Tree Seedling Survival Depends on Canopy Age, Cover and Initial Composition: Trade-offs in Forest Restoration Enrichment Planting

Recruitment of late-successional, shade-tolerant trees into early-successional plantings is a crucial step in long-term forest restoration, especially in urban areas where seed dispersal can be limiting. Enrichment plantings are an important strategy for diversifying the developing understory, but we lack clear guidance about when shade-tolerant species can be successfully introduced or how the initial canopy composition constrains the success of enrichment plantings. To address this knowledge gap, we conducted an experiment in a forest restoration project in a warm temperate forest region in New Zealand. We planted seedlings of six functionally-diverse species in restored sites that ranged from two to seven years since the initial restoration planting and analyzed 2-year seedling survival and growth rates as a function of planting age, canopy cover, and canopy composition. The canopy height of initial plantings grew at a linear rate of 1 meter per year. Within five years, initial plantings reached 80% canopy closure, a leaf area index of 5 m2 m−2, and a corresponding reduction of pasture weed cover to < 10%. Seedling survival was highest beneath young canopies dominated by tea tree (Myrtaceae) but also in older canopies with a more diverse composition. Our results show that the rate of canopy development is highly predictable despite differences in initial planting composition, but that enrichment planting seedling survival depends on both canopy age and composition. To enhance survival of enrichment plantings, we recommend using a balanced mixture of tea tree and broadleaved trees in the original planting to optimize seedling survival to facilitate long-term forest restoration.