Use of detrended correspondence analysis to evaluate factors controlling spatial distribution of benthic insects

Detrended correspondence analysis (DCA) was evaluated for its effectiveness in displaying factors controlling the spatial distribution of benthic insects in an oligotrophic stream where an experimental gradient (copper) that selectively affects population abundances was imposed. DCA proved to be highly sensitive to differences among samples and consistently provided ecologically meaningful species ordinations.Seasonality of taxa was the major gradient displayed by DCA prior to copper exposure when data for all sampling dates were included. Sensitivity of taxa to copper was a more important factor affecting community structure than was seasonality during periods of continuous exposure to copper (2.5 to 15 µg l^-1 Cu_T; approximately 12 to 75 ng l^-1 Cu²⁺. When pre-dose data for each sampling date were ordinated independently, substratum composition and biological interactions were the major gradients displayed in species ordinations. During periods of exposure, sensitivity of taxa to copper was the primary gradient. This gradient also reflected a generally greater sensitivity to copper of herbivorous than of detritivorous or predatory benthic insects. DCA revealed the persistence, eleven months after dosing ceased, of differences in community structure between the control and high treatment (5 and 10 µg l^-1 Cu_T) sections. Differences between sections were not evident on this sampling date from total biomass or total density (numerical) estimates.     

Effects of sample distribution along gradients on eigenvector ordination

In general, disproportionately heavy sampling of the ends of a gradient increases the interpretability of eigenvector ordinations. More specifically, correspondence analysis (CA) and detrended correspondence analysis (DCA) best reproduce the original positions of samples in simulated coenoclines when samples are clustered toward the ends of the axis. Principal components analysis (PCA) reproduces the original sample positions less well than either CA or DCA and shows no improvement as samples are increasingly clustered toward the ends of the axis. PCA and CA show less curvature of one dimensional data into the second axis when sampling favors the ends of the axis.     

Trends in structural compositional attributes of dune-interdune vegetation and their edaphic relations in the Indian desert

Vegetation of 127 sites on different aspects of dune-interdunes in the Indian Thar Desert was classified using TWINSPAN. TWINSPAN groupings of sites separated better vegetated dunes of the northeast form the poorly vegetated dunes of the northwest and the southwest. Of the different ordinations using noncentred, centred and centred and standardized principal component analysis, reciprocal averaging and detrended correspondence analysis (DCA), the site and species classes in DCA correlated well with ten edaphic and ten vegetational attributes of each site. Strong correlation of vegetation groupings with soil texture, moisture holding capacity and low correlation with pH and electrical conductivity revealed the possible importance of soil physical properties in affecting vegetation composition.The 11 species classes in TWINSPAN were regrouped into 18 species classes in DCA, which separated highly frequent species from those of less and least frequent species. Based on dominance-diversity attributes, Calligonum polygonoides-Lasiurus sindicus was brought out as bioedaphic climax stage. Correlation of ordination scores in different site groupings with vegetational attributes showed specific trends: From the zero of x, y and z axes to their maximum, the ordination scores of grasses and browse species declined while score of spinous species increased. The sites near the origin of the x, y and z axes were therefore least degraded and those at or near the maximum of x, y & z axes were most disturbed as was confirmed by the dominance diversity trends. Thus trends of compositional and functional attributes of vegetation of sites in different groupings helped in inferring a site's degradation status.Abbreviations RIV Relative importance value - TWINSPAN Two Way Indicator Species Analysis - PCA Principal Component Analysis - RA Reciprocal Averaging - DCA Detrended Correspondence Analysis - EC Electrical Conductivity - WHC Water Holding Capacity - BD Beta Diversity - DC Dominance Concentration - WWS Windward slope of the dune - LWS Leeward slope of the dune - ID Interdune Nomenclature: Bhandari, (1990)     

Species and plant community distribution in a Mojave-Great Basin desert transition

