bluuebunny/crossref_metadata_2025 huggingface dataset:
Problems of the solution: intervention into small‐scale irrigation for drought proofing in the Mekele Plateau of northern Ethiopia
Authors: Aberra Yohannes
Published: September 2004

The Mekele Plateau, located in the semi‐arid northern highland region of Ethiopia, is a drought‐prone, highly degraded area. It has long been neglected by policymakers and thus recurring famine, widespread poverty, armed conflicts and mass outmigration have contributed to the near collapse of the farming systems and the livelihoods of the population. Traditional stream diversion irrigation has been practised in this area for several centuries. The systems of water management in particular may be considered inappropriate owing to the need for thriftiness in water use imposed by the local environment. The systems of headworks and canals are crude and wasteful on water. Recent interventions to improve the small‐scale irrigation systems of the area have not been successful. Most interventions have preferred to introduce rainwater harvesting systems using micro‐dams rather than trying to upgrade the traditional diversion schemes. Evaluation of irrigation structures in the field, interviews with key informants in government, analysis of documents and discussions with irrigators indicate that both the Ministry of Agriculture and SAERT‐Commission's rainwater harvesting programmes have generally failed. The case of the most recent intervention by SAERT has failed because of myopic project planning, inadequate engineering studies, unsound designs, too short a time for implementation, and lack of full farmer involvement at different levels of project planning and implementation.
The karstlands of Trinidad and Tobago, their land use and conservation
Authors: Day Michael J , Chenoweth M Sean
Published: September 2004

The karst landscapes of Trinidad and Tobago, although restricted in area, present a previously undocumented array of tropical karst landforms, including karren, caves, springs, valley systems, and a range of dolines or sinkholes, including an area of polygonal cockpit karst. The karstlands are located primarily in the Northern and Central Ranges of Trinidad and in the southwestern portion of Tobago, and they have developed on carbonate rocks ranging in age from Jurassic to Quaternary. Associated with them is a range of vegetation types and faunal communities, many of which have been much altered by human land management, including forestry and agriculture. Quarrying has destroyed much of the karst in the western part of Trinidad, and urban development and tourism have taken their toll too, especially in the Lowlands of western Tobago. Little of the karst is incorporated within existing protected areas.
Age‐related decline in forest production: modelling the effects of growth limitation, neighbourhood competition and self‐thinning
Authors: BERGER UTA , HILDENBRANDT HANNO , GRIMM VOLKER
Published: September 2004

In growing forest stands, above‐ground net primary production peaks early in stand development and then declines. The causes for this decline are not yet well understood, but hypotheses include physiological and ecophysiological effects, as well as changes in stand structure due to local competition among neighbouring trees.

The majority of existing studies address mono‐causal explanations of this decline. Here we study the combined effects of intrinsic growth limitation of individual trees, growth limitation due to neighbourhood competition, and self‐thinning.

We use an individual‐based model to analyse forest wood production of a mangrove species described by a sigmoidal growth function, and two hypothetical species with exponential or linear growth. The model reproduces a decline for all species investigated, even when individual growth rates did not become limited.

We conclude that individual, sigmoidal growth curves are sufficient but not necessary to explain the production decline in natural forests where neighbourhood competition is appreciably active.

We show that the causes for production decline change during forest development. Whereas growth reduction through neighbourhood competition is the main process at the beginning, imbalanced wood loss dominates the later stage of the decline.
Occurrence of non‐mycorrhizal plant species in south Swedish rocky habitats is related to exchangeable soil phosphate
Authors: OLSSON PÅL AXEL , TYLER GERMUND
Published: September 2004

Plant species that do not typically form mycorrhiza are most likely to be successful under conditions where mycorrhizal fungi are not important for plant coexistence or where the costs of symbiosis outweigh the benefits. The relative occurrence of non‐mycorrhizal species was investigated in relationship to exchangeable soil P in herbaceous vegetation, where arbuscular mycorrhizal (AM) associations are generally common. We investigated a total of 439 sites in rocky habitats with sparse vegetation and 110 meadows with dense vegetation.

In both rocky habitats and meadows, soil pH was inversely related to exchangeable P. In rocky habitats plant species richness increased greatly between pH 3 and 5 and was also inversely related to P.

Plant communities in rocky habitats contained a relatively larger proportion of non‐mycorrhizal species than those in meadows. More non‐mycorrhizal species occurred at high soil P in the rocky habitats, but no such relation was found in meadows.

Non‐mycorrhizal species in rocky habitats were most common at low soil pH (high P availability). Plant species that thrived at extreme soil pH were often non‐mycorrhizal.

