共查询到20条相似文献,搜索用时 31 毫秒
1.
Violaine Nicolas Patrick Barrière Audrey Tapiero Marc Colyn 《Biodiversity and Conservation》2009,18(8):2043-2061
The aim of our study was to compare the shrew community diversity and structure in gradients of tropical forest degradation
and restoration. Four plots within each of six habitats of the Ziama Biosphere Reserve were surveyed, including primary forest,
secondary forest, cultivated fields, recently (less than 3 years) abandoned fields, young (10–12 years) forest restoration
plots, and old (34 years) restoration plots. From August to November 2003, we pitfall-trapped 2,509 shrews representing 11
species. Shrew species richness and composition was similar in the six habitat surveyed, while shrew species abundance varied
between habitats. Canopy height and cover, density of stems and trees and understorey density were shown to constitute important
parameters influencing the abundance of several shrew species. After clear-cutting, restoration of key attributes of the forest
vegetation structure was possible in 10–34 years, either by natural regeneration or by planting of seedlings. The relative
abundance of most shrew species was similar between restoring forest (i.e., young restoration plots or fallows) and primary
forest. Considering the advantages and disadvantages of these two methods of forest restoration, one of the most suitable
management practices to restore forest while preserving shrew biodiversity could be to perform an alternation of native seedling
plantation lines and fallows. 相似文献
2.
A. M. Jagodziński P. Horodecki K. Rawlik M. K. Dyderski 《Plant biology (Stuttgart, Germany)》2017,19(4):571-583
- One of the most important threats to peatland ecosystems is drainage, resulting in encroachment of woody species. Our main aim was to check which features – overstorey or understorey vegetation – are more important for shaping the seedling bank of pioneer trees colonising peatlands (Pinus sylvestris and Betula pubescens). We hypothesised that tree stand parameters will be more important predictors of natural regeneration density than understorey vegetation parameters, and the former will be negatively correlated with species diversity and richness and also with functional richness and functional dispersion, which indicate a high level of habitat filtering.
- The study was conducted in the ‘Zielone Bagna’ nature reserve (NW Poland). We assessed the structure of tree stands and natural regeneration (of B. pubescens and P. sylvestris) and vegetation species composition. Random forest and DCA were applied to assess relationships between variables studied.
- Understorey vegetation traits affected tree seedling density (up to 0.5‐m height) more than tree stand traits. Density of older seedlings depended more on tree stand traits. We did not find statistically significant relationships between natural regeneration densities and functional diversity components, except for functional richness, which was positively correlated with density of the youngest tree seedlings.
- Seedling densities were higher in plots with lower functional dispersion and functional divergence, which indicated that habitat filtering is more important than competition. Presence of an abundant seedling bank is crucial for the process of woody species encroachment on drained peatlands, thus its dynamics should be monitored in protected areas.
3.
Juli G. Pausas 《植被学杂志》1994,5(4):517-524
Abstract. Species richness was studied in the understorey of natural Pinus sylvestris forest in the eastern Pyrenees. Understorey plant species were grouped in three structural groups as woody species, herbs and mosses. The response curves of total species richness and species richness of each structural group were fitted against environmental and stand-structural parameters, using Generalized Linear Models. The results suggested that, to predict species richness, environmental parameters were more important than tree-canopy structural parameters, in particular incoming radiation and soil nutrient concentration. The species richness response curve was often humped in relation to soil nutrient concentration. Different patterns of species richness were found for each structural group. 相似文献
4.
K. Serge Bobo Matthias Waltert N. Moses Sainge John Njokagbor Heleen Fermon Michael Mühlenberg 《Biodiversity and Conservation》2006,15(13):4097-4117
Vegetation surveys were carried out at 24 sampling stations distributed over four land use types, namely near-primary forest,
secondary forest, agroforestry systems and annual crop lands in the northeastern part of the Korup region, Cameroon, to assess
the impact of forest conversion on trees and understorey plants. Tree species richness decreased significantly with increasing
level of habitat modification, being highest and almost equal in secondary and near-primary forests. Understorey plant species
richness was significantly higher in annual crop lands than in other land use types. The four land use types differed in tree
and understorey plant species composition, the difference being smaller among natural forests. Tree and understorey plant
density differed significantly between habitat types. Density was strongly correlated with species richness, both for trees
and understorey plants. Five tree and 15 understorey plant species showed significant responses to habitat. A 90% average
drop in tree basal area from forest to farmland was registered. Our findings support the view that agroforestry systems with
natural shade trees can serve to protect many forest species, but that especially annual crop lands could be redesigned to
improve biodiversity conservation in agricultural landscapes of tropical rainforest regions. 相似文献
5.
