Fallen retention aspen trees on clear-cuts can be important habitats for red-listed polypores: a case study in Finland

Green-tree retention is a relatively new forestry application, which aims at decreasing the negative effects of clear-cut logging on forest biodiversity. In this study, the value of retained aspens in maintaining diverse assemblages of wood-decaying fungi (polypores; Basidiomycota) on clear-cuts was investigated, after the retention trees had died, fallen and started to decay. A total of 110 fallen aspen trunks were investigated on clear-cuts and within old-growth forests in eastern Finland, southern boreal zone; and 499 records of polypores belonging to 46 species were made. The intermediately decayed trunks on a clear-cut area hosted more species and more red-listed species than did trunks within forests. Most of the polypore species with more than two records were found in both habitats. These results suggest that many aspen-associated polypores are able to live and reproduce in sun-exposed habitats, if the quality and quantity of dead wood fulfill the species-specific requirements. This unexpected result, however, may be partly due to the exceptionally great abundance of aspen in the study area. Furthermore, in the long term, the local benefits of fallen retention trees can be limited, unless the local continuity of large aspens, both living and dead, is ensured.     

Seasonal shifts in habitat selection of a large herbivore and the influence of human activity

Failure to recognize factors contributing to variation in habitat models like resource selection functions (RSFs) can affect their application for projecting probabilities of occurrence, and thereby limit their relevance for conservation and management. We compared seasonal RSFs (2006–2008) for 16 adult female moose (Alces alces) with home ranges located in western Algonquin Provincial Park (APP), Ontario, Canada, to those of 14 adult females located in provincial Wildlife Management Unit (WMU) 49, 40 km west of the protected area. Wildlife and habitat management practices differed between regions: hunting was higher in WMU 49 compared to APP, and APP preserved large tracts of old growth forest rarely found in WMU 49. Seasonal RSFs projected expected similarities in moose resource use between regions (e.g., responses to wetlands and stands of eastern hemlock, Tsuga canadensis [in winter]); however, we also observed differences consistent with the hypothesis that animals, through effects of hunting, would shift habitat use seasonally and in response to roads. We further observed evidence of functional responses in habitat selection due to underlying differences in forestry practices (e.g., responses to stands of old-growth hemlock forest). Given the close proximity and shared biogeographic region between study areas, we believe that observed spatial dynamics in RSFs were ultimately reflective of divergent management strategies between areas and ensuing differences in predation and hunting mortality risk, and functional habitat.     

Genetic evidence for the persistence of the critically endangered Sierra Nevada red fox in California

California is home to both the native state-threatened Sierra Nevada red fox (Vulpes vulpes necator), which historically inhabited high elevations of the Sierra Nevada and Cascade mountains, and to multiple low-elevation red fox populations thought to be of exotic origin. During the past few decades the lowland populations have dramatically expanded their distribution, and possibly moved into the historic range of the native high-elevation fox. To determine whether the native red fox persists in its historic range in California, we compared mitochondrial cytochrome-b haplotypes of the only currently-known high-elevation population (n = 9 individuals) to samples from 3 modern lowland populations (n = 35) and historic (1911–1941) high-elevation (n = 22) and lowland (n = 7) populations. We found no significant population differentiation among the modern and historic high-elevation populations (average pairwise F _ST = 0.06), but these populations differed substantially from all modern and historic lowland populations (average pairwise F _ST = 0.52). Among lowland populations, the historic and modern Sacramento Valley populations were not significantly differentiated from one another (F _ST = −0.06), but differed significantly from recently founded populations in the San Francisco Bay region and in southern California (average pairwise F _ST = 0.42). Analysis of molecular variance indicated that 3 population groupings (mountain, Sacramento Valley, and other lowland regions) explained 45% of molecular variance (F _CT = 0.45) whereas only 4.5% of the variance was partitioned among populations within these groupings (F _SC = 0.08). These findings provide strong evidence that the native Sierra Nevada red fox has persisted in northern California. However, all nine samples from this population had the same haplotype, suggesting that several historic haplotypes may have become lost. Unidentified barriers have apparently prevented gene flow from the Sacramento Valley population to other eastern or southern populations in California. Future studies involving nuclear markers are needed to assess the origin of the Sierra Nevada red fox and to quantify levels of nuclear gene flow.     

Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the , w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in T. cordata, and on , w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in P. tremula. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.     

Diameter class structure and sprouting characteristics of a northernmost Euptelea pleiosperma population in China: Implications for conservation

Euptelea pleiosperma (Eupteleaceae) is a class III threatened tree species in China. Investigations on structure, regeneration and population dynamics is of great scientific significance to properly conserve, develop and use the resources of this species. This study targeted a small and isolated northernmost E. pleiosperma population in China, which is distributed in Meidigou of Taikuanhe Nature Reserve in Shanxi province. The study aimed at seeking the answers: (1) is the E. pleiosperma population growing or declining? (2) what is the implications of sprouting for the E. pleiosperma population? It is shown through this study that sprouting is a main trait of the population with 49.4% of main stems having sprouted stems. The population is growing rather than declining thanks to the outstanding sprouting trait of E. pleiosperma. The population occurs only in a small shady and moisture valley area most likely because a moist environment is necessary for E. pleiosperma to survive, develop and regenerate. Mainly by rapid producing abundant sprouted stems, E. pleiosperma forms itself as an edifying species and demonstrates strong competition ability with other coexistent canopy species in the limited area. The sprouting characteristics of E. pleiosperma provide a mechanism for the expansion and maintenance of its population.     

Detection of species within the Xerocomus subtomentosus complex in Europe using rDNA–ITS sequences

Identification of species within the boletoid genus Xerocomus has relied heavily upon the macromorphological features of the basidiomes. However, the phenotypic plasticity of these features has resulted in considerable confusion over the delimitation of taxa. In this study, we examined collections attributed to the X. subtomentosus complex in Europe using morphological and rDNA–ITS sequence data. In total, 45 European collections from a wide range of geographical areas and ecological conditions were included in the study. In spite of detecting considerable genetic variation, even within individual basidiomes of X. subtomentosus, molecular data, spore size, flesh colour, and the colour of the basal mycelium allow for the recognition of four distinct taxa: two correspond to X. subtomentosus (13 collections) and X. ferrugineus (20); one X. chrysonemus sp. nov. (10), to date only found in the UK, is described as new; and the existence of another taxon (two; Italy and UK) is noted but left undescribed owing to lack of material. Eight collections from North America were also included in the study, from which two taxa with a close affinity to X. ferrugineus were recognised.     

Mapping potential habitats of threatened plant species in a moist tall grassland using hyperspectral imagery

We examined the capability of hyperspectral imagery to map habitat types of under-storey plants in a moist tall grassland dominated by Phragmites australis and Miscanthus sacchariflorus, using hyperspectral remotely-sensed shoot densities of the two grasses. Our procedure (1) grouped the species using multivariate analysis and discriminated habitat types (species groups) based on P. australis and M. sacchariflorus shoot densities, (2) used estimated shoot densities from hyperspectral data to draw a habitat type map, and (3) analyzed the association of threatened species with habitat types. Our identification of four habitat types, using cluster analysis of the vegetation survey coverage data, was based on P. australis and M. sacchariflorus shoot density ratios and had an overall accuracy of 77.1% (kappa coefficient = 0.71). Linear regression models based on hyperspectral imagery band data had good accuracy in estimating P. australis and M. sacchariflorus shoot densities (adjusted R ² = 0.686 and 0.708, respectively). These results enabled us to map under-storey plant habitat types to an approximate prediction accuracy of 0.537. Among the eight threatened species we examined, four exhibited a significantly biased distribution among habitat types, indicating species-specific habitat use. These results suggest that this procedure can provide useful information on the status of potential habitats of threatened species.     