Multivariate analyses were used to describe the vegetation characteristics of a transition from lowelevation Mojave desert to higher-elevation Great Basin desert. Vegetation data used were from Plutonium Valley in the Nevada Test Site. Data from forty nine releves were analyzed with two classifications (two-day indicator analysis or TWINSPAN and unweighted paried group cluster analysis or CLUSTER). Three ordinations, reciprocal averaging (RA), detrended reciprocal averaging (DCA) and non-metric multidimensional scaling (MNDS), were also used. A rotational correlation analysis was used to determine the vector direction of environmental gradients that correlate best with ordination results. Only token correspondence was found between multivariate classes generated by TWINSPAN and CLUSTER, and seven classes (plant communities) identified from field reconnaissance. The latter seven communities were based on differences in dominant species. Distribution of the vegetation was related more to beta diversity than alpha diversity. Individual species were much less diagnostic than the amount of plant cover, groups or guilds of species or differences in elevation and steepness of slope. Because of the high beta diversity the NMDS ordination gave results with the greatest ease of interpretation.     

Detrended correspondence analysis: An improved ordination technique

Summary Detrended correspondence analysis (DCA) is an improvement upon the reciprocal averaging (RA) ordination technique. RA has two main faults: the second axis is often an arch or horseshoe distortion of the first axis, and distances in the ordination space do not have a consistent meaning in terms of compositional change (in particular, distances at the ends of the first RA axis are compressed relative to the middle). DCA corrects these two faults. Tests with simulated and field data show DCA superior to RA and to nonmetric multidimensional sealing in giving clear, interpretable results. DCA has several advantages. (a) Its performance is the best of the ordination techniques tested, and both species and sample ordinations are produced simultaneously. (b) The axes are scaled in standard deviation units with a definite meaning, (c) As implemented in a FORTRAN program called DECORANA, computing time rises only linearly with the amount of data analyzed, and only positive entries in the data matrix are stored in memory, so very large data sets present no difficulty. However, DCA has limitations, making it best to remove extreme outliers and discontinuities prior to analysis. DCA consistently gives the most interpretable ordination results, but as always the interpretation of results remains a matter of ecological insight and is improved by field experience and by integration of supplementary environmental data for the vegetation sample sites.This research was supported by the Institute of Terrestrial Ecology, Bangor, Wales, and by a grant from the National Science Foundation to R.H. Whittaker. We thank R.H. Whittaker for encouragement and comments, S.B. Singer for assistance with the Cornell computer, and H.J.B. Birks, S.R. Sabo, T.C.E. Wells, and R.H. Whittaker for data sets used for ordination tests.     

Multivariate analysis of feeding relationships of fishes in blackwater streams

Synopsis The autumnal diets of 23 fish species in 9 families from South Carolina blackwater streams were characterized by rank order statistics and analyzed by detrended correspondence analysis (DCA). DCA ordinations of fishes and their food items indicate two primary gradients. The first axis contrasts fine particle feeders and omnivores (Catostomidae, some cyprinids) eating algae, detritus, microcrustacea and bivalves, with predators(Anguilla, Esox, largeAmeiurus) on decapods and vertebrates. The second is a gradient from benthic feeders (percid darters,Noturus catfishes) with varied invertebrate diets, to taxa (Gambusia, some cyprinids and centrarchids) feeding at the surface. With increasing size, fishes shift to larger prey and increased surface feeding. Much of the trophic differentiation within the assemblage reflects diversification at generic and family levels. DCA is an effective method for summarizing trophic relations in diverse assemblages.     

Ordination methods and chironomid species in stony bottom streams

Stony bottom streams in Italy were sampled in 369 sites in different years, and 174 chironomid species (or species groups) were identified.The species by sites matrix was submitted to different ordination methods, with the aim of outlining the major factors that are responsible of the observed species composition.The methods considered were principal component analysis (PCA), detrended correspondence analysis (DCA) and non-metric multidimensional scaling (MDSCAL). These methods assume different models describing the species-responses to the environmental gradients.PCA gave poor results, the best species ordination was given by MDSCAL. DCA gave results in agreement with MDSCAL. DCA results were useful as a starting point to perform MDSCAL. MDSCAL was performed with different options. At last Procrustean analysis was carried out to have a single fitted configuration, that summarized the results given by the different MDSCAL methods.All multivariate analyses emphasized that the first ordination axis can be interpreted as a crenonrithron-potamon gradient, probably an oxygen availability and/or a water temperature gradient, whereas the second axis can be considered a water speed gradient, separating lotic from lentic sites.The 3rd and 4th ordination axes had apparently no ecological meaning.Separated clusters of species were never evident, so well defined chironomid species assemblages could not be outlined in this analysis.     

