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The performance of seedlings is crucial for the survival and persistence of plant populations. Although drought frequently occurs in floodplains and can cause seedling mortality, studies on the effects of drought on seedlings of floodplain grasslands are scarce. We tested the hypotheses that drought reduces aboveground biomass, total biomass, plant height, number of leaves, leaf area and specific leaf area (SLA), and increases root biomass and root-mass fraction (RMF) and that seedlings from species of wet floodplain grasslands are more affected by drought than species of dry grasslands. In a greenhouse study, we exposed seedlings of three confamilial pairs of species (Pimpinella saxifraga, Selinum carvifolia, Veronica teucrium, Veronica maritima, Sanguisorba minor, Sanguisorba officinalis) to increasing drought treatments. Within each plant family, one species is characteristic of wet and one of dry floodplain grasslands, confamilial in order to avoid phylogenetic bias of the results. In accordance with our hypotheses, drought conditions reduced aboveground biomass, total biomass, plant height, number of leaves and leaf area. Contrary to our hypotheses, drought conditions increased SLA and decreased root biomass and RMF of seedlings. Beyond the effects of the families, the results were species-specific (V. maritima being the most sensitive species) and habitat-specific. Species indicative of wet floodplain grasslands appear to be more sensitive to drought than species indicative of dry grasslands. Because of species- and habitat-specific responses to reduced water availability, future drought periods due to climate change may severely affect some species from dry and wet habitats, while others may be unaffected.

Arable weeds are one of the most endangered species groups in Europe. Modern agriculture and intensive land-use management are the main causes of their dramatic decline. However, besides the changes in land use, climate change may further challenge the adaptability of arable weeds. Therefore, we investigated the response pattern of arable weeds to different water potential and temperature regimes during the phase of germination. We expected that endangered arable weeds would be more sensitive to differences in water availability and temperature than common arable weeds. To this end, we set up a climate chamber experiment where we exposed seeds of five familial pairs of common and endangered arable weed species to different temperatures (5/15, 10/20 degrees C) and water potentials (0.0 to -1.2 MPa). The results revealed a significant relationship between the reaction of arable weed species to water availability and their Red List status. The effects of reduced water availability on total germination, mean germination time and synchrony were significantly stronger in endangered than in common arable weeds. Therefore, global climate change may present a further threat to the survival of endangered arable weed species.

We report numerical and experimental results demonstrating accurate region-of-interest computed tomography (CT) reconstruction based on differential phase-contrast projection (DPC) images. The approach removes the constraint of covering the entire sample within the field of view of the image detector. Particularly for biomedical applications, the presented DPC-CT region-of-interest approach will allow for the visualization of previously inaccessible details deep inside an entire animal or organ. We envisage that this development will also be of interest for potential future clinical applications, because grating-based DPC-CT can be implemented with standard x-ray tube sources.

Living plant neighbours, but also their dead aboveground remains (i.e. litter), may individually exert negative or positive effects on plant recruitment. Although living plants and litter co-occur in most ecosystems, few studies have addressed their combined effects, and conclusions are ambivalent. Therefore, we examined the response in terms of seedling emergence and growth of herbaceous grassland and forest species to different litter types and amounts and the presence of competitors. We conducted a pot experiment testing the effects of litter type (grass, oak), litter amount (low, medium, high) and interspecific competition (presence or absence of four Festuca arundinacea individuals) on seedling emergence and biomass of four congeneric pairs of hemicryptophytes from two habitat types (woodland, grassland). Interactions between litter and competition were weak. Litter presence increased competitor biomass. It also had positive effects on seedling emergence at low litter amounts and negative effects at high litter amounts, while competition had no effect on seedling emergence. Seedling biomass was negatively affected by the presence of competitors, and this effect was stronger in combination with high amounts of litter. Litter affected seedling emergence while competition determined the biomass of the emerged individuals, both affecting early stages of seedling recruitment. High litter accumulation also reduced seedling biomass, but this effect seemed to be additive to competitor effects. This suggests that live and dead plant mass can affect species recruitment in natural systems, but the mechanisms by which they operate and their timing differ

Different scales of liquid mixing exist in bubble columns and it is very important to determine the prevailing mixing scale in each flow regime. Two independent parameters were found to exhibit a monotonous decline in the transition flow regime, which could be attributed to the decrease of the mixing length values L. In this work, a new parameter called ‘maximum number of visits in a region’ Nvmax and the Kolmogorov entropy (KE) were extracted from the gas holdup time series (60 000 points). The latter were recorded at a high sampling frequency (2000 Hz) by a wire-mesh sensor. The measurements were performed in a narrow bubble column (0.15 m in ID, clear liquid height = 2 m) equipped with a perforated plate distributor (14 holes, Ø 4 × 10-3 m). Both parameters were capable of identifying concordantly the two main transition velocities at Utrans = 0.022 and 0.112 m/s, which delineate the boundaries of gas maldistribution, transition and churn-turbulent regimes, respectively.

