bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geomorphic processes and spatial patterns of primary forest succession on the Bogue Chitto River, USA
Authors: Robertson Kevin M. , Augspurger Carol K. 
Published: March 2003

Summary 1 The Bogue Chitto River, Louisiana, USA, is a meandering river with 16 early successional tree species having > 1% relative dominance. We hypothesized that geomorphic processes associated with river planform promote predictable spatial patterns of tree species within each river bend by influencing the physical conditions under which the forest is initially established. We predicted that (i) species of trees differ in their location of recruitment along the point bar, and trees on older land reflect the spatial patterns of recent recruits; (ii) the location of recruitment differs among tree species in elevation, soil texture, herbaceous cover and light; and (iii) a specific pattern of elevation, soil texture and tree species in the earliest stage of forest succession characterizes multiple river bends in a reach. 2 On the floodplain interior to one bend, we mapped all trees and sampled seedlings and saplings, elevation, soil texture, herbaceous cover and light. In newly established forest adjacent to the point bars of eight bends, we surveyed spatial patterns of tree species, elevation and soil texture. 3 Trees of most species had characteristically become established along either the upstream or downstream segment of each point bar. Variation in tree species composition was gradual both along the point bar and along an axis of increasing age of land. The locations of recruitment and mature trees were the same for eight of 11 common species on the focal river bend. 4 The locations of tree recruitment and of mature trees of specific species on the focal bend were related to spatial patterns of elevation (corresponding to flood regime), soil texture, light, herbaceous cover and the inferred pattern of shear stress by flood water on the point bar. The same patterns of elevation and soil texture and locations of tree species characterized multiple river bends. 5 Geomorphic processes related to the river planform of the Bogue Chitto River promote spatially complex but predictable patterns of primary forest succession.
The role of resource interactions and seedling regeneration in maintaining a positive feedback in hemlock stands
Authors: Catovsky S. , Bazzaz F. A. 
Published: March 2003

Summary 1 When a species influences conditions at a site to promote recruitment of conspecifics, the community may remain dominated by that species until the next major disturbance. We examined the occurrence of possible mechanisms for such a positive feedback in one community where it may be important: forests dominated by eastern hemlock (Tsuga canadensis). 2 Intact soil divots were collected from a hemlock stand in central Massachusetts, USA, and placed in shade‐houses at either 1.5% or 60% full sun. Soil was amended with nitrogen (5 g m−2 year−1), lime (1 kg m−2 year−1; to increase soil pH) or left untreated. Emergence, survival and growth of seedlings arising from the soil seed bank were subsequently monitored. 3 Light availability had a very large influence on seedling regeneration dynamics. Birch (Betula spp.) seedlings were most sensitive of all species to light availability, with low light reducing emergence by 43% and substantially decreasing seedling growth (by 99%) and survival (by 94%). In contrast, light treatments had smaller effects on performance of other species [hemlock and white pine (Pinus strobus)]. Birch survival was also increased by nitrogen addition, but only in low light. Liming had only marginal effects on seedling performance. 4 Differential seedling responses to resource availability led to dominance of final seedling communities by hemlock in low light, unamended soils (i.e., under conditions naturally found in hemlock stands), but by birch in high light, as well as in low light, nitrogen‐amended soils. 5 Positive feedbacks in hemlock stands are mediated both by availability of light and nitrogen, and by species’ life‐history strategies.
Competition between roots and soil micro‐organisms for nutrients from nitrogen‐rich patches of varying complexity
Authors: Hodge A. , Stewart J. , Robinson D. , Griffiths B. S. , Fitter A. H. 
Published: March 2003

