How Does Disturbance Type and Frequency Affect Coastal Sage Scrub Recovery?

1. 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 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.. 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. 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. 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. 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