Establishment of Epiphytic Bromeliads in Successional Tropical Premontane Forests in Costa Rica

Plant community composition is the combined result of species-specific competitive abilities and the availability of propagules. For epiphytic plants, current hypotheses consider that dispersal-related factors are most important. By controlling seed dispersal constraints, we experimentally examined whether the community composition of epiphytic bromeliads in a tropical premontane area is determined during early phases of seedling recruitment. Also, we tested whether establishment success was related to eco-physiological traits of the species. A total of 7200 seeds were artificially affixed on several host trees in two secondary forest patches and in a mature forest stand. Four bromeliad species with differing physiological characteristics (CAM, C₃-CAM, and C₃) and habitat preference (secondary vs. primary forest) were selected. We found that differences in seed germination probability among habitats and species were not likely to influence community assembly. After 2 yr, seedling survival and plant development were relatively higher in the early-successional forest. Seedling establishment success was not associated with specific physiological and morphological adaptations or habitat preference of the studied species. Our results were not consistent with the described community composition and rates of population recruitment of the studied species in the same successional habitats. The results support the hypothesis that chance and historic events related to seed dispersal have an important influence on community assembly of epiphytic plants. In addition, differences in growth rates and reproductive turnover among species are expected to influence the relative abundance and recruitment rates in a particular habitat.     

A detailed linkage map of lettuce based on SSAP, AFLP and NBS markers

Molecular markers based upon a novel lettuce LTR retrotransposon and the nucleotide binding site-leucine-rich repeat (NBS-LRR) family of disease resistance-associated genes have been combined with AFLP markers to generate a 458 locus genetic linkage map for lettuce. A total of 187 retrotransposon-specific SSAP markers, 29 NBS-LRR markers and 242 AFLP markers were mapped in an F₂ population, derived from an interspecific cross between a Lactuca sativa cultivar commonly used in Europe and a wild Lactuca serriola isolate from Northern Europe. The cross has been designed to aid efforts to assess gene flow from cultivated lettuce into the wild in the perspective of genetic modification biosafety. The markers were mapped in nine major and one minor linkage groups spanning 1,266.1 cM, with an average distance of 2.8 cM between adjacent mapped markers. The markers are well distributed throughout the lettuce genome, with limited clustering of different marker types. Seventy-seven of the AFLP markers have been mapped previously and cross-comparison shows that the map from this study corresponds well with the previous linkage map.     

Does a warmer climate with frequent mild water shortages protect grassland communities against a prolonged drought?

While irrigation of farm dairy effluent (FDE) to land is becoming popular in New Zealand, it can lead to increased emissions of the greenhouse gas nitrous oxide (N₂O). This paper reports the results from trials on N₂O emissions from irrigation of FDE to two dairy-grazed pastures on two poorly drained silt-loam soils located at Waikato and Manawatu, New Zealand. These pasture soils were periodically irrigated with FDE under contrasting soil moisture conditions with water-filled pore-space (WFPS) ranging between 26% and 94%. Nitrous oxide emissions were measured from the FDE irrigated and unirrigated sites using large numbers of static chambers (12–20). Irrigation of FDE generally increased N₂O emissions compared to the control. N₂O emissions varied with changes in climatic conditions and soil WFPS. Overall N₂O emissions from effluent-derived N ranged between 0.01% and 4.93% depending on irrigation time and soil WFPS. Lower N₂O emissions from FDE were attributable to very low soil WFPS conditions during the dry seasons. Higher N₂O emissions were measured from application of FDE to a recently grazed pasture on wet soil. Our results suggest strategic application of FDE during dry summer and autumn seasons can reduce N₂O emissions from application of FDE. Delaying effluent-irrigation after grazing events could further reduce N₂O emissions by reducing the levels of surplus mineral-N.     

Influence of High Temperature on End-of-Season Tundra CO2 Exchange

The high-arctic terrestrial environment is generally recognized as one of the world's most sensitive areas with regard to global warming. In this study, we examined the influence of an isolated warm period on net ecosystem carbon dioxide (CO₂) exchange at high latitude during autumn. Using the Free Air Temperature Increase (FATI) technique, we manipulated air, soil, and vegetation temperatures in late August in a tundra site at Zackenberg in the National Park of North and East Greenland (74°N 21°W). The consequences for gross canopy photosynthesis, canopy respiration, and belowground respiration of increasing these temperatures by approximately 2.5°C were determined with closed dynamic CO₂ exchange systems. Under current temperatures, the ecosystem acted as a net CO₂ source, releasing 19 g CO₂-C m⁻² over the 14-day study period. Warm soils and senescing vegetation in autumn were unequivocally responsible for this efflux. Heating enhanced CO₂ efflux to 29 g CO₂-C m⁻². This effect was attributed to a 39% increase in belowground respiration, which was the main component of the carbon (C) budget. Gross photosynthesis, on the other hand, was not affected significantly by the simulated warming. Although the aftereffects of an isolated warm period on the C balance in early winter could be significant, simulations with a simple C budget model suggest that soil carbon pools are not affected to a great extent by such a climatic disturbance. The influence on atmospheric carbon, however, appears to be significant. Received 9 June 2000; accepted 20 December 2000.     

