Limited seed dispersal and microspatial population structure of an agamospermous grass of West African savannahs, Hyparrhenia diplandra (Poaceae)

We studied the microspatial population structure of the perennial tussock grass, Hyparrhenia diplandra (Poaceae), a facultative agamospermous species of West African savannahs. The microspatial population structure of H. diplandra was investigated by choosing two 100-m(2); quadrats at random from which all individuals were mapped. The genotype of every individual was determined using two highly polymorphic microsatellite loci. A chloroplast locus was also used to investigate the role of seed dispersal on the genetic structure of populations. The genetic diversity index (0.85) was high for a clonal species. Significant genetic differentiation over short distances was detected by F statistics, and spatial autocorrelation analyses within both quadrats showed significant isolation-by-distance patterns, both with the cytoplasmic locus and the nuclear loci. Some clones formed large patches (up to 5 m in diameter) whereas others were more scattered. However, the genetic differentiation between quadrats was much higher when studied with the cytoplasmic locus than with the nuclear loci, indicating that gene flow via pollen, but not seeds, may frequently occur between quadrats. The maintenance of genetic diversity in this facultative agamospermous species most likely results from several factors, such as low seed dispersal ability, nonnegligible gene flow through pollen, and selective pressures induced by regularly occurring fires in this ecosystem.     

流域尺度上河流水质与土地利用的关系

以苏子河流域内54个水质采样点为基点,生成6种尺度的河岸带缓冲区,并借助FRAGSTATS软件计算景观水平和类型水平上的8种景观指数.分别从景观空间格局与景观类型组成两方面,对景观指数与水质进行相关分析.结果表明:区域景观格局在不同缓冲区内对流域水质具有不同的效应.当缓冲区距离≤300 m时,旱地、建筑用地、水田为主要的景观类型组成,其面积比例、斑块数量、斑块密度、最大斑块指数、最大形状指数、景观斑块聚集度指数均较高,农田的连通性较高,对水质的影响较大.在距离河流较远的区域(缓冲区距离＞300 m),林地面积比例较高,林地聚集连通程度较好,对水质改善具有一定作用,但不明显.该流域耕地、建设用地等对水质有着关键的影响作用.     

Responses of an African tall-grass (Hyparrhenia filipendula stapf.) to defoliation and limitations of water and nitrogen

Summary Hyparrhenia filipendula stapf., a tall (1–1.5 m) perennial grass common in dry-subhumid African savannas, was collected from Serengeti National Park in Tanzania, propagated vegetatively, and grown in controlled environments simulating conditions in nature. Plants were subjected to a factorial experiment with combinations of watering frequency, nitrogen supply, clipping height (10 and 15 cm) and clipping frequency (7 and 14 d). Biomass yield and allocation to various tissue types, morphometric traits, and growth processes were measured.Watering frequency affected leaf elongation rate while nitrogen affected tiller number. Clipped yield was strongly correlated with leaf elongation rate but not tiller number, therefore it was primarily controlled by the activities of intercalary rather than apical meristems. There was a negative exponential relationship between tillering and clipped yield per tiller. Plants that received both high nitrogen and high water closely followed a-3/2 power law in this tradeoff. The fraction of total net photosynthate allocated to roots was not significantly related to any environmental treatment. Root and crown growths were not affected by defoliation treatment; leaf blade and stem growths were inhibited; and sheaths were inhibited only under low water.Despite a tall stature, H. filipendula tolerated herbivory by increased photosynthetic rate (Wallace et al. 1984), through continued production of young tissues by intercalary meristems balanced against tiller number, and by a statistically constant proportional allocation to roots. Clipped yield increased only when both nitrogen and water were abundant, and then, proportional clipped yield did not surpass an upper asymptotic limit.     

Linking rhizoplane pH and bacterial density at the microhabitat scale

Ion-selective microelectrodes and a novel micro-sampling technique were used to investigate the relationship in field soil between Brassica napus rhizoplane pH and bacterial density at a spatial scale approximating a microhabitat. Bacterial densities were observed to increase with decreasing pH, rhizoplane pH measurements varied by up to 1 pH unit over a distance of 1 mm and the mean pH of the rhizoplane at the root base varied by more than 1 pH unit between plants. These findings highlight the appropriateness of investigating the interactions between bacterial communities and their environment at the micro-spatial scale and the utility of the micro-sampling method.     