The occurrence of fewer non‐mycorrhizal plants in meadows than in rocky habitats supports the hypothesis that AM associations are more important in ecosystems with intense competition among plants. In rocky habitats, where abiotic stress may restrict photosynthesis more than nutrient limitation does, it is adaptive for plant species to utilize strategies other than mycorrhiza, particularly at low pH sites where P availability is likely to be adequate.
Height growth response of tree line black spruce to recent climate warming across the forest‐tundra of eastern Canada
Authors: GAMACHE ISABELLE , PAYETTE SERGE
Published: September 2004

The northward expansion of the boreal forest vegetation zone is generally predicted under a warmer doubled CO2, but the delay associated with vegetation development processes often has been overlooked. In the subarctic forest‐tundra of northern Québec, reforestation of tundra uplands appears currently limited by the poor reproductive capacity of shrubby black spruce (Picea mariana), and the development of erect stems through accelerated height growth should be the first registered response to 20th century climate warming. The subarctic forest‐tundra is characterized by small‐ and large‐scale heterogeneity in topography, vegetation structure and climate. This spatial heterogeneity, added to the complexity of tree growth–climate relationships, can cause various growth responses of subarctic tree line black spruce to 20th century climate change.

Twenty spruce populations at subarctic tree lines and seven isolated clones at the species limit were sampled along a > 300‐km latitudinal transect from the southern forest‐tundra to the shrub tundra. Height growth patterns of black spruce at tree line and above tree line were examined (i) over their life span, using dendrochronological dating of stem cross‐sections, and (ii) for the recent decades, using leader shoot elongation measurements. Indexed elongation chronologies were compared with regional climate data.

Height growth of tree line trees generally decreased with increasing latitude. However, tree line trees in the northern forest‐tundra have experienced an acceleration of height growth since the 1970s, with their growth comparable to that of trees in the southern forest‐tundra. Height growth response of spruce trees appeared increasingly delayed from the northern forest‐tundra to the species limit. Above the subarctic tree line, wind‐exposed conditions obscured the decrease in height growth with latitude observed for tree line trees.

Leader shoot elongation of spruce trees established on tundra hilltops appeared more controlled by summer heat sums than those at tree line all over the forest‐tundra, except at the arctic tree line. Winter precipitation also was linked to leader shoot elongation in some forest‐tundra sites. The increasing snow cover associated with recent warming appeared to have reduced the shoot elongation of spruce at forest margins showing the steepest slopes, hence subjected to snow overloading.

In the northern forest‐tundra sites, the recent increase in height growth and positive trend in leader shoot elongation, consistent with a 1990s’ increase in heat sums, point to the development of spruce krummholz into erect growth forms. In the southern forest‐tundra, reforestation of tundra hilltops and northward expansion of the boreal forest predicted under doubled CO2 conditions could be delayed, as suggested by suppressed height growth of spruce above tree line.
A simple mechanistic model of seed dispersal, predation and plant establishment: Janzen‐Connell and beyond
Authors: NATHAN RAN , CASAGRANDI RENATO
Published: September 2004

Although, in nature, seed dispersal usually declines with distance from the source, seedling establishment patterns are highly variable. An increase in seed survival can lead to either hump‐shaped (Janzen‐Connell (J‐C) pattern) or declining (Hubbell pattern) establishment with distance from seed source, but declining establishment can also be generated if survival decreases with distance (McCanny pattern). Pathogens and seed predators are considered to be major mortality agents structuring recruitment patterns, but it is unclear how well predation alone can explain variation in these patterns.

We introduce a simple mechanistic model showing that distance and density‐dependent seed predation can generate all of the observed recruitment patterns. Our approach provides the first mathematical reconstruction of conceptual models previously considered to be based on contrasting underlying mechanisms. Three easily measurable quantities (the proportion of seeds escaping predation at the source, and the mean distance from the source of dispersed seeds and of predators’ activity) can be used to test for consistency with the J‐C pattern. The association between recruitment patterns and plant (dispersal) and animal (predation) characteristics is robust with respect to parameter values and various functional forms.

The model shows that the J‐C pattern can occur only if the mean distance over which predators are active is lower than that over which seeds are dispersed, corresponding to a system with host‐specific, or immobile, seed predators (often invertebrates) that are restricted to areas of high seed density near adult plants, and therefore selecting for longer dispersal distances of seeds.

The Hubbell pattern is generated by the model when dispersal and predation distances are of equivalent magnitudes. The McCanny pattern emerges if more generalized, or more mobile, seed predators (often vertebrates) are attracted to the adult trees but also tend to forage farther away, thereby selecting for short dispersal distances that generate high densities needed to satiate seed predators.

The model also predicts that the total number of seeds surviving predation is lowest at intermediate distances, suggesting that distance‐dependent predation promotes either short or long dispersal distances, or both (dimorphism).
Dispersal potential in plant communities depends on environmental conditions
Authors: OZINGA WIM A. , BEKKER RENÉE M. , SCHAMINÉE JOOP H. J. , VAN GROENENDAEL JAN M.
Published: September 2004

Local plant communities can only function within a metacommunity context if they are connected by appropriate dispersal vectors, accommodating the transport of propagules between sites. The capacity for long‐distance dispersal may be a key factor in the survival of local populations, especially in fragmented landscapes, and hence may have a large impact on local species composition. Dispersal vectors with a large efficiency for long‐distance dispersal included in this study are: water, wind, large mammals and birds.

