The long-term effects of active management and landscape characteristics on carbon accumulation and diversity within a seasonal dry tropical ecosystem

Abstract

Reforestation is the largest natural climate solution, while potentially reversing the biodiversity crisis, especially in tropical countries. Dry tropical forests are of particular interest because they experienced greater historic loss, and offer large reforestation opportunities. This study addressed the potential of secondary forests in dry tropical systems regenerating after 60 + years in cattle pasture to accumulate carbon while increasing floristic diversity. Total carbon and woody species diversity were quantified within experimental treatments established 15 years ago. Initial active management practices included removing exotic grass with herbicide, excluding cattle by constructing live fences, and monitoring succession relative to proximity to forested riparian zones and slope position. Overall, carbon accumulated relatively slowly in this landscape (1.30 MgC Ha-1 Yr-1). Differences were seen between management practices and landscape characteristics. Lower slope plots, adjacent to forested zones, had significantly more carbon than upper slope plots, isolated from riparian zones. The initial application of herbicide decreased total carbon. However, natural regeneration of two valuable timber species with small seeds, Astronium graveolens and Cedrela odorata, benefited from this initial treatment. Live fences that were initially planted to exclude cattle significantly increased carbon of regenerating woody species. Lianas were abundant at this successional stage. Almost half of all inventoried trees (44%) had at least one liana climbing through them, with the most common species being Bauhinia glabra and Macherium microfolium. Some valuable timber tree species found within older, protected riparian forests were not yet regenerating in any treatment, such as the wind-dispersed species, Cieba pentandra, and the animal dispersed species, Hymenaea coubaril. These species may need to be actively planted at the mid-successional stage (~15–20 years) to restore biodiversity. A trade-off between timber value, diversity, and carbon sequestration must be considered in reforestation programs and this depends on landscape characteristics and management. Passive management, even with invasive grass species, is a practical option when sites are located near forested riparian zones due to low cost and high timber value of a suite of native species that naturally regenerate. No intervention is needed other than continual protection from fire and grazing. However, low-cost management, such cutting lianas and enrichment planting at the mid-successional stage, is likely to increase carbon accumulation and diversity. More active management, such as planting drought-tolerant, higher timber value tree species, is recommended at sites isolated from forested riparian zones on upper slopes where carbon accumulation is lower. Tropical forest regeneration mechanisms in proximity to forest fragments can be surprisingly resilient if chronic human impacts are removed.