Micaela Medina Magali Prez Flores Juan Francisco Goya Paula Ines Campanello Martin Alcides Pinazo Luis Javier Ritter Marcelo Fabian Arturi 《Austral ecology》2020,45(2):229-239
Deforestation is a global process that has strongly affected the Atlantic Forest in South America, which has been recognised as a threatened biodiversity hotspot. An important proportion of deforested areas were converted to forest plantations. Araucaria angustifolia is a native tree to the Atlantic Forest, which has been largely exploited for wood production and is currently cultivated in commercial plantations. An important question is to what extent such native tree plantations can be managed to reduce biodiversity loss in a highly diverse and vulnerable forest region . We evaluated the effect of stand age, stand basal area, as a measure of stand density, and time since last logging on the density and richness of native tree regeneration in planted araucaria stands that were successively logged over 60 years, as well as the differences between successional groups in the response of plant density to stand variables. We also compared native tree species richness in planted araucaria stands to neighbouring native forest. Species richness was 71 in the planted stands (27 ha sampled) and 82 in native forest (18 ha sampled) which approximate the range of variation in species richness found in the native forests of the study area. The total abundance and species richness of native trees increased with stand age and time since last logging, but ecological groups differed in their response to such variables. Early secondary trees increased in abundance with stand age 3–8 times faster than climax or late secondary trees. Thus, the change in species composition is expected to continue for a long term. The difference in species richness between native forest and planted stands might be mainly explained by the difference in plant density. Therefore, species richness in plantations can contribute to local native tree diversity if practices that increase native tree density are implemented. 相似文献
6.
Tree species composition and stand structural complexity are valuable indicators of sustainable forest management. This article
aims to investigate the relative influence of forest overstorey composition and structural attributes on understorey composition
and diversity, taking into account also site characteristics and broad-scale environmental variables. We sampled vascular
plant species composition and forest structure in 132 plots in the Cilento and Vallo di Diano National Park (southern Italy).
Spearman’s non-parametric correlation coefficients were calculated between overstorey and understorey diversity indices, beech
percentage, and altitude and environmental indices. A complete partitioning of the variation in understorey composition was
then performed through canonical correspondence analysis considering four sets of variables: (1) overstorey composition, (2)
structural attributes, (3) topography, and (4) landscape abiotic variables. Finally, we constructed a regression tree analysis
of understorey species richness using the same explanatory variables. Understorey diversity indices were positively correlated
with overstorey diversity indices and with environmental indices (i.e., light and soil heterogeneity). Overstorey and understorey
diversity indices were negatively correlated with both altitude and the dominance of beech in the overstorey. Compositional
variation was due primarily to overstorey composition and secondarily to structural attributes. Regression tree analysis revealed
that altitude, overstorey species richness, and structural attributes play an important role in determining understorey species
richness. According to our results, understorey composition and diversity are strongly related to overstorey composition and
structural attributes. Indeed, the latter proved to be effective indicators of understorey characteristics in the study area. 相似文献
7.
The goal of restoration is to accelerate ecosystem recovery, but in ecosystems that naturally regenerate rapidly restoration techniques need to be selected carefully to facilitate rather than impede natural recovery. We compared the effects of five restoration techniques, such as plowing the soil, removing grasses, adding forest litter, seeding, and planting nursery‐growing seedlings, on the regeneration of seasonal deciduous forest trees in four abandoned pastures in central Brazil. We monitored all woody stems immediately prior to treatments and again 14 months after the treatments. We recorded an average of 16,663 tree stems per hectare and a total of 83 species before implementing treatments. Planting strongly increased species richness; adding litter and seeding had weaker positive effects on richness; and plowing and grass removal had no effect. Plowing substantially reduced the density of naturally established stems. Despite the high survival of planted seedlings, stem density in planting treatments did not change because the tractor and digging holes to plant seedlings caused mortality of naturally regenerating seedlings. Tree stems grew more in the grass release plots than in the control plots. Our results suggest that early succession of seasonal deciduous forest in pastures in the region studied does not need to be stimulated once the perturbation is stopped and that intensive restoration efforts may actually slow recovery. We recommend only enrichment planting of seedlings that are not able to resprout. 相似文献
8.
