Catastrophic collapses of sensitive species, including the quiver tree (Aloidendron dichotomum), following fire in the arid Nama-Karoo, South Africa

Fire is rare in arid ecosystems owing to low fuel loads, and unadapted species face catastrophic population collapses if burnt. Slow-growing, rare species are particularly susceptible. Increased grassiness and drought-induced enhancement of flammability can promote fire in arid systems, and both may increase with climate change. A fire in the usually fire-free Nama-Karoo burnt quiver trees (Aloidendron dichotomum) in 2017 during severe drought. Vegetation comprises an understory of grass and shrubs, and a population of quiver trees. Desiccated shrubs provided fuel load. Steep slopes aided fire spread. Vegetation responses were recorded two years post-fire at twelve paired sites along the fire boundary. A 4-state mixture model demonstrated that the quiver tree population had developed through pulsed recruitment. Significant (P ≤ 0.05) fire-induced collapses in shrubs, dwarf-shrubs, succulent shrubs, and succulent dwarf-shrubs occurred, with six species eliminated. Fewer than 10% of quiver trees survived. Total canopy cover was lower post-fire (P < 0.001); only micro-grasses increased. Climatic conditions supporting germination are rare, shaded micro-environments provided by the understory have been destroyed, therefore the quiver tree population is unlikely to recover soon. Future increased grassiness and severe droughts may heighten future mortality risk of quiver trees as climate change progresses.