Temporal and spatial linkages between watershed land use and wetland vegetation response in the Elkhorn Slough watershed, Monterey County, California.

摘要

In Elkhorn Slough, California off-farm erosion has led to sedimentation in the slough and growth of sediment fans that filled salt marsh over several decades. This dissertation investigated relationships between watershed land use, disturbance processes, and ecological response in Elkhorn Slough over multiple spatial scales, temporal scales and frequencies through the use of remote sensing, GIS, and field sampling.; Historical aerial photo change detection revealed a shift in wetland species in response to sedimentation. Sedimentation caused arroyo willow ( Salix lasiolepis) encroachment into pickleweed-dominated ( Salicornia virginica) salt marsh on 11 sediment fans, with willow gain of 4.75 ha and salt marsh loss of 2.34 ha. A multiple-decadal analysis provided information about sequential processes acting upon habitat change and transitional stages of ecological response.; Vegetation zonation patterns of arroyo willow, cattail (Typha spp.), and pickleweed on sediment fans were studied by analyzing changes in salt marsh properties. Fan development increased elevation at least 2/3 meter above the marsh plain and increased topographic variability. Deposition of sandy sediment led to higher elevation, higher bulk density, lower salinity, lower soil moisture, and lower soil nitrogen, compared to reference sites. The high sand content of the sediment likely influenced changes in these soil properties. Willow expansion was limited by environmental thresholds, which included elevation of at least 1.8 meters NAVD88 where tidal influence existed, spring soil moisture less than 20% and salinity less than 3 dS/m year-round. Increased sedimentation within a hydrogeologic setting defined by highly erodible sandy soils and steep topography led to a shift in the type of wetland that could be supported downhill of agriculture.; Large catchment characteristics (>10 ha) were better predictors of present-day sediment fan size based on multiple regression models of catchments of different sizes. Historical landscape and land cover variables explained fan size in large catchments (R2 = 0.96), while historical landscape variables influenced 2001 percent riparian forest cover (R2 = 0.74). Salt marsh recovery potential increased with more grazing and less agriculture. Results indicated a time-lag between land use change and sedimentation disturbance and ecological response. With restoration, current forest cover should become a reminder of past land use.