Juniper expansion is occurring across western rangelands due to a variety of factors, including grazing, fire management practices and climate change. By examining historical imagery, the BLM Rawlins Field Office sought to understand juniper canopy changes during the last 71 years over a 250,000-acre study area in southern Wyoming. Black and white imagery from August 1938 was scanned, georeferenced, and used to manually map juniper stands into three density groups (low, moderate, high). The same technique was utilized using 2009 NAIP imagery, and stand areas were compared between 1938 and 2009. The study found high-density juniper stand area increased 3%, moderate-density stand area increased 14% and low-density stand area increased 30%. The latter category largely comprises juniper expansion into adjacent shrublands due to normal plant succession and wildfire suppression. Shrublands in this crucial mule deer winter range do not meet Rangeland Health Standards for heavy browse utilization and are declining in condition. Map products like this help identify juniper areas to thin as one action to improve plant community health. This case study illustrates how historical imagery, when scanned and georeferenced, can provide valuable vegetation trend data to assist land management.
An area in southern Wyoming depicted by 2009 NAIP color infrared imagery. All density classes of juniper stand area (1-100% combined) are depicted for 1938 (red) and 2009 (blue), illustrating the large expansion of juniper over time.