Spatial Variation of Soil
Characteristics with respect to
Hill Slope Position and
Vegetation Community
Christopher Shepard
REU Project
Summer 2010
Soil Formation
Soil forms from the
breakdown of parent rock
material.
Soil formation is affected
by climate, topography,
parent rock material,
erosion rates, biotic
community, and time.
Soil Creep and Mass Wasting
Soil moves downslope
under the influence of
gravity and erosion.
Soil can also be
transported by wind.
Soil movement can be
greatly affected by
vegetation community,
climate, and hill slope
position.
Slopes
• Slope aspect effects
microclimate in desert
environments (Monger
and Bestelmeyer
2006)
• Vegetation
composition changes
as hill slope position
changes (Bridge and
Johnson 2000)
Research Questions
• Do soil characteristics change as hill slope
position changes?
• Does soil creep behave as predicted in a
semi-arid climate? And what effect does
vegetation community have on soil creep?
• How does vegetation community and soil
formation interact at different hill slope
positions?
Research Location
Sevilleta National Wildlife Refuge
Los Piños Mountains
34 22'6.78"N 106 32'11.83"W
Cerro Montosa NPP Site
• 34 22’ 6.78"N 106
32’ 11.83"W
• Slope sequences
followed transects
previously setup by
Esteban Muldavin
• Bedrock
–
–
–
–
schist
gneiss
vein Quartz
quartzite
Methods and
Materials
• Hill Slope
• Soil Characteristics
• Vegetation Community
Hill Slopes
• Slope Sequences
– 5 different north
and south facing
slopes
– 4 soil pits dug on
each slope
– Total of 40 pits
– Aspect of each
slope was
measured
Crest
Shoulder
Backslope
Footslope
Soil Characteristics
•Pits dug to bedrock
•Ped type, size, and
grade
•% Gravel
•Consistence: Dry,
Moist, WetStickiness, and WetPlasticity
•Texture
•Color
•Carbonate
development
•Depth
Vegetation
Community
• Line Transects
– 40 transects
completed
– 11 m transects
– Parallel to
slope position
– Basal area
and Canopy
measured
Analysis
• Graphed as a function of hill slope position:
– Soil depth
– Soil Texture
– Soil Color
• Compared soil depth and slope aspect to:
–
–
–
–
% Grass Basal area
% Canopy- Tree
% Canopy- Shrub
% Canopy- Grass
Soil Depth along Slope Sequence 1, Northfacing
Crest
Hill Slope
Position
Backslope
Shoulder
Footslope
0
5
Soil Depth (cm)
10
B Horizon
(cm)
15
20
25
30
A horizon
(cm)
35
Soil Depth
Along Slope
Sequences
40
45
50
Soil Depth along Slope Sequence 3, North-facing
Hill Slope Position
Crest
Shoulder
Backslope
Footslope
0
5
Soil Depth (cm)
10
15
20
B Horizon
(cm)
25
30
35
40
45
50
A horizon
(cm)
Slope Aspect for both North and South Facing Slopes Along Hill Slope Position
35
Aspect (%)
30
25
20
N-Facing Aspect
15
S-Facing Aspect
10
5
0
Hill Slope Position
Slope Aspect as a Function of A horizon Depth
for both North and South Facing Slopes
16
14
Soil Depth (cm)
12
10
North-facing
South-facing
Linear (North-facing)
Linear (South-facing)
y = 0.0633x + 6.518
R² = 0.0066
8
6
4
y = -0.0746x + 5.9707
R² = 0.1255
2
0
0
5
10
15
20
Aspect (%)
25
30
35
Soil Texture
A horizons
were
primarily
Sandy Loam
and Loamy
Sand.
B horizons
were
primarily Silty
Clay Loam,
Clay Loam,
and Sandy
Clay Loam.
Soil Texture Along Hill Slope Position
Slope
Sequence
Hill Slope
Posit ion
C res t
Slope
Sequence 1
S houlder
Bac k s lope
Foot S lope
C res t
Soil Texture
Along Hill Slope
Position
Slope
Sequence 2
S houlder
Bac k s lope
Foot S lope
C res t
Slope
Sequence 3
S houlder
Bac k s lope
Foot S lope
C res t
Slope
Sequence 4
S houlder
Bac k s lope
Foot S lope
C res t
Slope
Sequence 6
S houlder
Bac k s lope
Foot S lope
Horizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A horizon
B H orizon
A horizon
B H orizon
A horizon
B H orizon
A horizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A H orizon
B H orizon
A horizon
B horizon
A horizon
B horizon
A horizon
B horizon
A horizon
B horizon
Nort hFacing
S andy L oam
S andy
L oam
ilty C lay
L oam
S andy L oam
L oam
L oamy S and
C lay L oam
S andy L oam
S andy L oam
Sout h
Facing
loamy
s andy s
c and
lay
loam
loamy
s and
s lity c lay
loam
s andy
loam
ilty c lay
loam
loam
s andy c lay
loam
S andy L oam
C layey L oam
L oamy S and
S andy loam L oamy S and
L oam
L oamy S and
L oamy S and S andy L oam
C layey L oam
S andy L oam S andy L oam
C layey loam
S andy L oam S andy L oam
C layey loam
S andy L oam L oam
L oam
S andy L oam S ilty loam
S andy L oam S andy loam
S andy loam L oamy s and
S andy loam L oamy s and
S andy c layey
loam
L oam
C layey loam
L oam
L oam
C layey loam
L oam
S
ilty c layey L oam
loam
L oam
S andy L oam
C layey L oam L oam
Soil Color Along Hill Slope
Position
• A horizon color
• Gray Brown 7.5 YR 5/3
- Found at all hill slope
positions
• Light Yellow Brownish Gray
10 YR 7/2 and 8/2
- Found primarily at Crest
and Shoulder
• B horizon color
• Gray Yellowish Orange 10
YR 4/3
• Reddish Brown 5 YR 5/8
• Gray Yellowish Brown 10 YR
6/3
• Found at multiple hill slope
positions
Discussion of Soil
Characteristics
• Two different soil types
– Both North- and South-facing slopes
– Slope Sequences 2 and 4
– Slope Sequences 3 and 6 (and possibly 1)
• Fine soil material from wind (eolian)
source
– Leads to higher water holding capacity
– Enables establishment by vegetation
Slope Sequences 2 and 4
• Primarily A horizons
• Soil very thin
• Textures
– Sandy Loam
– Loamy Sandy
• Soil likely wind blown deposits
Slope Sequences 3 and 6
• Both North- and Southfacing slopes
• Soil very deep (>25
cm)
– Well developed soil
column
– Thin A horizon
– Thick B horizon
– Strong CaCO3 film and
filament development
• Soil likely much older
than soil on Slope
sequences 2 and 4
Discussion of Soil
Characteristics
•
•
“Soil horizon development as determined
by landform age…” (McAuliffe 1994).