Bird community variation across <Emphasis Type="Italic">Polylepis</Emphasis> woodland fragments and matrix habitats: implications for biodiversity conservation within a high Andean landscape

The scattered and dwindling Polylepis woodlands of the high Andean global hotspot have been identified as being of particular importance to biodiversity conservation, and yet little is known of the make-up of their faunal communities, how these vary across landscapes, and how well species might tolerate matrix/edge habitats. We examined the bird communities and vegetation characteristics of Polylepis woodlands and the surrounding matrix habitats at three sites in the Cordillera Vilcanota, southern Perú (3,400–4,500 m). The vegetation structure of woodlands varied significantly across the three sites but all were dominated by two Polylepis tree species, with mossy ground cover. Matrix habitats were treeless and dominated by ground-level puna grass-steppe or boulder scree vegetation. Bird species richness and diversity, encounter rates and the number of globally-threatened and restricted-range bird species were consistently higher in the Polylepis forests, than in matrix habitat. We used canonical correspondence analysis (CCA) to identify habitat gradients across the landscape, and to classify bird species according to their association with Polylepis, the matrix or Polylepis-matrix interface. There were few matrix-restricted bird species, but around half the bird community, including fourteen threatened or restricted-range species, were Polylepis-dependant. Many of these species had very narrow niches. The Polylepis-matrix interface was dominated by species traditionally considered invasive ecological generalists. Our study illustrates the overriding importance of Polylepis interior habitats, indicating that conservation strategies for high Andean birds must focus on patch size maintenance/enlargement, enhancement of within-patch habitat quality, and efforts to safeguard connectivity of suitable habitat across what is essentially an inhospitable puna/scree matrix.     

广西石灰岩地区苦苣苔科植物一新种——北流报春苣苔

报道了广西石灰岩地区苦苣苔科报春苣苔属(Primulina Hance)1新种——北流报春苣苔(P. beiliuensis B. Pan & S. X. Huang)。该新种在形态上与黄花牛耳朵[P. lutea(Yan Liu & Y. G. Wei)Mich. Möller & A. Weber]较近,但叶宽卵形,叶基部近心形,叶缘具浅钝齿或呈浅波状齿,花冠紫色,花冠、花序梗、花梗、苞片及花萼均被紫色短柔毛而区别与后者; 分子生物学证据表明,在系统发育上与桂林小花苣苔[P. repanda var. guilinensis(W. T. Wang)Mich. Möller & A. Weber]近缘,但两者在形态学上相差较远。     

Devonohexapodus bocksbergensis is a synonym of Wingertshellicus backesi (Euarthropoda) – no evidence for marine hexapods living in the Devonian Hunsrück Sea

The Devonian Hunsrück Slate fossil Devonohexapodus bocksbergensis Haas, Waloszek & Hartenberger, 2003 has been interpreted as a stem-lineage representative of the Hexapoda, implying their marine origin and independent terrestrialisation within the ‘Atelocerata’. Devonohexapodus bocksbergensis was based on a single specimen embedded in a lateral position. Reinvestigation of that holotype and of all known specimens of the Hunsrück Slate arthropod Wingertshellicus backesi Briggs & Bartels, 2001 demonstrates that all this material represents a single species. The latter is redescribed, its taxonomic diagnosis is emended, and the name Devonohexapodus bocksbergensis is treated as a junior synonym of Wingertshellicus backesi. The phylogenetic position of W. backesi neither is that of a stem-lineage representative of Hexapoda, nor does it fall within the crown group Mandibulata. The Hunsrück Slate provides no evidence of an independent terrestrialisation within the ‘Atelocerata’ or of a marine origin of the Hexapoda.     

