Search

Themeda and Heteropogon are closely related grass genera frequently dominant in tropical C4 grasslands. Relationships between them are poorly resolved, impeding ecological study, especially of T. triandra with a broad distribution from Africa to East Asia, and H. contortus with a pantropical distribution. Our analyses of plastome and nuclear genomes with comprehensive sampling of Themeda and Heteropogon demonstrate that neither genus is monophyletic as currently circumscribed. Plastome and nuclear data place H. melanocarpus and H. ritchiei in Themeda. Nested in T. triandra are T. quadrivalvis and T. unica, demonstrating that this widespread species is more morphologically diverse than previously recognized. Heteropogon fischerianus is nested in H. contortus. The picture is more complex for H. triticeus that is sister to H. contortus in the nuclear analysis and to Cymbopogon in the plastome analysis. This incongruence between nuclear and plastid phylogenetic trees suggests hybridization between Cymbopogon-related genome donors and H. contortus. Plastome dating estimates the Themeda–Heteropogon crown age at c. 7.6 Myr, consistent with the Miocene C4 grassland expansion. Themeda triandra and H. contortus diversified 1–2 Mya in the Pleistocene. These results establish a foundation for studying the history of these ecologically significant widespread grasses and the ecosystems they form.

Grasses are an important part of the forage of many herbivorous mammals and their phytoliths have long been regarded as the most important agent of tooth wear. Recent work has challenged this “paradigm” in finding evidence 1. of native phytoliths to be much softer then tooth enamel and 2. indicating, that phytolith hardness is highly variable, 3. prone to methodology and 4. not easy to be related to habitat conditions. We conduct controlled silica-cultivations measuring SiO2 content in the common forage grass Themeda triandra. Phytoliths are extracted natively, and nano-indentation values are measured. Phytolith hardness in Themeda triandra is found to be independent of silicate availability in the substrate. We further investigate the phytolith ultrastructure of Hordeum vulgare phytoliths. Phytoliths are shown to be an anisotropic composite of at least 3 components, silica bodies, inter-body matrix (both mineralised) and globular inclusions (likely non-mineralised). It can be argued, that indentation will be largely influenced by the heterogeneity of the structure and thus nano-indentation measurements will largely reflect the matrix and its mechanical properties but not necessarily the silicate bodies, which make up the vast majority of a phytolith.

Polyploidy (the state of having more than two genome copies) is widely distributed in flowering plants and can vary within species, with polyploid races often associated with broad ecological tolerances. Polyploidy may influence within-species variation in seed development, germination and establishment. We hypothesized that interactions between polyploidy and the seed developmental environment would affect subsequent dormancy, germination and early growth traits, particularly in stressful environments. Using seeds developed in a common garden under ambient and warmed conditions, we conducted germination trials under drought and temperature stress, and monitored the subsequent growth of seedlings. The study species, Themeda triandra, is a widespread, keystone, Australian native grass and a known polyploid complex. Tetraploid plants produced heavier, more viable seeds than diploids. Tetraploids were significantly more dormant than diploids, regardless of seed developmental environment. Non-dormant tetraploids were more sensitive to germination stress compared to non-dormant diploids. Finally, tetraploid seedlings were larger and grew faster than diploids, usually when maternal plants were exposed to developmental temperatures atypical to the source environment. Seed and seedling traits suggest tetraploids are generally better adapted to stressful environments than diploids. Because tetraploid seeds of T. triandra are more dormant they are less likely to germinate under stress, and when they do germinate, seedling growth is rapid and independent of seed developmental environment. These novel results demonstrate that polyploidy, sometimes in interaction with developmental environment and possibly also asexuality, can have within-species variation in seed and seedling traits that increase fitness in stressful environments.

Lowland grassy woodlands in Australia’s south-east face reductions in native plant diversity because of invasion by non-native plants. We compared the relative abundance and diversity of plant species among sites dominated by the native Kangaroo grass (KG) Themeda triandra with sites co-dominated by the non-native African lovegrass (ALG) Eragrostis curvula and KG. We found significant differences in plant species composition depending on the dominant species. Furthermore, our results revealed differences in several diversity parameters such as a lower species richness and forb diversity on sites co-dominated by ALG and KG. This was the case despite the functional similarity of both ALG and KG—both C4 perennial tussock grasses of a similar height. Therefore, our results highlight the critical function of the native KG in maintaining and enhancing the target plant species composition and diversity within these grassy woodlands. Herbivore grazing potentially impacts on the abundance of the dominant grass and forb species in various ways, but its impact likely differs depending on their evolutionary origin. Therefore, disentangling the role of individual herbivore groups (native-, non-native mammals, and invertebrates) on the plant community composition of the lowland grassy woodlands is essential to find appropriate grazing regimes for ALG management in these ecosystems.

This paper presents insights from long-term monitoring into the dynamics of savanna plant species composition, specifically on the ground layer. Key questions are (1) how much variation is there from year to year in plant species abundance and composition? And (2) is there evidence of directional change in community composition? Five sites were located near Mareeba, north-eastern Australia, and sampled 18 times with fixed 500-m2 plots at the annual peak of plant species richness over 23 years. The sites were located in eucalypt communities across an altitudinal range of 460 m. Sampling involved systematic searching of all plant species visible above ground. The results show variation from year to year among satellite herbaceous species, with more than 50% of grasses and forbs observed at a given site only being detected at half or fewer of the 18 samplings. Herbaceous composition did not show clear signs of directional change relative to inter-annual variability. Over the 23 years, there were three very dry periods and 4 very wet years. Rainfall variation affected total cover more than plant species composition or richness. Several fires and episodes of grazing affected ground cover but did not have a lasting impact on cover or diversity. The cover and composition of the ground layer is consistently dominated by the core perennial grasses Themeda triandra Forssk. and Heteropogon triticeus (R.Br.) Stapf, with many species collectively contributing minimally to the overall cover but significantly to richness. Many herb species are persistent long-lived perennials which do not appear above ground every year. The number of naturalised plants in the ground layer was consistently low. Overall, the ground layer communities appear to be very stable in these woodlands that have been burnt 3–5 times in 23 years. High inter-annual variability of subdominant species is consistent with other studies, and emphasises the need for replication in time in ecological sampling.

Grass roots can harbor abundant endophytic N 2 -fixing microbes (diazotrophs), but their abundance and activity compared to those on legumes and in soil crusts is still unknown. Here, in a natural ecosystem, the Serengeti of East Africa, we explored whether herbivores and soil nutrients limited grass root endophyte diazotroph abundance and their root mass-specific and area-specific N 2 -fixation, as they often do for diazotrophs symbiotic with legumes and those free-living in soil. N 2 -fixation and copy number of the nitrogenase gene nifH was measured with stable isotope and molecular methods, respectively, for the dominant grass Themeda triandra , and legume, Indigofera volkensii , and in the top 5 cm of soil in a 16-year herbivore exclosure experiment across four sites that varied in mean annual rainfall and soil N, P, and moisture. T. triandra nifH gene copy number was highly variable across sites and individuals but often approached or exceeded that of I. volkensii roots and soils. T. triandra roots generally exhibited lower root mass-specific N 2 -fixation (activity), which was not reduced by herbivores and increased in drier soils. In contrast, I. volkensii activity was only reduced by herbivores and soil diazotrophs were mostly inactive. T. triandra exhibited greater area-specific N 2 -fixation than I. volkensii , due to its much greater root biomass, but this difference was reduced by herbivores. Grass-associated endophytic diazotrophs may fix far more N 2 in natural systems than previously realized, and may be limited by different factors those affecting symbiotic legume and free-living soil diazotrophs.

