The long-term decline of a succulent tree, Aloidendron pillansii, in the hyper-arid Richtersveld region of southern Africa

Abstract

With fewer than 9000 individuals, Aloidendron pillansii is listed as Critically Endangered, yet the timing and drivers of its decline remain poorly understood. We aimed to rectify this by examining long-term population dynamics at Cornell's Kop using repeat photography, ecological monitoring, simulation modelling, and climate analyses, and compared these with regional patterns of climate and vegetation change. Between 2004 and 2012, juvenile numbers (<1 m tall) were relatively stable (37–45 individuals), with recruitment slightly exceeding mortality. From 2013, juvenile mortality increased sharply, coinciding with the 2017–2019 centennial drought and, to a lesser extent, plant removal, resulting in a 51% decline by 2024. Adult numbers declined more slowly, from 42 plants in 2004 to 31 in 2024. Approximately half of adult losses occurred between 1937 and 2004, likely reflecting senescence of older cohorts. Range-wide size-class patterns indicate aging populations in the north and younger populations in the south, with Cornell's Kop intermediate. Since 1979, regional climate trends show declining rainfall, longer dry seasons, rising temperatures, and decreasing vegetation productivity. Overall, the species exhibits a dual decline: recent drought disproportionately affects juveniles, while adult losses reflect aging cohorts, with population replacement increasingly constrained by climate change and human disturbance. Our study examines the decline of a population of the Critically Endangered tree aloe Aloidendron pillansii on Cornell's Kop in the Richtersveld, South Africa, from 1937 to 2023, and demonstrates how drought has constrained juvenile recruitment while increasing mortality has occurred among ageing adult cohorts. • Juvenile numbers fell 51% from 2012 to 2024 due to severe drought and plant removal. • Adult losses largely reflect aging cohorts, with major declines before 2004. • Range-wide size classes show aging northern populations and younger southern ones. • Climate drying and reduced vegetation productivity threaten population recovery.