Macaranga and Mallotus species (Euphorbiaceae) as indicators for disturbance in the mixed lowland dipterocarp forest of East Kalimantan (Indonesia)

The indicator value (IV) of Macaranga and Mallotus species (Euphorbiaceae) for different types of disturbance in lowland dipterocarp forest was assessed by counting and identifying all individuals of species of these genera taller than 30 cm in 45 (10 m×300 m) plots at nine locations. Twelve Macaranga and nine Mallotus species were found. The main forest disturbance types (primary forest, secondary forest, selectively logged forest, forest burned once, and repeatedly burned forest used for shifting-cultivation) each had their own set of indicator species. The level of disturbance in the forest types was assessed by measuring nine forest structural parameters. The occurrence of Macaranga and Mallotus species was closely related to the level of disturbance in a forest. Most Macaranga species were characteristic of high disturbance levels, while most Mallotus species preferred intermediate to low levels of disturbance. However, both genera had species at both disturbance extremes. Using multiple regression analysis, combinations of Macaranga and Mallotus species were formed and used to predict the separate forest structural parameters and the general level of disturbance of a forest. The Macaranga and Mallotus species could be grouped into (1) primary forest ‘remnant’ species; (2) generalist pioneer species; and (3) high disturbance pioneer species.     

Development of SCAR markers distinguishing pure seedlings of the endangered species Morus boninensis from M. boninensis × M. acidosa hybrids for conservation in Bonin (Ogasawara) Islands

To support the recovery of Morusboninensis Koidz. (Moraceae), a criticallyendangered species distributed in the BoninIslands of Japan, we need to distinguishbetween pure M. boninensis and hybrid(M. boninensis × M. acidosa Griff.)seedlings within populations derived from M. boninensis mother trees. Therefore, wedeveloped three co-dominant SCAR (SequenceCharacterized Amplified Region) markers todistinguish between M. boninensis, M. acidosa and their hybrids. All threemarkers could clearly identify the species andthe hybrids, although we observed one case of aputative null allele or a backcross at onelocus (Mos0251-1). Hence, we suggest that pureand hybrid seedlings could be unambiguouslyidentified by the simultaneous use of the threemarkers. When we investigated a seedlingpopulation produced by a single mother treegrowing in the Iguma-bay population ofHaha-jima Island, the proportion of embryosformed by pollination from M. acidosapollen was about 13%. All hybrid seedlingsidentified according to the molecular markersshowed incisions in their leaves, amorphological character that has never beenobserved in pure M. boninensisindividuals. Our results suggest that M.boninensis can potentially be multiply byexcluding hybrids from the seedling pools,using incisions on the leaves as morphologicalmarkers to identify them.     

Biological properties of the endemic and threatened shrub in Iberia Vella pseudocytisus subsp. paui Gómez Campo (Cruciferae) and implications for its conservation

We present new data about chorology, seed bank, dispersal, seedling biology and population structure of the threatened endemic shrub Vella pseudocytisus subsp. paui Gómez Campo in Spain. In addition, we investigate human activities that are linked with the fine-scale distribution of the taxon to offer conservation recommendations.Currently, the plant occurs in two populations close to the city of Teruel (East central Spain), growing in an open shrubby community on patches of gypsum substrate surrounded by agricultural landscape. The plant has disappeared from two reported locations dated from early 20th Century likely due to human activities.The seed bank seems to be poor. A ballistic dispersal mechanism appears to move seeds between 0 and 4 m. We did not find a clear seasonal pattern of seedlings emergence although spring cohorts are the largest. Also, this period was where leaf seedling growth was more prominent. During the 3 years of our survey, the cumulative mortality was 94.2% of all seedlings plants. Plants are estimated to remain in the seedling stage for approximately 5 years. The demographic structure of both populations shows no apparent gaps in the distribution of size classes. Adult plants greater than 200 dm³ represent the most important stage for seed production and persistence.We point out two key factors for conservation purposes, first the weakness of some of the plant biological properties for natural establishment and second its ability to colonise artificial environments. We propose a mitigation of clonal spread in those sites and the protection of juvenile stages in more natural locations.     

Development of microsatellite markers for <Emphasis Type="Italic">Echinops setifer</Emphasis> (Asteraceae), an endangered grassland plant species in Japan

Echinops setifer is an endangered grassland plant species in Japan. We isolated and characterized nine microsatellite loci in this species. The number of alleles ranged from 3 to 7 and expected heterozygosities from 0.108 to 0.761. These markers described here will be useful for investigating the genetic diversity, genetic structure and gene flow, and planning for conservation of E. setifer.     