Plant chemical defense: monoterpenes and the growth-differentiation balance hypothesis

Recent studies of allocation to defensive chemicals in plants have provided insights into the ecological controls over plant defensive chemicals. Both developmental and ecological studies now suggest that we can understand the factors influencing allocation to defense by examining the relative availability of resources, external needs for chemical defense, and the internal demands for growth that plants face. These studies have also shed light on one of the more popular theories in plant evolutionary ecology, the growth-differentiation balance hypothesis of plant resource allocation.     

Relationships between culture density and the composition of three floating aquatic macrophytes

Eichhornia crassipes (Mart.) Solms, Pistia stratiotes (L.), and Lemna minor (L.) were cultured at four different densities each and analyzed for cell-wall fraction, crude protein, total available carbohydrate and ash. Cell-wall fraction increased and crude protein content decreased as density increased in Eichhornia and Pistia cultures. The cell-wall and crude protein content of Lemna did not change with increasing culture density. Differences in the trends of cell-wall and crude protein content of the three plants at increasing culture densities appear to be related to differences in growth form. There was no difference in the total available carbohydrate or ash content of the three species at different culture densities.     

Competitive interactions between grass and succulent shrubs at the ecotone between an arid grassland and succulent shrubland in the Karoo

Nearest-neighbour analysis was used to examine the competitive interactions between Stipagrostis brevifolia, a C₄ perennial grass, and two leaf succulent shrubs, Ruschia robusta and Leipoldtia pauciflora, at the ecotone between semi-arid grassland and succulent shrubland in the Karoo. The root distribution in the soil was also compared to assess the degree of overlap in the potential use of soil resources. Regressions between the combined sizes of interspecific, nearest-neighbour species and the distance between them showed significant positive correlations for S. brevifolia and R. robusta, which suggest the presence of competition. We infer from individual species regressions that the grass exerted a stronger competitive force on the shrub R. robusta than R. robusta on the grass. There was also evidence for strong intraspecific competitive relationships within S. brevifolia and R. robusta. There was no evidence of competition between S. brevifolia and L. pauciflora or among L. pauciflora individuals. S. brevifolia had the deepest root system, and was recorded at depths of 70 cm. Most of this root mass occurred between 10 and 40 cm. Ruschia robusta roots were recorded as deep as 55 cm, but more than 90% was found in the top 20 cm of the soil, creating a degree of overlap with the vertical root distribution of S. brevifolia. A clear separation in rooting depths occurred between S. brevifolia, and L. pauciflora which had only 3% of the total root mass below 10 cm. The partial overlap in the vertical root distribution between S. brevifolia and R. robusta may account for the observed competitive relationship, but each species dominates in a different layer, potentially minimising the net competition between S. brevifolia and R. robusta. Our findings demonstrate the possibility of a two-layer water-obtaining strategy in a semi-desert ecosystem, where the succulent shrubs seem to be playing the typical “grass” role described in most models of water partitioning between grass and woody plants. The stronger competitive effect of S. brevifolia on R. robusta at all the sites is of significance to species dynamics, and might be related to winter/summer rainfall dynamics at the climatic transition.     

Mammalian herbivores,grass height and rainfall drive termite activity at different spatial scales in an African savanna

Termites have a large influence on ecosystem functioning. Understanding what drives termite activity patterns improves understanding of nutrient cycling, productivity, and heterogeneity in savannas. We present a mechanistic framework that relates the interactive effects of rainfall, grassland structure, large herbivore presence, and soil factors to termite activity. To test this framework, we used grass litterbags to monitor termite activity at ten sites across Hluhluwe‐iMfolozi Park, South Africa. We assessed the effects of abiotic and biotic factors on termite activity at two scales: the large (landscape) scale, variation in bait removal among 300 m² plots that were distributed across the park and at the small (within‐plot) scale (1–300 m²). Half of our sites were located inside large herbivore exclosures to test for the effect of mammalian herbivore presence. At the landscape scale, termite grass removal declined towards higher rainfall and in the presence of mammalian herbivores. Removal did not depend on soil factors. At the small scale, removal declined with increasing grass height, particularly in the 1 m surrounding the bait bag. Resource quality did not affect bait removal. We suggest that competition for forage drives the negative effect of mammalian herbivores on termites, whereas lower bait removal in taller swards may be due to direct negative effects from rainfall, fire and/or competition with free‐living microbes. Ultimately, we suggest that the impact of termites on nutrient cycling is most pronounced when abiotic (rainfall) and biotic conditions (mammalian herbivory) limit grass removal by fire and decomposition by free‐living microbes.     

