Ecological and phylogenetic patterns of carbon isotope discrimination in the winter-rainfall flora of the Richtersveld,South Africa

Leaf tissues of vascular plant species from the arid Richtersveld of northern Namaqualand (Succulent Karoo), South Africa, show a great range of variation in carbon isotope discrimination. Analyses of values for species with obligate C₃ metabolism demonstrated that the water use efficiency indicated by these values varies between species and within a species population both between seedlings and mature shrubs and between sites along an aridity gradient. An expected pattern of increasing water use efficiency with greater longevity in these species was not present, however. Only among ephemerals were there taxa with high values associated with low water use efficiency, but these high values were balanced by other ephemeral species with low values comparable to those typical of moderate to long-lived shrubs. The extent to which putative CAM species in the succulent flora rely entirely on nocturnal carbon fixation is highly variable. Species of Crassulaceae and Aloe (Asphodelaceae) relied almost exclusively on CAM photosynthesis, although Crassulaceae from more mesic environments may be CAM-flexible. Succulent-leaved species of Senecio and leaves of stem-succulent Euphorbia were CAM-flexible in their values, while shrubby species of Ceraria (Portulacaceae) appeared to rely primarily on C₃ photosynthesis. Variable patterns of reliance on CAM photosynthesis were present in the ecologically dominant Mesembryanthema of the Aizoaceae. Shrubby species of the subfamily Mesembryanthemoideae, although capable of CAM-flexibility in less xeric habitats, relied largely on CAM fixation of carbon in our field sites. Within the subfamily Ruschioideae, there was a wide range of values indicating species with CAM, CAM flexibility, and primary reliance on C₃ photosynthesis. Low values indicative of typical CAM fixation were associated with species with greater longevity, suggesting that a positive correlation may exist between the degree of reliance on CAM photosynthesis and plant life span.     

The succulent flora of South Africa and Namibia: representation and distribution

Abstract

Taxonomic representation and regional distribution patterns of taxa, life forms, and defence mechanisms of the succulent flora of South Africa and Namibia are quantified and discussed in relation to physical and biotic factors. Habitats are investigated and for each region centres of succulent species richness are pointed out.     

Cissus quadrangularis L.及其它小型肉质植物标本制作中的压制前处理

以Cissus quadrangularis L.为材料用不同的技术处理并比较其收缩程度、色彩保真程度以及茎附属物的保留情况。试验了3种常规预处理方法及2种针对肉质组织的处理方法,并进行ANOVA（Analysis of variance）分析,结果显示3种常规植物标本预处理方法明显不同（P＜0.005）,用LSD（least significant difference）分析方法分析得出深冷冻法对植物标本造成的损害比汽油法（LSD＝1.708）和福尔马林法（LSD＝2.065）要小;汽油法和福尔马林法处理后得到的标本质量无明显差别。用ANOVA法分析醋酸法、纵切法得到的标本及未经任何处理的标本,它们的效果也有明显不同（P＜0.001）,醋酸法得到的标本比纵切法（LSD＝2.371）和未经任何处理得到的标本（LSD＝2.138）效果更好,而纵切法和未经任何处理得到的标本并无明显不同（LSD＝0.233NS）。醋酸处理法制作小型肉质植物标本其效果得到了植物标本馆工作者的肯定。     

In vitro propagation of <Emphasis Type="Italic">Huernia hystrix</Emphasis>: an endangered medicinal and ornamental succulent

An efficient and rapid method for in vitro clonal propagation of Huernia hystrix was developed, resulting in shoot regeneration within 3 weeks of culture. This endangered medicinal and ornamental succulent is in high demand. Multiple shoots were regenerated from stem explants (10 mm length) cultured on Murashige and Skoog (MS) medium containing 3% sucrose and supplemented with a range of NAA (0.00–8.06 μM) and BA (4.44–22.19 μM) concentrations. A 100% shoot response with a multiplication rate of four shoots per explant was obtained on MS medium containing 5.37 μM NAA and 22.19 μM BA. Callus produced at the base of the explant on the same medium showed root organogenic potential. The in vitro regenerated shoots produced roots when transferred to half strength MS medium with or without auxin. The micropropagated plants were easily acclimatized within 2 months under greenhouse conditions when potted in a soil and sand mixture (1:1; v/v) treated with a fungicide (Benlate, 0.01%). More than 95% survival with no observable morphological variations was obtained. The developed protocol provides a simple, cost-effective means for the conservation of endangered H. hystrix by clonal propagation within a short time.     

