Interactive effects of warming and increased nitrogen deposition on 15N tracer retention in a temperate old field: seasonal trends

The extent to which increased atmospheric nitrogen (N) deposition will drive changes in plant productivity and species composition over the next century will depend on how other influential global change factors, such as climate warming, affect the N retention of ecosystems. We examined the interactive effects of simulated climate warming and N deposition on the recoveries of ¹⁵N‐labeled ammonium and ¹⁵N‐labeled nitrate tracers added as a pulse to grass‐dominated, temperate old‐field plots at spring thaw. In addition to the year‐round warming treatment, a winter‐only warming treatment was applied to a set of plots to explore the contribution of this component of climate warming to the overall warming effect. By the end of the plant growing season, there was approximately twice as much ¹⁵N enrichment in the plant roots and bulk soil from ¹⁵NH₄⁺‐addition plots than from ¹⁵NO₃^?‐addition plots, but there were no effects of warming or N fertilization on ¹⁵N recovery. Over winter, approximately half of the excess ¹⁵N present in plant shoots was lost, which corresponded with large ¹⁵N losses from bulk soil in N fertilized plots and large ¹⁵N increases in bulk soil in nonfertilized plots. By the next spring, there was decreased ¹⁵N recovery in plants in response to N fertilization, which was largely offset by increases in plant ¹⁵N recovery in response to year‐round warming. However, ¹⁵N retention in bulk soil, where the major part of the ¹⁵N label was recovered, was approximately 40% higher in nonfertilized plots than in N fertilized plots. Overall, our results indicate that climate warming increases plant N sequestration in this system but this effect is overwhelmed by the overall effect of nitrogen deposition on ecosystem N losses.     

Hydrological and biogeochemical controls on the timing and magnitude of nitrous oxide flux across an agricultural landscape

Anticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N₂O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N₂O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha^?1 KNO₃‐N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N= 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N₂O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N₂O emissions (r²= 0.47; P= 0.013). Within individual soil columns, N₂O flux was a Gaussian function of water‐filled pore space (WFPS) during drainage (mean r²= 0.90). However, instantaneous maximum N₂O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N₂O flux rates occurred within a narrow range (?1.88 to ?4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N₂O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N₂O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N₂O fluxes across soils that differ in texture, structure and bulk density.     

Survival and recovery of perennial forage grasses under prolonged Mediterranean drought: I. Growth, death, water relations and solute content in herbage and stubble

Swards of Dactylis glomerata cultivars (cvs) KM2 and Lutetia and of Lolium perenne cvs Aurora and Vigor were grown under full irrigation or prolonged summer drought (80 d) in a field experiment in the South of France.
After irrigation was withheld, leaf extension rates of all cvs fell by 90% within 9–12 d, and rapid scorching of laminae followed. Tiller mortality at the end of the drought was very different in the cocksfoot cvs (4% for KM2 and 76% for Lutetia) and intermediate (41%) for both ryegrass cvs. Following re-watering, rates of herbage regrowth were closely correlated with tiller survival. Measured minerals contributed c . 0·52 MPa to osmotic potential in all treatments, whereas water-soluble carbohydrates (WSC) contributed 0·25 MPa under irrigation and 0·46 MPa during drought.
There was no systematic difference between the two species for summer survival under severe drought, but large differences between the cocksfoot cvs. The traits most strongly associated with superior survival were: (a) a deep root system and greater water uptake at depth; (b) low water and osmotic potentials in surviving laminae, i.e. better tolerance to dehydration; (c) large pool-size of WSC reserves (fructans having degree of polymerization >4) in entire tiller bases (stubble); (d) low accumulation of proline in stubble; (e) rapid nitrogen uptake after rewatering.     

The skeletal structure of Desmophyllum cristagalli: the use of deep-water corals in sclerochronology

Skeletal banding has been found in the deep-water scleractinian coral Desmophyllum cristagalli , an important animal in studies of climate change. This banding pattern sheds light on skeletogenesis and suggests methods by which the record of climate change contained within the coral skeletons may be interpreted. A central wall built of trabeculae forms the interior of the septa and rings the theca. Lamellae form a sheath over the trabecular frame, showing continuity from thecal edge to septum. Skeletal bands are added by the tissue layer, which overlaps and seals the internal coral and upper portion of the outer theca. Truncated inner bands on the outer theca indicate a pattern of skeletal deposition and dissolution dependent on the presence or absence of the live tissue layer. A long-term record will be difficult to collect from D. cristagalli since lamellae are less than 10 μm thick and band position is unpredictable. Density banding in shallow-water coral skeletons has long been recognized as a valuable paleo-oceanographic tool, and deep-water corals are now being used to reconstruct deep-ocean environments. Pattern of skeletal growth must be carefully considered if deep-water corals are to be used as proxy climate recorders.     

