Wood anatomical traits as a measure of plant responses to water availability: invasive Acacia mearnsii De Wild. compared with native tree species in fynbos riparian ecotones, South Africa

Riparian ecotones in the fynbos biome of South Africa are heavily invaded by woody invasive alien species, which are known to reduce water supply to downstream environments. To explore whether variation in species-specific functional traits pertaining to drought-tolerance exist, we investigated wood anatomical traits of key native riparian species and the invasive Acacia mearnsii across different water availability proxies. Wood density, vessel resistance against implosion, vessel lumen diameter and vessel wall thickness were measured. Wood density varied significantly between species, with A. mearnsii having denser wood at sites in rivers with high discharge. As higher wood density is indicative of increased drought tolerance and typical of drier sites, this counter-intuitive finding suggests that increased wood density was more closely related to midday water stress, than streamflow quantity per se. Wood density was positively correlated with vessel resistance against implosion. Higher wood density may also be evidence that A. mearnsii is more resistant against drought-induced cavitation than the studied native species. The observed plastic response of A. mearnsii anatomical traits to variable water availability indicates the ability of this species to persist under various environmental conditions. A possible non-causal relationship between wood anatomy and drought tolerance in these riparian systems is discussed.     

The potential for birds to disperse the seeds of Acacia cyclops,an invasive alien plant in South Africa

Rooikrans Acacia cyclops is an aggressive invasive tree that threatens natural resources in South Africa. The seeds of A. cyclops have a prominent aril which attracts birds that ingest the seeds and disperse them endozoochorously. Two biological control agents, a Seed Weevil Melanterius servulus and a Flower‐galling Midge Dasineura dielsi, were released on A. cyclops in 1991 and 2002, respectively. Together these agents have substantially reduced seed production and generally far lower numbers of seeds are now available to birds. A consequence of this transition from historically bounteous quantities of seeds to scanty seed availability is that birds may no longer associate with the trees and seed dispersal may be disproportionately reduced. To assess whether this has happened, seed attrition was measured by comparing the amount of seeds that disappeared from two groups of branches, one available to birds and the other enclosed in bird netting. Other types of granivores (mainly field mice) were excluded from both groups of branches with a plastic funnel placed around the stems. Mature seeds were also harvested and fed to caged bird species to determine gut retention times and germination rates of ingested seeds. Attrition rates of seeds showed that birds continue to remove seeds but that only a proportion of the crop is taken. Only two frugivorous species (Knysna Turaco Tauraco corythaix and Red‐winged Starling Onychognathus morio) and two granivorous species (Red‐eyed Dove Streptopelia semitorquata and Laughing Dove Streptopelia sengalensis) ingested A. cyclops seeds during feeding trials. Ingestion by birds enhanced seed germination except for those ingested by Laughing Doves. There were no apparent effects of length of gut passage time and avian body size on seed germination rates. Despite the diminished seed resource due to biological control agents, birds continue to disperse A. cyclops seeds.     

Stomatal length and frequency as a measure of ploidy level in black wattle, Acacia mearnsii (de Wild)

The length and frequency of stomata on leaf surfaces were examined as rapid techniques for future identification of ploidy level of Acacia mearnsii (de Wild). Diploid (2 n  = 2 x  = 26) and tetraploid (2 n  = 4 x  = 52) plants were germinated from chipped seed at 25°C and grown under nursery conditions. After one month, measurements showed that the mean stomatal length was 27.17 ± 0.474 µm for diploids and 40.24 ± 0.521 µm for tetraploids and these differed significantly from each other ( P  < 0.001). The frequency of stomata per leaf surface was shown to decrease significantly ( P  < 0.001) as the ploidy level increased, with a mean of 22.11 ± 0.495 for diploids and 10.26 ± 0.495 for tetraploids. It was concluded that stomatal length and stomatal frequency are rapid indirect methods to identify ploidy level in black wattle.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 141 , 177–181.     

