The Effect of Mechanical Disturbance on Soil Compaction and Soil Texture in
Coastal Sage Scrub
Karryssa Fenderson (Wilson High); Victoria Zamora (Polytechnic High); Marylynn (Wilson High); Jade Dean (LBCC); Dan Cardoza (CSULB); Jason Manack
(Polytechnic High); Jason Hazel (Polytechnic High); Christine M. Rodrigue (CSULB); and Dr. Paul Laris (CSULB)
Geoscience Diversity Enhancement Program (GDEP), California State University of Long Beach
Introduction
In 1784, the Spanish granted the first three land concessions
in California, one being Rancho San Pedro, to Juan José
Sepulveda (OAC 2009). Cattle and sheep ranchers of the
1920s and, later, farmers dominated the landscape, resulting
in the plowing of certain areas throughout the Peninsula. In
the early to mid twentieth century, Japanese farmers farmed
the southern slopes with such crops as beans and tomatoes,
and farmers of the northern slopes cultivated barley (Ranchos
Palos Verdes 2009). As a result, landscapes became barren
from overgrazing, and the plowing of native species
established the annual grasses and other exotics. Traditional
thought assumed that the weight of the plowing machines
and cattle would result in an increased soil compaction
(Brejda 1997). This study analyzed the differences in soil
resistance between sites of intact coastal sage scrub and
those dominated by exotic species on disturbed land.
graph courtesy of
Dr. Christine M.
Rodrigue
6
1
3
2
Methods
Quadrats Map Key
CSS Intact
Recent Brush
0.50
Cut
0.00
Restored
CSS Intact CSS
Lemonade Old Field Recent
Restored Berry
Brush Cut
CSS Intact
0.000
0.015
0.000
0.819
CSS Restored
5.790
0.005
0.293
0.000
Lemonade
2.451
2.883
0.000
0.054
Berry
Old Field
9.145
1.055
4.597
0.000
Recent Brush
0.229
4.832
1.945
7.657
Cut
ANOVA
F=
Dfb=
Dfw=
Prob=<0.001
Key
24.86
4.00
475.00
0.000
*Significant
relationships
are bolded
Re
ce
nt
Br
us
h
CS
S
Int
ac
CS
S
Quadrat Type
Cu
t
1.00
Ol
dF
iel
d
1.50
Berry
Old Field
Le
mo
na
de
Be
rry
2.00
Re
s to
red
Lemonade
t
2.50
Results
ANOVA produced a significant difference among the
penetrometer readings of the five quadrat types, meaning
the chances that particular differences among them were
random sampling error, was less than 5% (figure below).
Follow-up t-tests also showed that there were significant
differences among most pairs of quadrat types, shown in
bold.
CSS Restored
Soil Penetrometer Readings By Quadrat Type
Penetrometer Readings (kg/sq. cm)
Field Work: In a variety of sites on Palos Verdes Peninsula
(Fig. 1), Three Sisters (Fig. 2), and White Point (Fig. 3), 3 m by
3 m quadrats were assembled. These sites included: largely
intact coastal sage scrub (CSS), restored CSS, lemonade-berry
dominated CSS, old fields that had been plowed between the
1920s and 1940s, and areas that had been more recently
brush cut. The Durham Geo Pocket penetrometer was used
to measure the soil resistance in 30 separate readings, with 3
readings taken in each of 10 spots in the quadrats.
Lab Work: The compiled data from the field were then tested
for significant differences among the five classes of quadrats
as a group, using ANOVA. This was followed by t-tests of all
possible pairs of vegetation types. Probability values on
these tests that fell below 0.05 determined if there was a
significant difference.
Prairie
Old Entrance
4
Mustard
(Unplowed)
5
Discussion
Soil penetrometer readings tests among the five classes of quadrats
creates puzzling results (Fig. 4 and 5). CSS Intact and Recent Brush Cut
quadrats proved to be most similar by the highest soil compaction (kg/sq.
cm) means obtained; CSS Restored and Old Field also showed similarities
by having the lowest compaction readings. Lemonade berry was an
intermediate between the two extremes. Our expectation was that the
two categories of heavily disturbed vegetation (Old Field and Recent Brush
Cut) would share higher penetrometer readings due to compaction
associated with such disturbance (Brejda 1997). We expected CSS quadrats
to share lower penetrometer readings.
