Investigating the environmental and anthropogenic spatio-temporal patterns of plant health of Welwitschia mirabilis in the central Namib Desert
Environmental and anthropogenic patterns are the major factors that determine plant persistence across the landscape over a long-term period. The spatio-temporal patterns of vegetation distribution in the Namib Desert biome is attributed to the climatic conditions, geomorphology and edaphic features. Landscape patterns in the environmental conditions (e.g. rainfall and fog) of the Namib Desert have been described as a complex gradient that has a major influence on vegetation over time. Welwitschia mirabilis Hook. fil (Gnetales: Welwitschiaceae), commonly known as Welwitschia, is one of the Namib Desert endemic species. Many surveys have reported that desert plants, such as Welwitschia are well adjusted to environmental stressors. Nevertheless, cumulative impacts (potential impacts include interference with the water supply and deposition of dust) resulting from developments in the central Namib such as uranium mines might significantly affect the Welwitschia plants’ survival. These impacts and the changing baseline conditions might affect their physiological processes, thus their growth and reproduction. In this study, we measured chlorophyll a fluorescence (as an indicator of photosynthetic efficiency), leaf growth rate and cone dimensions to determine whether there are spatial and temporal differences among Welwitschia plants across catchments over time. The study showed a clear effect of topography, climatic conditions and anthropogenic effects, with plants in different catchments being clearly healthier than others during some months. We observed an increase in the photosynthetic efficiency and leaf growth rate accelerated by the episodic rainfall that occurred during January and April months across all catchments. Anthropogenic effects such as browsing by domestic animals probably caused the reduction in photosynthetic efficiency of plants located at the Welwitschia Wash catchment in December. Plants that were located at Welwitschia campsite catchment had the lowest photosynthetic efficiency and leaf growth rate throughout our investigation when compared to plants in other catchments. We have established a baseline study that can be used to develop a protocol to monitor the plant physiological status of Welwitschia. In this way, the results from the study will feed into a management strategy for this Welwitschia population. Apart from that, the findings may also aid restoration as well as rehabilitation measures such as transplantation and reintroduction of this unique plant by understanding its current functional health status across the landscape and over time.