Summary1 We usedLolium perenneplants grown in microcosms to investigate the responses of root demography, plant N capture, soil fauna populations and microbial community profiles to five organic patches containing the same amount of N but differing in their chemical and physical complexity and C : N ratio. All patches were dual labelled with15N/13C. Control patches contained the background sand : soil mix only.2 There was rapid decomposition in, and plant N capture from, the patches of lowest C : N ratio. Early in the experiment13C was detected in the soil atmosphere and15N in the shoots. No13C enrichment was detected in the plant material.3 The rate of root production was slowest in the most complex patch (L. perenneshoot material) but accelerated when patches were simpler and had lower C : N ratios. There was no difference in root mortality between treatments.4 Nitrogen concentrations of shoots and roots and shoot biomass were greater in the N‐containing patches than controls, except for the most complex patch, while root biomass did not differ with treatments.5 Total plant N capture was 45–54% of that initially added in patches that had a C : N ratio < 4. However, in the most complex patch (C : N ratioc. 21 : 1) plants captured only 11% of the N added.6 Biomass of microbial‐feeding protozoa was related to soil NO3–‐N concentration in the patch but not to numbers of microbial‐feeding nematodes. Patches of greater complexity increased the metabolic diversity of the microbial community (i.e. the number of substrates used in a Biolog GN plate) and altered the pattern of substrate utilization.7 At harvest, the amount of patch‐derived N estimated to be in the microbial biomass was much smaller (i.e. 7–13%) than in the plant tissues. Thus, plants were highly effective competitors with micro‐organisms when capturing N supplied in patches with a low C : N ratio.
Effects of resource competition and herbivory on plant performance along a natural productivity gradient
Authors: Van Der Wal René , Egas Martijn , Van Der Veen Adriaan , Bakker Jan 
Published: March 2003

Summary 1 The effects of resource competition and herbivory on a target species, Triglochin maritima, were studied along a productivity gradient of vegetation biomass in a temperate salt marsh. 2 Transplants were used to measure the impact of grazing, competition and soil fertility over two growing seasons. Three parts of the marsh were selected to represent different successional stages; Triglochin reached local dominance at intermediate biomass of salt‐marsh vegetation. At each stage, three competition treatments (full plant competition, root competition only, and no competition) and three grazing treatments (full grazing, no grazing on Triglochin, and no grazing on Triglochin or neighbours) were applied to both seedlings and mature plants. 3 Competition and herbivory reduced biomass and flowering of Triglochin. The impact of grazing was strongest at the stage with the lowest biomass, while both herbivory and competition had a significant impact at the stage with the highest biomass. When plants were protected from direct herbivory, competition operated at all three successional stages. 4 Grazing reduced light competition when vegetation biomass was low or intermediate, but at high biomass there was competition for light even when grazing occurred. Herbivore exclusion increased the effects of plant competition. Except at low biomass, the negative impact of plant competition on Triglochin performance was greater than the positive effect of not being grazed. 5 Grazing played a minor role in seedling survival and establishment which were largely controlled by competitive and facilitative effects. 6 Once established, the persistence of Triglochin will be determined largely by grazing. Intense grazing in the younger marsh and increasing competition for light in the older marsh will restrict the distribution to sites with intermediate biomass.
Ecological gradients, subdivisions and terminology of north‐west European mires
Authors: Wheeler B. D. , Proctor M. C. F. 
Published: March 2003

