Principal Gradient Research Articles

Arcto-Alpine Species at Their Niche Margin: the Western Carpathian Refugia of Juncus castaneus and J. triglumis in Slovakia

Juncus castaneus and J. triglumis are umbrella species of moist calcareous tundra and arctic-alpine initial fens (the Caricion atrofusco-saxatilis alliance), considered to be glacial relicts in the central-European Mts. Here we report new findings from the Western Carpathians (Slovakia). We analysed species composition of plant communities with the target Juncus species in a wide geographical context, using 20 available vegetation-plot records from the Carpathians, 711 records from other European regions, and formal definitions of fen alliances taken from a pan-European vegetation synthesis. The first gradient of non-metric multidimensional scaling (NMDS) differentiated alpine and arctic samples. The second principal gradient between Caricion atrofusco-saxatilis and other vegetation types was of similar importance. Records from the Western Carpathians assumed a marginal position in the gradient space, while records from the Eastern Carpathians (Romania) appeared to be similar to those from the Alps. Generally, Carpathian records did not match the formal definition of the Caricion atrofusco-saxatilis alliance. Although the arctic-alpine rush species persist occasionally in spring, fen and tundra microrefugia in the Carpathians, they do not belong in similar plant communities as in the Alps or in the Arctic.

Situating the default-mode network along a principal gradient of macroscale cortical organization

Understanding how the structure of cognition arises from the topographical organization of the cortex is a primary goal in neuroscience. Previous work has described local functional gradients extending from perceptual and motor regions to cortical areas representing more abstract functions, but an overarching framework for the association between structure and function is still lacking. Here, we show that the principal gradient revealed by the decomposition of connectivity data in humans and the macaque monkey is anchored by, at one end, regions serving primary sensory/motor functions and at the other end, transmodal regions that, in humans, are known as the default-mode network (DMN). These DMN regions exhibit the greatest geodesic distance along the cortical surface-and are precisely equidistant-from primary sensory/motor morphological landmarks. The principal gradient also provides an organizing spatial framework for multiple large-scale networks and characterizes a spectrum from unimodal to heteromodal activity in a functional metaanalysis. Together, these observations provide a characterization of the topographical organization of cortex and indicate that the role of the DMN in cognition might arise from its position at one extreme of a hierarchy, allowing it to process transmodal information that is unrelated to immediate sensory input.

Principal patterns of fractional-order differential gradients for face recognition

We investigate the ability of fractional-order differentiation (FD) for facial texture representation and present a local descriptor, called the principal patterns of fractional-order differential gradients (PPFDGs), for face recognition. In PPFDG, multiple FD gradient patterns of a face image are obtained utilizing multiorientation FD masks. As a result, each pixel of the face image can be represented as a high-dimensional gradient vector. Then, by employing principal component analysis to the gradient vectors over the centered neighborhood of each pixel, we capture the principal gradient patterns and meanwhile compute the corresponding orientation patterns from which oriented gradient magnitudes are computed. Histogram features are finally extracted from these oriented gradient magnitude patterns as the face representation using local binary patterns. Experimental results on face recognition technology, A.M. Martinez and R. Benavente, Extended Yale B, and labeled faces in the wild face datasets validate the effectiveness of the proposed method.

BHoG: binary descriptor for sketch-based image retrieval

Due to the popularity of devices with touch screens, it is convenient to match images with a hand-drawn sketch query. However, existing methods usually care little about memory space and time efficiency thus is inadequate for the rapid growth of multimedia resources. In this paper, a BHoG descriptor is proposed for sketch-based image retrieval. Firstly, the boundary image is detected from natural image using Berkeley boundary detector, and then divided into many blocks. Secondly, we calculate the gradient feature of each block, and find the principal gradient orientation. Finally, the principal gradient orientation is encoded to binary codes, which is proved to be efficient and discriminative. We evaluated the performance of BHoG on a large-scale social media dataset. The experimental results have shown that BHoG not only has a better performance on flexibility and efficiency, but also occupies small memory.