When dry soil is re-wet there is a pulse of C mineralization. It is likely that the pulse of mineralization is fuelled by soluble C that accumulates as soil is drying. When soil is drying soluble C could accumulate as products of exo-enzyme mediated depolymerisation (i.e. protein amino acids and small carbohydrates) in the extracellular fraction of the soil (i.e. adsorbed and in free solution). Alternatively there could be an accumulation of osmolytes within the microbial biomass. To test whether extracellular depolymerisation products and/or microbial osmolytes accumulate as soil dries, soil from a Themeda triandra grassland was dehydrated and then depolymerisation products and osmolytes were quantified by capillary electrophoresis-mass spectrometry and gas chromatography–mass spectrometry. A secondary aim of this experiment was to determine if the response of soil to drying is the same when soil is dehydrated rapidly in the laboratory as when an intact soil containing plants is slowly dehydrated by withholding water from large (200 L) mesocosms. The responses of soil to dehydration differed between lab incubations and mesocosms, despite involving the same soil being dehydrated to the same final water content. When soil was dehydrated slowly in 200-L mesocosms the accumulation of osmolytes was more quantitatively significant than accumulation of depolymerisation products, whereas when soil was dehydrated more rapidly in lab incubations there was negligible accumulation of osmolytes but large accumulation of depolymerisation products. This study has highlighted that when soil dries the accumulation of osmolytes within microbial biomass and depolymerisation products within the extracellular fraction of soil are both quantitatively important and likely underpin the flush of soil CO2 efflux when dried soil is re-wet.

Grassy woodlands worldwide are vertically structured by trees, a ground layer of grasses and forbs, and a variable mid-storey. In Western Sydney’s Cumberland Plain Woodland this mid-storey is increasingly dominated by the prickly shrub Bursaria spinosa Cav. subsp. spinosa. We investigated whether tree and shrub vegetation affects species richness and composition of ground layer microhabitats in this woodland, and whether fire frequency directly affects the ground layer in addition to any indirect effects via overstorey vegetation. Replicate sites were located in areas that had burnt with a frequency that was high, moderate or low. All sites had a similar time since last fire. Three microhabitats (open, under Bursaria, around tree) within each site were sampled for species richness and composition of native and exotic species. Native species richness was not significantly affected by either microhabitat or fire frequency, for total species, grasses or forbs; however the relative abundance of native grasses decreased significantly as fire frequency declined, while the relative abundance of native forbs increased. Exotic species richness was lowest at high fire frequency and significantly higher at low fire frequency. Species composition was significantly affected by both microhabitat and fire frequency combining independently. Planned burning at relatively short intervals can help managers retain grassy habitat for open patch species, habitat for ground layer species that do best under frequent fire, and a robust Themeda triandra Forssk. sward antagonistic to weeds.

The locally grown forage grasses are main fodder source for maintaining the indigenous livestock breeds like Dangi cattle and Sangamneri goat in Ahmednagar district. Most commonly observed fodder grass and tree species grazed by livestock were collected from the field. The nutritional quality of these grasses and tree species was determined by undertaking proximate analysis in the laboratory. The local fodder species include Dicanthium annulatum, Setaria intermedia, Themeda triandra, Heteropogon contortus, Indigofera cordifolia, Apluda mutica, Cymbopogon martini, Tephrosia purpurea. Among the tree species Melia azedarach, Azadirachta indica, Prosopis julifera, Grewia asiatica, Clerodendrum multiflorum were the major tree species commonly available as fodder source. The nutritional analysis indicated that crude protein content ranged from 5.61 to 15.46%, crude fiber 18.09 to 34.87%, ether extract 1.00 to 3.24%, Ash 6.44 to 21.56 % and silica was less than 5.31%. The grass and tree species found in hilly regions are showing tremendous variation in nutritional composition. This study highlighted the importance of local fodder species including grasses, legumes, bushes and trees in rearing the livestock with low inputs at field conditions.

We compared the results of a long-term (65 years) experiment in a South African grassland with the world's longest-running ecological experiment, the Park Grass study at Rothamsted, U.K. The climate is warm and humid in South Africa and cool and temperate in England. The African grassland has been fertilized with two forms of nitrogen applied at four levels, phosphorus and lime in a crossed design in 96 plots. In 1951, about 84% of plant cover consisted of Themeda triandra, Tristachya leucothrix and Setaria nigrirostris. Currently, the dominant species are Panicum maximum, Setaria sphacelata and Eragrostis curvula, making up 71% of total biomass. As in the Park Grass experiment, we found a significant (additive) interaction effect on ANPP of nitrogen and phosphorus, and a (marginally significant) negative correlation between ANPP and species richness. Unlike the Park Grass experiment, there was no correlation between ANPP and species richness when pH was included as a covariate. There was also a significant negative effect of nitrogen amount and nitrogen form and a positive effect of lime on species richness and species diversity. Soil pH had an important effect on species richness. Liming was insufficient to balance the negative effects on species richness of nitrogen fertilization. There was a significant effect of pH on biomass of three abundant species. There were also significant effects of light on the biomass of four species, with only Panicum maximum having a negative response to light. In all of the abundant species, adding total species richness and ANPP to the model increased the amount of variance explained. The biomass of Eragrostis curvula and P. maximum were negatively correlated with species richness while three other abundant species increased with species richness, suggesting that competition and facilitation were active. Consistent with the results from the Park Grass and other long-term fertilization experiments of grasslands, we found a positive effect of soil pH and a negative effect of nitrogen amount on species richness, a more acutely negative effect on species richness of acidic ammonium sulphate fertilizer than limestone ammonium nitrate, a negative relationship between species richness and biomass, and a positive effect on species richness of lime interacting with nitrogen.

The accumulation and subsequent release of microbial osmolytes in response to drying and rewetting are thought to be key players in C and N dynamics, yet studies on soils have failed to support this hypothesis. The aim of this experiment was to determine how low-molecular weight compounds, and osmolytes in particular, are affected by drying and rewetting. Water deficits were imposed slowly by withholding water for 21 weeks from large (200 L) mesocosms vegetated with a globally widespread grass Themeda triandra . A broad spectrum of small molecules in extracts was identified and quantified by capillary electrophoresis–mass spectrometry and gas chromatography–mass spectrometry. Compared with controls, drought-stressed mesocosms contained >10-fold larger amounts of known microbial osmolytes: ectoine, hydroxyectoine, betaine, proline-betaine, trigonelline, proline, trehalose, arabitol. The pool of osmolytes accounted for 3.6% of CHCl 3 labile TOC in control mesocosms and 17% of CHCl 3 labile TOC in drought-stressed mesocosms. There was no evidence that rewatering led to a large pulse of osmolytes in free solution. Instead osmolytes decreased to control concentrations within 1–3 h of rewatering – probably indicating rapid uptake by microbes and plants. Results of this study suggest that osmolytes can account for a substantial fraction of microbial C, and are at least one of the ways that soil microbes cope with water deficits.

