Combining Photogrammetry and Field Techniques to Determine Hypolith Colonization and Distribution Patterns

In drylands, photosynthetic microorganisms can occupy the underside of translucent to semi-translucent quartz rocks. These organisms (hypoliths) are dominated by photosynthesizing cyanobacteria that can provide critical ecosystem services such as soil stabilization and biogeochemical cycling. In some drylands where plant biomass is sparse, hypolithic microbial communities can account for a considerable fraction of the photosynthetic biomass. Thus, uncovering patterns in hypolith distribution has the potential to reveal critical information pertaining to ecosystem structure and function in drylands. Prior studies have focused on colonization of individual quartz rocks. However, these methods lack the capacity to address these patterns at larger spatial scales, and have limited potential for automation and scaling up to understand hypolith impacts. We developed novel methods for automated estimation of broad spatial scale hypolith distribution leveraging commercial off-the-shelf equipment. Twelve (4.5 m x 4.5 m) areas of interest (AOI) with one (0.5 m x 0.5 m) grid squares were chosen in two different land use areas of the Sonoran Desert in Arizona. Photographs were taken to analyze the relative location of each quartz rock within the AOI. GoPro footage was collected for Structure from Motion orthophotos and used to investigate local distribution of hypolithic communities with neural network-based 2D semantic mapping. Structural traits (physical dimensions and depth to soil horizon) of ~680 quartz rocks were measured in situ to validate automated processing. Each individual quartz rock was then photographed with a scale bar using a DSLR camera for colorimetry and further morphometric analysis. Ex-situ data analysis reveals that these methods are reproducible and compatible with multiple forms of analysis such as spectroscopic investigation of hypoliths. Preliminary results suggest a trend for colonization with regards to the soil horizon depth of individual quartz rocks. Understanding hypoliths at a broader scale provides insight to the relationships between soil quality, stability and the total surface area of translucent quartz rocks. Our methods show promise for providing a standard for surveying hypolithic communities in other drylands.