Differential Seedling Survival of Early-planted, Late-successional Trees: Results From Three Forest Restoration Experiments

Abstract

The recruitment of late-successional and canopy-dominant tree species into early-successional plantings is essential for long-term forest restoration. Introducing these species early enhances the stability of the emerging ecosystem and boosts its ecological functions and biodiversity. However, many genetic, ecological and environmental factors can alter the survival of late-successional trees in early-successional, open-field environments. Despite the critical role late-successional trees play in forming resilient and diverse forests, significant knowledge gaps remain regarding optimal planting strategies, including tree density and composition, and the impact of microhabitats on the establishment of these species. To address this, we employed aster models to analyse the survival of 1840 late-successional trees planted between 2019 and 2022 across three afforested and restored past agroecosystem sites as part of the Auckland University of Technology Living Laboratories programme. Our experiment revealed that four of the six late-successional tree species planted demonstrated high survival when planted simultaneously with early-successional trees, while the other two did not. We also showed that a spacing of 2 m between early-successional trees significantly improved the survival of late-successional trees compared to smaller or larger spacings. This 2-m spacing appeared to effectively balance the benefits of shelter provided by early-successional trees and the drawbacks of shading and competition. In addition, greater amounts of dead and live vegetation increased seedling survival, while soil chemical characteristics related to nutrient richness and solar radiation altered seedling survival in a site-specific way. These effects were likely driven by differences in species functional traits and ecological requirements and the environmental and ecological contexts of the experimental sites. Practical implication. Our study demonstrates that simultaneously planting both late- and early-successional tree species is effective for the establishment of late-successional trees on agricultural lands designated for restoration. Furthermore, it indicates that a 2-m spacing between early-successional trees is an optimal strategy to enhance late-successional trees' survival when planting them simultaneously.