Kathleen S. Knight Jacek Oleksyn rzej M. Jagodzinski Peter B. Reich Marek Kasprowicz 《Diversity & distributions》2008,14(4):666-675
The North American woody species, Prunus serotina Ehrh., is an aggressive invader of forest understories in Europe. To better understand the plant invasion process, we assessed understorey plants and Prunus serotina seedlings that have colonized a 35-year-old replicated common-garden experiment of 14 tree species in south-western Poland. The density and size of established (> 1 year old) P. serotina seedlings varied among overstorey species and were related to variation in light availability and attributes of the understorey layer. In a multiple regression analysis, the density of established P. serotina seedlings was positively correlated with light availability and understorey species richness and negatively correlated with understorey species cover. These results suggest that woody invader success is adversely affected by overstorey shading and understorey competition for resources. Simultaneously, however, invader success may generally be positively associated with understorey species richness because both native and invasive plant colonization respond similarly to environmental conditions, including those influenced by overstorey tree species. Identification of characteristics of forests that increase their susceptibility to invasion may allow managers to target efforts to detect invasives and to restore forests to states that may be less invasible. 相似文献
9.
Passive restoration methods offer great promise for tropical regions where resources are limited but the success of such efforts can be variable. Using trait-based theory, we investigated the likely trajectories of passive restoration efforts in a degraded Nigerian montane forest system recently protected from burning and cattle grazing. We quantified the density, species richness, and functional trait dispersion of dispersed seeds and seedling communities at increasing distances from the forest edge. We then determined which plant traits are responsible for colonisation by quantifying changes in functional-trait dispersion and relative frequencies of dispersal-linked traits with increasing distance from the forest. We found a rapid decrease in density and species richness, and significant species turnover in both seeds and seedlings just beyond the forest edge. This was mirrored by a significant decline in functional-trait dispersion and a shift in the relative frequencies of dispersal-linked traits. These findings suggest that the reassembly of plant communities adjacent to remnant forest is dependent on functional traits present in these remnant source populations, providing support for the incorporation of trait-based theory in restoration management. 相似文献
10.
Andrew G. Baker Claudia Catterall Kirsten Benkendorff Rod J. Fensham 《Austral ecology》2020,45(5):557-571
The expansion of rainforest pioneer trees into long‐unburnt open forests has become increasingly widespread across high rainfall regions of Australia. Increasing tree cover can limit resource availability for understorey plant communities and reduce understorey diversity. However, it remains unclear if sclerophyll and rainforest trees differ in their competitive exclusion of understory plant communities, which contain most of the floristic diversity of open forests. Here, we examine dry open forest across contrasting fire histories (burnt and unburnt) and levels of rainforest invasion (sclerophyll or rainforest midstorey) to hindcast changes in understorey plant density, richness and composition. The influence of these treatments and other site variables (midstorey structure, midstorey composition and soil parameters) on understorey plant communities were all examined. This study is the first to demonstrate significantly greater losses of understorey species richness, particularly of dry open‐forest specialists, under an invading rainforest midstorey compared to a typical sclerophyll midstorey. Rainforest pioneers displaced over half of the understorey plant species, and reduced ground cover and density of dry forest specialists by ~90%. Significant understorey declines also occurred with increased sclerophyll midstorey cover following fire exclusion, although losses were typically less than half that of rainforest‐invaded sites over the same period. Understorey declines were closely related to leaf area index and basal area of rainforest and wattle trees, suggesting competitive exclusion through shading and potentially belowground competition for water. Around 20% of displaced species lacked any capacity for population recovery, while transient seed banks or distance‐limited dispersal may hinder recovery for a further 68%. We conclude that rainforest invasion leads to significant declines in understorey plant diversity and cover in open forests. To avoid elimination of local native plant populations in open forests, fires should occur with sufficient frequency to prevent overstorey cover from reaching a level where shade‐intolerant species fail to thrive. 相似文献
11.
