Evaluating the success of direct seeding for tropical forest restoration over ten years

Abstract

Direct seeding is considered a feasible alternative for large-scale forest restoration, but little is known about the successional trajectory of tropical forests restored through direct seeding. To validate this method, we must understand how it changes forest structure and species richness and shapes the community trajectory as compared to the standard methods of natural regeneration and seedling planting. We evaluated restoration sites up to 10 years old in the Xingu river basin in Mato Grosso, Brazil. Our objectives were to: (i) describe changes in vegetation structure and richness in restored forests within the first decade; (ii) compare the outcome of restoration by direct seeding (broadcast seeding, sowing in rows, or placing seeds in holes) to seedling planting and natural regeneration; (iii) evaluate the influence of environmental variables (annual precipitation, soil phosphorus content, sand percentage, and soil base saturation) and the direct seeding technique on forest structure. We assessed species richness, density of individuals per size class, basal area, canopy height, aboveground biomass, and canopy cover. We sampled 72 direct-seeded sites between 1 and 10 years old, three seedling planting sites and six natural regeneration sites aged 7 to 9 years old. Community structure attributes changed over time, with the exception of the density of regenerating individuals. After four years, direct-seeded sites had formed a multi-layered canopy and were starting to be colonized by non-planted species. In general, direct-seeded sites had high aboveground biomass, but there was high variability among sites, especially after age six years. Sites with higher phosphorus content had more tree density, basal area, and biomass compared to the others. Broadcast seeding sites had higher seedling and sapling densities than sites that received other restoration methods. In conclusion, direct seeding was a successful method for the initial phase of forest restoration, promoting a structure that was more similar to resilient natural regeneration sites than to non-resilient natural regeneration sites and seedling planting sites.