Restoring tropical dry forests, where? how?: a global overview

Deforestation is the main threat to the tropical dry forest (TDF) worldwide, with agriculture and mining being the human activities with the greatest impact on this phenomenon. Rates of disturbance have reduced not only TDF areas, but also their ability to adapt and mitigate the effects of climate change. In some cases, natural regeneration is sufficient to reverse the effects of forest cover loss. However, when land use is intensive and chronic, soils deteriorate and sources of regeneration are depleted, then restoration effots require greater interventions to promotes the recovery of both forest cover and ecological functions. In this study we synthesize the published literature of over 30 years (January 1990 - February 2020) on the ecological restoration of TDF worldwide. We also conducted a meta-analysis to assess the effects of restoration strategies, previous land use and ecozone on restored sites and their soil properties, vegetation structure, biotic composition and ecological functions. We identified 196 studies that met our criteria, most of which took place in the Neotropic and Indomalaya ecozones. The most common restoration objective was the recovery of the vegetation structure, mainly after previous use for livestock, crops or mining. Planting seedlings was the most widely used restoration strategy and the evaluation of their success was generally based on assesing the survival of the plantings. Results showed that restoration treatments improved planted seedling survival (SMD = 0.51, 95%CIs = 0.15 to 0.87), with a significant effect in the Neotropics, Indomalaya and Oceania. Regarding the previous land use, no significant effect was observed on the survival of the planted seedlings. Derived from these results, it is necessary to highlight that survival, even though widely used to monitor restoration success, yield very high variation in its results. TDF ecosystems have specific characteristics throughout their distribution related both to diversity and environmental variables, as well as to their disturbances. Thus, collecting information and generating a reference frame on successful practices is urgent in order to achieve adequate restoration. In summary, through this synthesis we identified the main trends on restoration ecology of TDF at a global scale, pointing out aspects that must be considered in order to comply with the international commitments in terms of forest restoration.