Balaguer L Arroyo-García R Jiménez P Jiménez MD Villegas L Cordero I Rubio de Casas R Fernández-Delgado R Ron ME Manrique E Vargas P Cano E Pueyo JJ Aronson J 《PloS one》2011,6(8):e23004
Background
In the Peruvian Coastal Desert, an archipelago of fog oases, locally called lomas, are centers of biodiversity and of past human activity. Fog interception by a tree canopy, dominated by the legume tree tara (Caesalpinia spinosa), enables the occurrence in the Atiquipa lomas (southern Peru) of an environmental island with a diverse flora and high productivity. Although this forest provides essential services to the local population, it has suffered 90% anthropogenic reduction in area. Restoration efforts are now getting under way, including discussion as to the most appropriate reference ecosystem to use.Methodology/Principal Findings
Genetic diversity of tara was studied in the Atiquipa population and over a wide geographical and ecological range. Neither exclusive plastid haplotypes to loma formations nor clear geographical structuring of the genetic diversity was found. Photosynthetic performance and growth of seedlings naturally recruited in remnant patches of loma forest were compared with those of seedlings recruited or planted in the adjacent deforested area. Despite the greater water and nitrogen availability under tree canopy, growth of forest seedlings did not differ from that of those recruited into the deforested area, and was lower than that of planted seedlings. Tara seedlings exhibited tight stomatal control of photosynthesis, and a structural photoprotection by leaflet closure. These drought-avoiding mechanisms did not optimize seedling performance under the conditions produced by forest interception of fog moisture.Conclusions/Significance
Both weak geographic partitioning of genetic variation and lack of physiological specialization of seedlings to the forest water regime strongly suggest that tara was introduced to lomas by humans. Therefore, the most diverse fragment of lomas is the result of landscape management and resource use by pre-Columbian cultures. We argue that an appropriate reference ecosystem for ecological restoration of lomas should include sustainable agroforestry practices that emulate the outcomes of ancient uses. 相似文献12.
Mycorrhizal fungi are important symbionts for the majority of plant species, but their role in determining the susceptibility
of habitat to plant invasion is poorly understood. Hieracium lepidulum is an arbuscular mycorrhizal herb, currently invading the understorey of ectomycorrhizal Nothofagus solandri var. cliffortioides (mountain beech) forest in New Zealand. Mountain beech is solely ectomycorrhizal, and other plant species within the understorey
occur sporadically. Hieracium has been shown to establish preferentially in microsites with higher plant species richness at a scale of less than 1 m2 within mountain beech forest, and we tested the hypothesis that more diverse microsites (<1 m2) are associated with higher levels of arbuscular mycorrhizal fungal (AMF) inoculum. We found low levels of AMF inoculum across
all microsites, and over a third of samples contained no inoculum at all. Higher vascular-plant species richness (but not
biomass) was associated with higher AMF spore densities in field soil, and greater AMF colonization of H. lepidulum seedlings in a bioassay. Absence of AMF inoculum from much of the soil and the positive association of inoculum potential
with species richness provide a potential mechanism for the establishment of a positive diversity–invasibility relationship
in the mountain beech forest. 相似文献
13.
Oisín F. McD. Sweeney Mark W. Wilson Sandra Irwin Thomas C. Kelly John O’Halloran 《Biodiversity and Conservation》2010,19(8):2329-2342
This study compared the bird assemblages of native semi-natural woodlands and non-native Sitka spruce (Picea sitchensis) plantations in Ireland to identify what vegetation variables most influenced birds and to identify management targets in
plantations to maximise future bird conservation. Point counts were conducted in 10 Oak (Quercus spp.) and 10 Ash (Fraxinus excelsior) native woodlands and in five Mid-rotation (20–30 years old) and five Mature (30–50 years old) Sitka spruce plantations.
Ordination was used to characterise woodland types according to their constituent bird species. Total bird density (calculated
using Distance software) and species richness were assessed for the different woodland types. Oak and Ash woodland bird assemblages were
separated from Mid-rotation and Mature plantations by the ordination. There was no difference in total bird density between
any of the woodland types. Oak woodlands had significantly higher species richness than either Mid-rotation or Mature Sitka
spruce plantations. Ash had higher species richness than Mature Sitka spruce plantations. Understorey vegetation was negatively
associated with total bird density, which also varied with survey year. Understorey vegetation was positively associated with
species richness. Reasons for the relationships between vegetation and bird assemblages are discussed. Management should seek
to increase shrub and understorey vegetation in the Mid-rotation phase to improve the contribution of plantations to bird
conservation. 相似文献
14.