An Effective Neural Learning Algorithm for Extracting Cross-Correlation Feature Between Two High-Dimensional Data Streams

A novel information criterion for principal singular subspace tracking is proposed and a corresponding principal singular subspace gradient flow is derived based on the information criterion in this paper. The information criterion exhibits a unique global minimum attained if and only if the state matrices of the left and right neural networks span the left and right principal singular subspace of a cross-correlation matrix between two high-dimensional vector sequences, respectively. The proposed gradient flow can efficiently track an orthonormal basis of the principal singular subspace, and it has fast convergence speed, good suitability for data matrix close to singular matrix and excellent self-stabilizing property. The global asymptotic stability and self-stabilizing property of the proposed algorithm are analyzed. The simulation experiments validate the excellent performance of the proposed algorithm.

Object detection using voting spaces trained by few samples

A method to detect generic objects by training with a few image samples is proposed. A new feature, namely locally adaptive steering (LAS), is proposed to represent local principal gradient orientation information. A voting space is then constructed in terms of cells that represent query image coordinates and ranges of feature values at corresponding pixel positions. Cell sizes are trained in voting spaces to estimate the tolerance of object appearance at each pixel location. After that, two detection steps are adopted to locate instances of object class in a given target image. At the first step, patches of objects are recognized by densely voting in voting spaces. Then, the refined hypotheses step is carried out to accurately locate multiple instances of object class. The new approach is training the voting spaces based on a few samples of the object. Our approach is more efficient than traditional template matching approaches. Compared with the state-of-the-art approaches, our experiments confirm that the proposed method has a better performance in both efficiency and effectiveness.

Principal whitened gradient-projection algorithm for distribution control

In this paper, we use an information geometric algorithm to solve the distribution control problem. The new designed algorithm is called principal whitened gradient-projection algorithm. In the principal natural gradient step, we use principal whitened gradient algorithm to obtain an optimal trajectory of the weight vector on the B-spline manifold from the viewpoint of information geometry. In the projection step, we project the selected points on B onto M. The coordinates of the projections on M give the trajectory of the control input u.

Assessment of eutrophication effects on charophytes in Mediterranean ponds (North-Western Spain)

The distribution of charophyte vegetation in 57 ponds located in the basin of the River Duero in North-Western Spain was studied in relation to eutrophication. A total of 11 charophyte taxa were recorded in 40 ponds. However, only Chara fragilis Desv., Chara connivens Salzm. ex A. Braun and Nitella translucens (Pers.) C. Agardh were found in more than 17% of the ponds where charophytes occurred. Significant differences between ponds with and without charophytes were found for the following parameters: volatile suspended solids (VSS), total suspended solids (TSS), turbidity, chlorophyll-a, nitrate and total phosphorus (TP). These variables were significantly lower in ponds where charophytes were present. Charophytes were found over a wide range of TP concentrations (20―6884 μg L ―1 ). However, taxa showed differing sensitivities to phosphorus concentrations. Thus, Chara connivens and Chara fragilis showed the highest optimum values for TP concentration (1875 and 551 μg L ―1 TP, respectively) while Nitella translucens registered the lowest optimum value for TP (69 μg L ―1 ). The charophyte cover showed a significant negative correlation with VSS and chlorophyll-a. As for charophyte taxa, Chara connivens cover was positive and significantly correlated with orthophosphate (SRP), while Nitella translucens cover showed a significant negative correlation with TP and TSS. The ordination of charophyte taxa showed a principal gradient of mineralization and ecological condition and a secondary gradient of eutrophication.

A community‐level test of the leaf‐height‐seed ecology strategy scheme in relation to grazing conditions

AbstractQuestion: Is the assumption of trait independence implied in Westoby's (1998) leaf‐height‐seed (LHS) ecology strategy scheme upheld in a Mediterranean grazing system dominated by annuals? Is the LHS approach applicable at the community level?Location: Northern Israel.Methods: LHS traits (specific leaf area [SLA], plant height and seed mass), and additional leaf traits (leaf dry matter content [LDMC], leaf area, and leaf content of nitrogen [LNC], carbon [LCC], and phosphorus [LPC]), were analyzed at the species and community levels. Treatments included manipulations of grazing intensity (moderate and heavy) and protection from grazing. We focused on species comprising 80% of biomass over all treatments, assuming that these species drive trait relationships and ecosystem processes.Results: At the species level, SLA and seed mass were negatively correlated, and plant height was positively correlated to LCC. SLA, seed mass, and LPC increased with protection from grazing. At the community level, redundancy analysis revealed one principal gradient of variation: SLA, correlated to grazing, versus seed mass and plant height, associated with protection from grazing. We divided community functional parameters into two groups according to grazing response: (1) plant height, seed mass, LDMC, and LCC, associated with protection from grazing, and (2) SLA, associated with grazing.Conclusions: The assumption of independence between LHS traits was not upheld at the species level in this Mediterranean grazing system. At the community level, the LHS approach captured most of the variation associated with protection from grazing, reflecting changes in dominance within the plant community.

