Along the South Fork Eel River, white alders are found in linear patches of mature, even aged individuals growing at roughly the same elevation. Previous studies show the seasonal hydrograph in this Mediterranean climate decides this distribution through dispersal of seeds and providing moisture for germinating seedlings. This study models the survival and rooting depth of germinated white alder seedlings during the base- flow establishment period in the late spring and summer. Water availability was calculated to a depth of 15 cm at two locations with a one dimensional model (HYDRUS 1D) of sediment matric potential at a different elevations on a sand gravel bar during late spring and summer. Alder seedlings were grown in climate controlled growth chambers for 3 weeks under matric potentials from 0 to -0.7 MPa using polyethylene glycol. A logistic nonlinear mixed effects model was fit to predict root growth at different matric potentials. Seedling rooting depth in the field was calculated using the growth rate as a function of the daily average matric potential at depth of the root tip.
Results/Conclusions
Sediment moisture declines over the measurement period were driven by evaporative fluxes at the surface. As the stream receded, upward fluxes from the water table decreased. Alder seedlings’ roots continue to grow at reduced rates under lower matric potentials, but survive under these unsaturated conditions. Alder roots do not require a continuous connection with the receding water table as establishment models for other riparian species suggest. Seedlings that were deposited at higher elevations on the bar grew at a lower rate. Seedlings that are deposited at lower elevations grew longer roots. Alder seedlings' survival after germination was not limited by soil moisture availability. This study incorporates the unsaturated vadose zone in providing water to recently germinated seedlings, the physical processes that determine the sediment moisture availability, and the effect lower matric potential has on seedling root growth. The final rooting depth at the end of the establishment period may be important in resisting scouring by high flows in fall and winter. This method could be applied to find rooting depths of other riparian species on gravel-sand bars with knowledge of the sites' hydrological, atmospheric and sediment characteristics and the species growth rate response to reduced matric potential.