Different soil horizons and types
– Landform same age over Los Piños
– How are there such distinctly different soil
types?
1. Change in climate
2. Change in depositional regime
3. Effect of grazing
Average % Grass Basal Area
Along Hill Slope Position
Average % Grass Basal Area vs. Hill Slope Position on
both North and South Facing Slopes
Average % Grass Basal Area
14
12
10
8
N-facing
S-facing
6
4
2
0
Crest
Shoulder
Backslope
Hill Slope Position
Footslope
% Grass Basal Area vs. Soil Depth
60
% Grass Basal Area, S-facing
% Grass Basal Area, N-facing
Soil Depth (cm)
50
Linear (% Grass Basal Area, Sfacing)
40
Linear (% Grass Basal Area, Nfacing)
30
y = 0.7483x + 9.427
R² = 0.1564
20
y = 0.0423x + 13.938
R² = 0.0003
10
0
0
5
10
15
20
25
% Grass Basal Area
30
% Canopy- Tree vs. Soil Depth
60
Soil Depth (cm)
50
% Canopy- Tree, N-facing
40
% Canopy- Tree, S-facing
30
y = 0.1678x + 10.285
R² = 0.0903
Linear (% Canopy- Tree, Nfacing)
Linear (% Canopy- Tree, Sfacing)
20
y = -0.2244x + 18.913
R² = 0.1576
10
0
0
20
40
60
% Canopy- tree
80
100
% Grass Basal Area as a Function of
Aspect for both North and South Facing
Slopes
35
30
Aspect (%)
25
20
North-facing
South-facing
Linear (North-facing)
Linear (South-facing)
15
10
y = -0.4051x + 11.024
R² = 0.2889
5
0
0
5
10
y = -0.5624x 20
+ 11.126
15
R² = 0.1279
-5
% Grass Basal Area
25
30
Observed Trends between
Vegetation Community and
Soil
• The vegetation community may have an effect
on the soil of both north and south facing slopes.
• % Grass Basal Area vs. Soil Depth
– May lead to deeper soils on South-facing slopes
– No correlation on North-facing slopes
• % Canopy of trees vs. Soil Depth
– May lead to deeper soils on North-facing slopes
– May lead to shallower soil on South-facing slopes
• % Grass Basal Area vs. Aspect
– Grass prefer to live on shallower slopes
Soil-Geomorphic Template
Conclusions
• Soil characteristics changed with respect to hill
slope position
• Soil depth and aspect are likely linked to
vegetation community
• Vegetation may have an effect on soil transport
• North and South facing slopes appear to behave
differently, likely due to the differences in
vegetation community, slope aspect, and
microclimate.
Acknowledgements
•
•
•
•
Les McFadden, Esteban Muldavin
Jennifer Johnson
Amaris Swann
Summer REUs: Shayla Burnett, Harmony Lu,
Rick Duran, Mitch Nakai, Cynthia Malone,
Natasha Ribiero, Melissa Shaginoff, Antonio
Nevarez, Amanda Labrado, Elida Iniguez
• Summer Interns: Brenda Nieto
• National Science Foundation
References
• McAuliffe RJ. (1994). Landscape Evolution, Soil Formation,
and Ecological Patterns and Processes in Sonoran Desert
Bajadas. Ecological Monographs, Vol. 64, No. 2: 111-148.
• Monger HC and Bestelmeyer BT. (2006). The soil-geomorphic
template and biotic change in arid and semi-arid ecosystems.
Journal of Arid Environments. 65: 207-218.
• Daniels RB and Hammer RD. Soil Geomorphology. Wiley
(1992): New York.
• Bridge SRJ and Johnson EA. (2000). Principles of Terrain
Organization and Vegetation Gradients. Journal of Vegetation
Science, Vol. 11, No. 1: pp. 57-70
• Francis CF. Plants on Desert Hillslopes. Edited by: Athol D.
Abrahams and Anthony J. Parsons. Geomorphology of Desert
Environments. Chapman and Hall (1994): London.
Questions?