Decomposition in estuarine salt marshes: the effect of soil salinity and soil water content

The relation between decomposition rates and soil salinity and moisture conditions in tidal marshes of the Westerschelde estuary was investigated. In the first part of the study, these soil factors were experimentally manipulated in field plots which were either screened from rainwater or which received an additional weekly supply of freshwater from April to September 1989. These treatments had no clear effect on soil salinities and moisture conditions in a low marsh site. Decomposition rates of Spartina anglica leaves (kept in litterbags in the plots) also did not differ between treatments. In screened plots of a middle marsh site, decomposition rate of Elymus pycnanthus leaves decreased significantly. The effect of the experimental treatments on soil moisture content was variable, but comparatively high soil salinity values (up to 61.3) were consistently found in these plots. It is suggested that the elevated salinity levels induced the decrease in decomposition rate.In the second part of the study, cellulolytic decomposition, measured by loss of tensile strength of strips of cotton test cloth, was investigated in relation to a non-manipulated range of soil salinities (3.8–24.2), by exposing the strips in a series of tidal marshes along the salt gradient of the Westerschelde estuary. No correlation between decomposition rate and soil salinity was found. In addition, no relation was found between decomposition rate and soil water content. The results of both parts of this study lead us to the hypothesis that rate limitation of decomposition in estuarine tidal marsh soils is found at high soil salinities only.     

Estimating root lifespan of two grasses at contrasting elevation in a salt marsh by applying vitality staining on roots from in-growth cores

Contrasting soil conditions caused by different inundation frequenciesrequire different root growth strategies along the elevational gradient ofcoastal salt marshes. The objective of this study was to examine (1) if rootlifespan was shorter in Elymus pycnanthus, a relativelyfast-growing competitive species dominating high marshes, than inSpartina anglica, a relatively slow-growingstress-tolerating species dominating low marshes, and (2) if the species withlonger lifespan had higher tissue density (g cm^–3) and lowerspecific root length (m g^–1) than the species with shorterlifespan. Root production and mortality rates were established by samplingrootsin in-growth cores, and using triphenyltetrazolium chloride (TTC) staining todistinguish vital from dead roots. Root lifespan was estimated by dividing theliving root biomass (Elymus: 36 gm^–2, Spartina: 100 gm^–2) by root production (Elymus:0.28 g day^–1 m^–2,Spartina: 0.25 g day^–1m^–2) or root mortality rates(Elymus: 0.42–0.53 g day^–1m^–2). Spartina did not exhibitsubstantial mortality. Despite the present method only yielding rough estimatesof average root lifespan, it is evident that root longevity is much shorter inElymus than in Spartina. Rootlifespanranged between 10–19 weeks for Elymus but was closeto 1 year in Spartina, indicating thatElymus replaces it's roots continuously throughout thegrowing season, whereas Spartina maintains its roots overthe growing season. Fine roots of Elymus had slightlylowertissue density (0.094) than those of Spartina (0.139),whereas coarse roots of Elymus andSpartina had similar tissue density (0.100 gcm^–3). Fine roots of Elymus andSpartina had similar specific root length (195 mg^–1). However, coarse roots ofElymus (50 m g^–1) had higherspecific root length than those of Spartina (20 mg^–1) due to having smaller root diameter(Elymus: 548 m,Spartina: 961 m). We conclude thatpresentobservations on Elymus and Spartinasupport our first hypothesis that the competitive species fromthehigh marsh had short-lived roots compared to the'stress-tolerating'species from the low marsh. However, our result provide only weak support forthe existence of a positive correlation between root longevity and tissuedensity and a negative correlation between root longevity and specific rootlength.     

Tidal dispersal of salt marsh insect larvae within the Westerschelde estuary

In the Westerschelde estuary, salt marshes are present as isolated patches fringing the estuary. In the present paper tidal transport of stem-boring larvae of Agapanthia villosoviridescens (Coleoptera) from salt marshes of the upper reaches of the Westerschelde estuary to marshes of the lower reaches is demonstrated. The evidence for the origin of the larvae is based on comparisons of growth and development characteristics of larvae found in flood debris belts and resident larval populations. These characteristics are different on the various salt marshes along the Westerschelde, probably as a result of estuarine gradients. Additional evidence for the larval origin comes from the plant composition of the flood debris. The occurrence of upward tidal transport is discussed. Considering the comparatively large area of salt marshes in the upper estuary, tidal dispersal of larvae probably will be dominated by transport in seaward direction. So far, very little is known on the role of tidal currents with regard to the exchange between salt marsh populations. The present results suggest that tidal transport may not only be important for dispersal of aquatic organisms in an estuary, but also for organisms inhabiting the semi-terrestrial estuarine salt marshes.     

Memory,consciousness and neuroimaging.

Neuroimaging techniques that allow the assessment of memory performance in healthy human volunteers while simultaneously obtaining measurements of brain activity in vivo may offer new information on the neural correlates of particular forms of memory retrieval and their association with consciousness and intention. We consider evidence from studies with positron emission tomography and functional magnetic resonance imaging indicating that priming, a form of implicit retrieval, is associated with decreased activity in various cortical regions. We also consider evidence concerning the question of whether two components of explicit retrieval--intentional or effortful search and successful conscious recollection--are preferentially associated with increased activity in prefrontal and medial temporal regions, respectively. Last, we consider recent efforts to probe the relation between the phenomenological character of remembering and neural activity. In this instance we broaden our scope to include studies employing event-related potentials and consider evidence concerning the neural correlates of qualitatively different forms of memory, including memory that is specifically associated with a sense of self, and the recollection of particular temporal or perceptual features that might contribute to a rich and vivid experience of the past.     