Multiple scale analysis of factors influencing the distribution of an invasive aquatic grass

With the expected increase in the spread of invasive species, examination of factors controlling distributions at multiple spatial scales and ecological modelling of their potential distributions are important analyses for informed decision-making. The scale-dependence of mechanisms influencing invasion by non-native species has been shown previously, indicating that studies of key factors affecting invasive species distributions at multiple spatial scales are critical for successful management. Freshwater systems are particularly vulnerable to invasive species, yet few studies have examined the environmental factors influencing distributions of invasive species at multiple spatial scales. We examined the effect of environmental variables on the predicted distribution of the invasive aquatic grass Glyceria maxima over continental, regional and local scales in Australia. We undertook an initial critical evaluation of which predictor variables were most appropriate to use at each scale, largely considering prior knowledge. On a continental scale, climatic, topographic and hydrological variables predicted well the potential distribution of G. maxima, identifying temperate regions as most susceptible to invasion. The regional analysis found that dense, woody, riparian vegetation has a strong negative impact on the occurrence of G. maxima, especially at intermediate elevations. The invasive grass was found less often on biotite granite and on fluvial geology. At a local scale, occurrence of G. maxima was related positively to soil phosphorus and nitrogen, and negatively related to soil organic carbon. The identification of key factors affecting invasive species distributions at multiple spatial scales will inform prevention schemes, assist targeted field sampling for the development of monitoring programs, and allow prioritization of control methods.     

Suppression of nitrification and nitrous oxide emission by the tropical grass Brachiaria humidicola

Nitrification by soil nitrifiers may result in substantial losses of applied nitrogen through NO₃ ^– leaching and N₂O emission. The biological inhibition of nitrification by crop plants or pasture species is not well known. This study was conducted to evaluate the ability of three pasture species, Brachiaria humidicola, B. decumbens and Melinis minutiflora to inhibit nitrification. Plants were grown in a growth chamber for sixty days, fertilized with (NH₄)₂SO₄. After harvesting, the soil was incubated with (NH₄)₂SO₄ for 24 days. Ammonium oxidizing bacteria (AOB), NH₄-N levels, and N₂O emission were monitored at 4 d intervals. Among the species studied, B. humidicola inhibited nitrification and maintained NH₄-N in soil to a much greater extent than the other two species. This nitrification inhibition lasted for 12 days after initiation of soil incubation study (i.e. from 60 DAS when the plants were harvested). The AOB populations and N₂O emission from the soil were significantly lower in the soils where B. humidicola has been grown compared to the other two species. Root exudates and soil extracts of B. humidicola suppressed AOB populations, whereas those of B. decumbens and M. minutiflora did not. The results are in consistence with the hypothesis that B. humidicola suppressed nitrification and N₂O emissions through an inhibitory effect on the AOB population.     

Relationships between population density,fine-scale genetic structure,mating system and pollen dispersal in a timber tree from African rainforests

Owing to the reduction of population density and/or the environmental changes it induces, selective logging could affect the demography, reproductive biology and evolutionary potential of forest trees. This is particularly relevant in tropical forests where natural population densities can be low and isolated trees may be subject to outcross pollen limitation and/or produce low-quality selfed seeds that exhibit inbreeding depression. Comparing reproductive biology processes and genetic diversity of populations at different densities can provide indirect evidence of the potential impacts of logging. Here, we analysed patterns of genetic diversity, mating system and gene flow in three Central African populations of the self-compatible legume timber species Erythrophleum suaveolens with contrasting densities (0.11, 0.68 and 1.72 adults per ha). The comparison of inbreeding levels among cohorts suggests that selfing is detrimental as inbred individuals are eliminated between seedling and adult stages. Levels of genetic diversity, selfing rates (∼16%) and patterns of spatial genetic structure (Sp ∼0.006) were similar in all three populations. However, the extent of gene dispersal differed markedly among populations: the average distance of pollen dispersal increased with decreasing density (from 200 m in the high-density population to 1000 m in the low-density one). Overall, our results suggest that the reproductive biology and genetic diversity of the species are not affected by current logging practices. However, further investigations need to be conducted in low-density populations to evaluate (1) whether pollen limitation may reduce seed production and (2) the regeneration potential of the species.     

