The sagebrush steppe (Artemisia spp.) covers vast extents of the western United States, but has seen recent declines under a multitude of threats including increased wildfires and conversion to invasive annual grasses. The growth and survival of individual sagebrush plants drive restoration outcomes in sagebrush ecosystems and measuring these demographic rates plays a key role in current monitoring efforts. However, the vast scale of sagebrush restoration presents a challenge to efforts to measure sagebrush demographic rates with field data. In this study, we demonstrate how unmanned aerial system (UAS) flights can provide a solution by detecting demographic rates of individual plants from the air. We generated time series of UAS flights at two sagebrush common gardens in Idaho, USA to measure growth and survival of individual plants. We calculated cloud-to-cloud signed distances between UAS structure from motion (SfM)-derived point clouds to measure volumetric change in sagebrush plants between 2015 and 2019.
Results/Conclusions
For live sagebrush plants in the common garden, we detected growth in 146 of 207 plants (70%), with an average increase in canopy volume of 0.085 +/- 0.016 m3 over 4 years. Field surveys indicated that 37 plants died between our UAS flights and we detected decreases in canopy volume in 32 of those plants (86%). Overall, the 37 dead plants lost an average volume of 0.048 m3, with basin big sagebrush (Artemisia tridentata subsp. tridentata) losing more volume (0.081 +/- 0.035 m3) than Wyoming big sagebrush (A. tridentata subsp. wyomingensis) (0.030 +/- 0.018 m3) and mountain big sagebrush (A. tridentata subsp. vaseyana) (0.018 +/- 0.008 m3). We used image segmentation to detect phenological changes and development of canopy structure in a newly formed sagebrush common garden. Results demonstrate how advances in image analysis, including segmentation and structure from motion photogrammetry, will enable demographic rate measurements from UAS imagery. We describe how age of sagebrush stands and spacing of plants may alter the accuracy of measurements. We conclude with general recommendations for monitoring sagebrush demography with UAS flights.