Flowering in Merxmuellera disticha was more strongly stimulated by fire in summer. Flowering tillers produced between 9 and 11 months after fire were significantly more abundant than those produced 21 to 24 months thereafter. Growth in terms of size of individuals was considered insignificant for analysis. NeighboredHyparrhenia hirta showed, to a certain degree, an out-of-season growth, production and reproduction, whilstM. disticha and Themeda triandra on the same plots did not. The xylem water potential of H. hirta[-10.9(±0.29)], M. disticha [-14.6(±0.80)] and T. triandra [-13.8(±0.29)] on one stand differed significantly from one another (p < 0.0001), and that of H. hirta [-19.7(±0.81] and M. disticha [-34.5(±1.26)] on the other stand also differed significantly from each other (p < 0.0001). One way analysis of variance (ANOVA) thereof produced the following (F-ratio = 13.893; degrees of freedom = 29, p < 0.0001), and for H. hirta versus M. disticha, (F-ratio = 97.605; degrees of freedom = 23, p < 0.0001). Multiple range analysis tests revealed a significant difference between xylem water potentials of H. hirta from that of M. disticha and T. triandra but not between M. disticha and T. triandra. Key words: South Africa, Signal Hill, fire, flowering plants, impact.

The structure and composition of savanna vegetation is influenced by resource availability and disturbance. Grasses, a major component of savanna systems, influence the tree-grass balance by competing with trees for light, water and soil nutrients as well as providing fuel for fires. Overgrazing, and the ensuing decreased grass competition and fire effects resulting from diminished grass vigor and cover, are thought to be major drivers of bush encroachment. To investigate impacts of competition from grass on saplings of Acacia karroo Hayne. and Acacia nilotica (L.). Willd. ex. Del. under high and low soil fertility, six grass species, Aristida junciformis (Trin & Rupr.), Eragrostis capensis (Thunb.) Trin., Hyparrhenia hirta Stapf., Panicum maximum Jacq., Sporobolus africanus (Poir.) Robyns & Tournay and Themeda triandra Forssk., were planted in pots with a tree sapling as the central test species. The major competitive effects experienced by tree saplings were dependent on nutrient level and the identity of the competing grass species (P < 0.001). Aboveground A. karroo was more strongly affected by grass competition under high nutrient conditions, whereas an increase in nutrient level had little effect on the competition between A. nilotica and the grasses (P < 0.001). Acacia karroo was also strongly affected by grass competition belowground; however, the opposite trend was observed for A. nilotica (P < 0.001). Eragrostis capensis was one of the most competitive grass species, reducing sapling aboveground biomass by 85% and 65% under high and low nutrient conditions, respectively, and sapling belowground biomass by 71% on average. Increasing nutrient availability resulted in an increase in competition exerted on A. karroo by all grasses but little to no change for A. nilotica. Changes in savanna composition and structure are thus likely to be influenced by grass species composition and soil nutrient status.

Increased or fluctuating resources may facilitate opportunities for invasive exotic plants to dominate. This hypothesis does not, however, explain how invasive species succeed in regions characterized by low resource conditions or how these species persist in the lulls between high resource periods. We compare the growth of three co-occurring C4 perennial bunchgrasses under low resource conditions: an exotic grass, Eragrostis curvula (African lovegrass) and two native grasses, Themeda triandra and Eragrostis sororia. We grew each species over 12 weeks under low nutrients and three low water regimes differentiated by timing: continuous, pulsed, and mixed treatments (switched from continuous to pulsed and back to continuous). Over time, we measured germination rates, time to germination (first and second generations), height, root biomass, vegetative biomass, and reproductive biomass. Contrary to our expectations that the pulsed watering regime would favor the invader, water-supply treatments had little significant effect on plant growth. We did find inherent advantages in a suite of early colonization traits that likely favor African lovegrass over the natives including faster germination speed, earlier flowering times, faster growth rates and from 2 weeks onward it was taller. African lovegrass also showed similar growth allocation strategies to the native grasses in terms of biomass levels belowground, but produced more vegetative biomass than kangaroo grass. Overall our results suggest that even under low resource conditions invasive plant species like African lovegrass can grow similarly to native grasses, and for some key colonization traits, like germination rate, perform better than natives.

We examined if germination and seedling emergence of species from the soil seed bank of mesic grassland in South Africa differed in their response to smoke or heat treatments alone or combined. Soil seed bank samples taken from 0 to 5 cm depth of the topsoil were treated with smoke-water solution, heat and heat + smoke-water combined and subsequent emergence of seedlings monitored over 90 days. In total, 790 individuals from 11 different plant families representing 37 graminoid (Poaceae) and non-graminoid species of flowering plants were identified. The most abundant plant families that emerged were Poaceae followed by Asteraceae. Grasses contributed 18% of the germinable seed bank, while non-grass (forbs and trees) species contributed >80%. The most abundant grass species that emerged was Themeda triandra and the most common forb species was Centella asiatica. Compared to the control, smoke-water (SW) and/or smoke-water + heat (H + SW) treatment resulted in a 1.6-fold increase in mean number of seedling emergence. SW and H + SW treatment of soil seed bank also resulted in fourfold to fivefold greater biomass production. These results suggest that emergence of some smoke-responsive perennial grasses (e.g. Themeda triandra) from the soil seed bank can be enhanced using smoke techniques. Moreover, plant-derived smoke could potentially have a significant role in the restoration of degraded grasslands in South Africa and elsewhere.

Concerns over biodiversity loss and increasing biological invasion have forced interest on assessment of the effects on native plant species diversity in grassland community. To observe different patterns of grass emergence (dormancy/germination) in the warm tropical grasslands of India, time span of a seed from the seedling stage to a mature plant becomes very crucial for the community development. In the present study seed germination response of six dominant species of the selected study area were tested to record the various effects of dry storage conditions on seed germinability. The species selected were Apluda mutica L., Cenchrus ciliaris L., Chrysopogon fulvus (Spreng.) Chiov., Dichanthium annulatum (Forsk.) Stapf., Heteropogon contortus (L.) P. Beauv. ex Roem. and Schult. and Themeda triandra (R. Br.) Stapf. For this purpose, seed collection at mature seed stage, seed processing and dry seed storage were followed by the germination test system. Obtained results are exhibited in the form of different responses such as, species response patterns towards capacity for immediate germination, responses to dormancy, dry storage and temperature fluctuation. The extent of the requirement in breakage of primary dormancy was highly correlated with the timing of seed maturity, precursors of seed dormancy and seed viability. In present screening out of the six studied species Apluda mutica, Cenchrus ciliaris and Dichanthium annulatum showed dependable germination pattern to fluctuating temperature. The correlation between viability and germination suggests that the germination of Apluda mutica, Cenchrus ciliaris and Themeda triandra are linearly dependent on the viability that the seeds of these species have. As these species showed less influence with the relative fluctuating environment, they can be stored for longer period and frequently can be use for community regeneration in pasture development.