A fence-line contrast reveals effects of heavy grazing on plant diversity and community composition in Namaqualand,South Africa

Changes in plant species richness and community composition were investigated across a fence separating heavily grazed communal and lightly grazed commercial farming systems in Namaqualand, South Africa. No significant differences in plant species richness between communal and commercial farming systems were detected either locally within individual plots or overall across all plots. Within-plot, richness of species tolerant of grazing, such as annuals and geophytes, has increased, while the richness of large palatable shrub species has decreased on the communal rangeland. In terms of plant cover, species' responses to grazing were strongly associated with growth form. Annuals and geophytes formed the majority of grazing increasers, while large, presumably palatable, shrubs and leaf succulents were characteristic grazing decreasers. An investigation into population processes of five shrub species revealed that heavy grazing on the communal rangeland has resulted in: reduced size of palatable shrub species; reduced flower production and seedling recruitment of palatable species; increased density and recruitment of the unpalatable shrub, Galenia africana. Reductions in shrub volume, reproductive output and seedling recruitment were most marked in the palatable shrub Osteospermum sinuatum and were in the order of 90%. The results are further discussed in terms of their relevance to rangeland dynamics and the current land use practices of the region.     

Wide-band tracheids in genera of Portulacaceae: novel, non-xylary tracheids possibly evolved as an adaptation to water stress

Wide-band tracheids (WBTs) are novel tracheids with wide, lignified secondary walls that intrude deeply into the cell lumen when viewed in transverse sections. These tracheids are found in a few genera in related families in the order Caryophyllales: Aizoaceae, Cactaceae, and Portulacaceae. WBTs in these three families vary in (1) systematic occurrence (found in more highly derived genera in each family), (2) location in plant organs, and (3) structure and dimensions. In addition, an analysis was conducted of WBT cell walls to test the hypothesis that WBTs evolved as an adaptation to water stress (i.e., the wide secondary walls should prevent collapse of the primary wall during water stress). The cell wall data show that primary cell walls in WBTs cannot inwardly collapse to occlusion, thus providing support for the water stress hypothesis of WBT evolution. With consideration of their systematic occurrence, the molecular phylogenetic data, and data here showing support for a water stress adaptive origin, it is logical to assume that WBTs evolved in genera that were adapting to environments undergoing a rapid trend toward aridification.     

Clumping and dispersal of chloroplasts in succulent plants

Plants have evolved various photoprotective mechanisms to mitigate photodamage. Here we report the diurnal movement of chloroplasts in the leaves of succulent crassulacean acid metabolism (CAM) plants under combined light and water stress. In leaves of water-stressed plants, the chloroplasts became densely clumped in one or sometimes two areas in the cytoplasm under light and dispersed during darkness. The chloroplast clumping resulted in leaf optical changes, with a decrease in absorptance and an increase in transmittance. The plant stress hormone abscisic acid induced chloroplast clumping in the leaf cells under light. We suggest that the marked chloroplast movement in these CAM plants is a photoprotective strategy used by the plants subjected to severe water stress.Abbreviations ABA Abscisic acid - CAM Crassulacean acid metabolism     

Female striped mice (Rhabdomys pumilio) change their home ranges in response to seasonal variation in food availability

Animals may respond to seasonally changing environments withphysiological and behavioral strategies. Whereas migration isa behavioral strategy used by many taxa, it may not be an optionfor small mammals. However, small mammals can seasonally varythe area of habitat in which they are active. The striped mouseRhabdomys pumilio in the semiarid Succulent Karoo of South Africalives in a seasonal environment, characterized by hot, dry summerswith low food abundance and cold, wet winters, followed by highfood abundance in spring. We radio tracked a total of 28 femalesduring the 2004 dry season, the following breeding season inspring, and the following dry season in 2005 and tested theprediction that females shift their home ranges in relationto food availability. Females shifted their home ranges froman area characterized by evergreen succulent shrubs in the vicinityof a dry riverbed in the dry season to sandy areas that werecharacterized by new plant growth of annuals in spring. Homeranges during the breeding season in spring had a higher percentageof annuals than dry season home ranges measured in spring. Femalehome range size increased during the breeding season. We suggestthat female striped mice shift their home ranges seasonallyto gain access to protein-rich young plant material, which isimportant for breeding.     