Gdf6a is required for the initiation of dorsal–ventral retinal patterning and lens development

Dorsal–ventral patterning of the vertebrate retina is essential for accurate topographic mapping of retinal ganglion cell (RGC) axons to visual processing centers. Bone morphogenetic protein (Bmp) growth factors regulate dorsal retinal identity in vertebrate models, but the developmental timing of this signaling and the relative roles of individual Bmps remain unclear. In this study, we investigate the functions of two zebrafish Bmps, Gdf6a and Bmp4, during initiation of dorsal retinal identity, and subsequently during lens differentiation. Knockdown of zebrafish Gdf6a blocks initiation of retinal Smad phosphorylation and dorsal marker expression, while knockdown of Bmp4 produces no discernable retinal phenotype. These data, combined with analyses of embryos ectopically expressing Bmps, demonstrate that Gdf6a is necessary and sufficient for initiation of dorsal retinal identity. We note a profound expansion of ventral retinal identity in gdf6a morphants, demonstrating that dorsal BMP signaling antagonizes ventral marker expression. Finally, we demonstrate a role for Gdf6a in non-neural ocular tissues. Knockdown of Gdf6a leads to defects in lens-specific gene expression, and when combined with Bmp signaling inhibitors, disrupts lens fiber cell differentiation. Taken together, these data indicate that Gdf6a initiates dorsal retinal patterning independent of Bmp4, and regulates lens differentiation.     

Blooms of Pseudo-nitzschia and domoic acid in the San Pedro Channel and Los Angeles harbor areas of the Southern California Bight, 2003–2004

Abundances of Pseudo-nitzschia spp. and concentrations of particulate domoic acid (DA) were determined in the Southern California Bight (SCB) along the coasts of Los Angeles and Orange Counties during spring and summer of 2003 and 2004. At least 1500 km² were affected by a toxic event in May/June of 2003 when some of the highest particulate DA concentrations reported for US coastal waters were measured inside the Los Angeles harbor (12.7 μg DA L⁻¹). Particulate DA levels were an order of magnitude lower in spring of 2004 (February and March), but DA concentrations per cell at several sampling stations during 2004 exceeded previously reported maxima for natural populations of Pseudo-nitzschia (mean = 24 pg DA cell⁻¹, range = 0–117 pg DA cell⁻¹). Pseudo-nitzschia australis dominated the Pseudo-nitzschia assemblage in spring 2004. Overall, DA-poisoning was implicated in >1400 mammal stranding incidents within the SCB during 2003 and 2004. Ancillary physical and chemical data obtained during our regional surveys in 2004 revealed that Pseudo-nitzschia abundances, particulate DA and cellular DA concentrations were inversely correlated with concentrations of silicic acid, nitrogen and phosphate, and to specific nutrient ratios. Particulate DA was detected in sediment traps deployed at 550 and 800 m depth during spring of 2004 (0.29–7.6 μg DA (g sediment dry weight)⁻¹). The highest DA concentration in the traps was measured within 1 week of dramatic decreases in the abundances of Pseudo-nitzschia in surface waters. To our knowledge these are the deepest sediment trap collections from which DA has been detected. Sinking of the spring Pseudo-nitzschia bloom may constitute a potentially important link between DA production in surface waters and benthic communities in the coastal ocean near Los Angeles. Our study indicates that toxic blooms of Pseudo-nitzschia are a recurring phenomenon along one of the most densely populated coastal stretches of the SCB and that the severity and magnitude of these events can be comparable to or greater than these events in other geographical regions affected by domoic acid.     

