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Wettability of an Arizona
Chaparral Soil Influenced by
Prescribed Burning 1
John H. Brock and Leonard F. DeBano2
Arizona chaparral forms a discontinuous belt of vegetation extending
northwest to southeast across Arizona. It is dominated by fire-adapted
evergreen shrubs that have been classified into several climax vegetation
associations (Carmichael et al. 1978).
Chaparral has evolved in a climatic
regime that favors fire and, as aresult, has been exposed to repeated
wildfires during its evolution. Because fire has played such an important role in the evolution of chaparral, prescribed fire is used regularly
as an accepted management technique. Prescribed fire has been used
for improving wildlife habitat and
reducing fire hazard and, in conjunction with herbicides, for augmenting
water yield (DeBano et al. 1984, Hibbert et al. 1974.).
The continual accumulation of leaf
fall on the soil surface increases the
organic matter content of the surface
mineral soil layers under the shrub
canopies. This surface litter contains
organic substances that can induce a
resistance to water penetration in the
underlying mineral soil in unburned
stands (Brock and DeBano 1982).
'Poster paper presented at the conference, Effects of Rre in Management of
Southwestern Natural Resources (Tucson,
AZ, November 14-17, 1988).
2
Associate Professor of Environmental
Resources, School of Agribusiness and Environmental Resources, Arizona state University, Tempe: and Principal Soil Scientist,
USDA Forest Service, Rocky Mountain Forest
and Range Experiment Station, Tempe, AZ
85287-1304.
When a plant canopy and intact litter
layer are present, soil water repellency has little effect on infiltration. However, when these waterrepellent organic materials are volatilized during a fire and are translocated downward in the mineral soil,
they can form a severe water repellen( layer (DeBano 1981). Thus, in
cqntrast to the preburn condition,
this heat-induced water-repellent
layer presents problems for theresource manager because infiltration
is drastically reduced, surface runoff
is increased, and the potential soil
erosion greatly increased. This waterrepellent soil condition can also prevent microsi tes from becoming wet
during rainstorms, which reduces the
chances for successful seed germination and plant establishment during
postfire revegetation programs. Although water repellency in unburned
stands and its intensification by fire
have been reported earlier (DeBano
and Krammes 1966, Scho111975), no
attempt has been made to characterize its distribution both vertically
and horizontally in the mineral soil.
A three-dimensional characterization
of its distribution is important because of its effect on infiltration,
which is best modeled as a three-dimensional process. The objective of
this study was to describe the three. dimensional distribution of water
repellency in chaparral soil both before and after burning. This information can serve as the basis for developing future infiltration models that
206
better describe infiltration into
burned chaparral soils.
Methods
The study area was about 2 km
southwest of Goodwin, Arizona, a
townsite on the Prescott National
Forest. Shrub cover on the study area
was dominated by shrub live oak
(Quercus turbinella) and mountainmahogany (Cercocarpus montanus). The
soils on this site belong to the Lithic
Torriorthent great group and are derived from granite. The surface texture, determined by the hydrometer
method (Klute et al. 1986), is a sandy
loam containing 54% sand,36% silt,
and 10% clay.
The area was burned by an operational-scale prescribed fire in October
1979. Prior to burning, four 10-meter
transects were randomly located on
the area to be treated and marked
with metal pins. Along each transect,
four randomly selected points were
marked where two paired plots were
located for collecting prefire and
postfire soil samples. Each sample
plot was 0.25 m 2 and was subdivided
into 25 cells, each having an area of
100 cm2• Plots on the right side of the
transect line were designated as preburn sampling plots and those on the
left as postburn plots. Soil samples
were collected from each cell at 0-2,
2-5, and 5-10 em depths. This sampling scheme yielded 400 soil
samples per soil depth before bum-
ing and another 400 samples after
burning.
All soil samples were air-dried
and passed through a 2-mm sieve
before water repellency was measured by the water drop penetration
time method described by DeBano
(1981). Water drop penetration times
were terminated after 120 seconds
had elapsed. Organic matter content
was determined by the WalkeleyBlack method (Page et al. 1982) on
samples, but only for the 0-2 em surface soil layers before and after burning.
•
lsu
Data were analyzed and descriptive statistics developed using SAS
(Statistical Analysis System) computer programs. The experiment was
a 2-factor factorial (fire and depth) in
randomized complete block design.
