Restoring forest structure, composition and function is an important component of urban land management, but we lack a clear understanding of the mechanisms driving native tree regeneration beneath a planted forest canopy. We hypothesized that light availability, competition with exotic weeds, and microclimate mediate the establishment of the next generation of native trees in restored forests. We investigated relationships between environmental conditions and the plant community in 27 restored urban forests spanning 3 to 70 years in age and in both unrestored and remnant urban forests and used structural equation modelling to determine the direct and indirect drivers of native tree regeneration in the restored forests.
Compared to remnant forest, unrestored forest had significantly more light reaching the forest floor annually, exotic weed cover was greater, and there was marginally less native tree regeneration. In restored forests, light transmission through the canopy was reduced to levels found in remnant forests within 20 years of planting, and shortly thereafter herbaceous exotic weeds in the understorey declined and soil temperatures stabilised. Contrary to expectations, light availability was not a direct driver of tree regeneration, but canopy openness did regulate weed cover and soil temperature variation, which were the key drivers of native tree seedling regeneration. These results imply that ecosystem properties change predictably after initial restoration plantings, and that reaching critical thresholds in some ecosystem properties makes conditions suitable for the regeneration of late successional species, which is vital for restoration success and long-term ecosystem sustainability. Abiotic and biotic conditions that promote tree regeneration will likely be present in forests with a basal area ≥ 27 m2/ha. We recommend that urban forest restoration plantings be designed to promote rapid canopy closure to reduce light availability, suppress herbaceous weeds, and stabilize the microclimate.