Survey of moniliformin in wheat- and corn-based products using a straightforward analytical method

A straightforward analytical method was developed and validated to determine the mycotoxin moniliformin in cereal-based foods. Moniliformin is extracted with water and quantified with liquid chromatography tandem mass spectrometry, and its presence confirmed with liquid chromatography-Orbitrap-high-resolution mass spectrometry. The method was validated for flour, bread, pasta and maize samples in terms of linearity, matrix effect, recovery, repeatability and limit of quantification. Quantification was conducted by matrix-matched calibration. Positive samples were confirmed by standard addition. Recovery ranged from 77 to 114% and repeatability from 1 to 14%. The limit of quantification, defined as the lowest concentration tested at which the validation criteria of recovery and repeatability were fulfilled, was 10 μg/kg. The method was applied to 102 cereal-based food samples collected in the Netherlands and Germany. Moniliformin was not detected in bread samples. One of 22 flour samples contained moniliformin at 10.6 μg/kg. Moniliformin occurred in seven out of 25 pasta samples at levels around 10 μg/kg. Moniliformin (MON) was present in eight out of 23 maize products at levels ranging from 12 to 207 μg/kg.     

Warming affects different components of plant–herbivore interaction in a simplified community but not net interaction strength

Global warming impacts natural communities through effects on performance of individual species and through changes in the strength of interactions between them. While there is a body of evidence of the former, we lack experimental evidence on potential changes in interaction strengths. Knowledge about multispecies interactions is fundamental to understand the regulation of biodiversity and the impact of climate change on communities. This study investigated the effect of warming on a simplified community consisting of three species: rosy apple aphid Dysaphis plantaginea feeding on plantain, Plantago lanceolata, and a heterospecific neighbouring plant species, perennial ryegrass, Lolium perenne. The aphid does not feed on L. perenne. The experimental design consisted of monocultures and mixtures of L. perenne and P. lanceolata at three temperature levels. We did not find indication for indirect temperature effects on D. plantaginea through changes in leaf nitrogen or relative water content. However, experimental warming affected the life history traits of the aphid directly, in a non‐linear manner. Aphids performed best at moderate warming, where they grew faster and had a shorter generation time. In spite of the increased population growth of the aphids under warming, the herbivory rates were not changed and consequently the plant–herbivore interaction was not altered under warming. This suggests reduced consumption rates at higher temperature. Also plant competition affected the aphids but through an interaction with temperature. We provide proof‐of‐concept that net interactions between plants and herbivores should not change under warming despite direct effects of warming on herbivores when plant–plant interaction are considered. Our study stresses the importance of indirect non–trophic interactions as an additional layer of complexity to improve our understanding of how trophic interactions will alter under climate change.     

A simple method to vary soil heterogeneity in three dimensions in experimental mesocosms

Soil heterogeneity affects terrestrial plant communities both directly and indirectly. In nature, the exploration of the role of heterogeneity is made difficult because any co-varying factors (nutrients, soil depth, etc.) render it problematic to clearly link cause and effect. Attributing changes specifically to heterogeneity is facilitated if heterogeneity is varied in a controlled manner and other possible confounding factors are kept constant. The experiments conducted in such a way have up till now only considered heterogeneity in two dimensions, horizontally or vertically. In this methodological study, we present a novel technique that enables researchers to vary both qualitative and configurational heterogeneity in three dimensions by building up the soil cell by cell in experimental mesocosms. We illustrate the technique with an experiment where we test the effect of cell size (i.e. configurational heterogeneity) on the performance of grassland species that vary in nutrient preference (high N and low N species). Cell size did not affect aboveground biomass but modified species richness, both at the mesocosm and the patch scale, with most species being found when cells were small yet distinct (cell size 12 cm). High N species had significantly greater aboveground biomass and species richness than low N species, both on nutrient rich and nutrient poor cells. Remarkably, those differences disappeared when plants grew on the mesocosms with cell size close to zero. By allowing greater complexity in the design of experimental mesocosms, the 3-D approach can improve understanding of the interplay between soil heterogeneity and plant and ecosystem functioning.     

Leaf and canopy responses of Lolium perenne to long-term elevated atmospheric carbon-dioxide concentration

The relationship between leaf photosynthetic capacity (p _{n, max}), net canopy CO₂- and H₂O-exchange rate (NCER and E _t, respectively) and canopy dry-matter production was examined in Lollium perenne L. cv. Vigor in ambient (363±30 l· l^-1) and elevated (631±43 l·l^-1) CO₂ concentrations. An open system for continuous and simultaneous regulation of atmospheric CO₂ concentration and NCER and E _t measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCER_max of full-grown canopies was 49% higher at elevated CO₂ level, stimulation of p _n, max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO₂ (approx. 10% in one growing period examined). A larger amount of CO₂-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO₂ effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO₂ level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCER_max difference. While total water use was the same under high and low CO₂ levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO₂ concentration.Abbreviations and symbols C350 ambient CO₂, 363±30 l·l^-1 - C600 high CO₂, 631±43 l·l^-1 - c _a atmospheric CO₂ level - c _i CO₂ concentration in the intracellular spaces of the leaf - E_t canopy evapotranspiration - I _o canopy light compensation point - NCER canopy CO₂-exchange rate - p _n leaf photosynthetic rate - PPFD photosynthetic photon flux density - r _a leaf boundary-layer resistance - RD canopy dark-respiration rate - r _s stomatal resistance - WUE water use efficiency