Transects were considered as blocks
and plots were the exponential units.
Descriptive statistics were generated
across data within plots. Data were
subjected to analysis of variance and
correlation analyses. Differences
among means were tested with the
"F" statistic and considered significantly different if P ~ 0.05.
t
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Figure 1.-Average water penetration time
(sec) In soli from a chaparral community in
central Arizona prior to prescribed burning.
A surface (0-2 em), B 2-5 em depth, and
C 5-10 em depth.
=
=
=
Figure 2.-Average water penetration time
(sec) in soil from a chaparral community in
central Arizona after prescribed burning. A
=surface (0-2 em), B = 2-5 em depth, and
C = 5-10 em depth.
207
Results
Water Repellency
A resistance to wetting was found
in all three soil layers prior to burning (fig. 1A, 1B, 1C), although it was
most severe in the surface 0-2 em
layer (fig. 1A). The average water
drop penetration time of samples
from the 0-2 layer was 70.1 sec(±
2.69 SE). The average water drop
penetration time in the 2-5 em layer
averaged 47.7 sec(± 2.02 SE), which
was 32% less than in the 0-2 em layer.
The average water drop penetration
time decreased 71% between the surface and the 5-10 em layer, where the
time was 20.7 sec(± 1.93). The average water drop penetration times
were all significantly different at a P
=0.001. A large difference in water
repellency was present over the grid
surface at each soil depth (fig. 1A,
1B, 1C). For example, the coefficient
of variation was 76%, 110%, and
187% in the 0-2,2-5, and 5-10 em soil
depths, respectively.
The prescribed bum significantly
(P =0.001) increased water repellency in the underlying mineral
soils. The surface 0-2 em layer had an
average water drop penetration time
of 106.4 sec(± 1.73) (fig. 2A) after
burning which was a 151% increase
over the time required for water to
penetrate soil taken from this same
layer prior to burning (fig. 1A).
Average water drop penetration
time in the 2-5 em layer increased
210% over preburn times and was
100.5 sec(± 1.83 SE) (fig. 2B), but
was not significantly different (P =
0.05) than in the postburn 0-2 em
layer. Water drop penetration time
increased 337% over the prebum
times in the 5-10 em layer and averaged 69.6 sec (:±: 2.55 SE) after burning (fig. 2C). Less spatial variability
was also present among the postbum
samples at all soil depths. The coefficients of variation in the 0-2 and 2-5
em layers were both approximately
30%, whereas the 5-10 em layer was
73%.
Soil Organic MaHer
Burning was found to significantly
(P =0.001) increase organic matter in
the surface 0-2 em layer (fig. 3A, 3B).
The average soil organic matter content before burning was 9.0% (:±:
0.188 SE) and increased to 14.3% (:±:
0.289 SE) following fire. However,
the correlation between mean organic matter and water repellency
before and after burning in the surface layer was only 0.09 and was not
significant (P =0.65).
Discussion
The results from this study indicate that water repellency both before and following fire is much the
same in Arizona chaparral soils as
had been reported earlier in California (DeBano and Krammes 1966). Although water repellency is present in
unburned soils it probably does not
affect runoff and erosion substantially because it is mitigated by a protective plant and litter cover. Fire increases water repellency by volatilizing organic rna terials in the litter on
the soil surface, and these organic
substances move downward into the
underlying soil layers where they
condense to form a water-repellent
layer which is more severe than was
present before burning (DeBano
1966).
The increase in both organic matter and water penetration time following fire as measured in this experiment supports this theory. The
low correlation between organic matter content and water repellency was
caused by the unavoidable incorporation of some unburned organic
particles in the surface 0-2 em layer
sample before and after burning. It is
nearly impossible to separate ash
from the surface mineral soil following burning. The significant increases
in water repellency in the 2-5 and 510 em layers following fire further
confirm that organic materials are
transferred downward in the soil
during the combustion of surface litter and plant fuels.
The spatial variations of water repellency both horizontally in a specific layer and vertically between layers indicate that a careful sampling
pattern needs to be developed when
evaluating postfire water repellency.