Summary 1 The historical development of mire ecology and terminology is reviewed in relation to evolving concepts and perceptions, and the diverse schools and traditions of vegetation and habitat description and research. 2 Most ecological and floristic variation within north‐west European mire vegetation is accounted for by three ecological gradients: the acid base‐poor vs. neutral, base‐ and bicarbonate‐rich gradient; the gradient in fertility related to availability of the limiting nutrient elements N and P; and the water level gradient. Effects of salinity and the spring–flush–fen gradients are of more local significance, usually easily recognized. Land use is an important additional factor. 3 The mineral‐soil‐water limit between ombrotrophic and minerotrophically influenced sites is not sharp, and cannot be related to consistent differences in either vegetation or water chemistry. It should be abandoned as a general main division within mires. 4 The most important natural division is between ‘bog’, with pH generally < 5.0, low Ca2+, and Cl– and SO42– as the main inorganic anions, typically dominated by sphagna, ericoids and calcifuge Cyperaceae, and ‘fen’, with pH generally > 6.0, high Ca2+ and HCO3–, vegetation rich in dicotyledonous herbs and ‘brown mosses’. This division is reflected in a bimodal distribution of pH. 5 The terms oligotrophic, mesotrophic and eutrophic should refer only to nutrient richness (fertility, mainly N and P), not to base richness (metallic cations and pH). 6 It is recommended that ‘mire’ should embrace both wetlands on peat and related communities on mineral soils, that ‘bog’ (unqualified) should encompass both ombrotrophic and weakly minerotrophic mires, including ‘bog woodland’, and that ‘fen’ should be restricted to base‐rich mires but include both herbaceous and wooded vegetation (‘fen carr’). 7 Definitions are given for a range of broadly defined categories that should cover most vegetation types commonly encountered. The relation of these to British national vegetation classification types and to major units in European phytosociology is outlined.
Responses of the biennial forest herb Alliaria petiolata to variation in population density, nutrient addition and light availability
Authors: Meekins J. Forrest , McCarthy Brian C. 
Published: March 2003

Summary 1
Alliaria petiolata, a European biennial herb, is an important pest in temperate North American deciduous forests. Habitat resource structure has probably been important for invasion and proliferation of this species. 2
Alliaria was grown in an experimental garden at two densities (equivalent to 17 and 170 plants m−2), three nutrient levels (no, low, or high nutrient addition) and three light levels (ambient sunlight and two shading treatments) to determine the effects of environmental heterogeneity on growth, reproduction and resource allocation in both mature and rosette plants. 3 Overall, rosette growth and allocation patterns were significantly affected by all three variables tested. Low plant density, nutrient addition and high light availability resulted in plants with more leaves and greater dry weight biomass. Biomass allocation to shoots was greatest for plants in high‐density and low‐light treatments. Leaf chlorophyll content was significantly greater for plants in the two shaded treatments. 4 Mature plants also responded to environmental manipulation with significantly greater total dry weight biomass at low plant density, high light availability and with nutrient addition. Low density and high light availability resulted in significantly higher seed production. Plants in the lowest light treatment allocated significantly more biomass to shoot production and less to root production. Leaf chlorophyll content was lowest for plants in the highest light treatment and increased with nutrient addition at the two highest light treatments. 5 Plants growing at high density responded to nutrient addition and light attenuation in the same direction but with greatly reduced magnitude compared with plants growing at low density. The effect of irradiance was the most important determinant of all facets of growth and allocation patterns in both rosettes and mature plants. 6 Our results indicate that although density and site fertility may play important roles in Alliaria invasion and establishment, light availability may be the most important factor affecting subsequent growth and proliferation.
Influence of environmental factors on the growth and interactions between salt marsh plants: effects of salinity, sediment and waterlogging
Authors: Huckle Jonathan M. , Potter Jacqueline A. , Marrs Rob H. 
Published: March 2003

Summary 1 Artificial environmental gradients were established in a series of pot experiments to investigate the effect of salinity, sediment type and waterlogging on the growth, and interactions between Spartina anglica and Puccinellia maritima. In each experiment, one environmental variable was manipulated and plants grown in pairwise combinations to examine the effect of the environmental factor on the intensity of intra‐ and interspecific interactions, quantified using the Relative Neighbour Effect (RNE) index. 2 Puccinellia was found to exert an asymmetric, one‐way competitive dominance above ground over Spartina in experiments where gradients of sediment type and waterlogging were established. The intensity of the competition was highest in conditions with the least abiotic stress and lower or non‐existent where stress was increased. 3 The intensity of the above‐ground competition was greatest in loam and least in sand sediments. Reduction in competitive intensity in sand was accompanied by an increase in below‐ground Spartina biomass and it is suggested that the production of rhizomes is a potential mechanism by which this species can expand vegetatively into areas without competition. 4 Interspecific competition on Spartina from Puccinellia also varied in intensity in the waterlogging experiment, being more intense in non‐immersed treatments, where abiotic stress was reduced. 5 The competitive dominance of Puccinellia and the competition avoidance mechanism shown by Spartina in these experiments help to explain the successional interactions between the species along environmental gradients in natural salt marsh communities.
A modelling analysis of the genetic variation of phenology between tree populations
Authors: Chuine I. , Belmonte J. , Mignot A. 
Published: March 2003