The stable carbon isotope composition of animal tissues represents the weighted sum of the variety of food sources eaten. If sources differ in digestibility, tissues may overrepresent intake of more digestible items and faeces may overrepresent less digestible items. We tested this idea using whole blood and faeces of goats ( Capra hircus L., 1758) fed different food mixtures of C3lucerne ( Medicago sativa L.) and C4grass ( Themeda triandra Forssk.). Although blood and faecal δ13C values were broadly consistent with diet, results indicate mismatch between consumer and diet isotope compositions: both materials overrepresented the C3(lucerne) component of diets. Lucerne had lower fibre digestibility than T. triandra, which explains the results for faeces, whereas underrepresentation of dietary C4in blood is consistent with low protein content of the grass hay. A diet switch experiment revealed an important difference in13C-incorporation rates across diets, which were slower for grass than lucerne diets, and in fact equilibrium states were not reached for all diets. Although more research is needed to link digestive kinetics with isotope incorporation, these results provide evidence for nonlinear relationships between consumers and their diets, invoking concerns about the conceptual value of “discrimination factors” as the prime currency for contemporary isotope ecology.

Smoke, smoke-water and aerosols have a stimulatory effect on seed germination and growth vigour of many seedlings, making them potentially useful for different purposes, provided they do not pose a health risk. Therefore, the genotoxicity of two kinds of smoke-water and 3,7-dimethyl-2H-furo[2,3-c]pyran-2-one, a variant of the most active smoke compound (3-methyl-2H-furo[2,3-c]pyran-2-one) was evaluated using the Vitotox™ assay. Smoke-water extracts were obtained from burning leaves: Themeda triandra (smoke-water Tt) and a mix of Themeda triandra and Passerina vulgaris (smoke-water Kb). No genotoxic effect was observed for any of the three samples. However, the three samples are toxic at the highest concentrations (3,7-dimethyl-2H-furo[2,3-c]pyran-2-one, 2 ppm; smoke-water Tt, dilutions 1 : 1, 1 : 2, 1 : 4; smoke-water Kb, dilution 1 : 1) without addition of S9 mix. Both the butenolide 3,7-dimethyl-2H-furo[2,3-c]pyran-2-one and smoke-water Tt are also toxic at high doses in the presence of S9 (2 ppm and dilutions 1 : 1 and 1 : 2, respectively), but not smoke-water Kb. Thus, from these results, no genotoxicity of these three samples can be assumed, which is accordance with the previous tests performed with 3-methyl-2H-furo[2,3-c]pyran-2-one and a smoke-water. Copyright © 2010 John Wiley & Sons, Ltd.

Fire, natural or of anthropologic origin, is a recurrent phenomenon in South African mesic grassland. The species composition of these grasslands is sensitive to fire, particularly fire frequency. However, the mechanisms involved in influencing species composition are not fully understood. Currently there is a general suggestion that plant-derived smoke and smoke-isolated biologically active butenolide compound provide an important germination cue for a range of Poaceae species. Studies also show that these smoke solutions play a role in vegetative growth of many plants. We examined if this fire-response syndrome is related to the effect of plant-derived smoke-water (1:500 v/v) and smoke-derived butenolide compound (10 -8 M) on seed germination and seedling growth of six major constituent species of the grassland. In addition, the interaction of the smoke solutions with temperature was examined by incubating seeds at a range of temperatures. Treating seeds with smoke-water and butenolide, the germination rate and final germination percentage were greater in three of the six species. Themeda triandra Forssk. and Tristachya leucothrix Trin. ex Nees showed the greatest response, with final germination increased from 43% to 67% and 35% to 63%, respectively. With increasing temperature (> 30°C), Aristida junciformis Trin. & Rupr., Hyparrhenia hirta (L.) Staph, and Panicum maximum Jacq. responded positively to the test solutions. In nearly all the species tested, smoke-water–treated seeds produced significantly longer shoots or roots. However, the degree of response varied from species to species and across different temperatures. Findings from this study suggest that plant-derived smoke and its interaction with temperature may significantly influence the germination and seedling growth of the South African mesic grassland species, which can further alter the grassland composition.

Soil carbon (C) stocks generally decrease following land use conversion from pasture to conifer (pine) plantation. In exploring whether factors relating to fine root decomposition might be involved in that effect, a 1-year pot experiment in a controlled temperature glasshouse was designed to test the role of three factors on fine root decomposition. The factors were: fine root type (roots of grass— Themeda triandra , and pine— Pinus radiata ), soil source (top soil from a native grass pasture and top soil from an adjacent pine plantation), and species planted and its treatment (pine, uncut grass, defoliated grass, and soil-only control). The dry mass loss in 1 year, averaged across all factors, was 39% of the initial mass and the average decomposition rate constant ( k ) was 0.51 year −1 . Only the species planted had an effect on dry mass loss from decomposing roots, i.e. pine seedlings reduced the decomposition rate of fine roots. Carbon loss from decaying fine root was affected by the decomposing root type as well as the species planted in the soil. The C loss from pine root decomposition was slower than from grass root decomposition, and pine seedlings reduced the rate of C loss from decomposing root. All three factors affected N loss. Net N loss was found only from decaying grass roots, N being accumulated in decaying pine roots. Decaying pine fine root accumulated more N when in soil from the plantation (having lower N concentration) than when in soil from the native pasture (with higher N concentration). In conclusion, the findings have led us to a hypothesis for field testing that, in terms of carbon, pine roots decompose to soil humus slower than grass roots because of differences between ecosystems in both root litter quality and soil environment as determined by the plant species growing.

We examined the competitive ability and response to light (shaded or unshaded), nutrients (50% Hoaglands solution or tap water), water (frequent or infrequent watering) and cutting (cut or uncut) of four grass species that differ in their traits, habitat preferences and responses to management practices in KwaZulu-Natal. All species produced the greatest biomass in high-nutrient high-water treatments. Short to medium-height grass species, such as Themeda triandra and Aristida junciformis, produced nearly double the biomass of taller species such as Hyparrhenia hirta and Eragrostis curvula in the low-nutrient treatments, with the reverse being true in the high-nutrient treatments. Themeda triandra and A. junciformis produced the greatest root biomass both in the low- and high-nutrient treatments. The shorter species produced relatively greater root than shoot mass, with the reverse being true of the taller species. Tall species did not produce relatively less, when cut, or relatively more, when shaded, than the shorter species. However, A. junciformis and E. curvula were more shade-tolerant than the other species, suggesting that their increase in abundance with infrequent fire is due to the shading by litter in unburnt grassland. The competitive hierarchy among species reversed between low- and high-nutrient treatments, with A. junciformis and H. hirta having the greatest competitive effect on T. triandra in low- and high-nutrient treatments, respectively. These results contribute to an understanding of species distributions on fertility and burning gradients. Keywords: competitive ability, grass traits, habitat fertility, shoot/root ratio, trade-offsAfrican Journal of Range & Forage Science 2006, 23(2)

The botanical and chemical composition, intake and digestibility of local sheep (Ovis aries) and goat (Capra hirtus) diets were evaluated over the wet (growing) and dry (dormant) seasons. Diet botanical composition was related to the vegetation composition on the range. Commiphora riperia and Acacia tortilis were the most dominant tree species, while Duosperma kilimandscharicum and Premna hildebrandtii were the most dominant shrub species. Enteropogon macrostachyus, Cenchrus ciliaris and Chloris roxburghiana were the most abundant grass species, while Brepharis integriifolia, Commelina benghalensis and Macrotylomma axillare were the most dominant forb species. Grasses increased towards the end of the wet season and the beginning of the dry season, while the forbs decreased. Eragrostis caespitosa, Cenchrus ciliaris, Eragrostis superba, Enteropogon macrostachyus and Themeda triandra were the most dominant grass species in sheep diets during both seasons, accounting for over 82% of the diet. Acalypha frutico...

