Margaretha W. van Rooyen

Worldwide evidence of a unimodal relationship between productivity and plant species richness

Grassland diversity and ecosystem productivity The relationship between plant species diversity and ecosystem productivity is controversial. The debate concerns whether diversity peaks at intermediate levels of productivity—the so-called humped-back model—or whether there is no clear predictable relationship. Fraser et al. used a large, standardized, and geographically diverse sample of grasslands from six continents to confirm the validity and generality of the humped-back model. Their findings pave the way for a more mechanistic understanding of the factors controlling species diversity. Science, this issue p. 302 The humped-back model of plant species diversity is confirmed by a global grassland survey. The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

Fruit polymorphism in ephemeral species of Namaqualand. I. Anatomical differences between polymorphic diaspores of two Dimorphotheca species

Differences in germination behaviour between different diaspore types of Dimorphotheca sinuata DC. and Dimorphotheca polyptera DC. induced the morphological study of the above-mentioned species. The study reveals that the difference between the diaspore types lies in the structure of the pericarp. Pericarp characteristics such as presence of mucilage, tanniniferous substances and cuticles as well as extensive sclerenchymatous zones, are associated with those diaspore types with the poorest germination. Fruit polymorphism represents a survival strategy in these species from the arid Namaqualand.

A qualitative study of the nodulating ability of legume species: List 6

Ninety legume species were investigated for the presence of Rhizobium- type root nodules. Only three species, namely Xylia torreana Brenan of the Mimosoideae and Guibourtia conjugata (Bolle) J. Leonard and Piliostigma thonningii (Schumach.) Milne-Redh. of the Caesalpinioideae appear to lack the ability to nodulate. Fifty-nine of the species investigated were not studied previously in this regard. The nodulation observed on two Liparia species also represents the first nodulation reports for members of this genus.

A spatial and temporal analysis of Sand Forest tree assemblages in Maputaland, South Africa

Abstract Maputaland is a centre of plant endemism, within the Maputaland–Pondoland–Albany hotspot of biodiversity shared by Mozambique and South Africa. The Sand Forest vegetation is the most valuable vegetation type in this region due to the endemics it harbours, but it is currently under threat from growing animal population densities within conserved areas and from a growing human population outside. To improve our understanding of Sand Forest dynamics the present study investigated the woody plant assemblages in Tembe Elephant Park. A total of 59 plots were sampled and analysed using classification and ordination methods. The results indicate that Sand Forest in Tembe is a complex assemblage of at least three floristically and structurally distinct woody communities. An ordination of the combined results from the present study together with two other studies could link the floristics and structure of the Sand Forest to the level of disturbance by herbivores and humans. Large herbivores induce both structural and floristic changes in the Sand Forest of Tembe Elephant Park and a lack of disturbance in the nearby Tshanini Community Conservation Area leads to a state of equilibrium. The results therefore suggest that Sand Forest dynamics and complexity may be strongly linked to small- to medium-scale animal disturbance.

The effect of herbivores and humans on the Sand Forest species of Maputaland, northern KwaZulu-Natal, South Africa

Sand Forest in the Maputaland region of KwaZulu-Natal in South Africa is deemed the most valuable, but also probably the most complex vegetation type of this part of the Maputaland–Pondoland–Albany hotspot of biodiversity. However, Sand Forest is under threat from the current human population growth in that region as well as from uncontrolled increases in wild herbivore numbers in conservation areas. The present study compares the impacts of herbivores and humans on the state of woody resources between two sites under differing utilisation regimes. Sand Forest was found to be a complex association of tree assemblages defined by different canopy and subcanopy properties. Although marked differences in the abundance of selected species were noted at the two sites, the Sand Forest remained dominated by fine-grained species under both utilisation regimes. The fine-grained nature of Sand Forest implies that regeneration depends on the creation of small canopy gaps either by natural processes, humans or elephants, while the creation of large gaps could transform it into woodland. Management of conservation areas where Sand Forest occurs should therefore concentrate efforts on regulating animal populations to levels that provide gap properties that favour forest regeneration.