Analysis of T-RFLP data using analysis of variance and ordination methods: a comparative study

The analysis of T-RFLP data has developed considerably over the last decade, but there remains a lack of consensus about which statistical analyses offer the best means for finding trends in these data. In this study, we empirically tested and theoretically compared ten diverse T-RFLP datasets derived from soil microbial communities using the more common ordination methods in the literature: principal component analysis (PCA), nonmetric multidimensional scaling (NMS) with Sørensen, Jaccard and Euclidean distance measures, correspondence analysis (CA), detrended correspondence analysis (DCA) and a technique new to T-RFLP data analysis, the Additive Main Effects and Multiplicative Interaction (AMMI) model. Our objectives were i) to determine the distribution of variation in T-RFLP datasets using analysis of variance (ANOVA), ii) to determine the more robust and informative multivariate ordination methods for analyzing T-RFLP data, and iii) to compare the methods based on theoretical considerations. For the 10 datasets examined in this study, ANOVA revealed that the variation from Environment main effects was always small, variation from T-RFs main effects was large, and variation from T-RF × Environment (T × E) interactions was intermediate. Larger variation due to T × E indicated larger differences in microbial communities between environments/treatments and thus demonstrated the utility of ANOVA to provide an objective assessment of community dissimilarity. The comparison of statistical methods typically yielded similar empirical results. AMMI, T-RF-centered PCA, and DCA were the most robust methods in terms of producing ordinations that consistently reached a consensus with other methods. In datasets with high sample heterogeneity, NMS analyses with Sørensen and Jaccard distance were the most sensitive for recovery of complex gradients. The theoretical comparison showed that some methods hold distinct advantages for T-RFLP analysis, such as estimations of variation captured, realistic or minimal assumptions about the data, reduced weight placed on rare T-RFs, and uniqueness of solutions. Our results lead us to recommend that method selection be guided by T-RFLP dataset complexity and the outlined theoretical criteria. Finally, we recommend using binary or relativized peak height data with soil-based T-RFLP data for ordination-based exploratory microbial analyses.     

A coenocline of the high-ranked syntaxa of ruderal vegetation

Two sets comprising 1419 and 1350 phytosociological relevés of ruderal vegetation classified into 9 and 7 orders, respectively, from an area in the Podunajská ní zina Lowland, western Slovakia were ordinated using correspondence and detrended correspondence analyses. The paper describes a coenocline of the high-ranked syntaxa contained in the data sets, and discusses some issues of the classification of the syntaxa involved.The Bidentetalia and Potentillo-Polygonetalia should be considered a special category of synanthropic vegetation as habitat moisture (flooding and waterlogging) play the controlling role in the formation of structural and dynamical patterns within these communities. The latter factor complex is responsible for the clear separation of these orders from the other syntaxa included in the ordinations. The coenocline of terrestrial ruderal vegetation units has the following sequence along the CA axis 1: Poo-Polygonetalia, Sisymbrietalia, Eragostrietalia, Onopordetalia, Agropyretalia repentis, Artemisietalia vulgaris and Glechometalia hederaceae.Various complexes of soil factors and anthropogenic disturbance are operational along the concatenation (a series of portions composing the coenocline). The factor complex includes soil compaction, trampling disturbance, nutrient status, soil texture and moisture, and solar irradiation. Step-by-step ordination and interpretation of concatenated portions of the coenocline proved to be useful in the analysis of complex data sets.Abbreviations CA = correspondence analysis - DCA = detrended correspondence analysis - PCA = principal components analysis     