Genetic diversity and population structure of six South African Acacia mearnsii breeding populations based on SSR markers

Black wattle (Acacia mearnsii) has great economic value as a commercial source of tannins, timber and a source of firewood for local and international markets. It has been suggested that to maximize the genetic gain of A. mearnsii plantations in South Africa, the gene pool that exist within ICFR needs to be broadened via introduction of new genotypes with diverse traits. In this work, 282 A. mearnsii samples sourced from the ICFR breeding program were genotyped using 11 cross-species SSR markers. Our results showed low to moderate genetic differentiation (F_ST) among the six breeding subpopulations, with positive inbreeding (F_IS) values that could be attributed to an historical inbreeding event. Low levels of relatedness could however indicate some mechanism of inbreeding avoidance. The effects from a recent supplementation of genetic material from two native Australian populations were observed through genetic structuring analyses. Analysis of molecular variance (AMOVA) revealed that significant genetic variation was mainly distributed within populations (75%) and among individuals (23%). The results provide significant information on A. mearnsii population genetic diversity and structure, which can be used for conservation of the current subpopulations and future tree improvement programs.

     

黑荆（Acacia mearnsii）对几种林下植物种子萌发的化感作用

研究了入侵植物黑荆（Acacia mearnsii De Willd.）不同地上器官、不同浓度水浸提液对杜虹花（Callicarpa formosana Rolfe.）、紫薇（Lagerstroemia indica Linn.）、白花败酱（Patrinia villosa Juss.）、田麻（Corchoropsis tomentosa(Thunb.) Makino）4种植物种子的化感作用。结果表明黑荆茎、叶和豆荚水浸提液对4种植物的种子萌发均有不同程度的抑制作用。（1）黑荆不同地上器官对杜虹花、白花败酱、田麻的化感作用强度为叶>茎>豆荚,对紫薇的化感效应与浸提液的浓度有关,低浓度时茎>叶>豆荚,高浓度时豆荚>茎>叶。（2）同一器官的水浸提液对各植物化感效应不同,豆荚水浸提液对杜虹花、白花败酱、紫薇的种子萌发表现为较强的抑制作用,而对田麻则影响不显著。（3）高浓度茎水浸提液对种子萌发抑制作用的强度表现为田麻>杜虹花>紫薇>白花败酱。（4）随着浓度的增加,黑荆不同地上器官的水浸提液对杜虹花、白花败酱和田麻种子萌发的影响都呈现抑制作用增强的趋势。（5）黑荆的入侵可能通过化感作用抑制林下植物种子的萌发而对森林物种多样产生负面影响。     

The biology of Dasineura dielsi Rübsaamen (Diptera: Cecidomyiidae) in relation to the biological control of Acacia cyclops (Mimosaceae) in South Africa

Abstract  Acacia cyclops is an invasive Australian tree in South Africa and a target for biological control using seed-reducing agents. In southern Australia, two gall-forming Cecidomyiidae, Dasineura dielsi (Small Fluted Galler) and Asphondylia sp., develop on the flowers and seeds of A. cyclops , respectively. The larvae of D. dielsi form woody fluted galls on the ovaries of flowers and prevent the development of fruit. Immature Asphondylia sp. develop in the loculi of green fruit and destroy developing seeds. Dasineura dielsi was selected as a biological control candidate for A. cyclops in South Africa and was approved for official release after host specificity evaluation and consideration of potential conflicts of interest. Dasineura dielsi naturalised in South Africa in 2001 and after 3 years dispersed up to 450 km from a single population at Stellenbosch, Western Cape. At sites where D. dielsi has been present longest, high gall densities occur on A. cyclops during the peak flower season in summer. Four hymenopterans, ? Synopeas sp., Mesopolobus sp., Torymus sp. and an unidentified Platygastridae, were reared from D. dielsi galls and are suspected parasitoids of the cecidomyiid, with incidence levels less than 10%. Monitoring is required to evaluate trends in the population status of D. dielsi , its parasitoids and seed production of A. cyclops . Importantly, field monitoring should determine the extent and nature of possible competitive interactions between D. dielsi and an introduced seed-feeding weevil, Melanterius servulus .     

Effects of gall midge infestation on plant growth and growth regulating substances

The biological association between plant and gall-inducing insects can be characterized by growth and developmental abnormalities of the plant tissues caused by the feeding activities of the insects. The relationship between Wachtliella persicariae L. (Diptera: Cecidomyiidae) and Polygonum amphibium L. has been studied.The gall-making larvae settle only on the undifferentiated parts of the leaf and prevent the normal unfolding of the lamina; and there is a close developmental synchronization between the two organisms.The essential reaction of the leaf tissue to the continuous feeding activity of the larvae is primarily a disturbance of the normal development, characterized by an imbalance in growth-regulating substances. Bioassays showed greater amounts of auxin-like substances in the galled tissues. The way in which feeding behaviour, feeding activity and nutritional physiology possibly interfere with the growth-regulating factors of the leaf is discussed.The resulting abnormal leaf growth and development is essential for the normal growth and development of the larvae.

