Northern Colorado Plateau piñon‐juniper woodland decline over the past century

Abstract

The vegetation of Dinosaur National Monument and surrounding lands (∼298,000 ha) was reconstructed using General Land Office (GLO) survey records. This historical dataset was compared to two modern digital datasets to determine how vegetation has changed over 90 years. Net declines of 3–7% occurred in piñon‐juniper woodland and 12–19% in montane shrubland, along with corresponding 16–26% increases in sagebrush shrublands. Shorter natural and human‐caused fire rotations of 188–216 years appear to be driving woodland contraction. Piñon‐juniper expansion was concentrated, but not strictly confined to areas near historical piñon‐juniper‐sagebrush ecotones, and was particularly favored at elevations of 2000–2400 m and on slopes of 10–30%. Shorter recent fire rotations in sagebrush appear linked to cheatgrass expansion. Fire exclusion has had no net effect on fire rotations in the study area, as these rotations are generally shorter, not longer than historical rotations. If trends continue, a landscape outside the historical range of variability with significantly more early seral sagebrush and grassland and less piñon‐juniper woodland will emerge. Declines of piñon‐juniper in the study area are similar to the recent loss of woodland in Mesa Verde National Park, Colorado, and suggest that historical woodlands on the Colorado Plateau may be declining due to an excess of fire since Euro‐American settlement. If the goal is to maintain and restore the integrity of the natural vegetation and landscape of the study area, managers could suspend further resource‐management burning, reduce unintentional human‐set fires, and seek direct control of cheatgrass. Spatially complex patterns of woodland stability, recovery, contraction, and expansion show that century‐scale data are needed across large landscapes to discern net trends in landscape change needed for ecological restoration, management, and understanding impending future change.