Anadromous fish habitat in California's Mediterranean-climate watersheds: Influences of riparian vegetation, instream large woody debris, and watershed-scale land use.

摘要

Due to widespread declines in anadromous fish populations, significant resources are being invested in restoration of their freshwater habitat. To improve the effectiveness of these investments, restoration strategies must be targeted to the appropriate scale and tailored to the specific natural processes that create and maintain habitat within a region. This dissertation investigates several of the basic processes, operating across multiple spatial scales, which shape anadromous fish habitat in Mediterranean-climate streams of Northern California.; Although it is well established that large woody debris (LWD) provides critically important habitat values for salmonids in conifer-dominated watersheds, the relationship between LWD and fish habitat had not been previously investigated in the hardwood forests of California's Mediterranean-climate watersheds. Loading of LWD in hardwood streams was strongly related to characteristics of site- and reach-scale vegetation and management. Although debris loading was considerably lower than values reported from conifer streams, hardwood LWD provided similar habitat features: pool frequency was positively correlated with LWD loading, and LWD-formed pools had significantly higher shelter values than pools formed by other mechanisms. Debris jams were responsible for the majority of LWD-formed pools. Almost half of the channel-spanning jams contained a key piece composed of “living LWD”—a tree that entered the channel but remained rooted and living. Jams with a living key piece were significantly larger, more persistent, and more likely to cause a pool than jams without a living key piece. Living LWD potentially compensates for the smaller size and faster decay rate of LWD from riparian hardwoods.; I also investigated the relative influences of land use and land cover (LULC) at various spatial scales on levels of fine sediment within spawning gravels (embeddedness) in the Russian River basin. Agriculture, urban, and herbaceous LULC categories were positively correlated with embeddedness, while forest was negatively correlated. Watershed-scale variables for LULC consistently explained the most variation in embeddedness levels, while LULC values within the adjacent riparian corridor explained little variation. Thus, LWD and spawning gravels are primarily influenced by factors operating at disparate scales, indicating that effective restoration strategies for these key habitat elements must target the appropriate scale.