Landscape degradation may contribute to large-scale die-offs of Euphorbia ingens in South Africa

Abstract

Euphorbia ingens, a large succulent tree species native to southern African savanna ecosystems, has died in large numbers in recent years in some areas of South Africa. A previous study found that changes in climate (higher temperatures and lower or more variable rainfall) likely play an important role in causing mortality. However, anecdotal evidence suggests that stress due to habitat degradation may also contribute to E. ingens die-offs. In this study, we evaluated E. ingens die-offs in South Africa at 10 sites. Specifically, we aimed to examine the roles of both climate and landscape degradation in causing the die-offs. We used a combination of climate data, estimates of tree mortality and ratings of die-off symptoms (categories of grey discoloration and rotting associated with moth attacks), and proxies for landscape degradation associated with livestock grazing. We assessed which sites exhibited greater mortality and die-off associated symptoms, and whether they exhibited spatial auto-correlation (did distance between sites correlate with severity of E. ingens die-off?). We also used correlation analysis to compare tree mortality to proxies of savanna ecosystem degradation. These proxies were dung counts (livestock), woody debris counts, plant and bare soil cover, soil nutrients, and density of Dichrostachys cinerea (Fabaceae), a savanna plant that dominates when disturbance is high. Minimum and maximum temperatures as well as precipitation were compared among sites. There was no spatial auto-correlation between distance and die-off severity among sites, and sites with greater levels of tree mortality were associated with proxies indicating degradation. This suggests that die-offs of E. ingens are likely due to a complex of stressors, including both changes in climate and poor land-use practices. Our results indicate that sustainable rangeland practising of South African savannas may aid in conserving E. ingens and retaining this iconic tree on the landscape.