We tested the hypothesis that variation in dispersal traits across plant communities is related to the position of the communities along major environmental gradients. This hypothesis was tested for (i) separate long‐distance dispersal vectors and (ii) multiple dispersal vectors (the number of potential long‐distance dispersal vectors per species).

To quantify linkages between dispersal traits and environmental gradients, we coupled a data base containing dispersal attributes with another data base, containing 40 000 local vegetation descriptions aggregated into 123 plant communities. For each dispersal vector, the proportions of species that have access to this vector per community (weighted trait scores) were projected along three major environmental gradients: soil moisture, nutrient availability and light availability.

The potential importance of individual dispersal vectors showed clear differences along the three environmental gradients, with the greatest differences along the light availability gradient. The differences in dispersal traits probably reflect environmental constraints on the availability or efficiency of individual dispersal vectors.

The ability to be dispersed by multiple dispersal vectors is a common phenomenon in most plant communities (an average of 2.15 vectors per species). The mean number of potential long‐distance dispersal vectors per species increases with light availability. This probably implies that plant communities differ in their response to both habitat fragmentation and habitat restoration.

Despite differences in trait spectra among communities, all dispersal syndromes are represented in nearly all communities. An important consequence of this complementarity in dispersal traits is that species within the same community may experience different connectivity.

The results emphasize the need for dispersal models based upon multiple dispersal vectors that explicitly include parameters for habitat characteristics.
The influence of spatial patterns of damping‐off disease and arbuscular mycorrhizal colonization on tree seedling establishment in Ghanaian tropical forest soil
Authors: HOOD L.A. , SWAINE M.D. , MASON P.A.
Published: September 2004

Milicia regia (Moraceae) is a dioecious light‐demanding (Pioneer) timber tree of West Africa. Experiments in shadehouses were used to examine the influence of light and soil source (beneath and away from conspecific adults) on mortality and growth of its seedlings in relation to fungal pathogens and arbuscular mycorrhizal (AM) colonization.

The experiment in Ghana included treatments for light (2% and 20% of unshaded irradiance), three soil sources (under female, under male and 200 m distant from adults), and two different fungicide treatments.

The results showed significant effects of irradiance, soil source and fungicide on both seedling mortality and accumulated biomass. Biomass was greater and mortality reduced at higher irradiance. Fungicide application reduced mortality in 2% irradiance, but had no significant effects at 20%, although AM infection of seedlings grown in soils from beneath adults was severely reduced. Mortality was greatest in soil from beneath female trees and least in soil distant from adult conspecifics. However, biomass in 20% irradiance was greater in soil from beneath females, possibly because of enhanced AM colonization: seedlings had a significantly higher percentage root length colonized by AM fungi beneath adult conspecifics, and biomass accumulation was significantly correlated with root length colonized by AM fungi.

There were significant interactions amongst light, soil source and fungicide treatments that attest to the complexity of spatial effects on seedling establishment in tropical forest. Whilst there was clear evidence of pathogen‐mediated effects that provides support for Janzen‐Connell spacing mechanisms, and escape from the parent tree increases the chances of arriving in gaps, spatial differences in AM fungal communities may act in the opposite direction and enhance seedling growth close to adult conspecifics.

The complexity of interactions between abiotic and biotic factors and the way in which these factors can vary spatially to affect seedling recruitment may be an important factor contributing to the maintenance of tree species diversity in tropical forests.
Extensive hydrochory uncouples spatiotemporal patterns of seedfall and seedling recruitment in a ‘bird‐dispersed’ riparian tree
Authors: HAMPE ARNDT
Published: September 2004

Initial recruitment is by far the most variable and least predictable phase of the plant life cycle. Its dynamics remain little known, particularly in complex seed dispersal systems and over multiple reproductive cycles.

Seeds of the tree Frangula alnus are usually dispersed by frugivorous birds. Mediterranean populations, however, are mostly restricted to riparian forests, and their seeds may be re‐distributed during the winter by elevated stream water flow. Strength and frequency of peak discharges vary greatly from year to year. I investigated spatiotemporal variation of initial recruitment by monitoring fruit production, seedfall and seedling emergence over 4 years along permanent transects in two F. alnus populations growing along small mountain streams in southern Spain.

Seedling abundance varied greatly among years and independently of fruit or primary seedfall abundance. Spatial distributions of both seedfall and seedling emergence were highly consistent among years. Bird‐mediated seedfall density decreased exponentially with both increasing distance from the nearest fruiting tree and the decrease of its crop size. Seedling emergence was, however, uncoupled in space from the previous year's seedfall, both across single sample points and entire 60‐m transects. Dispersal experiments with seed mimics revealed that flooding caused secondary dispersal of much of the population seed pool over several scores of metres.