Biogeography of prostrate-leaved geophytes in semi-arid South Africa: hypotheses on functionality

Nowhere is the species diversity of geophytes greater than in the five mediterranean-climate ecosystems of the world. Of these, the Cape mediterranean zone of South Africa is the most speciose. While the relative diversity and importance of geophytes of all of the other four mediterranean regions of the world drops off sharply as one moves into adjacent winter-rainfall desert regions, geophytes in the semi-arid to arid Succulent Karoo (including Namaqualand) remain a very important component of the flora, both in terms of abundance and diversity (comprising 13 to 29% of the regional floras in this region). Apart from species richness, there are also a number of interesting geophyte growth forms in this region. One unusual growth form is geophytes with flattened leaves that lie prostrate on the soil surface. At least eight families (Amaryllidaceae, Colchicaceae, Eriospermaceae, Geraniaceae, Hyacinthaceae, Iridaceae, Orchidaceae and Oxalidaceae) exhibit this growth form. While this growth form is relatively common in many geophyte lineages in the Succulent Karoo biome and the Cape mediterranean zone (Fynbos biome), and occurs infrequently through the summer-rainfall temperate regions of Africa, it is virtually absent in other regions worldwide. A null hypothesis is that the prostrate leaved trait is a neutral characteristic, however biogeographical data do not support this. A neutral trait would be unlikely to show such a clear pattern of distribution. Several alternative hypotheses on the adaptive significance of this growth form are discussed. These include: avoidance of herbivory, reduction in competition from neighbors, creation of a CO₂ enriched environment below the leaves, reduction of water loss around the roots, reduction of water loss through transpiration, precipitation of dew on the leaves and maintenance of optimal leaf temperatures for growth.     

Plant species richness and composition a long livestock grazing intensity gradients in a Namaqualand (South Africa) protected area

The study described changes in floristic and vegetation structure in relation to livestock grazing intensity in a conservation area in the Succulent Karoo, South Africa. Grazing by goats and sheep is allowed in the Richtersveld National Park (a contractual National Park. which is also an area of high floristic richness and endemism. We used goat faecal pellet density, degree of trampling and percentage bare-ground at distances from the stock posts as surrogates for a gradient in grazing pressure. A stock post is the place where farmers keep, in most cases in an enclosure called a ‘kraal’, their animals at night and to which they return every evening after the day’s herding. Twenty-seven stock posts were located in the Richtersveld National Park; nine stock posts on flats, foot– slopes and mountain each. We measured plant species richness and diversity, and mean percentage cover of the various plant growth forms (including the number of species falling into each growth form category. in each of the five 10 m . 10 m plots (each 200 m apart. demarcated along a transect of one kilometre length from the centre of each stock post. The results showed that distance from the stock post does reflect grazing intensity use because densities in faecal pellets rapidly declined with increasing distances away from the stock post for all habitats studied. Faecal density was positively correlated with stocking density. Plant species richness and diversity was at a minimum near stock posts. Plants able to endure the effects of heavy grazing occurred near stock posts where declines in palatable plant species, assumingly sensitive to heavy grazing and trampling, were recorded. Grazing increased vegetation patchiness up to 800 m from the stock post for all the habitats. The degree to which this change in species composition occurred did not depend on stocking densities, suggesting that both grazing and landscape variability were responsible for vegetation changes in rangelands of that area of the Succulent Karoo biome.     