Evolutionary Modifications of the Spiralian Developmental Program

SYNOPSIS. The Spiralia, an assemblage of phyla united by theirstereotypic pattern of early embryonic cell divisions (spiralcleavage), is an interesting group in which to investigate theevolution of development. This paper examines modificationsof developmental mechanisms within the Spiralia with emphasison the basallybranching forms. Although demonstrating a notabledegree of evolutionary conservation, the equal quartet cleavagepattern, which appears to be the ancestral condition, nonethelessexhibits modifications within the various spiralian groups,such as unequal cleavage, changes in cell size and rate of division,formation of two rather than four quadrants (duet spiral cleavage),and in extreme cases the loss of any trace of the spiral pattern.While the cell lineages of spiralians are remarkably conserved,one can discern evolutionary changes, for example in the cellsthat give rise to mesodenn. Studies of blastomere specificationin many spiralian groups and analyses of axis determinationindicate that embryos with equal versus unequal cleavage typicallyuse different determinative mechanisms to establish cell fatesand the dorsoventral axis. These properties are establishedearly in species exhibiting unequal cleavage. While previousexperiments suggested that equal cleavage was associated withlate specification, there is now evidence of precocious specificationof quadrant fates in some equal-cleaving species, such as thenemerteans and the polyclad turbellarians     

Theoretical habitat templets, species traits, and species richness: aquatic macrophytes in the Upper Rhône River and its floodplain

• 1 The objective of this study, which is based on forty-two species of hydrophytes and helophytes, is to investigate: (i) relationships among species traits; (ii) habitat utilization by species; (iii) the relationship between species traits and habitat utilization; (iv) trends in species traits in the framework of spatial–temporal habitat variability, and if trends match predictions from the river habitat templet; and (v) trends in species richness in the framework of spatial–temporal habitat variability, and if trends match predictions of the patch dynamics concept.
• 2 Two data sets were used for this analysis: species traits (mainly reproductive and morphological characteristics) were documented from the literature; and species distribution across eight habitat types was from field surveys conducted in the floodplain of the Upper Rhone River, France. This information was structured by a fuzzy coding technique and analysed by ordination methods.
• 3 Several species traits, which are related to disturbances and reflect resistance (e.g. attachment to soil or substrate) or resilience (e.g. potential for regeneration of an individual), are closely related for aquatic macrophytes.
• 4 Habitat utilization by aquatic macrophytes separates the habitat types along a gradient of connectivity with the main channel, which corresponds to a gradient in flood disturbance frequency and the permanence of the different water-bodies.
• 5 The relationship between species traits and habitat utilization is highly significant, indicating that a particular set of habitat types is used by taxa with a particular set of species trait modalities.
• 6 Observations in one habitat templet (in which scaling of the templet is primarily based on water level fluctuations for the temporal variability axis and on substrate characteristics for the spatial variability axis) generally do not support predictions on trends in species traits but do support predictions on trends in species richness.
• 7 Observations in an alternative habitat templet (in which scaling of the templet is based on frequency of flood scouring for the temporal variability axis and on heterogeneity of the substrate for the spatial variability axis) support theoretical predictions on trends for about half of the species traits for which predictions were available. However, trends in species richness in this alternative habitat templet are only partly in agreement with predictions.

     

Occurrence of barley yellow dwarf virus (BYDV) isolates in different farmland habitats in western France and south-west England

The incidence and distribution of the three principal isolates of barley yellow dwarf virus (PAV, RPV and MAV) are described in winter cereal crops, cereal (stubble) regrowth and grasses from 11 sites in western France and south-west England during 1987 and 1988. Isolates were identified by indirect ‘sandwich’ ELISA using the monoclonal antibodies MAC91, MAC92 and MAFF2. More virus infection occurred in all localities and in most of the plant communities sampled, with the exception of perennial grass leys, in 1987 than in 1988. All three isolates were widespread. MAV was associated more with sites further north and PAV more with those further south. The geographical distribution of RPV was less variable. Underlying these trends, the relative abundance of isolates differed considerably between habitats. RPV always predominated in perennial grass leys and MAV in most cereal crops, although in the latter MAV was less prevalent in 1987 than in 1988. The greatest regional difference was found in stubble regrowth where PAV predominated in France but MAV predominated in England. Grasses from field margins (only sampled in England) were mainly infected by MAV and RPV. The implications of these findings for the epidemiology of BYDV are discussed, especially the roles of different host plant communities or habitats in the annual infection cycle of small-grain cereals.     

Further studies on a spiroplasma isolated from coconut palms in Jamaica

The pathogenicity of a spiroplasma isolated from coconut palms was tested by (1) transmission experiments to palms and other plants susceptible to infection by mycoplasmas, using the suspected vector of lethal yellowing, Myndus crudus, and vectors of the agents of other yellows diseases and (2) enzyme-linked immunosorbent assay (ELISA) to detect spiroplasma antigens in diseased palm tissues. Results of both these tests were negative and, as earlier attempts to repeat the isolations from lethal yellowing diseased palms had also been unsucessful, it was concluded that this organism was not the causal agent of lethal yellowing disease. Further analysis by serological tests and by polyacrylamide gel electrophoresis (PAGE) of spiroplasma proteins confirmed that the coconut isolates were related to members of the Spiroplasma citri serogroup but were distinct from other strains tested.     

Architecture of the leaf of the greater reed mace, Typha latifolia L

ROWLATT, U. & MORSHEAD, H., 1992. Architecture of the leaf of the greater reed mace, Typha latifolia L. The construction of the leaf of T. latifolia is that of a lightweight cantilever beam of sandwich type mounted vertically. A continuous leaf-handed twist starts at the base of the leaf, proceeds gradually throughout its length and becomes most pronounced toward the tip. It is argued that this construction prevents buckling under natural conditions and injury in storms.     

On the Irrelevance of the Segmentary Lineage Model in the Moroccan Rif

In attempting to analyze the precolonial Moroccan Rif in terms of the segmentary lineage model, David Hart overlooked the fact that violence in this region typically involved brothers and the sons of brothers in competition for inherited land, which was a commodity controlled by individuals, not lineages. What Hart depicted as a segmentary lineage system was actually a network of factions that cut across genealogical lines. Moreover, the political impact of the Moroccan state on the precolonial Rif was far more important than Hart suggested.