Seed re‐distribution by water appears to override patterns of primary (bird‐generated) seedfall and to determine spatial patterns of seedling recruitment. Seedling emergence is, however, more stable across years than would be expected given the unpredictability of rainfall (and thus stream flow), suggesting that seed deposition may be primarily determined by relatively stable microsite characteristics.

The dynamics of seed rain patterns are of critical importance for plant population demography and spatial structures. The finding that even complex, multistep dispersal systems may produce remarkably consistent year‐to‐year distributions of recruits has important implications for the interpretation of long‐term patterns of riparian plant regeneration at the landscape level and resulting conservation management.
Effect of spatial scale on factors limiting species distributions in dry grassland fragments
Authors: MÜNZBERGOVÁ ZUZANA
Published: September 2004

Distribution of plant species in fragmented landscapes is the result of both seed and site availability. Little is known about how their relative importance differs at different spatial scales.

I sowed seeds of eight dry grassland species into 22 localities that differed in occupancy by these species and followed seedling establishment over 3 years. I compared the number of emerging seedlings at three scales: between previously occupied and previously unoccupied localities, between occupied and unoccupied blocks within occupied localities, and between plots with and without seed addition within occupied blocks.

At the two larger scales, I also studied the relationship of the number of seedlings and of the distributions of adult plants to environmental factors.

Both seed and site availability are important in structuring the distribution of these species, but site availability becomes less important with increasing spatial scale. The intensity of this effect is, however, species specific.

The relationship between environmental factors and pattern of species distribution is also clearly scale dependent, and differs between seedlings and adults. Whilst abiotic factors are the main determinants of seedling distribution, the occurrence of adult individuals is best predicted by the occurrence of other species. This suggests that the present distribution of species in the landscape is determined mainly by historical factors.

Conclusions based on the importance of seed and site availability for species distribution in natural communities at one scale cannot be extrapolated to other scales. Only comparisons on multiple scales can provide a full understanding of factors affecting species distribution at landscape level.
An evaluation of alternative dispersal functions for trees
Authors: GREENE DAVID F. , CANHAM CHARLES D. , COATES K. DAVID , LEPAGE PHILIP T.
Published: September 2004

We compared three commonly used empirical seed/seedling dispersal functions for trees (lognormal, 2Dt, and two‐parameter Weibull) by analysis of published studies where the location of the source is known, as well as by inverse modelling within an old growth hardwood forest in southern Quebec. Almost all the species were wind‐dispersed.

For the discrete source studies, the lognormal was clearly superior, while for the inverse modelling the performance of the three dispersal functions was somewhat more even. We speculate that collisions with boles spuriously enhanced the likelihood of the 2Dt and the Weibull with inverse modelling, as both these functions assume that the greatest seed/seedling density will occur at the base of the maternal parent bole.

We conclude that the lognormal function is to be preferred because, as well as providing a framework for mechanistic interpretation, it tends to provide a closer approximation to observed dispersal curves.

We also argue that mean distances travelled by seed crops are far more extensive than indicated by previous studies that used the Weibull function.
Identifying and modelling the determinants of woody plant invasion of lowland heath
Authors: MANNING P. , PUTWAIN P. D. , WEBB N. R.
Published: September 2004

The invasion of Betula spp. represents the moment of transition between European lowland heath and scrub ecosystems. We hypothesized that Betula invasion was controlled by a multivariate threshold, comprising factors that may be subdivided into seed and safe‐site limitations, and that phosphorus availability was a key determinant of the Betula safe‐site.

We performed a multifactorial field experiment in which seed rain, P‐availability and disturbance were manipulated. All treatments had significant effects on Betula seedling densities, with seed availability proving the single greatest limitation. P addition emerged as having highly significant effects 4 and 12 months after germination. Disturbance had an initially large positive effect that dwindled across time.

Detailed descriptions of seedling density data were achieved by converting two factorial variables (seed rain and P availability) to a continuous form, including numerous covariates, and producing ‘minimum adequate’ generalized linear models (MAMs). These provided accurate descriptions of the data (explained deviance typically > 70%) and delimited site conditions where transition was likely.

Seed availability remained the most important factor, but the increased number of parameters in models describing densities of more mature seedlings suggested a temporal accumulation of recruitment limitations. Statistical modelling allowed for the subdivision of disturbance effects into those associated with a reduction in vegetation densities and the remaining ‘direct’ effects, e.g. soil disturbance, which shifted over time from being positive to negative. The results also support the hypothesis that soil phosphorus sorption capacity (PSC) determines heath–scrub transition as many of the identified determinants are controlled, either directly or indirectly, by PSC.

The study demonstrates that a combination of experimental and statistical modelling approaches can provide a detailed description of the factors controlling early stage invasion and may therefore have considerable utility in ecosystem management. Although site specific, the models illuminate the mechanisms by which many larger scale processes, e.g. burning and grazing, regulate the persistence of lowland heath ecosystems.
Plant community development is affected by nutrients and soil biota
Authors: DE DEYN G. B. , RAAIJMAKERS C. E. , VAN DER PUTTEN W. H.
Published: September 2004

Plant community development depends to a great extent on the availability of soil nutrients, but recent studies underline the role of symbiotic, herbivorous and pathogenic soil biota. We tested for interactions between these biotic and abiotic factors by studying the effects of additional nutrients and the removal of soil biota on the replacement of grassland plant species typical of different successional stages.

