Seedling growth responses to species-, neighborhood-, and landscape-scale effects during tropical forest restoration

Central to the success of restoration plantings within abandoned pastures is the appropriate selection of species that can establish and grow rapidly to form canopies to suppress grasses. However, species selection can be difficult, largely due to combinations of biotic and abiotic factors operating across multiple spatial scales that can affect seedling growth rates. Using a large replicated restoration experiment in Australia's Wet Tropics, we evaluated seedling growth rates of 24 native rainforest species commonly used in local restoration efforts over the first 31 months post-planting. We investigated the influence of landscape, site, and planting conditions on early-stage seedling growth and whether functional traits and surrounding neighborhood density and composition explain variation in seedling growth rates. Seedling growth rates were influenced by numerous stem-, species-, plot-, and climate-level factors, with the strength of these effects strongly dependent on the size of the seedling. Specifically, species with low wood densities and larger seeds grew faster. In response to plot-scale and climate factors, larger seedlings consistently displayed faster growth, demonstrating the benefits of initial seedling size for seedling success. Our study highlights that early-stage seedling growth can be influenced by many factors, operating across multiple spatial scales. Importantly, we demonstrate that planting larger seedlings may improve seedling growth and that developing strategies to increase the survival of fast-growing low wood density species is crucial for ensuring that plantings can achieve canopy closure quickly, improving early to mid-term trajectories of tropical forest recovery.