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Plant Recruitment and Plant Succession as the Major Soil Forming Factor for Young Terraces

in the Pino Wash during the Late Holocene

EPS 485L 001 Soil Stratigraphy and Morphology

Dr. Leslie McFadden University of New Mexico

Melanie E Magdalena 05 December 2013

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INTRODUCTION

Hans Jenny integrated the CLORPT approach into soil studies, or pedology. His

methodology for the study of soils entails the recognition of several factors, including climate

(CL), vegetation or organics (O), relief (R), parent material (P), and time (T), which must be

considered to interpret the development of soils. Fluvial processes and erosion have modified the

landscape producing new geomorphic features, such as terraces. During the late Holocene,

climate and vegetation in New Mexico have changed, and these changes likely affected soil

development. Plant recruitment and plant succession are linked to the “O” factor, and may be a

major soil forming factor influencing soils young terraces during the late Holocene. The purpose

of this study is to analyze the soils and vegetation communities of two terraces of the Pino Wash

to determine if plant recruitment and succession play a role in soil development during the late

Holocene.

STUDY AREA

Pino Wash lies on the western flank of the Sandia

Mountains and along the eastern edge of the Albuquerque

basin and Rio Grande rift (Connell & Love, 2009). The area

is characterized by ridges and valleys. The higher topography

breaks down due to weathering. Sediment is swept down the

slopes and it accumulates on the floodplain and alluvial fans.

These surfaces represent geomorphically stable areas where Figure 1: Granite is one of the

soil formation is favored. primary parent materials of the study area.

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Figure 2: Elena Gallegos Regional Open Space, NM, entrance to Pino Trail.

Figure 3: Location of the study area along the Pino Wash Trail at Elena Gallegos Regional Open Space, NM.

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Rocks in the Sandia Mountains include Phanerozoic sedimentary and Proterozoic

crystalline rocks (Connell, 1997). One of the most important and widespread rock type in this

area is granite, which forms as a plutonic rock well below the Earth’s surface when magma

solidifies. Minerals in granite strongly influence soil formation: feldspar, biotite, and quartz. The

second parent material in the area is limestone. Formed from calcite­rich organisms 300 million

years ago, New Mexico was submerged under ocean water (Connell & Love, 2009). Limestone

is rich in calcite, a mineral which effervesces when exposed to hydrochloric acid. Calcium

carbonate in arid soils crystallizes, from a dissolved form, as soil dries (Monger, 1991).

The study area has a warm, arid to semi­arid climate (Hacker, 1977). The mean annual

temperatures range from 4°C (~40°F) at the Sandia Crest, 10°C (~50°F) at the foothills, and

14°C (~57°F) in Albuquerque (Connell, 1997). Precipitation is concentrated in the winter;

moisture is supplied by the Gulf of Mexico circulation (Hacker 1977). Mean annual precipitation

is 178 to 254 mm (7 to 10 in) in Albuquerque, 254 to 356 mm (10 to 14 in) in the foothills, and

635 to 762 mm (25 to 30 in) in the mountains (Connell, 1997).

Vegetation, though sparse, consists of grasses which acidify the soil water (Bouteloua

gracilis, Bouteloua eriopoda, Bouteloua curtipendula, and Hilaria jamesii), juniper (Juniperus)

and piñon (Pinus edulis) trees, shrubs (Cercocarpus montana and Fallugia paradoxa), and cacti

(Opuntia polycantha). Piñon­juniper (PJ) woodland increases in density with elevation (Pieper,

2008). This plant community is typical of areas with 30­35 cm of rainfall a year. The PJ

community is also typical of semi­arid climate with an elevation of four to six thousand feet.

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Figure 4: Terrace plant communities (pre­Winter) within Pino Wash, Elena Gallegos Regional Open Space, NM.

In the foreground, Terrace 2, inset into an older terrace, has a visible plant community, primarily small­leaf shrubs.

There are three terraces in the study area; the surface in the far background is not included in this study.

inset into Terrace 1. The terraces are part of a fluvial landscape ­­ associated with river

and stream processes ­­ which when active can carry very large sediment loads, including

boulders during rare large flow events. The height of the terraces is related to their age: older

terraces occur at a greater height above the active channel due to a longer period of exposure to

incision by fluvial processes.

A total of three pits were excavated in Terrace 1 and one pit in Terrace 2. In order to gain

an accurate depiction of the soils in Terrace 1, test pits were placed along the flat top at

approximately 10m intervals. Terrace 2 is small, so one pit was placed at the center under the

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assumption that it is the least altered portion of the terrace away from any active flow in the

channel (the sides are weathering away). Pit depths ranged from 80 cm to 1 m.