Species representing each of the early, mid and target phases of secondary succession in a grassland community (four per phase) were grown in mid‐successional grassland soil. The mixed plant communities were grown in sterilized and non‐sterilized soil, at three nutrient supply levels. The distribution of shoot biomass over the different plant species, and the total root biomass, were determined, as well as the composition of nematode and microarthropod communities and soil decomposition rates.

The effect of nutrient supply on plant community composition depended on soil sterilization. In sterilized soil, the plant community was initially dominated by grasses that increased in dominance even without fertilization. In non‐sterilized soil, the plant community was more diverse and grass dominance decreased over time, except with high fertilization. Fertilization enhanced the productivity of the plant community in sterilized and, to a greater extent, in non‐sterilized soil.

The abundance of root‐feeding nematodes was positively related to increased root biomass, pointing to a strong bottom‐up control. Increased levels of nutrient supply were associated with reduced abundance of omnivorous nematodes, the cause of this reduction being less clear. Increased soil fertility altered the functional diversity of the soil nematode community, which might, in the longer term, also affect their feedback effects on the plant community.

Increased nutrient supply reduced soil decomposition activity in the non‐sterilized soil, but not in the sterilized soil.

Our results imply that soil biota may reduce the effects of nutrient supply on plant dominance. Incorporating the effects of soil biota on plant species interactions into studies on succession, plant species diversity and restoration may therefore considerably increase our understanding of the observed plant community patterns.
Effect of dispersal capacity on forest plant migration at a landscape scale
Authors: TAKAHASHI KANAKO , KAMITANI TOMOHIKO
Published: September 2004

We studied the effects of seed dispersal mode and seed mass on the migration patterns of woody and herbaceous forest species in an artificial pine forest band growing on a former sand dune. Seven sites in the artificial forest, at least 44 years old, were selected at different distances from an adjacent natural forest (0.1–17.4 km).

Both the species richness and the abundance of forest species decreased with increasing distance from the natural forest, indicating that the migration of forest species is limited by seed dispersal. Plants using different seed dispersal modes showed differences in migration rate.

Ingested and adhesive species migrated into the artificial forest with the most success. In contrast, almost all the species utilizing other dispersal mechanisms (wind, hoarding or no dispersal mechanism) migrated only into sites near to the natural forest. This is likely to be due to low dispersal capacities. Ant‐dispersed species were not found at all in the artificial forest.

Migration distances were calculated for 43 species with a frequency of ≥ 5% in at least one site in the artificial forest. Distances were based on the occurrence of the individual of each species furthest from the natural forest, and on the maximum abundance of that species in the artificial forest. The migration distances of the species did not correlate with their seed mass.

The dispersal efficiency is an important factor in migration of forest species on a landscape scale, and the migration ability is affected by dispersal mode rather than seed mass.
Consistency of plant species and trait responses to grazing along a productivity gradient: a multi‐site analysis
Authors: PAKEMAN ROBIN J.
Published: September 2004

Studies of the responses of communities to land use or climate change at a functional rather than species level have used species traits as response or effect variables to enable generalization between different communities and biogeographical regions. Data from 10 published experiments on semi‐natural and agricultural grasslands in Scotland, and from published information on plant traits, were used to determine whether (a) species and trait attributes behave consistently with respect to changing grazing intensity at sites of different productivity and (b) whether species and attribute responses to grazing were affected by site productivity.

Only 9 out of 22 species common enough to show a consistent response to grazing did so. For example, Deschampsia flexuosa and Molinia caerulea declined when grazing intensity increased, whilst Anthoxanthum odouratum and Cerastium fontanum increased. A similar proportion (12/29) of traits behaved consistently between studies.

Increased grazing intensity was accompanied by an increase in species with a ruderal strategy, an annual life history, seasonal regeneration by seed, flowering and seed dispersal early in the season, rosette habit, higher requirement for light and a lower minimum height.

The response of five species was modified by site productivity. In four cases, the effect was to modulate the rate of response to grazing change. In contrast, Nardus stricta increased with grazing at low productivity sites, but decreased at high productivity sites. Many more traits (22), including attributes related to life history, life form, and vegetative and sexual reproduction, showed a response to grazing modulated by productivity. Eight of these changed in a complex manner, akin to that of N. stricta.