A method for improving the performance of gradient systems for diffusion‐weighted MRI

The MR signal is sensitive to diffusion. This effect can be increased by the use of large, balanced bipolar gradients. The gradient systems of MR scanners are calibrated at installation and during regular servicing visits. Because the measured apparent diffusion constant (ADC) depends on the square of the amplitude of the diffusion sensitizing gradients, errors in the gradient calibration are exaggerated. If the error is varying among the different gradient axes, it will affect the estimated degree of anisotropy. To assess the gradient calibration accuracy in a whole-body MRI scanner, ADC values were calculated for a uniform water phantom along each gradient direction while monitoring the temperature. Knowledge of the temperature allows the expected diffusion constant of water to be calculated independent of the MRI measurement. It was found that the gradient axes (±x, ±y, ±z) were calibrated differently, resulting in offset ADC values. A method is presented to rescale the amplitude of each of the six principal gradient axes within the MR pulse sequence. The scaling factor is the square root of the ratio of the expected and observed diffusion constants. In addition, fiber tracking results in the human brain were noticeably affected by improving the gradient system calibration. Magn Reson Med 58:763–768, 2007. © 2007 Wiley-Liss, Inc.

AGRICULTURAL LEGACIES IN FOREST ENVIRONMENTS: TREE COMMUNITIES, SOIL PROPERTIES, AND LIGHT AVAILABILITY

Temperate deciduous forests across much of Europe and eastern North America reflect legacies of past land use, particularly in the diversity and composition of plant communities. Intense disturbances, such as clearing forests for agriculture, may cause persistent environmental changes that continue to shape vegetation patterns as landscapes recover. We assessed the long-term consequences of agriculture for environmental conditions in central New York forests, including tree community structure and composition, soil physical and chemical properties, and light availability. To isolate the effects of agriculture, we compared 20 adjacent pairs of forests that were never cleared for agriculture (primary forests) and forests that established 85-100 years ago on plowed fields (secondary forests). Tree communities in primary and secondary forests had similar stem density, though secondary forests had 14% greater basal area. Species composition differed dramatically between the two forest types, with primary forests dominated by Acer saccharum and Fagus grandifolia and secondary forests by Acer rubrum and Pinus strobus. Primary and secondary forests showed no consistent differences in soil physical properties or in the principal gradient of soil fertility associated with soil pH. Within stands, however, soil water content and pH were more variable in primary forests. Secondary forest soils had 15% less organic matter, 16% less total carbon, and 29% less extractable phosphorus in the top 10 cm than adjacent primary stands, though the ranges of the forest types mostly overlapped. Understory light availability in primary and secondary forests was similar. These results suggest that, within 100 years, post-agricultural stands have recovered conditions comparable to less disturbed forests in many attributes, including tree size and number, soil physical properties, soil chemical properties associated with pH, and understory light availability. The principal legacies of agriculture that remain in these forests are the reduced levels of soil organic matter, carbon, and phosphorus; the spatial homogenization of soil properties; and the altered species composition of the vegetation.

Forest river plants and water quality in Ghana

River plants and water chemistry in wet and dry seasons were sampled in a wide range of rivers with variable catchments in the forest zone of Ghana, in order to examine the relationships between the floristic composition within the river and river water and catchment environmental variables. Plant species occurring in the river or on its seasonally flooded banks along 500 m sample stretches were listed in 26 sites. Sample species richness was low (1–17 species per sample, mean 5.9). Only nine species were recorded that are confined to rivers. Cluster analysis defined five floristic groups which differed in catchment area, geology, turbidity and dissolved minerals. Analysis of water chemistry variables by principal components analysis revealed a strong principal gradient (42% of total variance) related to annual rainfall with low ionic concentrations under high rainfall. The second component described seasonal differences in water chemistry. Nitrate and chloride were more concentrated in the dry season, but sulphate and calcium were more concentrated in the wet season. Catchments with higher forest cover yielded more oligotrophic and less turbid water. Catchment geology influenced water chemistry so that sandstone areas yielded clear water, and ancient peneplain areas with bauxitic soil were characterised by exceptionally low silicon concentrations. The association of macrophyte composition with river chemistry and catchment conditions was somewhat tenuous, due to the low number of species, but the low turbidity, oligotrophic rivers under high rainfall and with a high percentage of species-rich forest cover were the more species rich. Prediction of river water quality at the scale of this study are more readily made from a knowledge of catchment rainfall and forest cover than from assessments of river plant composition.