Seven grass species were grown in monocultures and in multispecies mixtures along a gradient of total nutrient levels that ranged from 1/64 to 16 � the normal level of nutrient solution. The seven grasses represented three ecological groups: (i) three perennial species native to Australia ( Themeda triandra, Poa labillardieri and Danthonia carphoides ); (ii) two introduced annuals ( Vulpia bromoides and Hordeum leporinum ); and (iii) two introduced perennials ( Lolium perenne and Dactylis glomerata ). We hypothesized that the native grasses would prove less competitive when grown at increased nutrient levels than those introduced from Europe. Results supported the hypothesis. The native species were unable to compete in mixtures even at the lowest nutrient level, where T. triandra was the most productive species in monoculture. Lolium perenne and Dactylis glomerata dominated mixtures at intermediate nutrient levels. The responses of the annual introduced grasses differed in that Vulpia bromoides showed an optimum at intermediate nutrient levels in both monoculture and in mixtures, whereas Hordeum leporinum dominated at the highest nutrient levels in mixture but was suppressed by V. bromoides, L. perenne and D. glomerata at intermediate levels. The results are discussed in terms of predicting species responses in mixtures from their performance in monocultures as well as in terms of previous observations on the sequential changes in botanical composition of south-eastern Australian grasslands after 150 years of continuous grazing by sheep.

In several tropical forage species the neutral detergent extraction used to determine cell wall and cell contents fractions solubilised a substantial UV-absorbing polymeric fraction which appeared to be a soluble lignin-carbohydrate complex. A crude product was isolated from spear grass (Hetero-pogon contortus) with a yield of 30 g kg -1 . In other forages an acetyl bromide method was used to estimate lignin extractable by neutral detergent. For eight species of tropical grasses this gave the following values: Aristida calcyna, 38; Mitchell grass (Astrebla squarrosa), 37; Chloris gayana, 31; Eragrostis sp, 37; spear grass, 49; Iseilema membranaceum, 35; Panicum maximum, 36; Sorghum sp, 42; Themeda triandra, 79 g kg -1 . The levels in two legumes were: Alysicarpus bupleurifolius, 29; Macroptilium atropurpureum, 27 g kg -1 dry matter. Sequential extractions on spear grass and Mitchell grass showed that most of the 50% dioxan-soluble fraction was also soluble in neutral detergent, raising the possibility that 'dioxan lignin', on which studies of forage lignin structure in other species have been conducted, would be found in the cell contents fraction rather than in the fibre fractions on which lignin analyses have been performed. Addition of dioxan extractives reduced the in vitro neutral detergent fibre (NDF) digestibility of spear grass and Mitchell grass. Dans plusieurs especes de fourrages tropicaux, l'extraction au detergent neutre utilisee pour determiner les fractions solubles et fibreuses solubilise une fraction polymere qui apparait comme etant un complexe lignine soluble-carbohydrate. Cette fraction est mesuree dans differentes especes fourrageres tropicales. L'effet inhibiteur sur la digestibilite in vitro des fourrages est egalement observee.

Grasses (Poaceae) lack the complex biochemical pathways and structural defenses employed by other plant families; instead they deposit microscopic silica (SiO2) granules in their leaf blades (i.e. phytoliths) as a putative defense strategy. Silica accumulation in grasses has generally been considered an inducible defense; other research suggests silica accumulation occurs by passive diffusion and should therefore be closely coupled with whole plant transpiration. We tested the hypothesis that grasses increase leaf silica concentration in response to artificial defoliation in a common garden study in the Serengeti ecosystem of East Africa. Additionally, a watering treatment tested the alternative hypothesis that leaf silica was largely driven by plant water status. Leaf silica content of two dominant C4 Serengeti grass species, Themeda triandra and Digitaria macroblephara, was quantified after a 10-month clipping x water experiment in which defoliation occurred approximately every two months and supplementary water was added every two weeks. Themeda had greater silica content than Digitaria, and Themeda also varied in foliar silica content according to collection site. Clipping had no significant effect on leaf silica in either species and watering significantly increased silica content of the dominant tall grass species, Themeda, but not the lawn species, Digitaria. Our data, and those collected as part of a supplementary literature review, suggest that silicon induction responses are contingent upon a combination of plant identity (i.e., species, genotype, life history limitations) and environmental factors (i.e., precipitation, soil nutrients, grazing intensity). Specifically, we propose that an interaction between plant functional type and water balance plays an especially important role in determining silica uptake and accumulation.

Summary 1 The influence of botanical composition on annual phytomass production of a semi-arid grassland in response to precipitation was tested with a 19-year experiment. Three compositional states reflecting good, medium and poor rangeland condition, whose basal cover increased from poor through to good, were created in 1978. 2 Multiple regression models were developed for assessing whether phytomass was influenced by precipitation, composition, phytomass of the previous year, basal cover, abundance of individual species, or diversity. Composition and precipitation accounted for 66% of the variation in phytomass, but a separate model of basal cover by precipitation was equally successful. The linear effect of rainfall on phytomass was enhanced as composition improved from poor (0.076 g m−2 mm−1) through medium (0.197 g m−2 mm−1) to good (0.277 g m−2 mm−1). Phytomass increased for low precipitation if it had been high the preceding year. Phytomass was more variable over time with deteriorating condition. 3 Species’ complementarity ensured greater and more stable production. Setaria sphacelata, Eragrostis chloromelas and Digitaria eriantha increased phytomass on the good or medium condition grasslands during drier years, whereas Themeda triandra had this effect during wetter years. 4 Precipitation-use efficiency (PUE) was influenced mostly by composition and a linear and quadratic effect of precipitation (63% of variance). Optimum PUE of 0.308, 0.203 and 0.096 g m−2 mm−1 for the good, medium and poor condition grasslands, respectively, occurred at intermediate amounts (±680 mm) of precipitation. PUE was increased if phytomass had been high the previous year. 5 Species’ complementarity of PUE in response to precipitation was evident for all compositional states. Ten, mostly uncommon, species and their interaction with precipitation explained an extra 21–42% of the variance. Stability of production was related to PUE for medium and poor condition grassland. Uncommon species therefore ensured growth efficiency and stabilized production as condition deteriorated. 6 Diversity had no influence on phytomass or PUE except for a small to moderate effect, respectively, for the medium condition grassland. 7 Vegetation structure, through limiting runoff and promoting infiltration, is an important control on the amount and efficiency of plant production under variable precipitation, whilst composition further influences the amount and stability of production.