The response of some species and traits to grazing appears predictable. However, for some species and many traits either the rate or the direction of response is controlled by the productivity of the vegetation.
Pricing, Capacity Choice, and Financing in Transportation Networks
Authors: Verhoef Erik T. , Rouwendal Jan
Published: September 2004

Abstract.  This paper explores interrelations between pricing, capacity choice, and financing in transportation networks. We build on the Mohring‐Harwitz result on self‐financing of optimally designed and priced roads and investigate it in a network environment under various types of second‐best regulation. A small network model with endogenous car ownership demonstrates that optimal congestion pricing and capacity choice over an entire network may cause user prices to increase more in initially mildly congested areas compared to heavily congested areas. Furthermore, a flat kilometer charge under optimal capacity choice may result in first‐best efficiency gains.
The Spatial Distribution of Wages: Estimating the Helpman‐Hanson Model for Germany
Authors: Brakman Steven , Garretsen Harry , Schramm Marc
Published: September 2004

Abstract.  Using German district data we estimate the structural parameters of a new economic geography model as developed by Helpman (1998) and Hanson (1998, 2001a). The advantage of the Helpman‐Hanson model is that it incorporates the fact that agglomeration of economic activity increases the prices of local (nontradable) services, like housing. This model thereby provides an intuitively appealing spreading force that allows for less extreme agglomeration patterns than predicted by the bulk of new economic geography models. Generalizing the Helpman‐Hanson model, we also analyze the implications for the spatial distribution of wages once the assumption of real wage equalization is dropped. If we no longer assume real wage equalization we find support for a spatial wage structure as well as for the relevance of the structural parameters of the theoretical model.
Spatial Mismatch and the Structure of American Metropolitan Areas, 1970–2000
Authors: Martin Richard W.
Published: September 2004

Abstract.  This paper analyzes the impact of employment and population shifts in U.S. metropolitan areas from 1970 to 2000 on a spatial mismatch index to determine how metropolitan residents reacted to changes in metropolitan employment distributions. In particular, it seeks to determine whether suburban employment growth created new areas to which access is valued or whether it repelled metropolitan residents and sparked population growth in more distant suburban locations. The results show that residents tended to move away from areas gaining jobs. Black residents, on the other hand, appeared to be attracted to areas that are experiencing employment growth.
Employment Risk in U.S. Metropolitan and Nonmetropolitan Regions: the Influence of Industrial Specialization and Population Characteristics
Authors: Hammond George W. , Thompson Eric
Published: September 2004

Abstract.  A dynamic labor market model is used to motivate the inclusion of population characteristics and industrial structure as determinants of regional employment instability. We examine how these factors influence regional employment instability using data from both metropolitan and nonmetropolitan regions in the United States. We find that population characteristics are important determinants of employment volatility and that increased industrial specialization (reduced diversification) increases employment volatility, but the magnitude of that influence drops substantially once population characteristics are considered. We also find that the influence of population characteristics and industrial specialization varies significantly across metropolitan and nonmetropolitan regions.
Competitive Multifacility Location on Networks: the (r∣Xp)‐Medianoid Problem
Authors: Suárez‐Vega Rafael , Santos‐Peñate Dolores R. , Dorta‐González Pablo
Published: September 2004

Abstract.  We investigate the (r∣Xp)‐medianoid problem for networks. This is a competitive location problem that consists of determining the locations of r facilities belonging to a firm in order to maximize its market share in a space where a competitor is already operating with p facilities. We consider six scenarios resulting from the combination of three customer choice rules (binary, partially binary, and proportional) with two types of services (essential and unessential).

Known discretization results about the existence of a solution for the set of nodes are extended. Some examples and computational experience using heuristic algorithms are presented.
Early Ordovician bivalves from Dali, West Yunnan, China
Authors: Zong‐Jie Fang , Cope John C. W.
Published: September 2004

A bivalve fauna of Early Ordovician (late Arenig) age is described from the Hsiangyang Formation of the eastern part of West Yunnan, China. The fauna contains elements in common with Early Ordovician faunas of southern Gondwanan areas such as the Montagne Noire and Morocco and with those of Avalonia, together with several previously undescribed taxa. The following taxa are new: Biseriodonta simplex gen. et sp. nov.; Glyptarca sinensis sp. nov.; Trigonoglyptarca magna gen. et sp. nov.; Erhaiconcha xiangyangensis gen. et sp. nov.; Fasciculodonta impressa gen. et sp. nov.; Yunnanoredonia laevis gen. et sp. nov.; Daliella gen. nov.; Goniophorina (Goniophorina) contracta sp. nov.; Haidongoconcha radialis gen. et sp. nov.; Eopterinea aequiconcha gen. et sp. nov. The bivalve fauna includes the most diverse glyptarcid fauna and the earliest nuculanid and paracyclid known hitherto.  Eastern West Yunnan belonged to the Indochina terrane and it is concluded that the bivalve fauna represents a high‐latitude assemblage and that the Indochina terrane should be considered a component part of the Peri‐Gondwanan continent in the Ordovician; its bivalve faunas contrast with those of neighbouring terranes which have affinities with those of lower latitudes.
Chemico‐structure of the organophosphatic shells of siphonotretide brachiopods
Authors: Williams Alwyn , Holmer Lars E. , Cusack Maggie
Published: September 2004

The organophosphatic shell of siphonotretide brachiopods is stratiform with orthodoxly secreted primary and secondary layers. The dominant apatitic constituents of the secondary layer are prismatic laths and rods arranged in monolayers (occasionally in cross‐bladed successions), normally recrystallized as platy laminae. Sporadically distributed, interlaminar, lenticular chambers, containing apatitic meshes of laths and aggregates of plates and spherulites, probably represent degraded, localized exudations of glycosaminoglycans (GAGs) with dispersed apatite.

