Fig. 1: Study area in southeast Oregon, USA. At each pasture, two sites were established for a total of six treatment areas. At each of the six treatment areas, four paddocks (150 m x 150 m) were established; within each paddock, three transects are established annually. Each transect is sampled in at least three places with destructive harvesting and Structure-from-Motion volumetric estimates.
Field Data and Collection Protocol
Data were collected in June 2020 and June 2021, as supplement to existing protocol (Arispe et al. 2022) for collecting biomass data in each of the 24 experimental paddocks. For each of the 24 paddocks, three transects are established from a randomized central location. Along each transect, three 0.4 m by 0.5 m areas were photographed for Structure-from-Motion reconstruction and then destructively harvested. Vegetation was removed to as near to mineral soil as possible, and separated into annual grasses, perennial grasses, forbs, and litter prior to drying. Plots that contained shrubs were excluded, since shrubs are not destructively harvested for the larger project. Samples were oven-dried for 48 hours at 60° in a commercial drying hood and weighed. Approximately ⅓ of the plots were photographed again immediately after harvesting the vegetation but before disturbing the frames so as to collect the mineral soil surface microtopography.
Structure-from-Motion Protocol and Reconstruction
Two custom quadrat frames were constructed from extruded aluminum, containing custom 3D-printed 0.05 m cubes with coded targets (Fig. 2). These targets are automatically detected in Agisoft Metashape (version 1.6). We used a Sony a6000 with the stock kit lens (16-50 mm F3.5-5.6) to collect approximately 75 photographs per quadrat frame. These were taken approximately 1 m above the plot in a 5 x 5 sampling pattern repeated three times: once at nadir, once at approximately 20-degree tilt away from the photographer, and again tilted towards the photographer. Several additional images were taken at each plot from a greater height, and at several corner-on angles. Care was taken to stand so as to avoid casting a shadow on the plot. One of each of the six sites was collected per day. In total, approximately 216 plots were collected over the six treatment areas in both 2020 and 2021.
Our processing workflow in Agisoft was automated using Python; each plot was processed at high-quality image alignment and medium-quality dense cloud reconstruction. Resulting point clouds had typical densities of 0.5- to 4-million points per square meter with a minimum of 0.001 m spacing between points. After processing and quality assessment, there were 114 plots for 2020 and 174 plots for 2021.