The Ecology of Encroachment: Identifying the Species and Plant Ecological Strategies Driving African Savanna Vegetation Change.

Woody encroachment threatens African savanna and grassland biodiversity and ecosystem function. Prior studies have focused on the drivers and rates, yet encroacher ecology remains underexplored. We synthesized data across six dominant woody savanna genera (Combretum, Dichrostachys, Prosopis, Senegalia, Terminalia and Vachellia) to test whether encroachers represent a non-random subset of species with life history strategies and functional traits that facilitate establishment and dominance in changing ecosystems. Species were classified as encroachers based on documented ecosystem impacts and compared to non-encroachers across climatic niches, geographic range size and traits linked to survival in disturbance-prone ecosystems, including maximum height, plant habit, spinescence and capacity for nitrogen fixation. We identified 63 encroacher species, which occupied broader temperature (11°C-29°C vs. 17°C-27°C) and precipitation (50-2100 mm vs. 444-2300 mm) niches and larger geographic ranges (600,000 km² vs. 100,000 km²) than non-encroachers. Encroachers were generally taller and exhibited plasticity in habit at individual and community scales. Our results suggest that encroachment is driven by ecologically versatile woody species with wide environmental tolerances, large ranges and high phenotypic plasticity. These advantageous attributes may enhance responsiveness to global change factors characteristic of African savannas and grasslands, including altered fire and herbivory regimes, along with rising atmospheric CO₂. Understanding the ecology of the species driving encroachment could improve early monitoring and management.