Rebecca J. Cole Karen D. Holl Rakan A. Zahawi Philipp Wickey Alan R. Townsend 《Ecology and evolution》2016,6(15):5158-5168
Soil and litter arthropods represent a large proportion of tropical biodiversity and perform important ecosystem functions, but little is known about the efficacy of different tropical forest restoration strategies in facilitating their recovery in degraded habitats. We sampled arthropods in four 7‐ to 8‐year‐old restoration treatments and in nearby reference forests. Sampling was conducted during the wet and dry seasons using extractions from litter and pitfall samples. Restoration treatments were replicated in 50 × 50‐m plots in four former pasture sites in southern Costa Rica: plantation – trees planted throughout the plot; applied nucleation/islands – trees planted in patches of different sizes; and natural regeneration – no tree planting. Arthropod abundance, measures of richness and diversity, and a number of functional groups were greater in the island treatment than in natural regeneration or plantation treatments and, in many cases, were similar to reference forest. Litter and pitfall morphospecies and functional group composition in all three restoration treatments were significantly different than reference sites, but island and plantation treatments showed more recovery than natural regeneration. Abundance and functional group diversity showed a much greater degree of recovery than community composition. Synthesis and applications: The less resource‐intensive restoration strategy of planting tree islands was more effective than tree plantations in restoring arthropod abundance, richness, and functional diversity. None of the restoration strategies, however, resulted in similar community composition as reference forest after 8 years of recovery, highlighting the slow rate of recovery of arthropod communities after disturbance, and underscoring the importance of conservation of remnant forests in fragmented landscapes. 相似文献
15.
土壤螨类群落监测是生态恢复监测的重要内容之一,土壤螨类群落结构的变化可以反映退化环境生态恢复的效果.2014年按季节对贵州省毕节市朝营小流域中度石漠化生态修复区的土壤螨类进行调查,并与原生栓皮栎林和中度石漠化区的土壤螨类群落结构进行对比.结果表明: 中度石漠化生态修复区土壤螨类计有3目35科58属,以单翼甲螨属和威单翼甲螨属为优势属;土壤上层具有较高的属数、个体数量和个体密度;捕食性革螨群落以r对策者为主,甲螨群落结构以有翅孔背甲螨型(P型)为主.中度石漠化生态恢复区土壤螨类个体数量与个体密度分别高于中度石漠化区和栓皮栎林,属数、多样性指数和丰富度指数分别高于中度石漠化区,但低于栓皮栎林;该区在对照区基础上恢复和新增螨类共32属,占该区螨类总属数的55.2%.在研究区,螨类属数与土壤速效钾含量、丰富度指数与有机碳含量具有显著相关性,个体数量、个体密度、多样性指数、丰富度指数分别与速效钾含量以及丰富度指数与有机质含量具有显著相关性.表明石漠化的植被修复有利于土壤螨类群落结构的恢复和土壤环境的改善,且螨类优势类群、新增类群和恢复类群能对环境变化进行较好地指示,但土壤螨类群落结构与修复区植被和土壤理化因子变化的关系还有待进一步研究. 相似文献
16.
We used a long‐term fire experiment in south‐east Queensland, Australia, to determine the effects of frequent prescribed burning and fire exclusion on understorey vegetation (<7.5 m) richness and density in Eucalyptus pilularis forest. Our study provided a point in time assessment of the standing vegetation and soil‐stored vegetation at two experimental sites with treatments of biennial burning, quadrennial burning since 1971–1972 and no burning since 1969. Vegetation composition, density and richness of certain plant groups in the standing and soil‐stored vegetation were influenced by fire treatments. The density of resprouting plants <3 m in height was higher in the biennially burnt treatment than in the unburnt treatment, but resprouters 3–7.5 m in height were absent from the biennial burning treatment. Obligate seeder richness and density in the standing vegetation was not significantly influenced by the fire treatments, but richness of this plant group in the seed bank was higher in the quadrennial treatment at one site and in the long unburnt treatment at the other site. Long unburnt treatments had an understorey of rainforest species, while biennial burning at one site and quadrennial burning at the other site were associated with greater standing grass density relative to the unburnt treatment. This difference in vegetation composition due to fire regime potentially influences the flammability of the standing understorey vegetation. Significant interactions between fire regime and site, apparent in the standing and soil‐stored vegetation, demonstrate the high degree of natural variability in vegetation community responses to fire regimes. 相似文献
17.