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Zusammenfassung Die biologische Verbindung zwischen Pflanzen und gallenerzeugenden Insekten kann durch Wachstums-und Entwicklungsabnormalitäten der Pflanzengewebe gekennzeichnet werden, welche durch die Fraßtätigkeit der Insekten hervorgerufen werden. Hier wurde die Beziehung zwischen Wachtliella persicariae (Diptera: Cecidomyiidae) und Polygonum amphibium untersucht.Die gallenerzeugende Larve siedelt sich nur auf undifferenzierten Teilen des Blattes an und verhindert die normale Entfaltung der Lamina; es besteht eine enge Synchronisation der Entwicklung beider Organismen.Die wesentliche Reaktion des Blattgewebes auf die fortdauernde Fraßaktivität der Larve stellt zunächst eine Störung der normalen Entwicklung dar, die durch ein Ungleichgewicht der Stoffe charakterisiert ist, die das Pflanzenwachstum kontrollieren. Biotests ergaben in den vergallten Geweven größere Beträge auxinartiger Stoffe. Der Weg, auf dem Fraßverhalten, Fraßaktivität und Ernährungsphysiologie mit den wachstumkontrollierenden Faktoren des Blattes ineinandergreifen, wird diskutiert.Das entstehende anomale Blattwachstum und seine Entwicklung sind für das normale Wachstum und die Ontogenese der Larve wesentlich.

     

Deagrarianisation and Forest Revegetation in a Biodiversity Hotspot on the Wild Coast,South Africa

Deagraianisation is a worldwide phenomenon with widespread social, ecological and economic effects yet with little consensus on the local or higher level causes. There have been contested views on the causes and consequences of deagrarianisation on South Africa’s Wild Coast, which is an international biodiversity hotspot. Using GIS, household interviews and ecological sampling, we compared the perspectives of current and former cultivators as to why some have abandoned farming, whilst also tracking the uses and woody plant cover and composition of fields abandoned at different periods. The GIS analysis showed that field abandonment had been ongoing over several decades, with a decline from 12.5 % field cover in 1961 to 2.7 % in 2009. The area of forests and woodlands almost doubled in the corresponding period. There was a distinct peak in field abandonment during the time of political transition at the national level in the early 1990s. This political change led to a decrease in government support for livestock farming, which in turn resulted in reduced animal draught power at the household and community level, and hence reduced cropping. The study showed it is largely the wealthier households that have remained in arable agriculture and that the poorer households have abandoned farming. The abandoned fields show a distinct trend of increasing woody biomass and species richness with length of time since abandonment, with approximately three woody plant species added per decade. Most local respondents dislike the increases in forest and woodland extent and density because of anxiety about wild animals causing harm to crops and even humans, and the loss of an agricultural identity to livelihoods and the landscape.     

Response to nutrient additions by the plant growth forms of sand-plain lowland fynbos,South Africa

The growth response (foliage projective cover: FPC) of the plant growth forms of lowland fynbos, South Africa to a complete factorial fertilizer addition of nitrogen (N), phosphorus (P) and a mixture of all essential nutrients excluding N and P (M) was monitored for two years. Ordination by correspondence analysis revealed a successional trend as the vegetation aged, with increases in proteoid, and to a lesser extent restioid, and decreases in reseeding ericoid, graminoid, geophyte and annual plant FPC but no discernible effects due to fertilizer application. Analysis of covariance revealed significant increases in restioid, graminoid and annual plant FPC with N-addition, 1 and 2 yr after fertilizer addition and of total FPC with N-addition for only one year. Of the other nutrient treatments, only an increase in annuals with P-addition and a reduction in the rate of decline of reseeding ericoid with M-addition were found, both after two years. Thus nitrogen may often limit vegetative growth of at least the herbaceous species. The herbaceous growth forms are more plastic in their morphological growth responses to nutrient additions than the slow-growing, stress tolerant, evergreen shrub species. The vegetation appears to be resilient, at least in the short-term, to a disturbance of this magnitude. However, chronic nutrient applications and those of larger magnitude will probably result in long-term changes in species composition, with an increase in ephemeral, nutrient demanding species.Abbreviations FPC = Foliage Projective Cover     