Context‐dependent interactions between adult shrubs and seedlings in a semi‐arid shrubland

Abstract. Question: In semi‐arid systems, rainfall gradients can cause plant‐plant interactions to shift from negative to positive or vice versa. However, the importance of a second major abiotic factor, soil nutrients, has rarely been considered. We consider different combinations of both factors and ask: do net adult‐seedling interactions become less competitive and more facilitative with increasing overall abiotic harshness? Location: Succulent Karoo, Western Cape, South Africa. Methods: We examined the interactions between seedlings and adult shrubs at two sites. Sites differ in rainfall, and each contain two habitats: Nutrient‐rich mounds associated with underground termitaria and a relatively nutrient‐poor matrix. We carried out a spatial pattern analysis of community‐wide seedling‐adult associations. We then conducted field and greenhouse experiments to test the effects of soil and the presence of neighbouring shrubs on the growth and survival of six seedling species. Results: At the higher rainfall site, both competitive and facilitative effects of adults on seedlings were found but did not differ by habitat, despite the more benign conditions in the mound habitat. At the lower rainfall site, adult shrubs generally had neutral effects on seedlings in the matrix habitat. In the nutrient‐rich mound habitat, however, adult shrubs had strong and consistently competitive effects on seedlings. Conclusion: Seedling‐adult interactions could not be predicted by a simple overall gradient of abiotic harshness, demonstrating the need for more complex, mechanistic models to predict plant‐plant interactions. We suggest that rainfall and soil nutrients affect seedling‐adult relations through their interactive effects on the life‐history attributes of the species involved.     

Correlates of hyperdiversity in southern African ice plants (Aizoaceae)

The exceptionally high plant diversity of the Greater Cape Floristic Region (GCFR) comprises a combination of ancient lineages and young radiations. A previous phylogenetic study of Aizoaceae subfamily Ruschioideae dated the radiation of this clade of > 1500 species in the GCFR to 3.8–8.7 Mya, establishing it as a flagship example of a diversification event triggered by the onset of a summer‐arid climate in the region. However, a more recent analysis found an older age for the Ruschioideae lineage (17 Mya), suggesting that the group may in fact have originated much before the aridification of the region 10–15 Mya. Here, we reassess the tempo of radiation of ice plants by using the most complete generic‐level phylogenetic tree for Aizoaceae to date, a revised calibration age and a new dating method. Our estimates of the age of the clade are even younger than initially thought (stem age 1.13–6.49 Mya), supporting the hypothesis that the radiation post‐dates the establishment of an arid environment in the GCFR and firmly placing the radiation among the fastest in angiosperms (diversification rate of 4.4 species per million years). We also statistically examine environmental and morphological correlates of richness in ice plants and find that diversity is strongly linked with precipitation, temperature, topographic complexity and the evolution of highly succulent leaves and wide‐band tracheids. © 2013 The Authors. Botanical Journal of the Linnean Society published by John Wiley & Sons Ltd on behalf of The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174 , 110–129.     

Effects of Multispecies Clumping on Survival of Three Succulent Plant Species Translocated onto Mine Spoil in the Succulent Karoo Desert, South Africa

Salvaging from premining areas and translocation of succulent plants has been investigated for the revegetation of gypsiferous mine spoil in an arid region of South Africa. Given that facilitation effects are thought to outweigh competition effects in harsh environments, we hypothesized that the survival of translocated succulents would be higher when planted in multispecies clumps than alone and that the growth rate (measured as stem extension) and fruit‐set would be greater for plants in clumps than for those planted alone. Two leaf‐succulent species (Aridaria noctiflora ssp. noctiflora and Drosanthemum deciduum) and one stem‐succulent species (Psilocaulon dinteri) were salvaged from the area destined for mining and translocated onto the mine spoil. These plants were planted either in a multispecies clump of the three species together or alone. One year after translocation, 67% of the plants survived. It was also found that the succulents used in these experiments survived in higher numbers when planted alone. Due to the similar root morphology of D. deciduum and P. dinteri, they competed for resources instead of facilitating each other's establishment. The results were variable for each of the species used, and neither growth nor seed‐set was improved by clumping. These findings would also possibly vary from year to year with different abiotic conditions.     

