Abstract
Tropical moist forests are currently threatened by global changes, leading to an increase in degraded forest areas. A little-known yet widespread forest type in central Africa, the Marantaceae forests, presents a degraded appearance due to its open canopy and striking lack of tree regeneration caused by a dense understory dominated by giant herbs. These forests are believed to have remained stable for thousands of years, making them a potential example of arrested succession or of alternative stable state. As they appear to thrive under current global changes, it raises significant concerns for the future of central African forests. This thesis aims to investigate the external drivers and ecological mechanisms that explain the persistence and expansion of Marantaceae forests in northern Congo, a hotspot for this unique forest type.I used diverse approaches to explore the dynamics of Marantaceae forests, ranging from landscape scale to tree stand scale, and down to individual scale. First, I combined remote sensing and extensive field data to characterise the distribution and floristic and faunal composition of Marantaceae forests in the Sangha river region. I then focused on the community level to study the post-fire dynamics of these forests and the impact of logging on giant herbs and their reproductive strategy using extensive field inventories, soil and leaf measurements, seed experiments, drone acquisitions and genetic analyses. Finally, I examined the individual interactions between giant herbs and tree saplings in an 11-months nursery experiment to study competitive and potential allelopathic interactions, measuring the leaf and root traits of tree saplings in different treatments.My results show that open Marantaceae forests are significantly associated with drier areas and share a common flora with previously burned secondary forests, supporting the hypothesis of a specific link between Marantaceae forests and fire. I also found that fire had a positive impact on tree regeneration, but only in the short term, leading to a long-term maintenance of open Marantaceae forests, although fire also negatively affected giant herbs. Logging had a slight positive impact on giant herb abundance mainly thanks to asexual reproduction: some giant herbs grow as a waiting stage in undisturbed areas and colonise disturbed areas by clonality. Finally, the experiment showed that the maintenance of giant herbs is mainly due to their extreme competitiveness, mainly towards young pioneer species, without exerting allelopathy.This thesis proposes ecological mechanisms explaining why Marantaceae forests may indeed represent an arrested succession. The data and processes presented can inform models to test whether these forests are an alternative stable state to dense forests. Additionally, this work highlights the need for improved monitoring of this forest type, which is expected to thrive under current global change, with significant implications for both local and global populations.