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Deserts have been predicted to be one of the most responsive ecosystems to global climate change. In this study, we examine the spatial and demographic response of a keystone endemic plant of the Namib Desert (Welwitschia mirabilis), for which displacement and reduction of suitable climate has been foreseen under future conditions. The main aim is to assess the association between ongoing climate change and geographical patterns of welwitschia health, reproductive status, and size. We collected data on welwitschia distribution, health condition, reproductive status, and plant size in northern Namibia. We used ecological niche models to predict the expected geographic shift of suitability under climate change scenarios. For each variable, we compared our field measurements with the expected ongoing change in climate suitability. Finally, we tested the presence of simple geographical gradients in the observed patterns. The historically realized thermal niche of welwitschia will be almost completely unavailable in the next 30 years in northern Namibia. Expected reductions of climatic suitability in our study sites were strongly associated with indicators of negative population conditions, namely lower plant health, reduced recruitment and increased adult mortality. Population condition does not follow simple latitudinal or altitudinal gradients. The observed pattern of population traits is consistent with climate change trends and projections. This makes welwitschia a suitable bioindicator (i.e. a 'sentinel') for climate change effect in the Namib Desert ecosystems. Our spatially explicit approach, combining suitability modeling with geographic combinations of population conditions measured in the field, could be extensively adopted to identify sentinel species, and detect population responses to climate change in other regions and ecosystems. [ABSTRACT FROM AUTHOR]

One of the most recognizable icons of the Namib Desert is the endemic gymnosperm Welwitschia mirabilis. Recent studies indicated that climate change may seriously affect populations in the northern Namibia subrange (Kunene region), but their extinction risk has not yet been assessed. In this study, we apply IUCN criteria to define the extinction risk of welwitschia populations in northern Namibia and assign them to a red list category. We collected field data to estimate relevant parameters for this assessment. We observed 1330 plants clustered in 12 small and isolated stands. The extent of occurrence has a surface area of 214.2 km2 (i.e., <5000 km2) and the area of occupancy a surface area of 56.0 km2 (i.e. < 500 km2). The quality of habitat is expected to face a reduction of 69.47% (i.e., >50%) as a consequence of climate change predicted in the area. These data indicate a very high extinction risk for welwitschia in northern Kunene, hence, we classify these populations as endangered (EN) according to IUCN criteria. Similar assessments for other subranges are prevented by the lack of relevant data, an issue that deserves further research attention. Our results advocate the necessity of a management plan for the species, including measures for mitigating the impact of climate change on isolated populations across its fragmented range. • The extent of occurrence of Welwitschia mirabilis in northern Kunene has a surface of 214.2 km2 (i.e. < 5000 km2). • The same populations have an area of occupancy of 56.0 km2 (i.e. < 500 km2). • The quality of habitat is expected to face a reduction of 69.47% (i.e. > 50%) as a consequence of climate change. • Welwitschia mirabilis can be classified as endangered (EN) A4c; B1ab(iii)+2 ab(iii) in the northern Kunene. [ABSTRACT FROM AUTHOR]