A Soil Moisture-Heat Based Early Establishment Model of Riparian White Alder (Alnus rhombifolia)

Establishment of fluvially dispersed seeds on accreted gravel-sand bars is limited by water availability in streams. Past establishment models have used the stream/water table recession rate, and maximum root growth rate to determine the elevation limit of seedling establishment. This approach neglects the role of the saturated-unsaturated vadose zone in providing water to recently germinated seedlings, the physical processes that determine the soil moisture content, and the effect moisture deficit has on seedling root growth. This study combines a soil moisture-heat budget and a seedling root growth model that responds to soil moisture availability to find the elevation limit of establishment of white alder (Alnus rhombifolia) on vertically accreted bars along the south fork Eel River in the Angelo Coast Range Reserve, California. To establish successfully, seedling roots must maintain a connection with sufficient moisture to avoid water stress. This will depend on the elevation of the bar, the stream recession rate, the root growth rate, and the diurnal cycle of soil moisture. A one-dimensional moisture-heat budget of the top 15 centimeters of sediment was validated at two locations characterized by sand and clay-gravel textures respectively, using soil moisture and temperature measurements at 5, 10 and 15 cm, net radiation, air temperature, humidity, wind velocity and precipitation measured during spring-summer stream recession. Two patterns in soil water content were apparent: an average daily moisture decrease at each depth driven by stream/water table recession, and a diurnal pattern of isothermal liquid and vapour flux increasing soil water content in the upper 15 cm between 12:00 pm and 5:00 pm PDT. To determine seedling root growth rates, white alder seedlings were grown in growth chambers under a range of reduced matric potentials using polyethylene glycol. Root length measurements were made at 4 hour intervals and a quadratic equation was fit to the root length vs. time data at each matric potential. The derivative of each equation yields the growth rate at a time after germination at a specific matric potential. The soil moisture model and root growth rate are combined by calculating the root growth rate of a seedling at the surface soil water potential at germination. After a 4 hour time step, the depth of root penetration is determined and the root growth rate from the growth chamber experiments is adjusted to the water potential at that depth. These iterations continue over the recession period. The elevational limit to establishment on the bar occurs where the root tip experiences permanent wilting point (-1.5 MPa) for a 12 hour period. The results of this method show that the limit to establishment elevation of white alder on bars is limited by decreases in soil moisture driven by water table decline and the root growth response to moisture deficit. This decline is however mitigated by diurnal increases in soil moisture that are only apparent when capillary rise driven by isothermal pressure gradients is considered. This method could be applied to find establishment elevation limits for other riparian species on gravel-sand bars with knowledge of the sites' hydrological, atmospheric and sediment characteristics and the species growth rate response to moisture deficit.