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 WastingSoil 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 RefugeLos 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, Wet-Stickiness, and Wet-Plasticity•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

0

5

10

15

20

25

30

35

40

45

50

Crest Shoulder Backslope Footslope

Soil

Depth

(cm

)

Hill Slope Position

Soil Depth along Slope Sequence 1, North-facing

B Horizon (cm)

A horizon (cm)

Soil Depth Along Slope Sequences

0

5

10

15

20

25

30

35

40

45

50

Crest Shoulder Backslope Footslope

Soil

Depth

(cm

)

Hill Slope Position

Soil Depth along Slope Sequence 3, North-facing

B Horizon (cm)

A horizon (cm)

0

5

10

15

20

25

30

35

Aspe

ct (%

)

Hill Slope Position

Slope Aspect for both North and South Facing Slopes Along Hill Slope Position

N-Facing Aspect

S-Facing Aspect

y = -0.0746x + 5.9707R² = 0.1255

y = 0.0633x + 6.518R² = 0.0066

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30 35

Soil D

ep

th (

cm)

Aspect (% )

Slope Aspect as a Function of A horizon Depth for both North and South Facing Slopes

North-facing

South-facing

Linear (North-facing)

Linear (South-facing)

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 Position Horizon

North-Facing

South Facing

A Horizon Sandy Loam loamy sandB Horizon

sandy clay loam

A Horizon Sandy Loam loamy sandB Horizon

Silty Clay Loam

slity clay loam

A Horizon Sandy Loam sandy loamB Horizon Loam

silty clay loam

A Horizon Loamy Sand loamB Horizon Clay Loam

sandy clay loam

A horizon Sandy Loam Sandy LoamB Horizon Clayey LoamA horizon Sandy Loam Loamy SandB HorizonA horizon Sandy loam Loamy SandB HorizonA horizon Loam Loamy SandB HorizonA Horizon Loamy Sand Sandy LoamB Horizon Clayey LoamA Horizon Sandy Loam Sandy LoamB Horizon Clayey loamA Horizon Sandy Loam Sandy LoamB Horizon Clayey loamA Horizon Sandy Loam LoamB Horizon LoamA Horizon Sandy Loam Silty loamB HorizonA Horizon Sandy Loam Sandy loamB HorizonA Horizon Sandy loam Loamy sandB HorizonA Horizon Sandy loam Loamy sandB HorizonA horizon

Sandy clayey loam Loam

B horizon Clayey loamA horizon Loam LoamB horizon Clayey loamA horizon Loam LoamB horizon

Silty clayey loam

A horizon Loam Sandy LoamB horizon Clayey Loam Loam

Slope Sequence 1

Crest

Shoulder

Slope Sequence 2

Soil Texture Along Hill Slope Position

Crest

Shoulder

Crest

Shoulder

Back slope

Foot Slope

Back slope

Foot Slope

Back slope

Foot Slope

Slope Sequence 3

Crest

Shoulder

Back slope

Foot Slope

Slope Sequence 4

Slope Sequence 6

Crest

Shoulder

Back slope

Foot Slope

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 South-facing 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

0

2

4

6

8

10

12

14

Crest Shoulder Backslope Footslope

Avera

ge %

Gra

ss B

asal A

rea

Hill Slope Position

Average % Grass Basal Area vs. Hill Slope Position on both North and South Facing Slopes

N-facing

S-facing

y = 0.7483x + 9.427R² = 0.1564

y = 0.0423x + 13.938R² = 0.0003

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Soil D

ep

th (

cm)

% Grass Basal Area

% Grass Basal Area vs. Soil Depth

% Grass Basal Area, S-facing

% Grass Basal Area, N-facing

Linear (% Grass Basal Area, S-facing)

Linear (% Grass Basal Area, N-facing)

y = 0.1678x + 10.285R² = 0.0903

y = -0.2244x + 18.913R² = 0.1576

0

10

20

30

40

50

60

0 20 40 60 80 100

Soil D

ep

th (

cm)

% Canopy- tree

% Canopy- Tree vs. Soil Depth

% Canopy- Tree, N-facing

% Canopy- Tree, S-facing

Linear (% Canopy- Tree, N-facing)Linear (% Canopy- Tree, S-facing)

y = -0.5624x + 11.126R² = 0.1279

y = -0.4051x + 11.024R² = 0.2889

-5

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Asp

ect

(%

)

% Grass Basal Area

% Grass Basal Area as a Function of Aspect for both North and South Facing

Slopes

North-facing

South-facing

Linear (North-facing)

Linear (South-facing)

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?