Integrated management for sustainable cropping systems: Looking beyond the greenhouse balance at the field scale

Cover crops (CC) promote the accumulation of soil organic carbon (SOC), which provides multiple benefits to agro‐ecosystems. However, additional nitrogen (N) inputs into the soil could offset the CO₂ mitigation potential due to increasing N₂O emissions. Integrated management approaches use organic and synthetic fertilizers to maximize yields while minimizing impacts by crop sequencing adapted to local conditions. The goal of this work was to test whether integrated management, centered on CC adoption, has the potential to maximize SOC stocks without increasing the soil greenhouse gas (GHG) net flux and other agro‐environmental impacts such as nitrate leaching. To this purpose, we ran the DayCent bio‐geochemistry model on 8,554 soil sampling locations across the European Union. We found that soil N₂O emissions could be limited with simple crop sequencing rules, such as switching from leguminous to grass CC when the GHG flux was positive (source). Additional reductions of synthetic fertilizers applications are possible through better accounting for N available in green manures and from mineralization of soil reservoirs while maintaining cash crop yields. Therefore, our results suggest that a CC integrated management approach can maximize the agro‐environmental performance of cropping systems while reducing environmental trade‐offs.     

The ecological stoichiometry of toxins produced by harmful cyanobacteria: an experimental test of the carbon-nutrient balance hypothesis

The elemental composition of primary producers reflects the availability of light, carbon and nutrients in their environment. According to the carbon-nutrient balance hypothesis, this has implications for the production of secondary metabolites. To test this hypothesis, we investigated a family of toxins, known as microcystins, produced by harmful cyanobacteria. The strain Microcystis aeruginosa HUB 5-2-4, which produces several microcystin variants of different N:C stoichiometry, was cultured in chemostats supplied with various combinations of nitrate and CO₂. Excess supply of both nitrogen and carbon yielded high cellular N:C ratios accompanied by high cellular contents of total microcystin and the nitrogen-rich variant microcystin-RR. Comparable patterns were found in Microcystis -dominated lakes, where the relative microcystin-RR content increased with the seston N:C ratio. In total, our results are largely consistent with the carbon-nutrient balance hypothesis, and warn that a combination of rising CO₂ and nitrogen enrichment will affect the microcystin composition of harmful cyanobacteria.     

Relationships between structural development and the absorption of ions by the root system of Cucurbita pepo

Summary In both the seminal axis and lateral roots of Cucurbita pepo L. the formation of large central xylem elements and the commencement of secondary cambial activity occur 10–20 cm from the root tip. Concomitant with or slightly preceding these developments there are changes in the structure of the walls of endodermal cells where the lignified casparian band spreads along the radial wall and substances staining with Sudan IV are deposited in both radial and tangential walls. At distances more than 30 cm from the tip of primary roots the radius of the stele increases considerably causing splits in the cortex. The endodermis is stretched and the suberin becomes organized in a lamellar form.Against this background of anatomical change certain of the transport capabilities of the root are retained while others are lost. Using an apparatus for measuring the uptake of tracers by segments of intact roots it was found that neither the uptake nor translocation of potassium seem to be affected by the suberization of the endodermis or by secondary thickening, while the translocation of calcium is virtually eliminated when these processes begin. As the root ages its ability to absorb phosphate declines although the translocation of the phosphate absorbed is much less affected by structural development than that of calcium.The observed rates of potassium uptake by complete root systems could be predicted quite accurately from the average of segment uptake data suggesting that the method used gives reliable results.     

HDAC3 at the fulcrum of an epithelial-mesenchymal balance

In this issue of Molecular Cell, Wu et?al. (2011) reveal an essential role for a chromatin modifier, histone deacetylase 3 (HDAC3), in hypoxia-induced epithelial-mesenchymal transition (EMT); HIF-activated HDAC3 integrates with WDR5 to impose chromatin modifications that culminate in EMT.     

Ancestral transfer of symbionts between cockroaches and termites: an alternative hypothesis.

Closely related cellulolytic protozoa reside in the hindguts of extant woodroaches (Cryptocercidae) and termites (Isoptera). The evolutionary origin of these symbiotic relationships in the two lineages is uncertain. Transfer of protozoa between ancestors of modern Cryptocercus and termites remains a valid alternative theory to the established hypothesis of symbiont inheritance from a common ancestor. Nalepa's (Proc. R. Soc. Lond. B 246, 185 (1991] concerns regarding the protozoan transfer hypothesis focus on the biology of modern species, and neglect to consider the evolutionary framework of an ancestral dynamic postulated to occur among Palaeozoic insects. Legitimacy of the symbiont transfer theory removes the constraint of interpreting presence of cellulolytic protozoa as a synapomorphy between Cryptocercidae and Isoptera, with potential impact on objective resolution of dictyopteran phylogeny.