The distribution and species characteristics of the Culicoides biting midge fauna of South Africa

Abstract.  1. Monthly light trap collections over 2 years at 39 sites across South Africa captured over 3 000 000 Culicoides of 86 species, some of them known vectors of African horse sickness and bluetongue viruses.
2. To aid disease control and risk evaluation, cluster analysis and association measurements were carried out on species present at each site. Six species occurred at almost all sites. From the remaining 80 species, eight clusters were identified at > 48% similarity.
3.  Culicoides citroneus and C. krameri were characteristic of cluster 8, C. loxodontis of cluster 4, and two numbered, but un-named, species, C. #119 and C. #89 , of cluster 5.
4. Multiple anova and multiple discriminant analysis on the climatic and geographic parameters of sites in each cluster gave two significant discriminant functions that explained 91.5% of the variance between clusters. Function 1 contrasted sites with high cold cloud duration and high vegetation index with sites with high wind speeds. Function 2 contrasted upland clusters that also had high cold cloud duration with hotter, lower altitude sites.
5. Species associations were explained in terms of biotic provinces, based on ecogeographic factors that have previously been useful to describe communities of birds.
6. None of the clusters was ideal for siting an African horse sickness-vector free quarantine station as C. imicola and C. bolitinos , the main vectors, were found in all clusters but in very low abundance in cluster 5. The analysis provides a firm basis for studying the changing patterns of species distribution under varying climatic influences.     

Ethnobotanical plant uses in the KwaNibela Peninsula, St Lucia, South Africa

Ethnobotanical field studies were conducted for the first time in the KwaNibela Peninsula of southern Maputaland, KwaZulu-Natal, to document indigenous knowledge about useful plants. The vernacular names and uses of 82 plant species were recorded and compared to published Zulu and Swazi knowledge. Medicines for skin disorders, toothache, wounds, worms, chest and throat ailments, infertility and purgatives are still commonly used. Superstition and divination play a major role in the traditional knowledge system of the people of KwaNibela with 24 plants used for this purpose. Three KwaNibela medicinal plants (Erythroxylum delagoense, Putterlickia verrucosa, and Teclea natalensis) appear to be new records, not previously reported in the general scientific literature. The list also includes 61 novel uses of plants and another 15 new variations on known (published) uses. Ten previously unpublished vernacular names are presented, together with an additional 19 new variants of known names. These new additions to the scientific literature confirm that indigenous knowledge in KwaZulu-Natal is not yet completely recorded.     

Secondary succession in Acacia nilotica (L.) savanna in the Hluhluwe Game Reserve,South Africa

Analysis of aerial photographs indicates that woody plant biomass has rapidly increased in Hluhluwe Game Reserve over the last 40 years. Open Acacia nilotica savanna is being replaced by broadleaf species, especially Euclea spp. We were interested in whether this secondary successional shift was due to high numbers of seedlings establishing and growing to maturity under acacias (facilitation) or due to the release of already established, but suppressed individuals (gullivers) of the resprouting broadleaf species. We examined the recruitment patterns and size-class distributions (height, basal diameter) of important species in this savanna.Densities of euclea seedlings (<0.6 cm basal diameter) under Acacia nilotica were low (median of 0 and mean of 0.06 m²) below adult canopies and effectively zero in adjacent interspaces. No differences in numbers of other broadleaf species were found between open sites and under Acacia nilotica sites. Few large eucleas or other broadleaf species occurred under Acacia nilotica. Few Acacia nilotica recruits were found either under adults or in the open whereas Acacia karroo recruits were more common.The overall size class distribution for eucleas was dominated by individuals in the intermediate size class, suggesting that recruitment is not the dominant demographic process. We propose that the escape of intermediate sized eucleas from the fire trap has caused the increase in woody plants.Analysis of a time sequence of aerial photographs shows that invasion occurred rapidly between 1954 and 1975. A common feature for sites where woody plant invasion has taken place, was the presence of barriers to fire (especially roads). We suggest that the recent and rapid increase in woody vegetation is due to a decrease in the frequency of intense fires, rather than the recent absence of megaherbivores which allowed Acacia nilotica establishment. Few intense fires allow suppressed tree or shrub individuals, ('gullivers'), to escape the grass/fire layer and thereby become tall and fire-resistant. This release may explain the rapid rate of invasion by inherently slow growing broad-leaf species.     

