Forest restoration of Amazonian mine lands: Identifying factors driving tree recruitment

Abstract

El incremento de la deforestación en la Amazonía y sus impactos en los servicios ecosistémicos convierten a la restauración ecológica en una necesidad para garantizar la sobrevivencia de este bioma. En ese sentido, esta tesis estudió los factores que promueven el reclutamiento de árboles en la restauración de bosques. El estudio fue realizado durante la época seca de 2019 en 86 parcelas dispuestas en 6 minas cerradas, constituyendo una crono-secuencia de 27 años de áreas en proceso de restauración. Estas parcelas estuvieron ubicadas en la Amazonía del estado de Rondônia en Brasil. Se midieron diferentes factores asociados al manejo forestal y la dinámica ecológica, así como variables asociadas a los reclutas. Con los datos obtenidos se elaboraron modelos lineales mixtos en R para analizar el efecto de dichos factores en la riqueza de especies, composición específica y densidad poblacional de los reclutas. Este estudio evidencia los complejos procesos involucrados en el reclutamiento en sitios reforestados. Asimismo, informa sobre la importancia de la disponibilidad de semillas y expande la comprensión sobre el rol de la luz y suelo en el establecimiento de los reclutas, orientando alternativas silviculturales que incrementen la riqueza y densidad de reclutas y promuevan la sucesión ecológica.

Increasing deforestation in the Amazon and its potential impacts on ecosystem services make restoration necessary for the survival of this important Neotropical biome. Although mining accounts for only 9% of Amazonian deforestation, the damage it causes to the ecosystem is one of the most intensive. However, few studies focus on forest restoration on mined sites and most of them study sites less than 10 years old. Furthermore, one of the major challenges to ensure the long-term success of forest restoration is to re-establish forest dynamics (including tree recruitment). This thesis focuses on closing the knowledge gap about tree recruitment by assessing its ecological and management drivers in a 27-year-old chrono-sequence of surface mining sites in the Brazilian Amazon undergoing restoration. To achieve this, 86 monitoring plots were assessed in 6 mines in Southern Amazon, Rondônia state, Brazil, during the dry season of 2019. Linear mixed models were developed in R to analyse the effects of different management and ecological factors on recruit species richness, species composition and density. Ecological factors included successional age, canopy closure, litter depth, soil properties and amount of surrounding forest area. Management factors comprised mine zones and planted species richness, species composition and density. The results show that seed availability from planted trees and surrounding forests influenced recruit species richness, species composition and density. Light availability influenced recruit species richness and density, but it had no effect on species composition probably because all dominant species have similar light requirements. Mine zones and their associated soil properties influenced recruit species richness, species composition and density. This study evidences the complex processes involved in tree recruitment after tree planting, which is crucial for the long-term success of forest restoration efforts. It also provides suggestions about the importance of acknowledging seed availability. Finally, it expands the understanding of the roles of light and soil in establishment to inform management decisions in order to increase recruit species richness and density and avoid arrested succession pathways.