Assessment of the Distribution and Physical Structure of Litter Retention Elements in the Sonoran Desert Using 3D Photogrammetry

Plant litter plays an essential role in nutrient and carbon cycling in low-productivity semi-arid and arid ecosystems (drylands). Low vegetation cover in drylands allows for surface redistribution and heterogeneity of litter by wind and water. Litter retention elements (LREs) are abiotic or biotic structures that trap litter as it moves horizontally or vertically. Given the ecological importance of litter decomposition, it is essential to understand how the physical structure of LREs determines the amount of litter retained, especially in arid systems where there is low primary production and low decomposition rates that might be highly responsive to abiotic conditions in a microsite. Here, we present a methodology for large scale automated LRE mapping and analysis. This study assesses the spatial variability of litter distribution in drylands by mapping the spatial distribution of LREs and determining how much litter accumulates around and within them. We used time-lapse photogrammetry for automated mapping and analysis of LREs and the trapped litter. We established 10 m x 10 m plots at three sites, with fiducial markers on the ground for scale and 0.1 x 0.1 m subplots for litter collection. Sets of images were captured with overlap between frames with a GoPro Hero 8 by orbiting the plots, and plot metadata were recorded along with the time-lapse imagery. Agisoft Metashape was used to produce and analyze 3D models using structure-from-motion (SfM), as well as to produce orthomaps for semantic mapping. Our results show that SfM coupled with semantic mapping are viable tools for estimation of litter distribution at our study sites. We also found that LREs contain more litter than open areas, with an average mass of 11.13 g/m2 of litter collected from LRE subplots. Our methods for mapping litter using time-lapse imagery show promise for broader application of mapping micro-environmental effects on ecosystem processes.