Transcript 2009 poster - California State University, Long Beach

How Does Disturbance Type and Frequency Affect
Coastal Sage Scrub Recovery?
Jade Dean1, Karryssa Fenderson2, Marylynn Roun2, Victoria Zamora3, Daniel Cardoza1, Jason Manack3, Jason Hazel3, Brian Nagy1, Dr. Paul Laris1, Dr. Chrys Rodrigue1
Geosciences Diversity Enhancement Program, California State University Long Beach
1: California State University Long Beach, 2: Wilson Classical High School, 3: Long Beach Polytechnic High School
Discussion
Introduction
According to the data, plowing has the greatest and most negative impact
on CSS. It also has the most enduring impact; even a site that had been
plowed once over 90 years ago remains devoid of native plant cover
today. High fire frequency had the second greatest impact; however, the
frequently burned site still had intact patches of CSS suggesting that CSS
was either recovering or at least not completely destroyed by fire. Finally,
grazing alone appears to have the least negative impact on CSS. Although,
the three sites studied had a greater variation in diversity and percent of
native plant cover. For example, one site was dominated by mustard
(invasive), another by Lemonadeberry (native), and a third by a relatively
even mix of several native species (See Table 3).
The results support the conclusions of Davis (1994) and Hobbs (1983),
who found that mechanical disturbance has a long term and negative
impact on CSS. They also support the findings of Keeley and colleagues
(2005), who suggested that high fire frequency can result in the
replacement of CSS with nonnative annuals. In contrast to the findings of
Zink and colleagues (1995), there was no evidence that nitrogen levels
affected native plant cover. However, the tests performed to determine
nitrogen levels were suspect.
Although numerous studies find that CSS plants will recover after
disturbance from fire or overgrazing, a few studies have compared the
recovery rates of different disturbances. This study found that over 50
years after the end of grazing, there were obvious signs that native plants
were returning. Approximately 30 years after being subjected to a series
of fires, CSS has not been removed from the landscape. On the other
hand, this study finds that areas that mechanical disturbance has the
longest lasting and most degrading impact on CSS. Indeed there is little
evidence that native plants will ever recover on plowed sites. It is possible
that mechanical disturbance alone is not responsible, but that the
combination of plowing, soil texture, and slope prohibits native plants
from reestablishing.
California Coastal Sage Scrub (CSS) is one of the most threatened habitats
in the world. At least seventy percent of CSS has been lost and what
remains is under intense pressure from land developers and variety of
disturbances. Indeed, much of the remaining CSS is partially degraded
due to a variety of different human disturbances. Understanding the
combined impacts of anthropogenic disturbances and their long term
consequences is of great interest to land managers whose goal it is to
restore and manage native plant habitats.
Disturbances are characterized in terms of a disturbance regime, which is
defined in terms of type, frequency, severity, and extent (Laris 2009). It is
well documented that at least three disturbances are responsible for the
decline of CSS and the subsequent invasion of nonnative grasses and
forbes; these are grazing, fire, and nitrogen deposition. It is well known
that high fire frequency, intensive livestock grazing, and high levels of
nitrogen deposition favor invasive annuals over native CSS (Zink 1995;
Talluto and Sundig 2008). It has also been shown that a reduction in fire
frequency or grazing has resulted in recovery of CSS, yet there are large
areas in Southern California where CSS appears to be prevented from
reinvading nonnative grasslands (Hobbs 1983; Eckardt 2006). A fourth
and less studied factor is mechanical disturbance or plowing (Hobbes
1983; Davis 1994). It would be useful for those attempting to restore
degraded CSS habitat to understand the long term effects of different
disturbance regimes.
The objective of this study was to investigate the long term impacts of
three different disturbances: grazing, fire, and mechanical plowing or
disking on the Palos Verdes Peninsula. The study seeks to understand if
the type of disturbance, in addition to its intensity and frequency, affects
the recovery of CSS. Preliminary research during GDEP 2008 suggested
that mechanically disturbed areas have very low recovery rates for CSS,
but less is known about recovery from fire. This study hypothesized that
mechanically disturbed areas and those with high fire frequencies would
have the lowest recovery rates of all disturbance regimes.
Conclusion
Methods
The study area is located on land managed by the Palos Verdes Peninsula
Land Conservancy (PVPLC) shown in Figure 1. In order to isolate areas
with different disturbance histories, maps were generated using three
sources: historical aerial photographs (Ferris 2008), a dataset on fire
perimeters from the Fire and Resource Assessment Program (FRAP), and
historic photographs and documents of land use (See Table 1). The history
of mechanically disturbed areas was created by visually interpreting plow
marks in the aerial photographs. Fire frequency maps were derived from
the FRAP data using a GIS.
We selected areas with known high and low fire and plowing frequencies.
Fire frequency varied between 1 and 4 and plow frequency varied
between 0 and 4. Areas with the highest fire frequency and lowest return
interval (4 fires in a decade) were selected for the fire study. All areas
were assumed to have been grazed heavily prior to 1940 based on ground
and aerial photographs (See Figure_). We then used a GPS to navigate to
study sites and used a 3x3 m quadrat to gather data on vegetation type,
soil properties and general topography (See Table 1, Figure _).
