Climate change effects on desert ecosystems: A case study on the keystone species of the Namib Desert Welwitschia mirabilis

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.

The Welwitschia genome reveals a unique biology underpinning extreme longevity in deserts

The gymnosperm Welwitschia mirabilis belongs to the ancient, enigmatic gnetophyte lineage. It is a unique desert plant with extreme longevity and two ever-elongating leaves. We present a chromosome-level assembly of its genome (6.8 Gb/1 C) together with methylome and transcriptome data to explore its astonishing biology. We also present a refined, high-quality assembly of Gnetum montanum to enhance our understanding of gnetophyte genome evolution. The Welwitschia genome has been shaped by a lineage-specific ancient, whole genome duplication (~86 million years ago) and more recently (1-2 million years) by bursts of retrotransposon activity. High levels of cytosine methylation (particularly at CHH motifs) are associated with retrotransposons, whilst long-term deamination has resulted in an exceptionally GC-poor genome. Changes in copy number and/or expression of gene families and transcription factors (e.g. R2R3MYB, SAUR) controlling cell growth, differentiation and metabolism underpin the plant’s longevity and tolerance to temperature, nutrient and water stress.

Foliar fungi of the enigmatic desert plant Welwitschia mirabilis show little adaptation to their unique host plant

Foliar fungi, especially endophytic fungi, constitute an important part of the microbiome of plants. Yet little is known about the composition of these communities. In this study, we isolated fungi from leaf tissues of the desert plant Welwitschia mirabilis to determine the culturable diversity of the foliar fungal community. The isolated fungal taxa, which grouped into 17 distinct lineages, were identified by sequencing elongation factor 1 alpha, beta tubulin 1, beta tubulin 2 and the internal transcribed spacer region. The culturable community was mainly composed of cosmopolitan fungal genera despite the unique taxonomic position of the plant and its geographic isolation. To test for endemism in two of the common fungal genera, Alternaria and Aureobasidium, we built haplotype networks using a global data set. Even this broad data set showed little evidence for specialisation within this unique host or its geographical location. The data suggest that the culturable members of communities of leaf-associated fungi in habitats with little plant coverage, such as the Namib Desert, are mainly established by long-distance aerially distributed fungal inocula and few of these taxa co-evolve with the host within the habitat.

A baseline assessment of the photosynthetic potential of Welwitschia mirabilis using the JIP-test for monitoring and conservation purposes

Background: Welwitschia mirabilis is highly specialised to survive the harsh climate of the Namib Desert. Changes in land use, such as the expansion of mining activities, may endanger their survival.Objectives: The purpose of this study was to understand the photosynthetic potential of W. mirabilis plants to provide a baseline for future long-term monitoring, and for future comparison to determine plant health status after the onset of mining operations.Methods: The study was conducted in a population of W. mirabilis on the Welwitschia Plains. Chlorophyll a fluorescence data were used to measure plant photochemical potential and analysed using the JIP-test.Results: Significant differences in the photosynthetic potential was observed for W. mirabilis plants located in different catchments. The partial parameters of the PIABS values were also significantly lower, which indicated that all aspects of photosynthesis were influenced.Conclusion: PIABS values can serve as a baseline for future long-term monitoring studies to detect any changes in the health status of W. mirabilis that might result from land use change.

Welwitschia: Phylogeography of a living fossil, diversified within a desert refuge

Welwitschia mirabilis is one of the most extraordinary plant species on earth. With a fossil record of 112 My and phylogenetically isolated within the order Gnetales, the monotypic genus Welwitschia has survived only in the northern Namib Desert in Angola and Namibia. Despite its iconic role, the biogeography, ecological niche, and evolutionary history of the species remain poorly understood. Here we present the first comprehensive map of the strongly disjunct species range, and we explore the genetic relationships among all range fragments based on six SSR markers. We also assess the variation of the environmental niche and habitat preference. Our results confirm genetic divergence, which is consistent with the hypothetical existence of two subspecies within Welwitschia. We identify an efficient geographical barrier separating two gene pools at 18.7°S in northern Namibia. We also identify further diversification within each of the two subspecies, with several different gene pools in ten isolated range fragments. Given the presence of well-isolated populations with unique gene pools and the association with different bioclimatic variables, rock types, and habitats within arid river catchments, we can hypothesize that the present intraspecific diversity may have evolved at least in part within the present refuge of the northern Namib Desert.

The ‘fertile island effect’ of Welwitschia plants on soil microbiota is influenced by plant gender

ABSTRACT Desert and semi-desert plants are often associated with a distinct soil biota under the plants and close to the root system. We aimed to understand if similar effects could be found in the taxonomically isolated desert gymnosperm Welwitschia mirabilis in the Namib Desert, and whether this island effect varied with climate and with gender of plants. We took soil cores adjacent to the plants in environments ranging from extreme desert to arid shrubland, and in nearby control sites between the plants. Soil chemistry was analysed, and deoxyribonucleic acid was extracted and sequenced for the bacterial 16s region. Soil under the plants was richer in organic C, N and moisture. Despite the range of climates, the soil around Welwitschia plants was consistently associated with a particular bacterial community composition that was distinct from samples further away. Compared to unvegetated control patches, bacterial diversity close to the plants was reduced. In the plant-associated soil community, there was a clear gender effect across all sites with a distinct community composition and greater diversity under male plants. It is unclear what differences in the soil environment might be producing these gender-associated differences, which provide an additional dimension to the fertile island effect.

Very high extinction risk for Welwitschia mirabilis in the northern Namib Desert

One of the most recognisable icon 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 in the field 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 of 214.2 km2 (i.e. < 5000 km2) and the area of occupancy a surface 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 and 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.

Evidence of climate change impacts on the iconic Welwitschia mirabilis in the Namib Desert

Climate change represents an important threat to global biodiversity and African ecosystems are particularly vulnerable. Recent studies predicted substantial variations of climatic suitability for Welwitschia mirabilis under future conditions. Latitudinal/altitudinal range shifts are well-known responses to climate change but not coherent patterns were documented. This study aims to verify whether welwitschia populations are responding to climate change and if the assumption of a latitudinal/altitudinal shift is applicable. We collected field 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 climatic suitability under future scenarios. For each variable, we compared the observed pattern with the expected responses. Finally, we tested the presence of simple geographical gradients in the observed patterns. The realized thermal niche of welwitschia will be almost completely unavailable in the next 30 years in northern Namibia. Expected reductions of climatic suitability in the stand sites are strongly associated with indicators of negative population conditions. The same population conditions does not fit any simple latitudinal or altitudinal gradient. The observed pattern of population conditions mirrors the expected pattern of climate change effect but no simple geographical gradient was relieved. Overall, we observed negative population conditions in areas with stronger reductions of suitability. This makes welwitschia a suitable sentinel for climate change effect in the Namib Desert ecosystems. Our approach to detect population responses to climate change could be extensively adopted for selecting sentinel species in other regions and ecosystems.