Fruit polymorphism in ephemeral species of Namaqualand. II. Anatomical differences between polymorphic diaspores of Arctotis fastuosa and Ursinia cakilefolia

The different colour diaspore types of Arctotis fastuosa Jacq. and Ursinia cakilefolia DC. were examined. Pericarp and seed coat anatomy revealed only minor differences between fruit morphs. Diaspores of A . fastuosa are characterized, amongst others, by a lens-shaped pericarp operculum and bubble-shaped, air-filled cells in the outer epidermis of the seed coat. The latter may represent a mechanism to protect the embryo against excessive heat.

Fruit polymorphism in ephemeral species of Namaqualand. VI. Intermorphic competition among plants cultivated from dimorphic diaspores

Plants raised from disc and ray diaspores of Dimorphotheca sinuata, a polymorphic annual species from Namaqualand, were grown in replacement series. There were four plants per pot with disc to ray ratios of 4:0, 2:2 and 0:4. The plants were grown under different moisture and nutrient regimes. Disc plants were more aggressive than ray plants and thus the stronger competitors. Disc plants had a higher total dry matter in mixture than in monoculture under high moisture conditions. There was no difference in production between mixtures and monocultures of the ray plants. Ray plants were influenced more by disc plants than disc plants by ray plants. Both disc and ray plants were more successful when water and nutrients were freely available. Moisture was the main factor influencing intermorphic competition.

Evaluating patterns of wood use for building construction in Maputaland, South Africa

A questionnaire survey was conducted to evaluate the level of wood utilization for house building In the community of Manqakulane In Maputaland, northern KwaZulu-Natal, South Africa, In 2003. The questionnaires aims were to evaluate In both a descriptive and quantitative manner the utilization of wood In building construction. House building design In that community has abandoned the traditional round huts with thatched roofs of 20 years ago and now favours square or rectangular houses made with wooden walls covered with mud or cement and with a corrugated Iron roof. While wooden houses represented the majority of buildings, houses made of bricks were on the Increase. Square or rectangular houses with a corrugated Iron roof required significantly less elements for the roof beams than round thatched houses. The wall panels were the most wood-consuming part of a house. It was estimated that the average household would have to source 11 main posts, 10 main beams, 27 roof laths and 277 wall laths annually to replace old buildings. Between 40 and 50% of the material used In house building was obtained from species with an established sustainable harvesting potential. The high preference for Brachylaena huillensis as main posts Is, however, cause for concern as the current population of this species In the communal land of the Manqakulane people cannot sustain such high levels of harvesting.

Restriction of Dispersal Due to Reduction of Dispersal Structures

Antitelechory is the hindering of longdistance dispersal of diaspores by the initial placement of diaspores (at, near or below the soil surface) or by the morphological characteristics of the diaspores (Ellner and Schmida 1981). The number of publications on antitelechory is very limited in comparison to those on telechory. This may be due to the fact that most of the publications on dispersal were written by European authors. Central Europe has a temperate, moist climate with closed vegetation cover and competition for space and nutrients is severe. Diaspores of a perennial plant which stay in the near vicinity of the mother plant have little chance of finding suitable conditions for germination and growth, since the mother plant has already occupied the space. Under these circumstances it would be necessary for the species to develop mechanisms to transport the diaspores away from the mother plant.

Restriction of Dispersal in Time

Various terms have been suggested to describe the delay in release of mature seeds from the mother plant (Lamont et al. 1991). Michaux (1803) and Engelmann (1880) described serotiny (“serotinus” = late in occurrence) as the retention of seeds in temporarily or permanently-closed cones. The term serotiny has, however, become associated with the phenomenon where seeds are retained in cones that require fire to release their seeds. Fuel loads in true deserts are seldom high enough for fires to be a key environmental parameter and for this reason the term serotiny will not be used in this book.