A Study on Polymorphism in Aster tripolium L. (Sea Aster)

Abstract: Genotypic and environmental variation in Aster tripolium L. was studied in common garden experiments and in transplantation experiments in different saltmarsh sites along the estuarine gradient of the Westerschelde and along the elevation gradient of individual saltmarshes. Analysis of the variation in a number of morphologic characters of the inflorescences indicated that this variation is both environmentally as well as genetically controlled. Morphologic differences between an Aster tripolium population of a brackish marsh and a number of populations of a saline marsh were unchanged whether the plants were growing in their original habitat or in a transplant habitat. The population from the brackish marsh had smaller inflorescences with ray florets and a predominantly biennial life cycle, in contrast with the populations of the saline marsh, which had larger inflorescences usually without or with a few ray florets and showed a tendency to a perennial life cycle. We concluded that the variation between the populations of the saline and the brackish marsh is mainly genetically controlled, while the variation within the saline marsh is mainly environmentally controlled.     

Sample preparation and scanning protocol for computerised analysis of root length and diameter

Root length and diameter distribution are important characteristics to be considered when describing and comparing root systems. Root length and root-diameter distribution may be obtained in two ways: by microscopical measurements, which are laborious, or by computerised analysis, which is fast but sensitive to the scanning protocol. Although scanning protocols vary widely between laboratories, papers rarely report the details. Using two commercially available root analysis software packages (Delta-T Scan and WinRHIZO), we performed a sensitivity analysis of scanning protocols for estimating root length and diameter distribution. The results are shown graphically, rather than numerically, because the interactions between different parameters in the overall scanning protocol are most clearly illustrated by the shape of the curves. The present analysis clearly demonstrated the sensitivity of the two scanning methods with regard to staining period, maximum root density, scanning resolution and transformation threshold. For example, estimating the root-diameter distribution versus measuring root length, puts opposite constraints on the transformation threshold settings. We suggest the following settings for the most sensitive parameters: a staining period of 24 h, a root density of less than 0.5 mm root per mm² surface, a resolution of 400 dpi and the automatic threshold for WinRHIZO and a brightness of 200 for Delta-T Scan. According to this protocol, comparison of computerised analyses with microscopic measurements showed good total root length and diameter distribution agreement for three contrasting root systems. We suggest to always start with the present protocol when studying other species. If validation indicates that the proposed scanning protocol needs to be modified for other species, the present sensitivity analysis may be used as a guideline for changing the most critical parameters. Similarly, the use of another stain than neutral red may also require modifications of the scanning protocol. In general, a long staining period (e.g., 24 h) is recommended for all stains, as small differences in staining period have the least effect when the root tissue is saturated. To enhance comparability of results in the literature, the staining period, stain, (maximum) root density, scanner resolution and threshold should always be listed when root data obtained by scanning are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

When encoding yields remembering: insights from event-related neuroimaging.

To understand human memory, it is important to determine why some experiences are remembered whereas others are forgotten. Until recently, insights into the neural bases of human memory encoding, the processes by which information is transformed into an enduring memory trace, have primarily been derived from neuropsychological studies of humans with select brain lesions. The advent of functional neuroimaging methods, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), has provided a new opportunity to gain additional understanding of how the brain supports memory formation. Importantly, the recent development of event-related fMRI methods now allows for examination of trial-by-trial differences in neural activity during encoding and of the consequences of these differences for later remembering. In this review, we consider the contributions of PET and fMRI studies to the understanding of memory encoding, placing a particular emphasis on recent event-related fMRI studies of the Dm effect: that is, differences in neural activity during encoding that are related to differences in subsequent memory. We then turn our attention to the rich literature on the Dm effect that has emerged from studies using event-related potentials (ERPs). It is hoped that the integration of findings from ERP studies, which offer higher temporal resolution, with those from event-related fMRI studies, which offer higher spatial resolution, will shed new light on when and why encoding yields subsequent remembering.     

Coping with low nutrient availability and inundation: root growth responses of three halophytic grass species from different elevations along a flooding gradient

We describe the responses of three halophytic grass species that dominate the low (Spartina anglica), middle (Puccinellia maritima) and high (Elymus pycnanthus) parts of a salt marsh, to soil conditions that are believed to favour contrasting root-growth strategies. Our hypotheses were: (1) individual lateral root length is enhanced by N limitations in the soil but restricted by oxygen limitations, (2) the density of root branching within a species is inversely related to the length of the lateral roots, and (3) species from high elevations (i.e. the driest parts of a marsh) are the most responsive to changing soil conditions. Plant growth responses and soil parameters showed that the contrasting but uniformly applied soil treatments were effective. All three species showed a small but significant shift towards a finer root diameter distribution when N was limiting, partly because of the finer diameters of the laterals (Elymus and Spartina) and partly because of increased length of individual 1st-order laterals (Elymus and Puccinellia). The increased length of the 1st-order laterals of Elymus and Puccinellia grown under low N indicates that the first part of hypothesis 1 may be true. However, lack of effect of flooding and reduced soil conditions lead us to reject the second part of hypothesis 1. Hypothesis 2 was rejected for these three halophytes, as the branch density of 1st- and 2nd-order laterals appears to be controlled by other factors than length of individual laterals. Hypothesis 3 may be true for specific root characteristics (e.g. length of individual 1st-order laterals), but cannot be generalised (e.g. branch density and topological index). In conclusion, the present data on root growth in contrasting but homogeneous soil conditions indicate that morphological responsiveness of the root systems of these halophytic grass species is limited, regardless of their location along the elevational gradient.