Summary 1 The phenology of temperate woody plants is commonly assumed to be locally adapted to climate. 2 However, the high gene flow expected in forest tree species, the high between year variance of thermal conditions at a given place and the high plasticity of phenology regarding temperature, lead us to hypothesize that genetic variation of phenology between populations is likely to be insignificant for many lowland tree species. 3 Using phenological models, we investigated variation in the timing of flowering between locations for four European clonal trees and between different populations of a further five species. 4 Models were also used to study the responses of the different populations to climate change by simulating transfers of each population to different locations. 5 While clinal variations can be observed in the phenological response to temperature between populations, only one species (Corylus avellana) showed significantly different responses between populations and even then only one of three populations could be separated from the others. 6 Hypothetical transfers show that the differences observed between populations depend on the thermal conditions at the location of transfer, and that these differences are less marked in warmer conditions. 7 Our results indicate that local adaptation will probably not be a serious constraint in predicting the phenological responses of temperate lowland tree species to global warming.
Nitrogen retention by Hylocomium splendens in a subarctic birch woodland
Authors: Eckstein R. L. 
Published: March 2003

Summary 1 Nutrient dynamics and growth of the feathermoss Hylocomium splendens were studied in a subarctic birch woodland. My aims were to estimate the species’ nutrient use strategy (and its determinants: mean residence time (MRT) and annual nutrient productivity (aNP)) and to unravel possible traits related to nutrient conservation and their implications for ecosystem nitrogen flux. Three methods to estimate nutrient losses in bryophytes were evaluated: a conventional growth analysis technique, a retrospective analysis and a 15N‐tracer approach. 2 Estimates for nitrogen retention varied between 3 and 10 years as a result of the different N pools considered by the three methods. Growth analysis results depended on the distinction between live and dead tissues, whereas retrospective analysis gave valuable information on N release from decaying segments but did not measure MRT within the living segments. Valid estimates of MRT were obtained by a 15N‐tracer approach. 3 MRT and aNP of H. splendens were similar to values typically found in woody evergreen vascular plants. Efficient nutrient recycling and a relatively long segment life span were responsible for the long residence time of nitrogen. Feathermosses show efficient nutrient acquisition, nutrient recycling and acropetal transport; nutrient losses will therefore be small. 4 Dominant bryophytes may retard the nutrient turnover at the forest floor through their production of acidic nutrient‐poor organic matter and their negative effect on soil temperature, and they may therefore function as autogenic ecosystem engineers.
Shifts in trait‐combinations along rainfall and phosphorus gradients
Authors: Fonseca Carlos Roberto , Overton Jacob McC. , Collins Bronwyn , Westoby Mark 
Published: March 2003