The role of domestic herbivores in endozoochorous plant dispersal in the arid Knersvlakte,South Africa

Herbivores can act as dispersal vectors by purposely or accidentally ingesting ripe fruits, and thus endozoochory is one determining factor for plant distribution patterns. The objective of our study was to investigate to what extent plants of major taxonomic groups of the Knersvlakte (Succulent Karoo, South Africa) are endozoochorously dispersed. On three different farms in the central Knersvlakte, dung of domestic herbivores was collected and analysed by the seedling-emergence method. The resulting species composition was compared to the standing vegetation of thirty-four 1000-m² plots each recorded on one hundred 400-cm² subplots. Our results show that domestic livestock facilitated the dispersal of taxa characteristic of the Knersvlakte, in particular Aizoaceae. Among the taxa of this family, the local endemic dwarf shrub Drosanthemum schoenlandianum emerged with the highest frequency in dung (14.5% of all seedlings). For the Asteraceae, which are frequent in the standing vegetation of the Knersvlakte, however, endozoochorous dispersal by livestock was only of minor importance. Conservation planning should consider these dispersal patterns on behalf of future population dynamics. The complete exclusion of livestock might change current processes and thus alter vegetation patterns.     

Long-term population changes in the Giant Quiver Tree, Aloe pillansii in the Richtersveld, South Africa

With probably fewer than 3000 individuals alive in the biodiversity hotspot of the Succulent Karoo in southern Africa, populations of the endemic, Giant Quiver Tree, Aloe pillansii, are thought to be declining and thus threatened with extinction. Using repeat photography and field data we investigated the long-term changes in one population of A. pillansii at its type locality, the roughly 100 ha Cornell’s Kop in the Richtersveld, South Africa. There are currently 75 individuals alive at this site. Of these, 44% are <1 m in height (seedlings), 4% are 1–3 m (juveniles) and 52% are >3 m (adults). An analysis of 14 repeat photographs shows that since 1937 an average of 1.4% of the plants >3 m in height has died annually. At this rate all the remaining 39 plants on Cornell’s Kop in this size class will be dead in 71 years. The relative paucity of plants in the 1–3 m size classes could be explained by several factors including plant theft, animal damage and unfavourable recruitment conditions during the first 80 years of the 20th century. Annual growth rates decrease as plants age. Individuals <1 m in height grow at 42.5 mm yr⁻¹ while plants 1–3 m and those >3 m grow at 31.0 and 16.4 mm yr⁻¹ respectively. At 8 m, the tallest plant on Cornell’s Kop could be as old as 382 years and thus to maintain itself at this site, A. pillansii would only need to recruit relatively infrequently. The relatively high proportion of seedlings suggests that conditions have recently been favourable for recruitment at this site. Seedling ages, estimated from their heights, indicate that over 50% of the plants <1 m in height germinated 5–10 years ago. This is consistent with local rainfall records which show that rainfall was consistently above the long-term annual average of 75 mm during this period. However, the loss of six seedlings from the population in the last 5 years, probably due to grazing or theft, suggests that without intervention this species will not survive on Cornell’s Kop.     

Biodiversity,habitat and range-size aspects of a flora from a winter-rainfall desert in north-western Namaqualand,South Africa

We examined patterns of richness and endemism in relation to phylogeny (family membership), growth form and habitat in the flora of an arid (<60 mm annual rainfall), sandy coastal belt between Port Nolloth and Alexander Bay on the north-west coast of Namaqualand, South Africa. This region forms part of the species-rich Succulent Karoo biome. In an area of 750 km₂, 300 plant species were recorded with 24% endemnicity. This level of diversity and endemism is exceptionally high for a desert region. Plant distribution is strongly influenced by habitat type. Diversity, especially of endemic species, was concentrated on rocky or unusual (lag-gravel pavements) habitats. Endemics were not random phylogenetic nor biological assemblages. Dwarf leaf-succulent shrubs, especially Mesembryanthemaceae, were significantly over-represented amongst the endemic component. The results are discussed in terms of a model for plant diversification in an arid environment with relatively predictable sources of moisture.     

