Influence of tree clumps on vegetation dynamics and resilience in a west African humid savanna submitted to annual fires

Annual fires shape humid savanna dynamics, yet their interaction with heterogeneous vegetation structures remains poorly understood. This study examines how tree clumps influence vegetation dynamics in the fire-prone savanna of Lamto (Côte d’Ivoire), testing the hypothesis that these formations locally alter fire regimes, creating micro-habitats that facilitate species survival and forest tree establishment. We sampled fifty tree clumps (>9 trees each), comparing understory (internal) zones with adjacent external areas (5 m perimeter). Vegetation structure, fire behavior, tree health, and plant community composition were quantified. Results reveal a protective “umbrella effect” of tree clumps. Understory zones exhibited lower grass height and biomass, leading to significantly reduced flame heights compared to external areas. This fire mitigation enhanced species richness beneath tree clumps, supporting savanna species (Piliostigma thonningii, Crossopteryx febrifuga), along with juvenile regeneration of forest and exotic species (Elaeis guineensis, Ceiba pentandra, Azadirachta indica). External zones were dominated by pyrophilic grasses (Hyparrhenia sp. 88%), while understories featured less flammable assemblages. Tree health (Bridelia ferruginea) was superior under tree clumps, confirming their protective function. Tree clumps function as critical fire refuges, attenuating fire intensity and creating niches for fire-sensitive species within the savanna matrix. They serve as resilience nuclei and potential drivers of forest expansion. However, exotic species colonization within refuges raises concerns about invasion vulnerability. Preserving these keystone structures is essential for maintaining biodiversity and ecosystem processes in West African savannas under global change.