Single point sampling over depth is
not sufficient to adequately characterize the distribution of water repellency in the mineral soil, particularly after burning. The complications that this variable spatial distribution of water repellency has on the
development of infiltration models
has not yet been resolved. However,
it is apparent that infiltration rates
based primarily on textural information will have to be reduced substantially following fire. Coarse-textured
soils, which normally have high infiltration rates, should receive special
attention because they are very sensitive to heat-induced water repellency
(DeBano et al. 1970).
Management Implications
Land managers need to be aware
of the potential reductions in infiltration that can occur in chaparral soils
as a result of burning-either by
wildfires or prescribed burns. Waterrepellent layers form in much the
same way in Arizona chaparral as
has been reported in California. The
largest problems associated with
heat-induced water repellency are in
coarse-textured soils containing less
than 10% clay. However, on all soils,
the loss of the plant canopy and litter
layer can lead to increases in raindrop splash and surface erosion. Water repellency on burned areas can be
assessed by using the water drop
penetration time method. However,
a single point sample is usually not
sufficient for characterizing the dis- tribution of water-repellent layers
because the vertical and horizontal
extent of this layer has been found to
vary widely. It is recommended that
the average of several samples taken
208
in different locations and soil depths
be used as the basis for assessing the
extent and intensity of fire-induced
water repellency.
Literature Cited
Brock, J. H.; DeBano, L. F. 1982. Runoff and sedimentation potentials
influenced by litter and slope on a
chaparral community in central
Arizona. In: Proceedings of the
symposium on dynamics and
management of Mediterraneantype ecosystems. Gen. Tech. Rep.
PSW-58. Berkeley, CA: U.S. Department of Agriculture, Forest
Service, Pacific Southwest Forest
and Range Experiment Station.
p. 372-377.
Carmichael, R. S.; Knipe, 0. D.; Pase,
C. P.; Brady, W. W. 1978. Arizona
chaparral: plant association and
ecology. Res. Pap. RM-202. Fort
Collins, CO: U.S. Department of
Agriculture, Forest Service, Rocky
Mountain Forest and Range Experiment Station. 16 p.
Figure 3.-Average organic matter content
In the surface 0-2 em layer of a chaparral
community in central Arizona. A= preburn
and B = postburn.
DeBano, Leonard F. 1966. Formation
of non-wettable soils .. .involves
heat transfer mechanism. Res.
Note PSW-132. Berkeley, CA: U.S.
Department of Agriculture, Forest
Service, Pacific Southwest Forest
and Range Experiment Stn. 8 p.
DeBano, Leonard F. 1981. Water repellent soils: A state-of-the-art.
Gen. Tech. Rep. PSW-46. Berkeley,
CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment
Station. 21 p.
DeBano, L. F.; Brejda, J. J.; Brock, J.
H. 1984. Enhancement of riparian
vegetation following shrub control
in Arizona chaparral. Journal of
Soil and Water Conservation. 39:
317-320.
DeBano, L. F.; Krammes,]. S. 1966.
Water repellent soils and their relation to wildfire temperatures.
International Association Scientific
Hydrology Bulletin. 11: 14-19.
DeBano, L. F.; Mann, L. D.; Hamilton, D. A. 1970. Translocation of
hydrophobic substances into soil
by burning organic litter. Soil Science Society America Proceedings.
34: 130-133.
Hibbert, Alden R.; Davis, Edwin A.;
Scholl, David G. 1974. Chaparral
conversion potential in Arizona.
Part I: Water yield response and
effects on other resources. Res.
Pap. RM-126. Fort Collins, CO:
U.S. Department of Agriculture,
Forest Service, Rocky Mountain
Forest and Range Experiment Station. 36 p.
Klute, Arnold, ed. 1986. Methods of
Soil Analysis. Part 1. Physical and
mineralogical methods. 2d ed.
Agronomy Series No. 9. Madison,
WI: American Society of Agronomy.1188p.
Page, A. L.; Miller, R. H.; Keeney, D.
R., eds. 1982. Methods of soil
analysis. Part 2. Chemical and
microbiological properties. 2d ed.
Agronomy Series No. 9. Madison,
WI: American Society of .Agronomy, Inc., Soil Science Society of
America, Inc. 1159 p.
Scholl, David G. 1975. Soil wettability in Arizona chaparral. Soil Science Society America Proceedings.
39: 356-361.