Soil profiles were described using the Soil/Stratigraphy Profile Worksheet, provided by

UNM Office of Contract Archaeology. Soil horizons were determined on the basis of structure

(type, size, and grade), Munsell color (dry and moist), percentage of gravel, consistence (dry and

moist), plasticity and stickiness when wet, texture, clay films, and carbonate morphology. Depth

for the upper and lower boundary were recorded, including distinctness and topography for the

lower boundary. Vegetation around the pits were recorded. Clayey argillic horizons of

well­developed soils can hold more water than sandy layers, which can influence to the plant

community: redistribution of water allows the growth of shrubs and trees rather than only

grasses. (McAuliffe). Along with parent material, these two factors have a large impact on soil

development.

Figure 5: Small­leaf plants from both terraces studied at Pino Wash, Elena Gallegos Regional Open Space, NM.

On right, specimen is found on both terraces. On left, specimen is from Terrace 1.

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Terrace 1 is dominated by juniper and pinon trees and large boulders ranging from the

size of a fist to half a meter in diameter, primarily granite. The surface color (dry) is 10YR 2/1.

Terrace 2 has a plant community dominated by small­leaf shrubs and granite gravel. The surface

color (dry) is 10YR 4/3. Facing south, there is a third terrace, not included in this study, which

has a grass­dominant plant community and a thick, visible Bk horizon.

RESULTS

Terrace 1

Located at 2035 m above sea level, Terrace 1 is dominated by large piñon and juniper

trees and spotted with prickly pear cacti, yucca, and moss. Soil pits were spread out across the

top three meters apart in a line. Avoiding large granite boulders and wildlife, at the center was

the one meter pit flanked by 80 cm pits on either side. As seen in Tables 1, 2, and 3, TA has an O

horizon 10YR 2/1 in color (both dry and moist) with fine­grained subangular blocky structure.

The soil is very friable when moist and has slightly sticky and slightly plastic consistency. The

horizon ends smoothly and abruptly at 3 cm.

The easternmost pit “Wonky” (see Table 1) has an A/C horizon over a C/A. Coarse­

grained and granular structure throughout, neither of these horizons is sticky or plastic. Both wet

and dry, they retain a non­coherent consistency and are more gravelly than sandy. Gravel

percentage is around 80% and there is no evidence of clay films or visible carbonate reactions

with hydrochloric acid. The loamy sand A/C horizon starts at 3 cm and ends gradually with a

wavy lower boundary at around 70 cm; it has a color of 7.5YR 3/3 dry and 7.5YR 3/2 wet. The

C/A horizon is 5YR 4/4 dry and 5YR 3/2 wet, starting at around 70 cm. Granite boulders and

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roots prevented digging past 80 cm. With depth, the amount of fine­grained material increased,

likely silt, and the undersides of rocks at least 5 cm in size had visible reactions to HCl on their

undersides.

The westernmost pit “Monkey” (see Table 2) was named due to the extensive amount of

maneuvering through roots and tree branches while digging. Located directly next to a juniper

tree trunk, this pit was 85 cm in depth with an A/C horizon over a B horizon. Starting at 3 cm,

the A/C horizon is identical to the A/C horizon at “Wonky” in every aspect except color. It is a

loamy sand with a gradual and wavy lower boundary. It has a color of 10YR 3/4 dry and 10YR

3/3 moist. A reddish­yellow silt is prominent and well­mixed into the 80% gravel dominated

coarse­grained granular horizon. Beneath the A/C, at 65 cm, is a sandy clay B horizon colored

7.5YR 4/3 dry and 7.5YR 2.5/2 moist. The structure of Bb is fine­grained subangular blocky

with no visible clay films. It is friable when moist and slightly plastic when wet.

The pit at the center named “Rocky” (see Table 3) had four identifiable horizons beneath

the 3 cm O horizon. A mere 2 cm was identified as an A horizon composed of coarse­grained

granular sand with a small amount of silt. The A horizon is 80% gravel with no coherence,

stickiness, plasticity, clay films, or carbonates. The lower boundary is smooth and abrupt. The A

horizon is 7.5YR 4/3 (dry) and 7.5YR 3/4 (moist) in color. The A/C horizon has about 50%

gravel, has a 7.5YR 3/4 (dry) and 7.5YR 3/3 (moist) in color, with a silty texture increasing with

depth. In terminates with an abrupt lower boundary. The C/A horizon is over 70% gravel, 7.5YR

3/4 (dry) and 7.5 YR 3/2 (moist) in color, with more silty texture than A/C. A weakly developed

Bb horizon lies beneath the churned A and C horizons. At 68 cm, the silty clay has less than 10%

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Figure 6: Terrace A test pit “Wonky” Figure 7: Soil samples from Terrace A test pit “Monkey”

gravel, 7.5YR 3/4 (dry) and 10YR 3/2 (moist) in color. It is weakly coherent (dry) and friable

(moist). The soil is sticky and slightly plastic with no evidence of clay films. Igneous rocks

encountered have calcium carbonate coatings but peds have no visible effervescence.

