How reliable are our vegetation analyses?

Abstract. Two sets of 40 relevés, made independently by two observers on the same 5m x 5m sample plots, were compared to estimate the sampling error and to assess the effect of this sampling error on (1) estimates of species richness and diversity (2) results of multivariate analyses, and (3) estimation of species turnover in repeated sampling. The relevés were made according to the standard Braun-Blanquet method. The sampling error was estimated for (1) recording of species in sample plots and (2) visual estimation of the degree of cover (or of the general population size). Despite the fact that the sample plots were searched thoroughly for 30 - 40 min, the number of overlooked species was high with a discrepancy of 13% between corresponding relevés. Regarding multivariate analysis, the error caused by missing species was at least as important as the error in visual estimation of species cover. The estimates of degree of cover using the Braun-Blanquet scale are sufficiently reliable for use in multivariate analysis when they are subjected to ordinal transformation. When average cover values are used, the patterns detected are based solely on dominants. Species richness and species diversity could be reliably estimated from the relevés, but the estimates of equitability are very unreliable. The classical relevé method remains one of the most efficient survey methods for recognition of vegetation types on the macro-community and landscape scales.     

Data-processing in phytosociology,retrospect and anticipation

Summary The existence of the Working Group for Data-Processing in Phytosociology spans about six years, 1969–1975. Its activities are summarized and a series of papers to appear in Vegetatio, presenting results of various numerical treatments, mainly of salt marsh vegetation, is introduced. A survey is given of seven releve selections to which treatments were applied. Five main fields of activities are discussed: systematical data-processing, rearrangement of phytosociological tables, development of classification and ordination techniques, numerical syntaxonomy, and theoretical phytosociology.The establishment of the Working Group for Theoretical Phytosociology and its first symposium on theoretical aspects is announced.Contribution from the Working Group for Data-Processing in Phytosociology, International Society for Vegetation Science.     

Regional interpretation of environmental gradients which influence Trachypogon savannas in the Orinoco Llanos

Multivariate structural data describing Trachypogon savanna relationships were used to characterize the sources of variation among savannas as a function of the environmental characteristics of the Orinoco Llanos. Twenty-six savannas including major physiognomic types of the Orinoco Llanos ranging from herbaceous to bush island savannas were ordinated on a regional scale. Results of Detrented Canonical Correspondence Analysis (DCCA) using floristics variables indicated that savannas were ordinated along two complex gradients. A soil physical and chemical gradient as expressed by changes in bulk density and magnesium concentration were evident after the analysis of the first DCCA axis. The second axis was a climatic gradient of decreasing annual precipitation and incresing monthly precipitation during the dry season. The impact of human disturbance on the savanna composition was also an explanatory variable of the second DCCA axis. Savanna sites overlapped considerably in composition and most species were widely distributed, with aboveground phytomass abundance depending upon the site.     

Analysis of the disintegrating group and gradient structure in Swiss riparian forests

The differentiation of river flood plain vegetation is predominantly determined by the inundation frequency and the type and age of the substrate. A comparison of data published in 1958 with recent relevés from all parts of Switzerland indicate that the hitherto known gradient- and group structure is disintegrating into multiple floristically isolated stands as a result of waterflow management constructions. In view of further vegetation mapping, a two-phase classification is proposed where the core of the gradients and groups is isolated and analysed first. The outliers are then re-attached to serve as possible indicators of the existence of variation not sufficiently described by the data. To structure the tedious analytical calculations involved, a hierarchical procedure has been developed.Abbreviations AOC = Analysis of concentration - CA = Correspondence analysis - PCA = Principal components analysis     

Vegetation-environment relations of a Middle Zambezi floodplain

Detrended correspondence analysis was used to study the relationships between environmental factors and the species composition of vegetation on Zambezi River alluvium downstream of the Kariba hydroelectric dam. Grass, sedge and woody species were recorded in 73 stands in Mana Pools National Park, Zimbabwe. Grass and sedge species composition was related to the soil moisture regime, as indexed by soil texture and flooding frequency. The first woody plant ordination axis was related to a stand development gradient; Acacia albida was a pioneer species on lowlying sandbanks and the woody species richness of stands increased with their height above the Zambezi River. Two-way indicator species analysis identified 7 vegetation types which could be separated on the basis of their topsoil texture and flooding frequency. The types were: sandbanks; young A. albida woodland; A. albida woodland; A. albida dominated mixed woodland; mixed riverine woodland with understory; mixed riverine woodland; and grassland on clay soils.     