A mesoscale analysis of floristic patterns in the south-west Finnish Archipelago

Aim To identify the ecological gradients based on the flora on a mesoscale in an archipelago. To interpret the results of the ordination and the classification of a grid cell‐based botanical data set, with several environmental and geographical attributes. To compare the mesoscale distribution patterns of vascular plants with patterns previously observed on an island scale, and to develop a floristic zonation of the study area. Location The south‐west Finnish Archipelago. Methods Vascular plant species‐lists from over 1500 localities were assigned to 5 × 5 km grid cells. The grid cell‐based floristic data were subjected to both unconstrained [detrended correspondence analysis (DCA)] and constrained [detrended canonical correspondence analysis (DCCA)] gradient analyses. The results of DCA were interpreted with calculated weighted averages of Ellenberg's indicator values for vascular plants, the number of occurring taxa and indices for the strength of human influence and the occurrence of limestone. The results of DCCA were interpreted with geographical attributes of the grid cells and the occurrence of limestone. The grid cells were clustered using two‐way indicator species analysis (twinspan ). Results Both the unconstrained and the constrained ordinations gave consistent and interpretable results. The main ecological gradient runs from the grid cells containing species‐rich islands with high human impact to grid cells containing species‐poor islands with low human impact. This gradient also represents the continuum from areas with large islands near the mainland, to the outermost areas at the edge of the open sea. The secondary gradient was shown to be a gradient of soil reaction. twinspan gave a clustering primarily based on the location of the grid cells on an inner–outer archipelago gradient, but the occurrence of limestone also influenced the classification. The archipelago was divided into five non‐homogeneous areas based on the twinspan clusters. The detected gradients correspond well with the gradients detected in a similar island‐level analysis. Main conclusions The two major ecological gradients in the study area seem to be robust, which is indicated by the similar results obtained both on an island and on a mesoscale. A shift from local and regional processes to broader geographical gradients probably starts to occur at the applied scale. The distribution patterns are strongly affected by the inner–outer archipelago gradient and the occurrence of limestone.     

Environmental relationships and vegetation associates of columnar cacti in the northern Sonoran Desert

The environmental distribution, habitat segregation, and vegetation associates of the columnar cacti Carnegiea gigantea, Stenocereus thurberi, and Lophocereus schottii were examined in Organ Pipe Cactus National Monument, Arizona. Three primary environmental gradients were identified with principal components analysis of environmental data: soil texture, elevation/nutrients, and xericness (based on slope aspect and angle). Environmental influents of spatial variation in density were modeled with ordinary least squares regression analysis, and common associates were identified with two-way indicator species analysis for each cactus. Of the three cacti, Carnegiea gigantea occurred over the broadest ecological range of habitats, but was densest on coarse, granitically derived alluvial soils of flat upper bajadas and basin floors, where it was associated with Larrea tridentata, Ambrosia deltoidea, and Opuntia fulgida. Stenocereus thurberi reached its maximum densities on coarse sandy soils of steep, south-facing granitic slopes, with Encelia farinosa, Jatropha cuneata, and Opuntia bigelovii as associates. Lophocereus schottii was restricted to very coarse, granitically derived alluvial soils in the southern part of the monument, where it occurred along wash banks with Beloperone californica, Hymenoclea salsola, Acacia greggii, and Opuntia arbuscula.Abbreviations DCA Detrended correspondence analysis - OPCNM Organ Pipe Cactus National Monument - OLS Ordinary least squares - PCA Principal components analysis - RA Reciprocal averaging - TWINSPAN Two-way indicator species analysis     

GeoChip-based analysis of the functional gene diversity and metabolic potential of soil microbial communities of mangroves

