Spatial opportunity, rather than conserved traits, is associated with encroachment among African Vachellia species

Abstract

Woody plant encroachment is transforming savanna ecosystems, but the factors determining why some woody species proliferate while others remain at low densities remain poorly understood. We tested whether documented encroachment among African Vachellia species is associated with geographic range size, available functional traits, or phylogenetic structure. We combined a systematic review of 134 studies published between 1995 and 2025 with species-level occurrence records, 23 leaf, structural, allocation and reproductive traits from the TRY database, and a reconstructed Vachellia phylogeny. Among 36 African Vachellia species examined, encroachers occupied substantially broader geographic ranges than species not documented as encroaching. Encroachment status showed moderate phylogenetic structure but was not restricted to a single clade. Pooled trait comparisons suggested that encroachers had higher leaf nitrogen and phosphorus concentrations, enriched δ15N values and greater seed dry mass, but most trait differences weakened after accounting for phylogenetic relatedness. Seed dry mass showed the clearest difference in the pooled comparisons. These results indicate that Vachellia encroachment is not explained by a single conserved trait syndrome. Instead, encroachment appears to arise when spatially widespread and evolutionarily labile species encounter environmental and disturbance conditions that permit recruitment, survival, and population increase. Incorporating geographic range size, species-level functional variation and evolutionary lability may improve predictions of woody encroachment under changing fire, herbivory, rainfall and atmospheric CO2 regimes.