Hierarchical processes define spatial pattern of avian assemblages restricted and endemic to the arid Karoo, South Africa

Abstract Aim To identify and quantify biotic and abiotic factors associated with the regional gradients in the distribution and abundance of bird communities restricted and endemic to the Succulent and Nama Karoo biomes of South Africa.Location The arid Nama and Succulent Karoo biomes in South Africa.Methods The quarter degree grid cell (QDGC) was used to extract environmental data, while the bird data previously atlased, was linked to the same geo‐referenced system, using a geographical information system (GIS). Bird species were grouped into different life‐history assemblages. A quantitative, systematic analysis of the different bird communities spanning the Karoo was undertaken to examine contributions of broad‐ and local‐scale physical environmental and biotic factors to regional variations in the species composition, using multivariate statistical and spatial analytical tools. These included two indirect gradient methods; principal components analysis (PCA) and detrended correspondence analysis (DCA), and two direct gradient methods; canonical correspondence analysis (CCA) and redundancy analysis (RDA).Results Principal components analysis results showed that the selected environmental variables accounted for about 85% of the variation in the region. The first two principal gradients defined regional temperature seasonality and variability especially in winter and summer. The third principal gradient mainly defined summer rainfall areas in association with the coefficient of variation of rain and regional primary production, while the fourth gradient defined winter rainfall areas, growth days and elements of landscape structure. CCA/RDA analysis produced shortened hierarchically ranked explanatory variables for each bird assemblage. Stepwise gradient analysis results showed summer rain, rainfall concentration, topographic heterogeneity and annual evapotranspiration, as the most important climate variables explaining species occurrence. Landscape, in terms of percentage transformation, morphology, coefficient of variation of primary productivity and distance between suitable habitat patches, were also important, but to a lesser degree. Total variation explained (TVE) by the supplied variables was between 23 and 37% of variation. Less than 20% of TVE was the intrinsic spatial component of environmental influence, indicating that any unmeasured factors were independent of spatial structuring. For all the eight bird assemblages, climate contributed most to TVE (24–57%). Landscape characteristics (human‐induced transformation, vegetation in terms of size if grassy clumps and the average distances between them) contributed theleast to TVE for all the different assemblages (0–6%), especially the granivorous assemblage where it was not significant at all (0%). Seasonal extremes and variability were more important in explaining species gradients than were annual climatic conditions, with the exception of annual potential evapotranspiration.Main conclusions This study was able to synthesize species environment relations at the broad scale and demonstrated the association of arid zone endemic species occurrence with climate extremes and seasonality. Given the predicted climate change scenarios for South Africa, this regional gradient study provides a quantitative ecological basis for finer scale modelling and analysis, developing regional strategies for conserving biodiversity as well as predicting and planning for the effects of global climate change. However, most importantly, it clearly showed that bird species restricted and endemic to the arid Karoo biome may be more sensitive to climate rather than vegetation structure as previously thought.

Colonisation of gravel lakes by Chironomidae

The Environment Agency (England and Wales) has constructed a flood relief channel through gravel substrata for the River Thames to bypass the towns of Maidenhead and Windsor. As they were dug, sections of channel flooded with groundwater to create a series of lakes. Colonisation of lakes was monitored by collecting floating pupal exuviae discarded by emerging adult Chironomidae. The same technique will be used for future monitoring of the new channel. Six lakes were sampled four times during 2000. Lake age at the time of the first sample ranged from four months to 3 years, with surface area from 0.2 to 7.0 ha. Despite a contiguous groundwater source, the lakes exhibited significant chemical differences. Sixty-eight species were identified, including some atypical of the region. In common with other lake colonisation studies, there were high proportions of predators although detritivores were dominant. Species diversity was unrelated to lake age, however, the oldest lakes could be discriminated from the youngest lakes on the basis of their species composition. The principal gradient of species change between lakes was correlated with alkalinity. This had been demonstrated in surveys of other lakes in England and Wales, although conductivity was a better discriminator of lake type. Weighted average regression and calibration of chironomid data from established lakes provided good estimates of conductivity for the new lakes. Results suggest these new lakes were rapidly colonised to an extent that predictive models based on mature lakes were applicable.