Mangroves are unique and highly productive ecosystems and harbor very special microbial communities. Although the phylogenetic diversity of sediment microbial communities of mangrove habitats has been examined extensively, little is known regarding their functional gene diversity and metabolic potential. In this study, a high-throughput functional gene array (GeoChip 4.0) was used to analyze the functional diversity, composition, structure, and metabolic potential of microbial communities in mangrove habitats from mangrove national nature reserves in China. GeoChip data indicated that these microbial communities were functionally diverse as measured by the number of genes detected, unique genes, and various diversity indices. Almost all key functional gene categories targeted by GeoChip 4.0 were detected in the mangrove microbial communities, including carbon (C) fixation, C degradation, methane generation, nitrogen (N) fixation, nitrification, denitrification, ammonification, N reduction, sulfur (S) metabolism, metal resistance, antibiotic resistance, and organic contaminant degradation. Detrended correspondence analysis (DCA) of all detected genes showed that Spartina alterniflora (HH), an invasive species, did not harbor significantly different microbial communities from Aegiceras corniculatum (THY), a native species, but did differ from other species, Kenaelia candel (QQ), Aricennia marina (BGR), and mangrove-free mud flat (GT). Canonical correspondence analysis (CCA) results indicated the microbial community structure was largely shaped by surrounding environmental variables, such as total nitrogen (TN), total carbon (TC), pH, C/N ratio, and especially salinity. This study presents a comprehensive survey of functional gene diversity of soil microbial communities from different mangrove habitats/species and provides new insights into our understanding of the functional potential of microbial communities in mangrove ecosystems.     

Mapping of arthropod alpha and beta diversity in heterogeneous arctic-alpine ecosystems

Assessing biodiversity in arctic-alpine ecosystems is a costly task. We test in the current study whether we can map the spatial patterns of spider alpha and beta diversity using remotely-sensed surface reflectance and topography in a heterogeneous alpine environment in Central Norway. This proof-of-concept study may provide a tool for an assessment of arthropod communities in remote study areas. Data on arthropod species distribution and richness were collected through pitfall trapping and subjected to a detrended correspondence analysis (DCA) to extract the main species composition gradients. The DCA axis scores as indicators of species composition as well as trap species richness were regressed against a combined data set of surface reflectance as measured by the Sentinel-2 satellite and topographical parameters extracted from a digital elevation model. The models were subsequently applied to the spatial data set to achieve a pixel-wise prediction of both species richness and position in the DCA space. The spatial variation in the modelled DCA scores was used to draw conclusions regarding spider beta-diversity. The species composition was described with two DCA axes that were characterized by post hoc-defined indicator species, which showed a typical annidation in the arctic-alpine environment under study. The fits of the regression models for the DCA axes and species richness ranged from R² = 0.25 up to R² = 0.62. The resulting maps show strong gradients in alpha and beta diversity across the study area. Our results indicate that the diversity patterns of spiders can at least partially be explained by means of remotely sensed data. Our approach would likely benefit from the additional use of high resolution aerial photography and LiDAR data and may help to improve conservation strategies in arctic-alpine ecosystems.     

瑞典河漫滩草甸植被的数量分类和排序

本文用目前各国学者广泛使用的一些数量分类和排序方法对瑞典河漫滩草甸样地资料进行了分类和排序。所用的方法包括多元等级聚合分类(TABORD程序),多元等级分划分类(TWINSPAN程序),PCA排序(ORDINA程序),RA和DCA排序(DECORANA程序)。研究结果表明可以把28个样地分为6个群落类型,它们的分布格局是与土壤水分梯度密切相关的。此外本文还对数量分类和排序方法在植物群落学研究中的应用以及所用方法的比较进行了讨论。     

The inselberg flora of Atlantic Central Africa. I. Determinants of species assemblages

