Natural regeneration of Norway spruce under pioneer trees outperforms conventional reclamation on post-mining sites

Afforestation is one of the feasible ways to restore post-mining land. Previous studies have shown that, under favorable conditions, forests can spontaneously establish through natural regeneration on post-mining sites developing by spontaneous succession, often colonized by pioneer tree species. However, the establishment of late-successional tree species is an important step in forest development. Here, we evaluate the growth of Norway spruce (Picea abies) in the understory of pioneer trees in successional sites developed through spontaneous succession, in comparison with spruce seedlings planted on conventionally reclaimed sites. The comparison of 14-year-old spruce trees shows that those growing in the understory of pioneer species are significantly taller, have higher chlorophyll and phosphorus content in their needles, a greater proportion of mycorrhizal root tips, and a higher content of ergosterol in the soil beneath them. However, the density of volunteer spruce trees on successional sites is significantly lower than the density of planted seedlings on reclaimed sites. Topsoil in reclaimed sites is alkaline (pH ∼8), whereas in successional sites, the pH is significantly lower (<7.5)—likely due to the greater accumulation of organic matter on the soil surface. Soil pH is negatively correlated with tree height, as well as needle phosphorus and soil ergosterol contents. We propose that the decrease in pH on successional sites is a major factor increasing their suitability for tree establishment. Surface leveling and subsequent tree planting, common in conventional reclamation practice, may result in less favorable site conditions than those arising from spontaneous succession and on-site natural seed germination.