Impacts of forest restoration on multifaceted bird diversity and community assembly in the Loess Plateau of China

Restoring forests is crucial for safeguarding biodiversity and providing ecosystem functions on degraded lands globally. Despite significant restoration efforts over a half-century in the Loess Plateau of China's prominent dryland region, the impact of restored forests on biodiversity remains less understood. Unlike the abundant studies in the tropics, such understanding is urgently needed for dryland regions given that forest restoration is being scaled up. Here, we assessed the alpha- and beta-diversity of birds from taxonomic and functional dimensions in restored forests, and compared them to that of relatively undisturbed primary forests. We conducted rigorous point-count surveys and acoustic recordings during the avian breeding season in 2021 and 2023. We found that plantations have limited support for bird taxonomic and functional diversity compared to secondary and primary forests. Notably, the biodiversity inference drawn from our acoustic recordings generally supported similar conclusion of bird diversity across studied forest types. Additionally, secondary forests exhibited greater functional richness compared to plantations and contributed complementary functional trait space relative to primary forests. While nestedness dominated functional beta-diversity, spatial turnover dominated the taxonomic beta-diversity between the bird communities of restored forests and primary forests. Importantly, spatial proximity positively influenced the nestedness component of taxonomic beta-diversity, indicating that the distance of plantations to local primary forests (i.e., dispersal limitation) influenced bird community assembly during forest recovery. Overall, our findings provide insights on preserving intact primary forests, delivering biodiversity benefits of forest restoration in dryland regions, and prioritizing restoration locations only when tree planting is necessary.