Geomorphic Factors Impact Groundwater Levels More Than Harvesting in a Coast Redwood Forest

Groundwater is a major source of stream baseflow during the summer and can supply additional water to plant transpiration, which may be particularly important in Mediterranean environments where the growing season is decoupled from wet season precipitation. However, little is known about how forest management may affect groundwater level. We used a paired watershed design in a northern California coast redwood forest to study how intrinsic and extrinsic factors as well as harvesting affect groundwater level and the connectivity between soils, groundwater, and streams. Geomorphic variables such as soil depth, well depth (or depth to the confining layer), and slope were influential factors in our random forest model on groundwater level, and basal area was of moderate importance. However, harvesting did not have a significant effect on either wet or dry season groundwater level, which may be due to the large drought that occurred post-harvest. In this coast redwood forest, it appears that factors that affect vertical infiltration and lateral flow of rainfall, such as slope, soil depth, and well depth, may be more important than vegetative characteristics like stand density.