Aims To identify the relative contributions of environmental determinism, dispersal limitation and historical factors in the spatial structure of the floristic data of inselbergs at the local and regional scales, and to test if the extent of species spatial aggregation is related to dispersal abilities. Location Rain forest inselbergs of Equatorial Guinea, northern Gabon and southern Cameroon (western central Africa). Methods We use phytosociological relevés and herbarium collections obtained from 27 inselbergs using a stratified sampling scheme considering six plant formations. Data analysis focused on Rubiaceae, Orchidaceae, Melastomataceae, Poaceae, Commelinaceae, Acanthaceae, Begoniaceae and Pteridophytes. Data were investigated using ordination methods (detrended correspondence analysis, DCA; canonical correspondence analysis, CCA), Sørensen's coefficient of similarity and spatial autocorrelation statistics. Comparisons were made at the local and regional scales using ordinations of life‐form spectra and ordinations of species data. Results At the local scale, the forest‐inselberg ecotone is the main gradient structuring the floristic data. At the regional scale, this is still the main gradient in the ordination of life‐form spectra, but other factors become predominant in analyses of species assemblages. CCA identified three environmental variables explaining a significant part of the variation in floristic data. Spatial autocorrelation analyses showed that both the flora and the environmental factors are spatially autocorrelated: the similarity of species compositions within plant formations decreasing approximately linearly with the logarithm of the spatial distance. The extent of species distribution was correlated with their a priori dispersal abilities as assessed by their diaspore types. Main conclusions At a local scale, species composition is best explained by a continuous cline of edaphic conditions along the forest‐inselberg ecotone, generating a wide array of ecological niches. At a regional scale, these ecological niches are occupied by different species depending on the available local species pool. These subregional species pools probably result from varying environmental conditions, dispersal limitation and the history of past vegetation changes due to climatic fluctuations.     

Use of Plant Strategy Ordination,DCA and ANOVA to elucidate relationships among habitats of Salix planifolia and Salix monticola

Abstract. Detrended Correspondence Analysis (DCA) was used on a data set of 112 species from 14 stands dominated by Salix planifolia and/or S. monticola. Environmental variables were determined which might be important to explain differences in stand vegetation composition, and willow stature (height), in willow carrs in the Rocky Mountain National Park. Correlation of environmental variables with DCA stand scores indicate that peat depth and soil redox potential are highly related to the primary DCA axis. Soils in stands of S. planifolia are more reduced in the early summer and have greater peat depths, than soils in stands of S. monticola. The second DCA axis is related to a water chemistry gradient of hydrogen ion, sulphate, and ammonium concentration. Four environmental variables selected by correlation analysis were regressed on stand scores from the first two DCA axes. Regression coefficients of August soil redox potential and soil water pH were significantly non-zero on the primary DCA axis. In addition to overall vegetation relationships elucidated by DCA, the relationships between willow stature and environmental variables were tested by simultaneous ANOVAs. ANOVA results and DCA ordinations indicate that S. planifolia and S. monticola plants achieve the greatest stature in the least reduced soils, and in shallow peat. Competitive, stress tolerant, and ruderal plant strategy theory applied to the vegetation data produced an arrangement of stands highly correlated to the DCA Axis 1 stand arrangement. These correlations indicate that the primary axis of vegetation structure can be extracted either by species composition, or species morphology/physiology relationships. Concurrent use of DCA and plant strategy theory is suggested as a tool for the prediction of community composition.     

Quantitative analysis of endangered Acanthopanax senticosus communities in Dongling Mountain of Beijing

Based on the survey of community plots, a quantitative analysis of endangered Acanthopanax senticosus communities in Dongling Mountain was performed with two way indicator species analysis (TWINSPAN), detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA). The communities of A. senticosus were classified into 9 types by TWINSPAN and the results were validated by DCA. On the DCA graph, the first axis reflected the gradient of altitude and the second axis reflected the aspect and slope. Most of A. senticosus were distributed in the thick forests at a high altitude with little light. With the exception of being a dominant species of shrub layers in a few communities, A. senticosus has a relatively scarce distribution. In accordance with DCA, the results of CCA also show the trend that the distribution of A. senticosus communities varied along with the gradient change of environmental factors. Altitude and light are the main factors affecting A. senticosus growth. __________ Translated from Chinese Journal of Ecology, 2007, 26(3): 314–318 [译自: 生态学杂志]     

Alterations in flood frequency increase exotic and native species richness of understorey vegetation in a temperate floodplain eucalypt forest