Terrace 2

Located at 2028 m above sea level, Terrace 2 is barely noticeable. The small­leaf shrubs

and prickly pear cacti obscure it from view. The surface is immediately different. Test pit #4, or

“Zonky” (see Table 4), revealed little to no soil development. Terrace 2 is composed of

weathered granite. Krotovina (see Figure 9) mix the finer­grained granite with coarser grains and

large granite pieces (see Figure 8).

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Figure 8: Granite gravel parent material. Figure 9: Close up of test pit “Zonky” soil profile.

Figure 10: Soils recovered from Terrace 2 test pit “Zonky.”

color changes (see Figure 10): C1 (0­10 cm) is 10YR 4/3 (dry) and 7.5YR 4/3 (moist), C2 (10­50

cm) is 10YR 5/4 (dry) and 7.5YR 4/4 (moist), C3 (50­70 cm) is 10YR 5/3 (dry) and 7.5YR 5/3

(moist), and C4 is 10YR 6/3 (dry) and 7.5YR 4/3 (moist). A total of two non­granite rocks were

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recovered from C3 and C4, each having a visible effervescent reaction. Around the test pit were

many large pieces of granite and very little evidence of any other possible parent material

present.

DISCUSSION & CONCLUSIONS

A semi­arid ustic regime can form A horizons in several decades or centuries. Currently,

Terrace A has an incipient A horizon, bioturbated with the parent material (C horizon). Prior to

the active paleochannel, which incised the study area, a developed soil was near the surface. This

clay­rich soil is fertile and the larger plants grow where it is located. Juniper trees were

concentrated on Terrace A and none were on Terrace B. This interpretation of the observed

abundance of active vegetation on the terrace with a water­rich soil is supported by studies J. R.

McAuliffe (2000).

Active transport in the channel of large sediment loads and boulders are easily depositing

weathered granite from the eroding pediment to the east onto Terrace 2. Terrace 2 lightens with

depth suggesting soil forming processes are at work but not enough time has passed to see any

development with the unaided eye.

Parent materials affect what plant communities can thrive in the area. Climate influences

the magnitude of mineral breakdown in the parent materials. The fluvial deposits composed

mainly of granitoid material is breaking down into soil and has provided a home for plant

communities requiring less moisture. The buried soil is providing extra water for larger plants,

such as the PJ community. The presence of grasses and shrubs only on Terrace 2 is evidence that

this plant community can adapt to the lack of soil development. The clayey­rich Bb horizon on

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Terrace 1 may mean that extra moisture in the soil permits this plant community’s success.

Future research should look for Terrace 1’s Bb as a possible match in the older terrace

across the channel and study the plant community to see if the tendency for increased plant

abundance and size continues. Research may reveal that the primary factor influencing plant

success is time rather than the presence of clayey rich soils. This is not to mean that increased

moisture is not beneficial, but that there may be multiple factors at play. Future study of the

terraces and more in depth study of the deeper soils and plant communities on terraces at Pino

Wash will increase the collective knowledge available to the scientific community about the late

Holocene.

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REFERENCES

Birkeland, P.W., 1999. Soils and Geomorphology: New York, Oxford University Press, 429 p.

Connell, S. D., 1997. Geology of the Alameda 7.5­Minute Quadrangle, Bernalillo and Sandoval

Counties, New Mexico. New Mexico Bureau of Mines and Mineral Resources, Open­File

Report DGM­10, 11p.

Connell, S. D., Love, D., 2009. Geology and Landscape of the Albuquerque Region. in Greer

Price, L., Bland, D., Johnson, P. S., and Connell, S. D. eds., Water, Natural Resources,

and the Urban Landscape: The Albuquerque Region. New Mexico Bureau of Geology

and Mineral Resources. p. 10­17.

Hacker, L.W., 1977. Soil survey of Bernalillo County and parts of Sandoval and Valencia

County, New Mexico: U.S. Department of Agriculture, Soil Conservation Service.

Jenny, H., 1994. Factors of Soil Formation: A System of Quantitative Pedology. New York,

Dover Publications, Inc., 191 p.

McAuliffe, J. R., 2000. Desert Soils. A Natural History of the Sonoran Desert. University of

California Press/ASDM Press.

Monger, H. C., et al, 1991. Microbial precipitation of pedogenic calcite. Geology 19. p.

997­1000.

Pieper, R. D., 2008. Ecology of Piñon­Juniper Vegetation in the Southwest and Great Basin.

Ecology, Management, and Restoration of Piñon­Juniper and Ponderosa Pine

Ecosystems: Combined Proceedings of the 2005 St. George, Utah and 2006 Albuquerque,

New Mexico Workshops. U.S. Department of Agriculture, Forest Service: Proceedings

RMRS­P­51. p. 3­10.