EXPLORING THE RELATIONSHIP BETWEEN CANOPY HEIGHT AND BEDROCK DEPTH IN THE CRITICAL ZONE USING SEISMIC REFRACTION AND LIDAR

The depth to fresh bedrock defines the bottom of the critical zone (CZ) and is the location where chemical and physical weathering begins. Weathering, often initiated by water passing through fractures in the bedrock, creates and maintains CZ structure. Therefore, being able to quantify CZ structure over large spatial areas (100s to 1000s of km) is an important step forward in improving our understanding of CZ processes. In weathered and crystalline rocks, where the porosity of unweathered bedrock is close to zero, seismic velocities estimated by seismic refraction are controlled by chemical and physical weathering. Large scale seismic refraction data sets in weathered and fractured rocks can reveal complex heterogeneity that would otherwise go unnoticed. In this study, 2.6-line km of seismic refraction data were collected on a weathered and fractured granite in the Laramie Range, Wyoming. The data were centered over a small (0.1 km²) watershed with an ephemeral stream. The seismic refraction data show that bedrock, defined by 4000 m/s, comes to within 5 m of the surface under the valley bottoms and is undetectable (greater than 40 m deep) under two hilltops that bound each side of the drainage. High-resolution (1 m) LiDAR data was used to estimate canopy heights and reveal that the tallest canopy heights (> 10 m) occur at locations where the geophysical interpreted bedrock is the shallowest. Although, canopy height is not a direct metric for species, forest age, or health, the observation of taller canopy over shallow bedrock suggests the ecosystems at the surface could be tightly coupled to processes occurring near the bedrock boundary. Our data suggests that either the shallow bedrock forces water to pool in the thin CZ above bedrock at the drainage bottom that makes water more readily available, or the shallow bedrock might be providing important bedrock derived nutrients to the plant community—both ideas require further interdisciplinary measurements. However, the large-scale seismic refraction observation of CZ new and exciting opportunities to explore new relationships over large spatial scales.