World's Largest Regreening Project Promotes Healthy Tree Growth and Nutrient Accumulation Up to 40-Years Post Restoration

Abstract

Mining and smelting degraded landscapes are characterised by heavily eroded, acidic soils that are contaminated with toxic metals and depleted of essential nutrients. Restoring forests on these landscapes has been highlighted to support carbon (C) mitigation measures and protect biodiversity. Understanding how tree growth and aboveground nutrient accumulation changes following restoration will be essential to planning future forest restoration projects. In this study, we assessed aboveground biomass (AGB) and aboveground nutrient (calcium (Ca), magnesium (Mg), nitrogen (N), phosphorous (P), potassium (K), and C) pools across as series of sites ranging in age from 15 to 42 years old; to determine the effects of erosion on AGB and AGB nutrient pools each site was categorized as 'stable' (less than 10% bedrock cover) or 'eroded' (greater than 30% bedrock cover). Both AGB and AGB nutrient pools increased with time since restoration at rates similar to coniferous plantations grown in areas unimpacted by centuries of mining and smelting practices. Individual tree growth and nutrient accumulation did not differ between stable and eroded sites; however, stable sites had a higher stem density leading to overall higher AGB and AGB nutrient pools. Future N limitation of the regreening forests does not appear to be a concern as aboveground N pools are six times larger than applied N, indicating additional N is entering into the system whether through residual soil organic matter or the establishment of N-fixing species. Conversely, aboveground P concentrations are decreasing with time since tree planting and the 40-year-old study sites have aboveground P concentrations below values for 'healthy' trees. This study shows that the regreening efforts have led to a massive addition of 1, 144, 588 Mg of AGB (550, 547 Mg C) onto the landscape, and capable of sustaining healthy tree growth up to 40-years post restoration. However, as the regreening stands age, nutrient limitation may impact future tree growth. Future studies should continue to investigate nutrient cycling within these remediated forests particularly nutrients of concern such as P.