A picture is worth a thousand plants: Using historic and repeat photography to quantify long-term change in drylands on the Colorado Plateau

Abstract

This study analyzes over a century of plant community change on the Colorado Plateau (CP) using repeat photography (1910–2016) and various GIS (Geographic Information Systems) data. We examined 338 repeat photo pairs across three dominant vegetation communities: CP Blackbrush-Mormon-tea Shrubland (CP Blackbrush), Inter-Mountain Basins Semi-Desert Grassland and Big Sagebrush Shrubland (IM grasslands) and CP Pinyon-Juniper Woodland/Shrubland (PJ Woodland). Vegetation changes, including plant counts, plant size, patch shifts, patch size and live plant cover, were evaluated against photo pair time span and spatial covariates, including climate, topography, soil characteristics and land-use history using logistic regression. Responses varied significantly by community type, driven by interacting factors such as time since domestic grazing cessation, climate conditions, and geomorphic setting. CP Blackbrush and IM grasslands showed recovery with longer time since grazing cessation, with increases in plant numbers, patch size, and cover. PJ Woodlands showed mixed, context dependent responses. All communities exhibited increased growth on alluvial landforms, likely from deeper soils and better water availability. Human disturbances lead to a higher probability of patch shifts in CP Blackbrush and IM grasslands. These findings underscore utility of long-term datasets for understanding dryland ecosystem dynamics and highlight how land-use history, climate and geomorphology can inform conservation efforts on dryland ecosystems.