Bradley J. Cosentino Robert L. Schooley Brandon T. Bestelmeyer Alison J. McCarthy Kevin Sierzega 《Molecular ecology》2015,24(24):6120-6133
Genetic founder effects are often expected when animals colonize restored habitat in fragmented landscapes, but empirical data on genetic responses to restoration are limited. We examined the genetic response of banner‐tailed kangaroo rats (Dipodomys spectabilis) to landscape‐scale grassland restoration in the Chihuahuan Desert of New Mexico, USA. Dipodomys spectabilis is a grassland specialist and keystone species. At sites treated with herbicide to remove shrubs, colonization by D. spectabilis is slow and populations persist at low density for ≥10 years (≥6 generations). Persistence at low density and low gene flow may cause strong founder effects. We compared genetic structure of D. spectabilis populations between treated sites and remnant grasslands, and we examined how the genetic response to restoration depended on treatment age, area, and connectivity to source populations. Allelic richness and heterozygosity were similar between treated sites and remnant grasslands. Allelic richness at treated sites was greatest early in the restoration trajectory, and genetic divergence did not differ between recently colonized and established populations. These results indicated that founder effects during colonization of treated sites were weak or absent. Moreover, our results suggested founder effects were not mitigated by treatment area or connectivity. Dispersal is negatively density‐dependent in D. spectabilis, and we hypothesize that high gene flow may occur early in the restoration trajectory when density is low. Our study shows genetic diversity can be recovered more rapidly than demographic components of populations after habitat restoration and that founder effects are not inevitable for animals colonizing restored habitat in fragmented landscapes. 相似文献
18.
Collins Jared Amwatta Mullah Kari Klanderud Ørjan Totland Bernard Kigomo 《African Journal of Ecology》2014,52(1):77-87
In recent years, there have been considerable efforts to restore degraded tropical montane forests through active restoration using indigenous tree species. However, little is known about how these species used for restoration influence other species. In this study, two potential restoration species, Albizia gummifera and Neoboutonia macrocalyx, are investigated with regard to the relationship between their density and the abundance and richness of other plant species. The study was conducted in a degraded forest consisting of disturbed transition zones and secondary forest. Our results show positive relationships between the density of A. gummifera and the abundance of tree seedling and sapling richness in the transition zones and in the secondary forest. Shrub richness was negatively related to the density of A. gummifera. Abundance and richness of tree saplings and shrubs were positively related to N. macrocalyx density both in the transition zones and in the secondary forest. Herb species richness declined with N. macrocalyx density in the transition zones but increased with N. macrocalyx density in the secondary forest. The positive relationships between the density of the two tree species and species richness of other woody species suggest that both A. gummifera and N. macrocalyx can be suitable for active restoration of degraded mountain forests within their natural range. 相似文献
19.
Non-native plant species richness may be either negatively or positively correlated with native species due to differences
in resource availability, propagule pressure or the scale of vegetation sampling. We investigated the relationships between
these factors and both native and non-native plant species at 12 mainland and island forested sites in southeastern Ontario,
Canada. In general, the presence of non-native species was limited: <20% of all species at a site were non-native and non-native
species cover was <4% m−2 at 11 of the 12 sites. Non-native species were always positively correlated with native species, regardless of spatial scale
and whether islands were sampled. Additionally, islands had a greater abundance of non-native species. Non-native species
richness across mainland sites was significantly negatively correlated with mean shape index, a measure of the ratio of forest
edge to area, and positively correlated with the mean distance to the nearest forest patch. Other factors associated with
disturbance and propagule pressure in northeastern North America forests, including human land use, white-tailed deer populations,
understorey light, and soil nitrogen, did not explain non-native richness nor cover better than the null models. Our results
suggest that management strategies for controlling non-native plant invasions should aim to reduce the propagule pressure
associated with human activities, and maximize the connectivity of forest habitats to benefit more poorly dispersed native
species. 相似文献
20.
Liba Pejchar Travis Gallo Mevin B. Hooten Gretchen C. Daily 《Diversity & distributions》2018,24(6):811-819