Lidar-derived soil surface roughness as a measure of biological soil crust disturbance and recovery within cool and hot deserts

Abstract:

The soil surface is a dynamic interface between the ground and atmosphere that reflects complex biophysical interactions. In drylands these interactions are driven by environmental conditions such as low precipitation that limit vascular plant coverage and promote colonization of biological soil crusts (BSC) in the top few mm of the soil surface between plants. BSC are communities of bryophytes, lichens, and microbiota that increase soil surface roughness promoting nutrient capture and retention and water infiltration and storage. However, soil surface disturbance and subsequent declines in BSC can alter these functions by increasing soil compaction and reducing surface roughness. In this study we evaluated the linkages between BSC and soil surface morphology by monitoring surface roughness change within two desert climates following disturbance. We measured surface roughness on soil plots using mm-resolution terrestrial lidar within cool and hot desert climates under undisturbed conditions and within 0.5 to 2.5 years following removal of the upper soil surface. Using roughness calculated from point clouds at seven sub-meter focal scales, we related differences in surface morphology after disturbance with observed topographic change, biota cover, aggregate stability, and soil chlorophyll a to interpret relative contribution of associated biophysical processes, like erosion, desiccation cracking, and BSC growth. Supporting previous work, undisturbed cool desert soils were up to two times rougher than hot desert soils with no significant changes over the two-year period. Concurrently, disturbed soils increased in roughness > 8-fold within cool desert soils. Using multi-model inferences, we found that increased surface roughness between the 0.04 m and 0.1 m focal scales provided the best evidence for changes in surface morphology associated with increased aggregate stability, an indicator of BSC growth, at 95% confidence. Roughness at greater scales showed stronger relationships with topographic changes. These findings demonstrate a novel approach for using scale-dependent roughness to attribute mechanisms of surface change and provide insights to the scale and magnitude of soil recovery, quantification of disturbance impacts, and potential identification of surface stabilization ages.