Northern Arizona Ecological Conservation​: Assessing Quaking Aspen Health in Northern Arizona Using Earth Observations​

Quaking aspen (Populus tremuloides) forests in Northern Arizona provide critical habitat for endangered bird species and support regional tourism and recreation. However, aspen populations are declining due to a combination of abiotic and biotic stressors. In response, land managers implemented strategies, such as ungulate exclusion fencing, prescribed burns, and stand thinning, but efforts are constrained by limited data on where and when to intervene. This project worked in collaboration with the U.S. Forest Service, the National Park Service, the Arizona Department of Forestry and Fire Management, and Northern Arizona University School of Forestry. The team utilized Earth observations (EOs) and geospatial analysis to assess long-term trends in aspen extent. We applied a random forest classification and phenological analysis using Landsat 8 Operational Land Imager (OLI) imagery (2014–2024), deriving the Normalized Difference Vegetation Index (NDVI), Normalized Difference Yellowness Index (NDYI), and Modified Normalized Difference Water Index (MNDWI). The team sourced elevation, slope, and aspect data from the U.S. Geological Survey’s USGS Shutter Radar Topography Mission Digital Elevation Models (SRTM DEM). A 2017 aspen presence map was produced with an overall accuracy of 79.15%, classifying approximately 41.41 square miles of aspen forest. Time-series analysis of vegetation and climate indicators revealed an overall decrease in aspen extent between 2017 and 2024. These results provide partners with a scalable, repeatable tool to identify regeneration hotspots and inform restoration priorities. While our classification was limited by the availability of known aspen training data, this study demonstrated the feasibility of leveraging EOs to enhance field-based forest monitoring and decision-making.