Songbird population trajectories diverge under simulations of conifer encroachment versus removal in a sagebrush ecosystem

Woody plant encroachment into grasslands and shrublands is a global phenomenon that negatively impacts ecosystem services and wildlife populations. North American sagebrush ecosystems have experienced widespread degradation from encroaching conifers, leading to losses of sagebrush-obligate wildlife. Removal of encroaching trees is a primary restoration method, but whether management actions can influence wildlife populations at management-relevant scales is rarely investigated. We studied a local Brewer's Sparrow in the Medicine Lodge Valley of southwest Montana to understand how their territory occupancy and nest success were impacted by tree and shrub cover. From 2019 to 2022, our data collection resulted in 1161 mapped territories and 449 nests, which we used to build models of territory occupancy and reproductive productivity relative to tree and shrub cover. We then used tree and shrub cover extracted from historical imagery to estimate population size and productivity 70 years in the past. Finally, using models of tree growth and expansion, we estimated population size and reproductive productivity under two simulated scenarios 30 years in the future: with and without restoration through conifer removal. We observed that tree cover has more than tripled at the study site since 1954, which our models predicted has caused an almost 25% decrease in the local population size and a 35% decline in offspring production. In a future scenario where tree removal is conducted in areas with <20% tree canopy cover after 30 years, we predict population size and offspring production will likely remain stable. Alternatively, if tree cover is allowed to increase unabated, our simulations predict a potential population decline of 60%, with similar losses to offspring production. We observed a stark divergence in the potential futures of the local Brewer's Sparrow population and implications for the species at large. Continuing tree encroachment drastically decreases the available habitat and causes a steep decline in population size. However, proactive and continuing management of encroaching trees can alleviate further losses in a species that has already experienced significant range-wide declines. We also highlight the importance of encroaching trees as a previously underappreciated conservation risk for sagebrush avifauna.