Comparing diversity‑carbon tradeoffs across reforestation strategies at a long-term floodplain experiment

Due to historical drainage and conversion, restored forested floodplains have depleted soil organic carbon (SOC) and vegetation carbon (C) stocks. However, it is difficult to predict C stock recovery because flooding has confounding effects on SOC dynamics and vegetation composition. Moreover, restoration managers must weigh C gains against biodiversity losses because factors that enhance C accrual often reduce plant community richness. Thus, to improve restoration predictability and decision-making, we compared the effects of five increasingly expensive reforestation treatments on SOC recovery and diversity‑carbon tradeoffs at a long-term floodplain restoration experiment. Twenty-five years after restoration, we collected data from an adjacent reference wetland and each reforestation treatment: seedbank, acorns, seedlings, bareroot trees, and balled-and-burlapped trees. After conducting tree and understory surveys, we collected soil cores to measure SOC concentrations and fractions along with multiple soil edaphic properties. Despite having sandier soils and smaller aggregates, the reference wetland maintained higher SOC stocks than all reforestation treatments, underscoring the legacy effects of drainage and cultivation. Additionally, the natural recolonization (seedbank) treatment had much greater SOC stocks than treatments with tree planting by favoring an invasive species that enhanced litter inputs, aggregation, and organo-mineral associations. While this demonstrates the strong impacts of contrasting reforestation methods on SOC dynamics, treatments with tree planting had greater ecosystem C stocks due to their more abundant woody material and soil carbonates. Since tree planting also increased understory richness by excluding shade-intolerant invasive species, high-density seedling planting could have the most potential to support biodiversity conservation and carbon sequestration.