The shells of Helmersenia and Gorchakovia are perforated by canals with external depressions (antechambers) that possibly contained chitinous tubercles in vivo. The immature shell of Siphonotreta and most other siphonotretids is similarly perforated and pitted; but the mature part bears recumbent, rheomorphic, hollow spines that grew forward out of pits. Internally, spines pierce the shell as independent structures to terminate as pillars in GAGs chambers. Spines and pillars were probably secreted by collectives of specialized cells (acanthoblasts) within the mantle.

The shell of the oldest siphonotretide, Schizambon, is imperforate but the ventral valve has a pedicle foramen that lies forward of the posterior margin of the juvenile valve. This relationship characterizes all siphonotretides, suggesting that the pedicle, in vivo, originated within the ventral outer epithelium and not from the posterior body wall as in lingulides.
Taphonomy and origin of an accumulate of soft‐bodied cephalopods in the Oxford Clay Formation (Jurassic, England)
Authors: Wilby P. R. , Hudson J. D. , Clements R. G. , Hollingworth N. T. J.
Published: September 2004

An exceptionally well‐preserved fauna from a new exposure in the Peterborough Member (Oxford Clay Formation) of southern England, equivalent to the famous and now inaccessible Lagerstätte at Christian Malford, Wiltshire, is described. It comes from a single bed and includes coleoid cephalopods with phosphatized soft tissues, and fully articulated fish. The level is unusual in that it lacks permanent benthos and is dominated by ‘rain‐out’ from the upper water column. It was deposited on a sea floor that experienced prolonged periods of anoxia and which was overlain by a water column that was at least intermittently stratified. It is postulated that the coleoids formed large shoals that were killed en masse, together with other elements of the associated fauna, in one or more catastrophic mass mortality events that affected a significant area of the Peterborough Member sea. During the event(s), many of the coleoids preyed upon moribund fish and other coleoids, sometimes of the same species, before becoming overcome themselves. Crucially for the phosphatization of their soft tissues, the substrate surface was ‘soupy’ and allowed them to sink to a zone of rapid diagenesis where levels of dissolved phosphorus were greatly augmented by the large number of associated decaying carcasses.
New ophiuroids from the Permian/Triassic boundary beds of South China
Authors: Chen Z. Q. , Shi G. R. , Kaiho K.
Published: September 2004

A new genus of ophiuroid, Huangzhishania, is created based on new material from the Permian/Triassic boundary beds at the Huangzhishan section, South China. The age of the new genus is constrained as earliest Griesbachian by means of faunal correlation of the associated bivalves and stratigraphical correlation with the Mixed Fauna Beds of the neighbouring Meishan section. Taphonomic and palaeoecological evidence suggest that the collapse of the ophiuroid association was related to a catastrophic event, and Huangzhishania was rapidly buried in life position.
A new pachyostotic squamate reptile from the Cenomanian of France
Authors: Rage Jean‐Claude , Néraudeau Didier
Published: September 2004

Cenomanian beds from western France have yielded a new pythonomorph squamate (Reptilia) that is described here as Carentonosaurus mineaui gen. et sp. nov. This fossil is referred to non‐mosasaurid mosasauroid lizards, i.e. to ‘aigialosaurs’. It has been found in sediments of marine origin and its anatomy confirms that it was an aquatic lizard. It is characterized by a combination of characters that has not been reported, thus far, for squamates. Moreover, its vertebral column includes non‐pachyostotic cervical vertebrae and highly pachyostotic dorsal vertebrae as in several other ‘aigialosaurs’. This new taxon is perhaps restricted to the upper Cenomanian. It lived in shallow and rather warm water of the inner shelf. It is worth mentioning that nearly all pachyostotic squamates are concentrated in the Cenomanian and/or lower Turonian deposits of the present European‐North African‐Middle East portion of the Tethys. A parallel is drawn between the high percentage of pachyostotic squamates and the fact that this period corresponds to both the largest transgression of the Phanerozoic and the warmest period in the whole of the Mesozoic and Caenozoic.
Allocrioceras from the Cenomanian (mid‐Cretaceous) of the Lebanon and its bearing on the palaeobiological interpretation of heteromorphic ammonites
Authors: Wippich Max G. E. , Lehmann Jens
Published: September 2004

