Ultra-high-resolution mapping of biocrusts with Unmanned Aerial Systems

Abstract

Biological soil crusts (biocrusts) occur in drylands globally where they support ecosystem functioning by increasing soil stability, reducing dust emissions and modifying soil resource availability (e.g. water, nutrients). Determining biocrust condition and extent across landscapes continues to present considerable challenges to scientists and land managers. Biocrusts grow in patches, cover vast expanses of rugged terrain and are vulnerable to physical disturbance associated with ground-based mapping techniques. As such, remote sensing offers promising opportunities to map and monitor biocrusts. While satellite-based remote sensing has been used to detect biocrusts at relatively large spatial scales, few studies have used high-resolution imagery from Unmanned Aerial Systems (UAS) to map fine-scale patterns of biocrusts. We collected sub-centimeter, true color 3-band imagery at 10 plots in sagebrush and pinyon-juniper woodland communities in a semiarid ecosystem in the southwestern US and used object-based image analysis (OBIA) to segment and classify the imagery into maps of light and dark biocrusts, bare soil, rock and various vegetation covers. We used field data to validate the classifications and assessed the spatial distribution and configuration of different classes using fragmentation metrics. Map accuracies ranged from 46 to 77% (average 65%) and were higher in pinyon-juniper (average 70%) versus sagebrush (average 60%) plots. Biocrust classes showed generally high accuracies at both pinyon-juniper plots (average dark crust = 70%; light crust = 80%) and sagebrush plots (average dark crust = 69%; light crust = 77%). Point cloud density, sun elevation and spectral confusion between vegetation cover explained some differences in accuracy across plots. Spatial analyses of classified maps showed that biocrust patches in pinyon-juniper plots were generally larger, more aggregated and contiguous than in sagebrush plots. Pinyon-juniper plots also had greater patch richness and a lower Shannon evenness index than sagebrush plots, suggesting greater soil cover heterogeneity in this plant community type.