Spatial Patterns Between Regolith Thickness and Forest Productivity in the Southern Sierra Critical Zone Observatory.

Soil and underlying weathered bedrock constitute what is referred to as regolith, which is a substrate that actively contributes water and nutrients to above ground biomass.  As a result, regolith thickness is an important regulating factor of forest health and drought tolerance in the Sierra Nevada. This study examines the relationships between terrain attributes, regolith thickness, and tree productivity within a sub watershed of the Southern Sierra Critical Zone Observatory. We hypothesized that tree productivity will increase with increasing regolith thickness and that regolith thickness will be controlled by the allocation of water across the landscape as a function of landscape and terrain attributes. Data was collected in the summer of 2015 at one-hundred sites within a 543-ha watershed averaging 1180m in elevation with a MAP of 80cm and a MAT of 11C. Regolith was sampled by hand augering to either hard bedrock or a maximum depth of 7.56 m, which ever was shallower.  Biomass measurements were made for all conifers (DBH>20cm) within a 10m radius of the primary auger hole. Tree age was measured from a representative tree for each species within the site. Preliminary results indicate that erosional processes and incision into the paleo-surface are controlling patterns in regolith thickness within this watershed. It also appears that tree productivity is controlled by multiple factors that can confound the overall model. Sites that receive a higher effective precipitation as computed by a compound topographic index (CTI) tend to have shallow regolith thicknesses limiting water storage capacity.  As climate change continues it will become increasingly important for land managers to evaluate forest health, productivity and drought tolerance.