Picture worth a thousand words: Updating repeat photography for 21st century ecologists

Anthropogenic climate change is altering every ecosystem on Earth. Understanding these changes requires quality baseline measurements of ecosystem states. While satellite imagery provides a coarse baseline for regional-scale changes in vegetation, landscape-scale observations are lacking. Ground-based repeat photographic points (RPP) can provide this finer baseline. As precise visual records of ecosystems at a particular time, RPP provide rich data for diverse uses. Current methodology for establishing RPP, developed in the era of film cameras, requires placement of permanent markers in a landscape to provide accurate repeats over time. Another form of RPP involves relocating sites of historic photographs, to assess change between historic and present-day photographs. Through a three-year field survey, we synthesized these techniques to modernize repeat photography for the 21st century ecologist.We established 100 RPP in the Peloncillo Mountains of New Mexico, recapturing 86 RPP in the three years (2015-2017) of the study. During our study, a large (>16,000 ha) complex of wildfires burned more than half of the RPP sites we established in the prior month, providing a unique opportunity to assess method accuracy after dramatic landscape disturbance by comparing burned, unburned, pre-, and post-fire RPP image recapture precision.Our method produced 92% mean similarity for 86 RPP between original and repeated photographs, with no difference between burned and unburned sites. Interval between photographs did not cause a decline in similarity.Our updated methods can be practically applied to nearly all terrestrial study systems. Landscape changes driven by human (e.g., effects of anthropogenic climate change, land use) and natural activities (e.g., wildfires, phenology, and hydrologic events) are especially well suited to our updated methods. Modern smartphones include the technology necessary (e.g., camera, GPS, and compass) to employ our method and provide a means for low-cost deployment of the technique in diverse landscapes. We encourage broad adoption of this technique to establish baseline RPP of ecosystems across the globe, and the formation of a centralized database for repeat photography.