Specimens of the mid‐Cretaceous heteromorphic ammonite Allocrioceras cf. annulatum (Shumard, 1860) from the Upper Cenomanian lithographic limestones of the Lebanon provide an in situ record of aptychi and interpreted remnants of internal soft tissue and stomach content. Based on this exceptionally well‐preserved material, Allocrioceras is interpreted as a pelagic, aperture‐upwards drifter that probably fed on comatulid crinoids.
A new Xinjiangchelyid turtle (Testudines, Eucryptodira) from the Jurassic Qigu Formation of the southern Junggar Basin, Xinjiang, north‐west China
Authors: Matzke Andreas T. , Maisch Michael W. , Ge Sun , Pfretzschner Hans‐U. , Stöhr Henrik
Published: September 2004

A new eucryptodiran turtle, Xinjiangchelys qiguensis sp. nov. from the Upper Jurassic (Oxfordian — ?Kimmeridgian) Qigu Formation of the southern Junggar Basin (north‐west China) is described. The type material consists of a partial skeleton, including the complete carapace, plastron, nearly all cervical vertebrae, both scapulae, the pelvis and one ulna. It is clearly identifiable as a basal eucryptodire since it lacks the mesoplastron. It is distinguished from other species of Xinjiangchelys by several autapomorphies of the carapace and plastron, such as the first and fifth vertebrals extending on the peripherals, the plastron with three pairs of gulars, and an intergular which does not contact the hyoplastron. In the postcranium, the scapula with a long acromial and a small scapular process, the pelvis with a short ilial shaft and the elongated cervical vertebrae are characteristic. A new phylogenetic analysis of the in‐group phylogeny of the Xinjiangchelyidae is proposed and discussed, resulting in a new classification of the family. Xinjiangchelys (Toxocheloides) narynensis is regarded as a nomen dubium. Shartegemys is referred to Xinjiangchelys, whereas the holotypes of ‘Plesiochelys’chungkingensis and ‘P’. latimarginalis are excluded from the genus Xinjiangchelys but included in the Xinjiangchelyidae.
Percutaneous absorption of gold sodium thiosulfate used for patch testing
Authors: Möller Halvor , Schütz Andrejs , Björkner Bert , Bruze Magnus
Published: September 2004

Contact allergy to gold has been shown to be correlated to wearing of golden jewellery as well as to the presence of dental gold. The prerequisite for sensitization and elicitation of an allergic contact dermatitis to gold is ionization and percutaneous absorption of the allergen. The present study was undertaken with the aim to demonstrate gold in blood after an occlusive patch testing with gold sodium thiosulfate (GSTS). Patients were patch tested with GSTS 0.5, 2.0 and 5.0% in petrolatum. Venous blood was sampled before testing as well as on day 3 (D3) or day 7 (D7). Blood gold levels were analysed by inductively coupled plasma mass spectrometry. In all 66 patients, the blood level of gold was significantly higher in the 2nd blood sample than in the 1st (median 0.34 versus 0.03 µg/l). The increase was similar in patients allergic and not allergic to gold. There was no difference between gold levels on D3 and on D7 after epicutaneous application. Presumably, other allergens may also be taken up in blood during patch testing.
Fondaparinux: a suitable alternative in cases of delayed‐type allergy to heparins and semisynthetic heparinoids? A study of 7 cases
Authors: Jappe U. , Juschka U. , Kuner N. , Hausen B. M. , Krohn K.
Published: September 2004

Hypersensitivity to unfractionated and low‐molecular‐weight heparins and semisynthetic heparinoids is increasingly common. 7 female patients between 30 and 74 years with delayed‐type allergy to heparins and semisynthetic heparinoids were investigated for (cross)‐reactivity to fondaparinux, a new pentasaccharide with selective factor Xa inhibition. All patients showed delayed‐type reactions to heparins and some additional cross‐reaction to a heparinoid on intracutaneous testing. 6/7 tolerated fondaparinux on intradermal testing as well as on subcutaneous challenge testing. However, the 7th patient developed a characteristic delayed‐type reaction to both skin tests with fondaparinux. Fondaparinux is a new synthetic pentasaccharide with a molecular weight of 1.728 Da. In some patients with cross‐reactivity between various heparins and semisynthetic heparinoids, lepirudin, a recombinant hirudin, may be a safe and effective alternative. However, combined allergy to hirudin and heparins has been reported. Sometimes, intravenous administration of heparins or heparinoids may be tolerated. However, these patients are at risk of developing a systemic reaction. The pathogenesis of heparin hypersensitivity is not fully understood. Heparins may act as haptens by binding to dermal and/or subcutaneous structural proteins. The chemical structures of heparins and fondaparinux are different concerning their α‐ and β‐configuration and the molecular weight. However, some of their functional groups are nearly identical and therefore similar chemical and pharmacological reactivity is to be expected. Fondaparinux seems to be a valuable alternative in most cases of heparin and hirudin hypersensitivity. The clearly rare cross‐reaction between fondaparinux and heparins, now confirmed by us, may be due to differences in the response to haptens.