Aluminium toxicity was investigated by analyzing the reaction of hydroponically-grown plants of two ecotypes of T. triandra to Al (0, 25, 50, 100 μM; pH 4.1; for 28 days). The Al-tolerant 'Berg ecotype responded to the Al treatments by increasing root extension growth, D.M. production and they also controlled water-use over the full range of Al stresses. In contrast, Al inhibited root growth and D.M. production in the Al-sensitive Zululand ecotype and they also failed to control water-use at 100 μM Al. Microscopical investigation showed that the root cells of both ecotypes were generally resistant to Al with cellular disorganization being confined to the Zululand plants at the highest Al stress. The root caps of both ecotypes responded to Al in ways (amyloplast numbers, cap volume, secretions) which coincided with the contrasting growth responses observed in 'Berg and Zululand plants. Aluminium tolerance involved the accumulation of Al in the roots of 'Berg plants and was coincident with increases in root...

Aqueous extracts of smoke, derived from Themeda triandra, a fire-climax grass, and Passerina vulgaris, a fynbos plant, stimulated the growth of primary root sections of tomato roots in suspension culture. The optimal dilution for both extracts was 1:2000. Several of the fractions obtained from TLC separation of the Themeda and the Passerina extracts significantly promoted primary root growth. The auxins naphthaleneacetic acid (NAA), indolebutyric acid (IBA) and indoleacetic acid (IAA) were found to stimulate the growth of the primary root axis, with IAA and NAA significantly promoting lateral root number. Similarly, the naturally occurring cytokinins, zeatin and its derivatives (zeatin-O-glucoside; dihydrozeatin and zeatin riboside) stimulated primary root length. Zeatin and dihydrozeatin promoted secondary root growth, but only at very low concentrations.

Degradation in arid and semi-arid regions can be ascribed primarily to environmental conditions such as extreme changes in temperature and erratic rainfall patterns. Seed germination and seedling growth of three grass species (Anthephora pubescens, Heteropogon contortus and Themeda triandra) indigenous to arid and semi-arid regions of southern Africa were assessed under controlled conditions that simulated drought stress using polyethylene glycol (PEG) 6000. Exposure to water potentials below − 0.3 MPa significantly decreased germination rates and shoot lengths for all three species, despite slight differences in their sensitivities to osmotic stress. More dramatic differences between the species were evident when the effects of water stress on root growth were evaluated. These findings form the basis for future trials involving the use of indigenous grasses in the restoration of rangelands.

The influence of micro‐site (shaded, exposed, buried) on the seed longevity and seedling emergence of Themeda triandra was tested using experimental seed banks. Deep shade considerably reduced soil surface temperature. Seeds did not persist in any micro‐site for longer than one year. The fate of most seeds in the first year was to germinate but most buried seeds died before emergence

The influence of water stress on the photosynthetic rate of the C 4 grasses Themeda triandra Forsk. and Eragrostis lehmanniana Nees was determined for the vegetative and reproductive phases. Gas exchange was determined with an infra-red gas analyser, white leal water potential was used to quantify water stress. An open system of gas flow was used. The rate of photosynthesis was 0.3131 mg CO 2 m - 2 s - 1 and 0.6287 mg CO 2 m - 2 s - 1 respectively for T. triandra and E lehmanniana (P ≤ 0.001) if water was not limiting. The rate of photosynthesis began to decline at a leaf water potential of − 1 927 kPa and − 1 625 kPa lor T. triandra and E. lehmanniana respectively. This decline in rate of photosynthesis was significantly (P ≤ 0.001) correlated with leaf water potential, and linear relationships with correlation coefficients of 0.936 and 0.938 were obtained for T. triandra and E. lehmanniana respectively.

Summary The interaction between temperature and diet quality can affect the life-history of ectotherms. The rate and ratio at which protein and carbohydrate are obtained from food are an important aspect of diet quality, and insects have a well-developed capacity to adjust their feeding behaviour and postingestive physiology to regulate intake and allocation of these nutrients. If the supply of protein and carbohydrate varies with temperature (e.g. via effects on intake, digestion or metabolism), then herbivorous insects can use thermoregulatory behaviour to help achieve nutritional homeostasis. When fed the host grass Triticum aestivum, Locusta migratoria nymphs absorbed and allocated protein and carbohydrate to growth with the same efficiency at 38°C as at 32°C, however, at the higher temperature, they ingested more food. In contrast, when feeding on Themeda triandra, the nymphs absorbed carbohydrate with higher efficiency at 32°C and protein at 38°C. Using synthetic diets, we induced either a protein or a carbohydrate deficiency in experimental insects and showed that locusts placed in a thermal gradient following a meal of T. triandra selected 32°C when deprived of carbohydrate, and 38°C when protein-deficient. This capacity to use thermoregulatory behaviour to redress an imposed nutritional imbalance improved with experience of feeding on T. triandra. As predicted, locusts fed T. aestivum always chose higher temperatures, irrespective of nutritional state. Our results have consequences for understanding host plant choice by herbivores and interpreting the effects of changed environmental temperatures and microclimate on animal-plant interactions.

Selective grazing by livestock inevitably results in patch grazing. This study was initiated to determine the effects of patch grazing on (1) the vigour of a single grass species, Themeda triandra, and of the sward, and (2) sward species composition. The generally held idea that a full season's rest followed by an early spring burn would prevent preferential grazing of patch grazed areas which had developed in the seasons before the rest was also tested. The vigour of 71 triandra was estimated from etiolated growth of marked tufts while sward vigour was indexed by above‐ground herbage production (AGHP). Etiolated growth of T. triandra and AGHP of the sward within patches were negatively affected by three seasons of grazing, but a full season's rest appeared sufficient to restore both the etiolated growth of T. triandra and the AGHP of the sward to a level similar to that in the non‐patches. A full season's rest followed by spring burning did not, however, prevent preferential grazing of grazed pa...

Smoke is now widely recognized as a germination cue for fire-dependent and non-fire-dependent plant species. Although the identity of the stimulant(s) and its mode of action remain unknown, it needs to be established whether burning or heating of all plant materials generate the active compound(s). In view of the variation in fire intensity in the wild it must also be established at what temperatures the active compound(s) is produced. Heating dry Themeda triandra leaves at a range of temperatures showed that the active component(s) is produced between 160 and 200°C. At higher temperatures activity is lost, apparently due to volatilization of the stimulatory compounds. Aqueous smoke extracts prepared from a range of plants, and extracts prepared by heating agar and cellulose, contained compounds that stimulated the germination of light-sensitive lettuce seeds. Chromatographic evidence obtained by thin-layer chromatography and high-performance liquid chromatography suggest that the same active compound(s) is produced from T. triandra leaves, agar and cellulose used in this study.