Seedling establishment is influenced by litter cover and by seed predators, but little is known about interactions between these two factors. We tested their effects on emergence of five typical grassland species in a microcosm experiment. We manipulated the amounts of grass litter, seed sowing position and earthworm activity to determine whether: (i) the protective effect of litter against seed predation depends on cover amount and seed sowing position, i.e., on top or beneath litter; (ii) seed transport by earthworms changes the effect of seed sowing position on seedling emergence; and (iii) seeds transported into deeper soil layers by earthworms are still germinable. Litter cover and presence of earthworms lowered seedling emergence. The impact of seed position increased with seed size. Emergence of large-seeded species was reduced when sown on the surface. Additionally, we found an important seed position x earthworm interaction related to seed size. Emergence of large-seeded species sown on top of the litter was up to three times higher when earthworms were present than without earthworms. Earthworms also significantly altered the depth distribution of seeds in the soil and across treatments: on average 6% of seeds germinated after burial. In contrast to the seed position effect, we found no size effect on mobility and germinability of seeds after burial in the soil. Nevertheless, the fate of different-sized seeds may differ. While burial will remove large seeds from the regeneration pool, it may enhance seed bank build up in small-seeded species. Consequently, changes in the amount of litter cover and the invertebrate community play a significant role in plant community composition.

Jung LS, Eckstein RL, Otte A & Donath TW (2012). Above- and below-ground nutrient and alkaloid dynamics in Colchicum autumnale: indications for optimal mowing dates for population control or low hay toxicity. Weed Research52, 348-357. Summary In some Central European regions, the conservation of seminatural grasslands is jeopardised by management intensification or abandonment, caused by high densities of the toxic weed Colchicum autumnale. We investigated two possibilities to deal with C. autumnale: (i) reducing population densities by mowing when nutrient contents are high in leaves and capsules and low in the storage organs, that is, when the ratio between both is highest, or (ii) reducing alkaloid content in hay by mowing when alkaloid content of leaves and capsules of C. autumnale is low. To identify the optimal mowing point, we analysed the dynamics of nutrients, starch and alkaloids of naturally grown plants in two biogeographical regions. In the colder region, the maximum nutrient ratio between above-ground and storage organs, as well as alkaloid content in leaves and capsules, occurred significantly later. Compared with the common first mowing date (15 June), alkaloid content decreased significantly until 5 July in both regions. On both dates, it was on average 1.8 times higher in the colder region. Our results suggest the following time for the two management options: (i) mowing at about 25 cm plant height (late April/early May) to reduce C. autumnale densities or (ii) delayed mowing in late June/early July when the plant has turned brown and dry and alkaloid content has declined.

We report on a setup for differential x-ray phase-contrast imaging and tomography, that measures the full 2D phase-gradient information. The setup uses a simple one-dimensional x-ray grating interferometer, in which the grating structures of the interferometer are oriented at a tilt angle with respect to the sample rotation axis. In such a configuration, the differential phase images from opposing tomography projections can be combined to yield both components of the gradient vector. We show how the refractive index distribution as well as its x, y, and z gradient components can be reconstructed directly from the recorded projection data. The method can equally well be applied at conventional x-ray tube sources, to analyzer based x-ray imaging or neutron imaging. It is demonstrated with measurements of an x-ray phantom and a rat brain using synchrotron radiation.

Here we review our recent progress in the field of X-ray dark-field and phase-contrast imaging using a grating interferometer. We describe the basic imaging principles of grating-based phase-contrast and dark-field radiography and present some exemplary results obtained for simple test objects and biological specimens. Furthermore, we discuss how phase-contrast and dark-field radiography can be combined with the concept of computed tomography, and yield highly detailed three-dimensional insights into biomedical sample. Exemplary results obtained with standard X-ray tube sources and highly brilliant synchrotron sources are presented.

Udgivelsesdato: Oct. 17

We studied seedling emergence in four familial pairs of floodplain herbs in response to the experimental manipulation of soil moisture and litter cover to analyse (i) whether the effect of litter changes from negative under humid to positive under dry conditions, and (ii) whether the response to changing water and light conditions with increasing litter cover varies among species and plant families. We carried out a controlled pot experiment using four levels of litter cover (0 g, 2 g, 4 g and 8 g litter per pot, corresponding to 0 kg m(-2), 0.2 kg m(-2), 0.4 kg m(-2) and 0.8 kg m(-2)) and two levels of water-addition, leading to constantly humid substrate or intermittently dry topsoil. Across water-additions, percentage emergence reached a peak at low levels of litter cover (0.2 kg m(-2) and 0.4 kg m(-2)). There was a significant litter x water-addition interaction in six species, with positive effects of litter under intermittently dry conditions and negative or neutral effects under constantly humid conditions. Litter lowered maximum temperature as well as amplitude, and alleviated soil humidity under low water supply, while imposing increasingly shaded conditions. Analysis of species- and family-specific responses suggested that germination under a litter cover of 0.8 kg m(-2) was significantly reduced in smaller-seeded species (i.e. those that tend to have higher light demands for germination). Our results suggest that transfer of seed-containing plant litter can aid restoration projects if applied at 0.2-0.4 kg m(-2). Below these levels, establishment of most species may be inhibited by drought, while higher amounts will increasingly suppress seedling emergence, especially of small-seeded species. In addition to facilitation effects observed between living plants, dead plant remains may also exert positive effects on establishment. The sign of the litter effect on seedling emergence depends on soil humidity, with negative effects seen above a threshold amount