Practicum Project | 2010

Edaphic Properties of Stable, Shrinking and Expanding Coastal Sage Scrub Boundaries (1990-2009) in Cheeseboro Canyon, Santa Monica Mountains National Recreation Area

Students: Monica Pacheco, Erin Mulberg, Suza Khy, Maggie Olsen, Andrew J. Price, Prodipto Roy, Eduardo Jimenez

Client: National Park Service, Santa Monica Mountains National Recreation Area

Advisor: R. Travis Brooks

The coastal sage scrub (CSS) vegetation of southern California is an ecologically important habitat that is threatened due to losses from urban development and type-conversion in adjacent wilderness by historic land use practices and decreased fire return intervals. Many areasthat have been historically grazed and subject to frequent burning have been replaced by a dominant non-native annual grass cover (NAG). Together CSS and NAG occur in vegetation mosaics in the landscape, including in Cheeeseboro Canyon, part of the Santa Monica National Recreation Area managed by the National Park Service. Although fire history has been documented to be an important consumer pressure on CSS communities, a prior photointerpretation study in Cheeseboro Canyon found that from the early 1920s to 2003, parameters of fire history did not directly correlate with the gain or loss of CSS cover (Eckhardt 2006). CSS-NAG boundaries may be responding to other environmental factors, including edaphic factors related to soil moisture availability. It is known CSS and NAG have differing soil hydrologic patterns, NAG being dryer that CSS due to plant water utilization (Davis 1985). Therefore, we posited that edaphic boundary conditions between CSS and NAG may be correlated with the relative stability of CSS-NAG boundaries. Over a 19-year period (1990-2009) we did not find that the relative stability of individual CSS boundaries (stable, expanding or shrinking) correlated with parameters of fire history; however, we found from a stratified random sample of the edaphic properties across these boundaries that soil texture was almost always different between the CSS and NAG sites. For differences between boundary stability types, we only found a significant difference between CSS and NAG across expanding boundaries at a depth of 0-20 cm (p<0.001). We interpret this finding to indicate that while soil texture alone may not consistently explain the trajectory of expanding or shrinking boundaries, the observation that soil texture was different across these vegetation boundaries suggests that it may be interacting with another unmeasured environmental factor, which is ecologically important in determining whether a particular site would promote or suppress CSS colonization. Future research and restoration projects should consider edaphic factors in attempting to understand the long-term stability of CSS communities in the landscape.