The delivery of environmental flows for biodiversity benefits within regulated river systems can potentially contribute to exotic weed spread. This study explores whether exotic plants of a floodplain forest in Victoria, Australia, are characterised by specific functional groups and associated plant traits linked to altering hydrological conditions over time. Permanently marked 20 × 20 m² plots from five wetland sites in Eucalyptus camaldulensis floodplain forest were sampled twice, first in the early 1990s (1993–1994) and then 15 years later (2007–2008). Species cover abundance data for understorey vegetation communities were segregated by season and analysed using ordination analysis. Exotic species richness was modelled as a function of site flooding history and native species richness using general linear models. Site ordinations by detrended correspondence analysis showed differential community compositions between survey dates, but native and exotic species were not clearly differentiated in terms of DCA1 scores. Most exotics belonged to functional groups containing annual species that germinate and reproduce under drier conditions. Exotics reproducing under wetter conditions were in the minority, predominantly perennial and capable of both sexual and asexual reproduction. Site flooding history and native species richness significantly predicted exotic species richness. Vegetation changes are partially structured by reduced flood frequency favouring increased abundance of exotic, sexually reproducing annuals at drier sites. Sites of low flood frequency are more sensitive to future exotic weed invasion and will require targeted management effort. Flow restoration is predicted to benefit propagule dispersal of species adopting dual regeneration strategies, which are predominantly natives in this system.     

Clustering of ant communities and indicator species analysis using self-organizing maps

To understand the complex relationships that exist between ant assemblages and their habitats, we performed a self-organizing map (SOM) analysis to clarify the interactions among ant diversity, spatial distribution, and land use types in Fukuoka City, Japan. A total of 52 species from 12 study sites with nine land use types were collected from 1998 to 2012. A SOM was used to classify the collected data into three clusters based on the similarities between the ant communities. Consequently, each cluster reflected both the species composition and habitat characteristics in the study area. A detrended correspondence analysis (DCA) corroborated these findings, but removal of unique and duplicate species from the dataset in order to avoid sampling errors had a marked effect on the results; specifically, the clusters produced by DCA before and after the exclusion of specific data points were very different, while the clusters produced by the SOM were consistent. In addition, while the indicator value associated with SOMs clearly illustrated the importance of individual species in each cluster, the DCA scatterplot generated for species was not clear. The results suggested that SOM analysis was better suited for understanding the relationships between ant communities and species and habitat characteristics.     

Understorey vegetation development in North Finnish Picea forests after disturbance: re‐analysis of Sirén's data

Abstract. Sirén (1955) studied understorey species composition, tree stand properties and humus‐layer thickness in 64 unlogged forest stands on topographically and pedologically comparable sites. The stands were of even age (6 – 300 yr), stocked with the first or second tree generation after wildfire. The view of Sirén and several authors after him, that the vegetation of old‐growth boreal Picea forests is homogeneous on a broad scale, was examined by applying, in parallel, the partial variants of two ordination methods (DCA and PCA) to Sirén's vegetation data. Two main vegetation gradients were found: a major gradient running from recently burnt plots with prominence of pioneer species to plots with stand age > 100 yr, a well stocked tree layer and a thick humus layer, dominance of feather‐mosses and ample occurrence of shade‐tolerant as well as light‐preferring vascular plant species, and a second gradient along which first‐ and second‐generation plots segregate. The more prominent element of Betula trees in first‐ than in second‐generation stands < 100 yr contributed to the latter. A minor third gradient related to humus‐layer thickness was recovered by partial DCA only. The main vegetation gradient reappeared in separate ordinations of data from 47 mature forest stands (> 100 yr), but without being correlated with forest age. Variation among mature‐forest stands in the importance of pioneer species is considered mainly to be brought about by fine‐scale disturbance processes such as tree uprooting. Increasing importance of factors operating on within‐stand scales [development of a varied gap structure and stronger gradients in tree influence (radiation at ground level), soil moisture, soil depth and nutrient availability] with time is also reflected in the second and third mature‐forest ordination axes. Possible implications of the results for conservation of biological diversity and monitoring of changes in boreal forests are discussed.