The interactive effects of increased carbon dioxide (CO 2 ) concentration and ultraviolet-B (UV-B, 280-320 nm) radiation on Acacia karroo Hayne, a C 3 tree, and Themeda triandra Forsk., a C 4 grass, were investigated. We tested the hypothesis that A. karroo would show greater CO 2 -induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C 4 grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol -1 ) and elevated (650 μmol mol -1 ) CO 2 , combined with ambient (1.56 to 8.66 kJ m -2 day -1 ) or increased (2.22 to 11.93 kJ m -2 day -1 ) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO 2 had no effect on biomass of A. karroo, despite increased net CO 2 assimilation rates. Interaction between UV-B and CO 2 on stomatal conductance was found, with conductances decreasing only where elevated CO 2 and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO 2 . Elevated UV-B caused only an increase in foliar carbon concentrations. In T triandra, net CO 2 assimilation rates were unaffected in elevated CO 2 , but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO 2 , and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO 2 . Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO 2 -induced growth responses of the species therefore do not support the hypothesis of CO 2 -driven woody encroachment of C 4 grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO 2 response of these species.

Aqueous extracts of smoke, derived from Themeda triandra, a fire-climax grass, and Passerina vulgaris, a fynbos plant, stimulated root initiation and development in the hypocotyls of mung beans. Immersion time and the dilution of the smoke extracts were important in optimising the rooting response.

Tuft vigour of important sourveld grasses was indexed by measuring the regrowth, during a full season's rest, of tufts of Themeda triandra Forssk. and Tristachya leucothrix Nees, which (1) had not been grazed, (2) had been leniently, or (3) had been severely grazed during the previous two growing seasons. A sequential harvesting technique was applied and regrowth was expressed in terms of above‐ground herbage‐mass accumulation and etiolated growth. Patterns of regrowth were similar for both species. Severe grazing during the previous two years significantly reduced topgrowth yields during the first 27 to 31 weeks of rest compared to tufts which had previously been leniently or not grazed. Etiolated growth was greater on leniently and ungrazed tufts than on severely grazed tufts for 13 to 15 weeks after the commencement of spring growth, whereafter there were no significant differences. The regrowth patterns of tufts for all intensities of defoliation suggests that the vigour of severely grazed tu...

SUMMARYThe seasonal rate and extent of dry matter (DM) and neutral detergent fibre (NDF) degradation of the African sourveld grasses Alloteropsis semialata, Andropogon appendiculatus, Cynodon dactylon, Elionurus muticus, Eragrostis plana, Harpochloa falx, Heteropogon contortus, Hyparrhenia hirta, Microchloa caffra, Themeda triandra and Tristachya leucothrix in the rumen were measured using the nylon bag technique at the Döhne Agricultural Development Institute, South Africa, in 1993. The size of the soluble fraction (SF) was markedly different (P < 0·05) between species, being largest in A. semialata (22·6%) and smallest in E. plana (13%). Over all species, the mean SF was highest (P < 0·01) in spring (21·6%) and lowest in winter (11·8%). Species differed (P < 0·05) in the size of the potentially digestible fraction (PDF), with H. contortus and T. leucothrix having the largest (71%) and M. caffra the smallest (53%) PDF. Mean PDF declined (P < 0·01) from spring (77·3%) through to winter (55·8 %). The rate of DM degradation (kd) in the rumen also differed (P < 0·01) between species, with C. dactylon and A. semialata having the fastest (0·054) and E. plana and A. appendiculatus the slowest (0·039) degradation rates. Mean kd values were lower (P < 0·05) in winter (0·039) than in the other seasons (0·048).Neutral detergent soluble (NDS) content was highest in A. semialata (29%) and lowest in E. plana (21·5%). Species similarly varied in the amount of potentially digestible cell wall (PDCW), with E. plana having the highest (57%) and C. dactylon the lowest (45·1%) PDCW content. Mean PDCW content was highest in spring (59·5 %) and lowest in winter (44·6%). Cell wall degradation (kc) rates were fastest (0·04) in C. dactylon and slowest in M. caffra (0·03). Overall, kc declined from spring (0·039) through to winter (0·029). In general, A. semialata, T. triandra and H. hirta appeared to have the most favourable, and E. plana, A. appendiculatusand M. caffra the least favourable, ruminal degradation characteristics. These differences suggest that species composition is likely to have a major impact on potential animal production on these low quality grasslands.

The percentage of phenolic containing cells (PCC) and total phenolics in the leaves of different ecotypes of three ecological important grass species were determined during cultivation under uniform environmental conditions. Results obtained clearly indicate that ecotypes of these species exhibit a significant degree of variation with regard to percentage PCC and total phenolics. Furthermore, no anti-herbivory function can be assigned to phenolic compounds in these species, as ecotypes with Decreaser-species ecological status (highly palatable) have higher levels of total phenolics, compared with other less palatable ecotypes with lower levels of total phenolics. Percentage PCC and total phenolics represent two of various ecological important characteristics to be used in conjunction with other ecological characters to construct a usable, special purpose classification system. This system will assist rangeland ecologists in identifying functional ecotypes within grass species, thus facilitating the understanding and interpretation of vegetation dynamic models and rangeland condition assessments.

The relative effects of drought and heavy grazing on the floristic composition, population size and and structure, and basal cover of an African savanna grassland were differentiated by comparing changes over eight years over eight years, which included a severe drought year, across a gradient of grazing history. Drought had an overriding effect on community change, but grazing history had an additional effect. Severe drought in combination with a history of severe grazing transformed grassland of predominantly palatable, perennial grasses (Themeda triandra, Setaria incrassata, Heteropogon contortus) to grassland dominated by the unpalatable perennial Aristida bipartita, annual grasses and forbs. Palatable species were almost eliminatedfrom the sward between tree canopies, but residual populations were protected beneath certain woody species. The most lightly grazed grassland maintained its character of platable, perennial grass species, but was changed considerably in the relative proportion of these species. After the drought, tuft size of palatable species was smaller than any previously recorded but recovered thereafter. A numbr of annual and perennial grass species were recorded for the first time subsequent to the drought. Consideration of the life history attributes of species was useful for predicting species' responses. Grazing management during drought episodeswould seem critical for determining the direction of community change. The notion that savanna grasslands are insensitive to grazing because of their disequilibrium behaviour was rejected.Keywords: basal cover; botany; disequilibrium; drought; floristic composition; Gazankulu; grassland; grazing; life history; population dynamics; population structure; savanna; South Africa; transformationAfrican Journal of Range and Forage Science 1995, 12(2): 53–60