Succulent plants on arid inselbergs

Succulent plants in arid areas are believed to be often associated with high altitudes, except where they form the dominant, zonal vegetation such as in the Succulent Karoo Biome of southern Africa. To test this hypothesis the contribution of perennial succulents to inselberg floras was investigated on twenty-two isolated mountains (inselbergs) at four study sites (Etendeka–Barab: 19°34′ S/13°42′ E; Spitzkoppe: 21°49′ S/15°09′ E; Namibrand: 25°17′ S/15°47′ E; and Sperrgebiet: 27°43′ S/16°04′ E) along a nearly 1000 km bioclimatic gradient in the Namib Desert and adjacent areas, from the Succulent Karoo in the south to the northern Namib, and along altitude gradients.The contribution of succulents to the flora clearly increased with altitude at the two central study sites, thus showing a hump-shaped trend along the bioclimatic gradient. No such correlation was found at the northern and southern end of the bioclimatic gradient. The surrounding (rocky) lowlands likely resulted in a more even distribution of succulents throughout the landscape in the north, while the position in the heartland of the succulent flora in southern Africa at the southern-most site, levelled out potential patterns related to altitude. Nevertheless, even here, a fair number of succulents restricted to mountain habitats occur. This is the case in all other study sites, where succulents restricted to mountain habitats are always present.Some perennial succulents, such as Aridaria noctiflora, Euphorbia gummifera and Tetragonia reduplicata indicate an upward movement with increasing distance from the Succulent Karoo, by growing on plains and lowlands in the Succulent Karoo, but only on inselbergs at study sites north of the Succulent Karoo.Many of the mountain specialist succulents are sought after by plant collectors, such as species of Conophytum, Crassula and Lithops. Protecting and monitoring vulnerable populations as well as growing these in horticulture may help to alleviate the pressure on some of these rare species in nature. Preserving genetic material of isolated populations from mountain tops ex situ is another important conservation measure, and particularly important in the view of anticipated climatic change.     

Field surveys and revegetation experiments show that simulation of topographical habitat features could improve the chances of successful restoration for the threatened succulent Juttadinteria albata

Species with highly restricted ranges are disproportionately at risk of extinction, particularly where habitat loss occurs as a result of mining. Postmining restoration of rare species populations is considered as an appropriate response to counter such threats, but requires a careful, evidence‐based, and information‐driven approach. The economically important diamond mining at Sendelingsdrif in the southern Namib Desert occurs in the highly diverse Succulent Karoo Biome and threatens a significant part of the population of the narrow endemic succulent plant species Juttadinteria albata. To decrease the inherent risks of restoring such a rare species, we studied the habitat features of premining J. albata populations and experimentally tested whether some features could assist future reintroductions in postmining substrates. Plots where J. albata occurred were mostly south‐ to west‐facing and had among others higher rock cover and steeper slopes than plots where J. albata plants were absent. A revegetation experiment, with J. albata cuttings that were established on postmining substrate mounds, revealed that plants on steeper slopes, facing south to west, grew faster than plants on other slopes and aspects. Slope and aspect are therefore important habitat properties to recreate when restoring J. albata populations. These, and other preferred habitat properties such as higher levels of organic C, should now be tested in larger‐scale field trials. Validation of habitat requirements of J. albata through the revegetation experiment has decreased the risks at least partially and provides additional empirical evidence of the importance of establishing reference conditions to enhance ecological restoration.     

Do geographic distribution,niche property and life form explain plants' vulnerability to global change?

We modelled the future distribution in 2050 of 975 endemic plant species in southern Africa distributed among seven life forms, including new methodological insights improving the accuracy and ecological realism of predictions of global changes studies by: (i) using only endemic species as a way to capture the full realized niche of species, (ii) considering the direct impact of human pressure on landscape and biodiversity jointly with climate, and (iii) taking species' migration into account. Our analysis shows important promises for predicting the impacts of climate change in conjunction with land transformation. We have shown that the endemic flora of Southern Africa on average decreases with 41% in species richness among habitats and with 39% on species distribution range for the most optimistic scenario. We also compared the patterns of species' sensitivity with global change across life forms, using ecological and geographic characteristics of species. We demonstrate here that species and life form vulnerability to global changes can be partly explained according to species' (i) geographical distribution along climatic and biogeographic gradients, like climate anomalies, (ii) niche breadth or (iii) proximity to barrier preventing migration. Our results confirm that the sensitivity of a given species to global environmental changes depends upon its geographical distribution and ecological proprieties, and makes it possible to estimate a priori its potential sensitivity to these changes.