A host of hypercholesterolaemic homozygotes in South Africa.

From 1972 to 1979 34 patients with homozygous familial hypercholesterolaemia were seen in one clinic in Johannesburg. All were Afrikaners and most lived in Transvaal Province. Their epidemiological, genetic, clinical, and biochemical characteristics were studied. The course of the disease varied considerably among the 34 patients, with no fewer than six surviving into their fourth or fifth decades. In some patients arterial atheroma was severe while cutaneotendinous xanthomas were slight and vice versa. Coronary heart disease was common but peripheral and cerebral arterial disease was rare. Another prominent finding was high concentrations of low-density lipoprotein cholesterol coupled with low high-density lipoprotein cholesterol values. The prevalences of homozygotes and heterozygotes with familial hypercholesterolaemia in Transvaal Afrikaners, calculated from this group of patients, were 1 in 30,000 and 1 in 100 respectively. These figures are the highest ever reported and may help to explain why South African whites have the highest death rate from coronary heart disease in the Western world.     

Effect of insecticide application and host plant resistance on parasitization of sorghum midge,Contarinia sorghicola coq.

Sorghum midge (Contarinia sorghicola Coq.) (Diptera: Cecidomyiidae) is one of the most important pests of grain sorghum. The effects of different insecticides applied at the complete‐an‐thesis stage for insect control, and host plant resistance on the parasitization of sorghum midge, were studied. Tetrastichus spp. and Eupelmus popa are the major parasites of sorghum midge larvae. Of the several insecticides tested, monocrotophos reduced midge parasitization by half compared with the untreated control. Midge parasitization was lower on midge‐resistant genotypes. Amongst the lines resistant to sorghum midge, the levels of parasitization were higher on the moderately resistant lines IS 10132 and PM 9760 compared to those on IS 3461, IS 7005, IS 9807, IS 19512 and AF 28, etc. Parasite activity closely followed the midge population density across sowing dates. The role of these factors on the effectiveness of midge parasites in integrated pest management are discussed.     

Differential climatic conditions drive growth of Acacia tortilis tree in its range edges in Africa and Asia

Premise

Tree growth is a fundamental biological process that is essential to ecosystem functioning and water and element cycling. Climate exerts a major impact on tree growth, with tree species often requiring a unique set of conditions to initiate and maintain growth throughout the growing season. Still, little is known about the specific climatic factors that enable tree growth in savannah and desert tree species. Among the global tree species, Acacia tortilis occupies one of the largest distribution ranges (crossing 6500 km and 54 latitudes), spanning large parts of Africa and into the Middle East and Asia.

Methods

Here we collected climate data and monitored Acacia tortilis tree growth (continuous measurements of stem circumference) in its southern and northern range edges in South Africa (SA) and Israel (IL), respectively, to elucidate whether the growth–climate interactions were similar in both edges.

Results

Growth occurred during the summer (between December and March) in SA and in IL during early summer and autumn (April–June and October–November, respectively). Surprisingly, annual growth was 40% higher in IL than in SA. Within the wide distribution range of Acacia tortilis, our statistical model showed that climatic drivers of tree growth differed between the two sites.

Conclusions

High temperatures facilitated growth at the hot and arid IL site, while high humidity permitted growth at the more humid SA site. Our results confer an additional understanding of tree growth adaptation to extreme conditions in Acacia's world range edges, a major point of interest with ongoing climate change.     

Interactions between Verticillium dahliae and its host: vegetative growth,pathogenicity, plant immunity

Verticillium dahliae is a soil-borne phytopathogenic fungus that causes vascular wilt diseases in a wide variety of crop plants, resulting in extensive economic losses. In the past 5 years, progress has been made in elaborating the interaction between this hemibiotrophic fungus and its host plants. Some genes responsible for the vegetative growth and/or pathogenicity in V. dahliae have been identified. Plants have accrued a series of defense mechanisms, including inducible defense signaling pathways and some resistant genes to combat V. dahliae infection. Here, we have reviewed the progress in V. dahliae–plant interaction research.