Due to the topography of the peninsula, there was a large degree of
distortion in the aerial photographs. To correct this issue, photographs
were georectified in ArcMap using at least 30 ground control points per
image and a 3rd order polynomial equation with the Nearest Neighbor
option. The RMS error for all images was less than 20. Using the 2006
orthorectified image, each subsequent aerial photograph was re-sampled
to fit the orthorectified image. To map plowed areas, polygons were
created using on-screen digitizing in rcMap. Polygons were then combined
in a GIS to create a frequency map. To create the fire frequency map, fire
perimeter data was summed for the study area and transformed in a GIS.
8/7/09
Table 1, Historical Data Sources and Field Data Sources.
Historical Data
Field Data (10 Quadrats)
Aerial Photographs, 1928, 1945, 1963, Vegetation: Species Percent Cover
1972, 1980, 2006 (Satellite Image)
FRAP Dataset, 1936-2005
Soil: Compaction, Texture, Nitrogen
Historical Photographs circa 1930s
Topography: Slope
Table 2, Summary of Field Data.
Variables
Plowed
Native Plant Cover (%)
3
Soil Compaction (kg/sq-cm) Low (0.56)
Nitrogen
Trace
Slope
Relatively Flat
Soil Texture
Silty Clay Loam
High Fire
13
Low (0.56)
Trace
Fairly Steep
Sandy Silty Loam
Only Grazed
56
Low (1.6)
Trace
Varied
Varied
Table 3, Common Native and Nonnative (Invasive) Species found at PVPLC
Sites.
Native Species
Nonnative (Invasive) Species
Coastal Sage Scrub (Artemisia
californica)
Lemonadeberry (Rhus integrifolia)
Black Mustard (Brassica nigra)
California Sunflower (Encelia
californica)
Purple Sage (Salvia leucophylla)
Fennel (Foeniculum vulgare)
Ashy Leaf Buckwheat (Eriogonum
cinereum)
Wild Oats (Avena fatua)
Field Mustard (Brassica rapa)
Ripgut Brome (Bromus diandrus)
Results
A summary of the results if shown in Table 2. As the table indicates, there is a dramatic difference between with the percentage of native
plant cover in the areas that have a high fire frequency or were plowed, than those that were only grazed. Soil compaction was relatively low
regardless of disturbance history. Soil samples from all sites had very low nitrogen levels (trace). Not surprisingly, plowed areas were
relatively flat and had a soil texture of silty clay loam. The areas that had a high fire frequency were located on steeper, rockier slopes and
had a soil type of sandy silty loam. The “Only Grazed” sites varied between relatively flat and fairly steep with a variety of soil texture.
As seen in Chart 1, there is a higher diversity of species and a higher percentage and number of native species on unplowed plots. A summary
of the most common native and nonnative species is shown in Table 3. Indeed, there were very few native plants observed on plowed areas.
Conversely, fire frequency does not seem to be a key determinate of overall species diversity or native species count; there were only two
more species found on plots with low fire frequency (See Chart 2).
The number of times a site was plowed appears to have no impact on percent native plant cover. All plowed sites had extremely low native
plant cover, including the site that had been plowed only once in 1928 (PHOTO of paul on trail). Sites with the lowest native plant cover were
also relatively flat sites with silty clay loam. In general, more native plant cover was found on rockier soils.
This study found that the type of frequency does affect CSS recovery.
Plowing has the greatest long term impacts of all disturbances. It also
finds that frequency of disturbance is important when considering fire,
but that a single plowing event is sufficient for converting CSS into
nonnative grassland. Therefore, that habitat restoration specialists should
classify previously plowed areas as sites for intensive restoration efforts.
Although this study finds that plowing has the greatest impact, plowed
areas correlate with areas that have a gradual slope and silty clay soils.
Therefore, more research needs to be done to examine the possible
relationship between these three factors, perhaps in areas with a less
complex disturbance history. Finally, the mechanism(s) by which CSS is
prevented from recovering remains undetermined. Perhaps a better
understanding of the succession of CSS plants and the alleopathic
properties of invasives, particularly Brassica spp..
Acknowledgements
I would like to thank: The National Science Foundation Award #0703798
for funding GDEP Track 2, the Palos Verdes Peninsula Land Conservancy
for allowing us to use their land, Brian Nagy for technical support, Jason
Manack for creating the maps, and Drs. Paul Laris and Chrys Rodrigue for
helping me with my project.
References
Davis, C.M. 1994. Successional changes in California shrub communities following mechanical anthropogenic disturbance. M.S.
Thesis. San Diego State University.
Eckardt, S. W. 2006. Assessment of Wildfire Frequency in Coastal Sage Scrub Vegetation Dynamics in the Santa Monica Mountains
of Southern California. M.A. Thesis. California State University Long Beach.
Hobbs, E.R. 1983. Factors Controlling the Form and Location of the Boundary Between Coastal Sage Scrub and Grassland in
Southern California. Dissertation. University of California, Santa Barbara.
Keeley, J.E., Fotheringham, C.J., Baer-Keeley, M. 2005. Determinants of postfire recovery and succession in Mediterranean-climate
shrublands of California. Ecological Applications, 15:1515:1534.
Talluto, M.V. and Suding, K.N. 2008. Historical change in coastal sage scrub in southern California, USA in relation to fire frequency
and air pollution. Landscape Ecology, 23:803-815.
Zink, T.A., Allen, M.F., Heindl-Tenhunen, B. and Allen, E.B. 1995. The effect of a corridor on an ecological reserve. Restoration
Ecology, 3:304-310.