Summary1 If different factors inhibiting plant growth, e.g. low rainfall or low soil nutrients, were to select for species that have similar constellations of traits, then the unfavourable factors might usefully be grouped together as ‘stress’.2 We offer a method for assessing this issue. A species mixture at a site is described by a point on a plane with two traits as axes. Change along an environmental gradient is then represented as a trajectory across the trait‐plane. Trajectories along different environmental gradients are compared.3 We measured leaf width, specific leaf area (SLA) and mature canopy height for the 386 perennial species found at 46 sites spread along rainfall and soil total phosphorus gradients in south‐eastern Australia. Each trait was lognormally distributed across species within sites, hence the mean of log10(trait) satisfactorily described the species mixture at each site.4 Combinations of assemblage‐mean leaf width with SLA followed similar trajectories as rainfall and soil total P decreased. For these traits in this setting, the method indicated that low rainfall and low soil P favour similar trait‐combinations.5 Mature plant height also decreased along both rainfall and soil P gradients, and thus was positively correlated with leaf width and SLA at the level of assemblage means. The rainfall trajectories involving height behaved differently from the soil P trajectories, especially at rainfalls belowc. 400 mm year−1, where assemblage‐mean height declined much further than at low soil P.6 Across all species, traits were only very loosely correlated (r2from 0.04 to 0.17). For leaf width and SLA, evolutionary divergences were positively correlated, both before and after cross‐correlation with divergence in rainfall and soil P was removed. This latter measures evolutionary divergence correlation within habitat. For height the picture was more complicated. Considering these within‐habitat divergence correlations, species that were taller at maturity tended to have lower SLA and leaf width. This pattern is the reverse of the broad geographical correlation of assemblage means, showing that the patterns across assemblages result from species being selectively sifted from the regional flora into sites, not from evolutionary or cross‐species correlations.7 The trait‐combination trajectory approach showed some commonalities between low soil nutrient and low rainfall habitats with regard to traits favoured in species occurring there, but also some differences. The approach has potential for clarifying which environmental factors can usefully be grouped together as ‘stress’, and which trait combinations can usefully be regarded as part of a syndrome favoured by stress.
Phylogeny and the niche structure of meadow plant communities
Authors: Silvertown Jonathan , Dodd Mike , Gowing David 
Published: March 2003

Summary

Because related species tend to be ecologically similar, it has long been recognized that the phylogenetic relationships between species in a community are likely to influence its structure. Previous investigations of the influence of phylogeny on community structure have generally shown that congeneric species are more frequent than would be expected if communities were randomly assembled. Because, under most theories of coexistence, stable coexistence requires ecological differences between species, the apparent excess of coexisting congeners suggests that trait differences important to coexistence must be recently evolved, arising near the tips of phylogenetic trees.
We test this hypothesis in two meadow plant communities for which we have good evidence of niche segregation along axes defined by hydrological conditions.
Niche overlaps were calculated and compared against null models for (i) all species in the community, (ii) species within genera, (iii) genera within families, (iv) families within the eudicots and monocots, and (v) eudicots and monocots.
Against expectation, we found that community structure was the cumulative result of niche separation arising at a range of phylogenetic levels and therefore that niche differences have been accumulated through the evolutionary history of species.
A comparison of a Bayesian vs. a frequentist method for profiling hospital performance
Authors: Austin Peter C. , Naylor C. David , Tu Jack V. 
Published: March 2003

AbstractThe objective of this study was to compare the classification of hospitals as outcomes outliers using a commonly implemented frequentist statistical approach vs. an implementation of Bayesian hierarchical statistical models, using 30‐day hospital‐level mortality rates for a cohort of acute myocardial infarction patients as a test case. For the frequentist approach, a logistic regression model was constructed to predict mortality. For each hospital, a risk‐adjusted mortality rate was computed. Those hospitals whose 95% confidence interval, around the risk‐adjusted mortality rate, excludes the mean mortality rate were classified as outliers. With the Bayesian hierarchical models, three factors could vary: the profile of the typical patient (low, medium or high risk), the extent to which the mortality rate for the typical patient departed from average, and the probability that the mortality rate was indeed different by the specified amount. The agreement between the two methods was compared for different patient profiles, threshold differences from the average and probabilities. Only marginal agreement was shown between the Bayesian and frequentist approaches. In only five of the 27 comparisons was the kappa statistic at least 0.40. The remaining 22 comparisons demonstrated only marginal agreement between the two methods. Within the Bayesian framework, hospital classification clearly depended on patient profile, threshold and probability of exceeding the threshold. These inconsistencies raise questions about the validity of current methods for classifying hospital performance, and suggest a need for urgent research into which methods are most meaningful to clinicians, managers and the general public.