A Comparison of Pampas Grass Cortaderia jubata Coyote Bush Bacharis pillularis and Poison Oak Toxicondendron diversilobum on Soil Cohesion in Big Sur California Alexander Ford Abstract An infestation of pampas grass along the California coast has displaced native plants This change in vegetation has dramatically effected the local soil s stability and cohesive properties Previous studies in the Oregon Coast Range show sharp declines in soil stability after the introduction of new species If soil cohesion is weakened by the invasion of pampas grass land sliding may occur more frequently Root tensile forces rooting depth and root morphology of pampas grass coyote bush and poison oak were measured to establish each species contribution to soil stability The roots of native coyote bush and poison oak have penetration depths of 3 5 meters root diameters of 2 4 cm and nodular rooting morphologies The roots of invasive pampas grass have an average penetrance of 1 3 meters an average diameter of 0 3 cm and a radially symmetric rooting morphology Roots act as fibers to reinforce the soil and as such are important to soil stability The number of roots per unit area for coyote bush is on average 4 roots m2 Poison oak has an average 15 roots m2 Pampas grass has 60 roots m2 To adequately compare cohesion added by each species a slope stability model was applied to the data gathered This model predicts cohesion values for soils based on three main parameters root tensile strength root diameter and number of roots per unit area Based on this model coyote bush contributes 0 82 Kpa poison oak contributes 0 78 Kpa pampas contributes 1 34 Kpa cohesion Introduction Pampas grass Cortaderia jubata has infested the California coast from San Diego to north of Mendocino Lambrinos 2001 C jubata is a major pest along the Big Sur Coastline where it has colonized the steep ocean side cliffs and road cuts Native vegetation has evolved to live in the rapidly uplifting and frequently sliding terrain of Big Sur Pampas grass invasions displace native vegetation D antonio 1991 possibly changing the stability of these slopes C jubata s shallow roots may not penetrate deeply enough to add the soil strength a deep rooted native would Landscapers in Big Sur use C jubata to stabilize steep or loose slopes La Loma Landscaping No scientific study has been performed to assess the merits of such practices Steep cliffs along the Big Sur Coast roads may become unstable when infested with C jubata endangering motorists The purpose of this study is to determine how pampas grass affects soil cohesion in Big Sur The working hypothesis is that pampas grass contributes less soil cohesion than the native species it displaces Native species such as coyote bush Baccharis pillularis and poison oak Toxicodendron diversilobum are found growing within and around pampas infested areas on steep slopes in Big Sur Until now no data has been published about their impact on soil cohesion A major paper by Schmidt et al 2000 documented a significant loss of soil cohesion as a consequence of clear cutting forests in Oregon Soil root interactions affect slope stability Wu 1976 According to the Mohr Coulomb slope failure criteria Selby 1983 1 Ssr cs cr tan In this equation Ssr is the soil root shear strength cs is effective soil cohesion cr is the cohesion added by roots is the normal stress from the weight of soil and water is the soil pore water pressure and is the effective internal friction angle of the soil Soil root shear strength is directly proportional to root cohesion This means a soil with high root cohesion will increase the soil shear strength adding to slope stability Roots of plants increase soil cohesion by binding to soil particles Waldron 1982 The number depth size and growth patterns of roots affect the soil cohesion Wu 1976 A few models quantify the cohesion added to soils from roots Schmidt et al 2000 employ the model 2 cr 1 2 Tri Ari As where cr is the cohesion added to soil Tri is the root breaking tensile strength and the rootarea ratio Ari As is root cross sectional divided by area of exposed soil The root area ratio is included in this model to address the variation in number size and distribution of roots for each species The focus of this research is on that of cr the strength added to soil from roots To address this tensile strength root diameter and root density were measured for various species Materials and Methods Site Description The study site is located 10 miles south of Monterey in a steep southfacing swale two miles up Palo Colorado Canyon Big Sur The colluvium in the hollow is a loose aggregate of small 1 20mm xenolithic angular rocks covered with half a meter of rich topsoil Native vegetation dominated the landscape until 8 years ago when a landscaping company planted C jubata to stabilize the slope below a residential driveway Down slope of the initial planting a dense population of C jubata now exists Procedure A large flat shovel a potting shovel two meter sticks pruning shears a Piesola tensiometer 0 40 kg vice grips wire calipers 0 10 cm and a cotton towel were all used to perform the following procedures On November 17 2002 eight cross sections of soil 1m wide and 1m deep were excavated perpendicular to the slope face and centered on the plant of study Excavations were done on previously exposed landslide scarps All exposed roots 1mm were tested Each root was clamped with vice grips and pulled on with the Piesola scale Tensile strength sampling procedures were performed according to Schmidt et al 2000 However to prevent root breakage from the vice compression and better simulate the soil s pull a small towel was wrapped around the root at the place of vice grip attachment thereby distributing the clamping force over a larger surface area Root breakage was generally near the soil root interface The maximum force reached before breakage was recorded Diameter of root at the point of breakage was measured using calipers Fifteen individual root measurements were taken on two plants of each species All roots from each cross section were identified Rooting depth was then measured perpendicular to the soil surface with a flexible metal measuring tape Root morphologies were sketched photographed and then classified according to branching pattern and symmetry Results Pampas Grass Poison Oak Coyote Bush Mean Root Strength 6 Kn 13 Kn 18 Kn Mean Root Diameter 0 3 cm 1 2 cm 3 5 cm Mean Root Density 62 roots m2 15
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