A scoping review of tropical pioneer trees' roles for restoration and conservation management: Harungana madagascariensis (Hypericaceae) a widespread African species as a model.

Background: Globally, biodiversity is declining rapidly, including tropical forests ecosystems in particular. To stop and reverse this trend, governments worldwide signed up to international agreements and initiatives, but success to date has been limited. In this context, reviewing pioneer trees' ecology, particularly the most widespread species, can help gauge their pros and cons and guide their judicious use for cost-effective ecological restoration projects.

Objectives: This study aims to review the potential of pioneer tree species for biodiversity conservation and forest restoration and identify eventual knowledge gaps, using a widespread species from Africa, Harungana madagascariensis Lam. (Hypericaceae), as a model. Our specific objective was to synthetize information on the distribution and habitat of H. madagascariensis; its documented interspecific ecological interactions; and its potential for forest restoration.

Methodology: A scoping review was conducted using multiple databases to identify relevant papers, supplemented by the Global Biodiversity Information Facility database (GBIF) to extract distribution records of H. madagascariensis. Following the PRISMA screening process for scoping reviews, 398 publications and 4,379 records from GBIF (2023) were used in the final analysis out of a total of 1,159 unique articles and 5,230 records originally retrieved.

Results: We show that H. madagascariensis, which is native to tropical Africa, Madagascar, and some islands of the Mascarenes, is a key component of young secondary forests, wetland areas, and grasslands. At least 125 species were found to interact directly with H. madagascariensis, including through commensalism, mutualism, and herbivory. It is recognized as a tool for restoration regionally, and considered as invasive in Australia where it has been introduced and, by some, in Mauritius where it is native. The benefits it provides for restoration include its capacity to improve degraded soil fertility, its ability to compete with invasive alien species mostly due to its heliophilous and fast-growing nature, and its good nurse tree potential along with its ecological interactions that support numerous species including threatened ones.

Conclusion: The widespread African pioneer tree H. madagascariensis plays a critical role in vegetation dynamic and holds great potential for fostering forest restoration and biodiversity conservation in its range of nearly 13 M km². Its greater use in restoration projects could significantly accelerate ecological restoration, decrease its costs, and increase benefits to biodiversity, leading to larger areas being restored, contributing effectively to national and international objectives. However, a number of aspects deserve further studies, such as the species' role in multitrophic interactions and its precise interactions, and their strengths, with species in each of its specific geographical contexts and through different temporal scales.