Recovery of White Alder forests in Northern California streams: Influence of climate and disturbance history.
DiVittorio, Christopher*,1, Dietrich, William1, Power, Mary1, 1 University of California, Berkeley, Berkeley, CA
ABSTRACT- We examined the age structure of populations of White Alder (Alnus rhombifolia) along the South Fork Eel River in Northern California to document spatial and temporal patterns of historic tree recruitment. Populations along tributaries and the main stem exhibit unimodal or multimodal age distributions, indicating that recruitment is episodic. Even-aged stands on the main stem established during periods of low rainfall. This contrasts with recruitment on tributaries which appears to be unrelated to rainfall. Low rainfall may promote recruitment on the main stem by dispersing Alder seeds to suitable germination sites, and by reducing the severity of scouring winter floods. Reduced flood severity may allow Alder saplings to escape in size and withstand normal flood regimes after about the third growing season. Alternatively, recruitment on tributaries appears to be controlled more by the disturbance history and geomorphology of the particular channel than by interannual climate variability. Alder recruitment on tributaries occurs where light gaps are created in an otherwise closed canopy due to landslides, stream incision, or tree death. In this system as the size of the stream increases, light availability and flood severity increase as well, leading to a shift in limiting conditions from light to disturbance. Thus, we predict Alder to recruit rapidly along smaller tributaries as long as direct sunlight and local seed sources are available. We predict Alder recovery along larger channels to proceed much slower, on decadal time scales, and to be episodic, since establishment requires a combination of suitable seed dispersal and drought.
Key words: recruitment, Alnus, dendrochronology, riparian
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