Summary A study of net CO2 assimilation rate and water vapour exchange of Panicum maximum that grows predominantly beneath savanna tree canopies, and Themeda triandra that grows primarily in adjacent open grassland sites, was carried out on both sites in Nairobi National Park. Panicum maximum exhibited less water stress, had lower stomatal conductance and transpiration, and had higher water use efficiency than T. triandra under tree canopies. This indicated that P. maximum should accumulate more biomass with less water loss than T. triandra under tree canopies, as both grasses had similar rates of net CO2 assimilation in the shade. More grazing selection pressure for P. maximum than T. triandra in the open grassland may also help to explain why P. maximum is more dominant in shaded habitats; whereas, T. triandra dominates open habitats. Resume On a menea deux endroits du Parc National de Nairobi une etude du taux net d'assimilation de CO2 et de l'echange de vapeur d'eau chez Panicum maximum qui pousse surtout sous la canopee des arbres de savane et chez Themeda triandra qui pousse d'abord dans les endroits ouverts qui leur sont voisins. Panicum maximum presente moins de stress liea l'eau, une conductance et une transpiration stomatiques moindres et montre une utilisation de l'eau plus efficace que T. triandra sous la canopee. Ceci montre que P. maximum devait accumuler plus de biomasse, avec une perte d'eau moindre que T. triandra sous la canopee puisque les deux herbers presentent des taux comparables d'assimilation nette de CO2a l'ombre. La pression exercee par un pâturage selectif sur P. maximum plus que sur T. triandra dans les prairies ouvertes peut aussi expliquer pourquoi P. maximum est plus dominant dans les zones ombragees tandis que T. triandra domine dans les zones ouvertes.

Plant-derived smoke and aqueous extracts of smoke can overcome certain types of seed dormancy. Successful smoke (aqueous) pre-treatment of seed of the fire-climax grass Themeda triandra Forssk. is reported. The promotive effect of smoke on seed germination was retained when pre-treated seed was dried and stored for up to 21 days. Seed pre-treatment with smoke had no detrimental effect on subsequent seedling growth.

Plant-derived smoke and aqueous extracts of smoke stimulate germination of dormant seed of the important fire-climax grass Themeda triandra (redgrass). The positive germination response to smoke increased as the state of seed imbibition increased. Furthermore, aqueous smoke extracts significantly increased seed germination at optimum and sub-optimum germination temperatures. Ethrel and ethylene, at a wide range of concentrations, failed to increase T. triandra seed germination, indicating that ethylene is not the active component of plant-derived smoke. Similarly, ash or aqueous ash extracts failed to stimulate germination. In contrast, aqueous smoke extracts prepared from burning fynbos vegetation and grass leaf material stimulated T. triandra seed germination. The bioactive component of plant-derived smoke may originate from a commonly occurring source, possibly being a thermal breakdown product of hemicellulose or cellulose.

A population cytogenetic study was undertaken across a contact zone between Themeda triandra populations in the Transvaal Bushveld to the north of the Magaliesberg, and Highveld populations to the south. Fifteen collection points were chosen and three plants collected at each point. The northern-most collection point contained only tetraploids and those on the Highveld only hexaploids. In the vicinity of the Magaliesberg Mountain range, contact between the tetraploids and hexaploids produced pentaploid and decaploid hybrids. Meiotic pairing association analyses showed that both the tetraploids and hexaploids formed apomictic clones, of which some were widely distributed and others were more limited. This indicates that facultative apomixis occurs among both the tetra- and hexaploids and gives rise to genotypic and genomic segregants thus producing variation and forming new apomictic clones. The possible evolution of Themeda triandra in southern Africa is discussed.

Background and aims The success of C4 plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C4 grasses are not necessarily less sensitive to drought than C3 grasses and are proposed to respond with greater metabolic limitations, while the C3 response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C3 and C4 NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response. Methods Six species of locally co-occurring grasses, C3) species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C4 (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO2 concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types. Key results Leaves of C4 species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C4 photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO2 assimilation, whereas, in the C3 species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C4 plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C3 plants which recovered a greater proportion of photosynthesis through increased stomatal conductance. Conclusions Within the Panicoid grasses, C4 (NADP-ME) species are metabolically more sensitive to drought than C3 species and recover more slowly from drought.

A two-phase grassland mosaic was described using species association measures and pattern analysis techniques on local frequency data from six transects of contiguous quadrats. One phase consisted of almost mono-specific patches of Setaria incrassata, which was negatively or not associated with ten common species. The other phase was dominated by Themeda triandra which was positively associated with several species. PQV analyses revealed similar, significant, anisotropic scales of contagion of Setaria and Themeda, with phases alternating over 9 m up-slope and 4 m along the contour. Setaria was associated with a tree canopy and deeper soils of higher organic carbon content than Themeda, which was negatively associated with tree canopies. Excavation of Setaria rhizomes indicated mean densities of 20.5 putative genets/m2 and 3.6 ramets per genet. Dense clonal growth, seedling recruitment and a competitive exclusion mechanism may contribute to patch maintenance.

The embryo sac development of 53 collections of Themeda triandra Forssk. was studied. All the diploids and two of the tetraploids had sexual embryo sacs of the Polygonum -type, whereas the remaining polyploids possess 4-nucleate aposporic embryo sacs. Some of the collections are facultative apomicts, while others (in particular the penta- and hexaploids) are essentially obligate apomicts. Interesting variations in the type of apospory was encountered in the different apomicts. This variation might be due to environmental influences. On the whole it would appear from the results that Themeda triandra is a young and therefore still actively developing agamic complex in southern Africa.

The second youngest fully-expanded leaf on the main tiller of vegetative Themeda triandra plants was exposed to 14CO2. Thereafter, quantitative analysis of partitioning, storage and remobilization after defoliation of 14C-labelled assimilate was conducted in serially-harvested plants over a 27-day period. In undefoliated plants, 50% of assimilated 14C was ultimately lost through respiration, while that remaining in the plant reached its final destination within two days of assimilation. A small, but significant, portion of assimilated 14C was invested into long-term storage in roots and stem bases. These reserves were remobilized after defoliation, and a portion of the remobilized 14C was incorporated into new growth. Partially-defoliated plants regrew more rapidly than totally-defoliated plants, but they both made equal demands on 14C reserves. Reserve depletion took place from both roots and stem bases. Keywords: 14c; assimilation; defoliation; growth; labile carbon reserves; leaves; partitioning; plant physiology; regrowth; reserves; respiration; roots; stem; storage; structural carbon; themeda triandra; tiller

The effect of defoliation during wilting on the production of Themeda triandra and Eragrostis lehmanniana was studied over four years (1977 to 1980). Defoliation whilst the plants were at temporary wilting point in the early growing season (before 1 January) decreased production from both species in the subsequent season (P < 0.01). Defoliation over the last half of the growing season, during wilting, had little effect on growth in the following season (P > 0.05). The effect of a defoliation treatment on production was not necessarily neutralised after one season's rest. Themeda triandra produced more (P < 0.01) than E. lehmanniana for each defoliation. The seasonal soil-water extraction pattern in the soil profile was also determined. As the season progressed, plants showed visible signs of wilting at lower soil-water content than earlier in the season. Most water extraction over the season takes place to a soil depth of 600 mm.Keywords: botany; defoliation; drought; eragrostis lehmanniana; free state; grassland; growth; season; semi-arid; soil depth; soil water; soil water content; south africa; sydenham; temporary wilting point; themeda triandra; themeda-cymbopogon veld; wiltingAfrican Journal of Range & Forage Science, 10(3): pp. 113-117