The Jungle Cat <Emphasis Type="Italic">Felis chaus</Emphasis> in Indochina: a threatened population of a widespread and adaptable species

The Jungle Cat Felis chaus is widespread in India and neighbouring countries but is known by only one historical specimen from Cambodia, Laos or Vietnam (Indochina), widely published as from Vietnam, but in fact from Cambodia. All but two of the recent Indochinese records come from extensive natural lowland habitat dominated by deciduous dipterocarp forest in northeast Cambodia. The species probably occurred more widely in Indochina, largely through additional use of secondary habitats, where hunting pressure is now very heavy. Suggestions of decline in Indochina are corroborated by more conclusive evidence from Thailand. In Indochina, all other small and medium-sized cats are recorded much more frequently than Jungle Cat: closed evergreen forest supports source populations of them, but there is no evidence that Jungle Cat uses extensively such forest. The open forests of northern and eastern Cambodia are highly significant for conserving threatened biodiversity, notably large waterbirds, vultures and ungulates, groups where species formerly widespread across Indochina have contracted in range and declined steeply. These taxa were better collected than small cats and it seems plausible that Jungle Cat showed a similar change. Jungle Cat conservation in Indochina needs extensive habitat retention with intensive anti-poaching activities, because suitable habitat is easily accessible to hunters. The habitat adaptability shown elsewhere by Jungle Cat could allow a much healthier regional conservation status if hunting (including trapping) can be greatly reduced in scrub and agricultural habitats, but changing culturally ingrained hunting practices will take a long time.     

The ‘home advantage’ is necessary for a full winner effect and changes in post-encounter testosterone

Winning aggressive contests can both enhance future winning ability and change post-encounter hormones; however, it remains unclear if the context of a fight also influences such winner effects and hormone changes. We investigated this issue by using California mice (Peromyscus californicus) to test if the effect of residency status is necessary to improve future winning ability and alter post-encounter hormones. Male mice were subjected to an aggressive contest and their blood was collected 45 min after the fight. Upon contest initiation, focal mice had a ‘home advantage’ and three prior winning experiences, only one of these factors, or neither factor. Only individuals with a ‘home advantage’ and prior winning experience showed a full winner effect. Post-encounter changes in testosterone and progesterone resulted from an interaction between residency status and winning experience. These data indicate that in male California mice a ‘home advantage’ is necessary to produce the full winner effect and that residency status helps regulate inter-individual variation in the expression of post-encounter testosterone pulses and progesterone changes. Furthermore, these respective behavioral and physiological phenomena might be modulated in a context-specific manner, in part by the surrounding physical environment.     

Infectivity of Chlorella species for the ciliate Paramecium bursaria is not based on sugar residues of their cell wall components, but on their ability to localize beneath the host cell membrane after escaping from the host digestive vacuole in the early infection process

Summary. Paramecium bursaria cells harbor several hundred symbiotic algae in their cytoplasm. Algae-free cells can be reinfected with algae isolated from algae-bearing cells or cultivated Chlorella species through the digestive vacuoles. To determine the relationship between the infectivity of various Chlorella species and the nature of their cell wall components, algae-free P. bursaria cells were mixed with 15 strains of cultivated Chlorella species and observed for the establishment of endosymbiosis at 1 h and 3 weeks after mixing. Only 2 free-living algal strains, C. sorokiniana C-212 and C. kessleri C-531, were maintained in the host cells, whereas free-living C. sorokiniana C-43, C. kessleri C-208, C. vulgaris C-27, C. ellipsoidea C-87 and C-542, C. saccharophila C-183 and C-169, C. fusca var. vacuolata C-104 and C-28, C. zofingiensis C-111, and C. protothecoides C-150 and C-206 and the cultivated symbiotic Chlorella sp. strain C-201 derived from Spongilla fluviatilis could not be maintained. These infection-incapable strains could escape from the host digestive vacuole but failed to localize beneath the host cell membrane and were eventually digested. Labeling of their cell walls with Alexa Fluor 488-conjugated wheat germ agglutinin, GS-II, or concanavalin A, with or without pretreatment with 0.4 N NaOH, showed no relationship between their infectivity and the stainability with these lectins. Our results indicate that the infectivity of Chlorella species for P. bursaria is not based on the sugar residues on their cell wall and on the alkali-insoluble part of the cell wall components, but on their ability to localize just beneath the host cell membrane after escaping from the host digestive vacuole. Correspondence and reprints: Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8512, Japan.