Numerical modeling of saline water transport in the lower Nam Kam Basin, Amphoe That Phanom, Changwat Nakhon Phanom, Thailand

The objective of the project is to establish a conceptual groundwater model over the lower Nam Kam Basin in order to apply a numerical technique for the prediction of the impact of saline water transport due to the proposed weir across the Nam Kam River. Hydrogeological investigations including mapping, drilling, piezometer installations and monitoring were systematically conducted during 1997 to 1998. Brackish groundwater is saturated under the area with a depth of 30-60 m. Groundwater regionally flows from the south (the Phu Phan Range) to the north and discharges to the Nam Kam River. Another direction is from the northern region to the southern region, discharging to the central region. A two-dimensional model was constructed along the principal gradient in the NW-SE direction. There are several local recharge and discharge areas across the Nam Kam floodplain. A local groundwater flow is active within the depth of 2 m to 30 m below the ground surface within the sand and gravel unit. Simulations were calibrated with hydraulic heads and salinity of groundwater in the piezometers. It is found that the recharge and evapotranspiration rates are 1% to 40% of the rainfall and 10% to 15% of a pan evaporation, respectively. The ranges of horizontal hydraulic conductivity to vertical hydraulic conductivity are 0.1 to 0.01. The possible longitudinal dispersivity values of the hydrostratigraphic units are 20 m to 500 m, but the transverse dispersivity is less than the longitude by one order of magnitude. The comparison of calculated heads and measured heads give a root mean square error of less than 1 m. The different salinity concentrations are still in a range of 2000-5000 mg/l. Ten year simulation of saline water transport indicates that the reservoir ponding with water level at +140.5 m above mean sea level may divert groundwater flow and discharging to the northern boundary of the reservoir at Ban Don Kao.

Studies of weak electric quadrupole interaction in57,60CoFe using modulated adiabatic passage on oriented nuclei

The electric quadrupole interactions at57Co and60Co in co-diffused57,60CoFe single crystal have been measured using Modulated Adiabatic Passage on Oriented Nuclei (MAPON). The quadrupole splittings ΔνQ are + 18(2) kHz for57Co and +9(2) kHz for60Co, corresponding to a principal electric field gradient (efg) tensorVzz=2.0(5)×1019 Vm−2 and 2.5(6)×1019 Vm−2, respectively, in broad agreement with previous MAPON results for CoFe. The distributions of the efg’s are very similar for the two isotopes, verifying that previously reported differences in58CoFe and60CoFe could be attributed to different host preparations. The measurement of such weak efg’s which are not spectroscopically resolved, allows determination of new nuclear electric quadrupole moments, not accessible by other techniques. Applications to other systems are discussed.

Effects of fire, topography and year-to-year climatic variation on species composition in tallgrass prairie

Native unploughed tallgrass prairie from Konza Prairie, Kansas, USA is described with respect to plant species compositional changes over a five year period in response to fire and topography. The principal gradient of variation in the vegetation is related to time since burning. Species show an individualistic response in terms of relative abundance to this gradient. Both the percentage of and cover of C4 species and all grasses decrease as the prairie remains unburnt. Forb and woody plant species numbers and abundance increase along this gradient. A secondary gradient of variation reflects topography (i.e. upland versus lowland soils). Upland soils support a higher species richness and diversity. Upland and lowland plant assemblages are distinct except on annually burnt prairie. The interaction between burning regime, topography and year-to-year climatic variation affects the relative abundance of the plant species differentially. The most dominant species overall, Andropogon gerardii, was affected only by year-to-year variation (i.e. climate). Its position at the top of the species abundance hierarchy was unaffected by burning regime or soil type. The other dominant species showed a suite of varying responses to these factors.

NQR Zeeman Effect in 2, 4, 6 Tribromophenyl-p-Toluene Sulfonate

NQR Zeeman effect studies in 2, 4, 6 Tribromophenyl-p-toluene Sulfonate for 79 Br isotope were carried out using single crystals. The value of the asymmetry parameter and the directions of the principal field gradient X , Y , Z axes were determined. The values of the various contributions to the bond character are calculated.