below the water repellent soil (Dekker & Ritsema, 2000). Consequently, water repellency may impair seeding success by decreasing moisture availability to levels that
are not adequate for seed germination and seedling survival.
Potentially the cycle of seedling emergence and desiccation that was observed in
this study may also occur in the field, which would result in a loss of residual or
aerially-seeded species from the seed bank. Once soil water repellency has dissipated
after a fire, and seed propagules of desired species are no longer present, the site may
be primed for weed invasion. In general, the susceptibility of a site to weed invasion
can be explained in part, by the fluctuating resource hypothesis which states that ‘‘a
plant community becomes more susceptible to invasion whenever there is an increase
in the amount of unused resources’’ (Davis et al., 2000, p. 529). Therefore, we
hypothesize that soil water repellency negatively affects germination of seeded
species and increases invasibility by annual weeds, such as B. tectorum, that can
establish on the resource rich subcanopy areas after soil water repellency dissipates.
Results of this study are limited to the specific site in which soil was obtained from
the study (i.e., subcanopy region within a phase III Pinus-Juniperus woodland). The
variables that determine the occurrence, severity, persistence, and spatial distribution
of water repellency across the landscape are likely associated with various ecological
attributes such as soil properties, topography, and woodland stand characteristics
(Lewis et al., 2006). For areas where soil water repellency is limiting vegetation recovery, these results indicate that wetting-agents can ameliorate post-fire water repellent
soil and subsequently help restore ecohydrologic function in conjunction with reseeding efforts. Application of wetting-agents in this study significantly decreased water
runoff and increased the amount of available soil moisture, which increased plant density, and above and below ground biomass. These results are also consistent with previous studies within the California chaparral which showed that wetting-agents
improve seedling densities on post-fire water repellent soil (e.g., Osborn et al., 1967;
Krammes & Osborn, 1969; Debano & Conrad, 1974). This research, along with previous studies in other ecosystems, provides evidence that this technology should be
further field-tested in wildland systems impaired by post-fire water repellent soil.
Conclusions and Management Implications
The effects of large, high-severity wildfires are a significant management concern
for rangeland ecosystems. After a fire, the ability of an ecosystem to recover is
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dependent on the extent to which ecological processes have been altered, both prior
to and as a result of the fire. This study indicates that the modification of the soil
through the development of a post-fire water repellent layer is one alteration that
may significantly limit site recovery.
Restoration approaches that focus on ameliorating water repellency could
potentially improve the success of native plant materials in post-fire reseeding
efforts. This study provides evidence that the use of wetting-agents can significantly
improve ecohydrologic properties required for plant growth within a post-fire
Pinus-Juniperus woodland. While these results are promising, future work is needed
to verify these results in the field, and to determine if the application of this soil
amendment would be practical for post-fire wildland revegetation. The application
of soil amendments is typically not practical for the reclamation of rangeland systems due to large areas and difficult topography that require treatment, as well as
the perceived low economic value of the land. Further research could be performed
in developing new techniques for efficiently applying wetting-agents such as by aircraft in a granular form, or directly coated on the seed itself, thus providing a cost
effective post-fire restoration treatment.
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