We attempted to investigate other characteristics of the soil to clarify
these results by returning to the field to collect soil samples from 15 sites:
Our original 10 sites in Portuguese Bend, and an additional 5 from Three
Sisters and Filiorum to the east of PB. Using the British Rural Development
Service (BDRS 2006) method, we identified the textures of the 15 samples
brought back to the lab (Figure 6). Although the samples were too small to
analyze statistically, the CSS quadrat samples do seem to cluster on welldrained loamy soil types and exotic-dominated quadrats on heavy clayey
soils. Zamora et al. (2009) supported these results with a grain size analysis
on 6 soil samples taken from the original 10 sites.
References
Conclusions
The associations among mechanical disturbance, such as grazing and disking, and soil compaction are not as straightforward as
literature led us to expect. Other factors must be important on the Palos Verdes Peninsula, such as the underlying geology,
geochemistry, and topography differences that can exist among the soil types these factors can create. Analysis of soil texture (Fig. 6)
suggests that it may be an important factor affecting with vegetation, disturbance history, and compaction.
The vegetation communities we see in the Palos Verdes Peninsula today may reflect the land’s potential for agriculture. Farmers may
have avoided the more compacted and/or rockysoils of CSS. An opening for the introduction of exotic species, such as Brassica nigra
(black mustard) and Foeniculum vulgare (fennel) may have been created when the farmers selected softer and deeper soils for their
fields. It is possible that the breaking up of the soil by plowing it for agriculture destroyed shrub roots of CSS, and thereby allowed
annual grasses and other exotics to invade. The low soil compaction readings of restored CSS in comparison with intact CSS may have
been a result of the areas having been used for agriculture because of their softer soils and then later restored.
Our study of the effects of the compaction did not directly reveal the reasons for those results, but requires additional research and
analysis. Suggested work could focus on the effects of plant communities on a landscape over time, and the effects of the geology of
the landscape on its soil conditions. Further work is needed to isolate each possible factor and its effects. Because the native cover of
the peninsula has been significantly decreased due to disturbance, the understanding of the differences in soil compaction may be
important for future knowledge to sustain native species.
Brejda, John J. 1997. Soil Changes following 18 Years of protection from Grazing in
Arizona Chaparral. The Southwestern Naturalist. 42, 4: 478-487.
British Rural Development Service (BRDS). 2006. Soil texture. Technical Advice Note 52.
Online Archive of California (OAC). 2009. Rancho San Pedro Reference Collection, 19022004. <http://www.oac.cdlib.org/findaid/ark:/13030/kt3s20214f/.>
Ranchos Palos Verdes: The Birth of a City."The City of Ranchos Palos Verdes. 2009. The
City of Ranchos Palos Verdes. 31 Jul 2009
<http://www.palosverdes.com/rpv/news/content/City_history.cfm>.
Zamora, Victoria; et al. 2009. Disturbia: The real dirt on the Palos Verdes Peninsula.
GDEP Symposium.
Acknowledgements
A special thanks to Geoscience Diversity Enhancement Program at Cal State University of
Long Beach for funding this experience through the National Science Foundation Award
#0703798. I would like to thank Dr. Christine M. Rodrigue and Dr. Paul Laris for their
direction and, the assistance from graduate student Daniel Cardoza and high school
science teachers Jason Manack and Jason Hazel, in the field and the lab. Thank you to
Ann Dalkey at the Palos Verdes Land Conservancy, as well as Friends of the Colorado
Lagoon and the Bolsa Chica Land Conservancy. Furthermore, the peer support and
assistance of Victoria Zamora, Jade Dean, and Marylynn Roun were very much
appreciated.