汾河河口湿地植被数量分类与排序

在群落样方调查基础上,采用双向指示种分析（TWINSPAN）和除趋势对应分析（DCA）对汾河入河口湿地植被群落进行了数量分类和排序。TWINSPAN将78个样方分为18个群丛,并论述了各群丛的群落学特征。DCA排序结果反映了植物群落类型与环境梯度水分之间的关系,表明影响群丛分布格局的主导生态因子为土壤水分。     

广西热带石灰岩季节雨林分类与排序

本文利用二歧指示种分析法TWINSPAN将广西热带石灰岩季节雨林的30个样地划分两大类共6个群落：洼地季节雨林(1)火焰花+萍婆群落; (2)海南风吹楠+人面果群落,石山季节雨林(1)东京桐+广西顶果豆群落; (2)假肥牛树+安南牡荆群落;(3)肥牛树群落; (4)肥牛树+蚬木群落。文章还利用无偏对应分析法DCA对群落进行了排序,指出由洼地至山坡水分条件由湿到干是影响群落变化的主导因素。环境的封闭程度对群落变化也有一定影响。     

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.     

Prairie plant guilds: a multivariate analysis of prairie species based on ecological and morphological traits

An ecomorphological analysis of the tallgrass prairie of central North America divided representative species of the native grassland flora into eight guilds or groups of species with similar life-form, phenology, and ecology. The guilds, segregated by multivariate analysis, are: (1) warm-season graminoids with Kranz anatomy and the Hatch-Slack photosynthetic pathway (C4 grasses); (2) cool-season graminoids without Kranz anatomy, but with the common Calvin or C3 photosynthetic pathway (C3 grasses and sedges); (3) annuals and biennial forbs; (4) ephemeral spring forbs; (5) spring forbs; (6) summer/fall forbs; (7) legumes; and (8) woody shrubs. The study was based on 158 plant species indigenous to three upland prairie sites in northeastern Kansas. Each species was scored for 32 traits which fall into five broad categories: plant habit, leaf characteristics, stem structures, root structures, and reproductive traits, including phenology. A multivariate, detrended correspondence analysis sorted the 158 species into the eight principal groups or guilds. These groups were further supported by a cluster analysis and discriminant function analysis of the same data set. The discriminant function analysis determined that 94.3% of the species were correctly classified in their respective guilds, and that the guilds were statistically different. Results indicate that guild analysis offers a basis for detailed classification of grassland vegetation that is more ecologically focused than species composition, as the myriad of species (about 1,000 prairie species on the central plains of North America) vary in presence, cover, and importance with their individualistic distribution.Abbreviations C3= C3 photosynthesis - C4= C4 photosynthesis - LSD= least significant difference     

Succession in a South Swedish deciduous wood: A numerical approach

Summary Successions in a South Swedish deciduous wood, which is a national park and has been almost undisturbed since 1918, are studied using semipermanent plots. These plots were originally established in 1935 and reestablished in 1969–70 and 1975–76. Each plot is represented by a record, which combines fieldlayer data from spring- and summer aspects. The accuracy of the approach of semipermanent plots is evaluated from a slightly displaced sample.Vegetational and successional gradients are revealed in a classification of the entire dataset. Successional trends are analysed through ordinations of site- period data. A converging successional trend is found. The vegetational and successional gradients are found to coineide. A considerable reduction in the variation and diversity of the fieldlayer is evident.An analysis of the development in smaller groups of corresponding quadrats shows consistent, and in some cases rather drastic, changes in cover and/or frequency for many species. Characteristic indicator values are used to connect these changes to environmental factors, and changes in light regime and nitrogen status are indicated. The spatial-temporal pattern is analysed through maps.Nomenclature of species follows Lid (1974), except for Crataegus oxyacantha, and for mosses Nyholm (1954–1969). Viola silvestris (= V. reichenbachiana) and V. riviniana are treated collectively as Viola sp. Dactylis glomerata includes D. aschersoniana, Crataegus oxyacantha includes C. lagenaria, Eurhyngium striatum includes E. Zetterstedtii and E. praelongum includes E. Swartsii.I wish to thank Prof Nils Malmer, Institute of Plant Ecology, Lund, for his support and valuable advice, Dr Eddy van der Maarel, University of Nijmegen, The Netherlands, for valuable advice and for providing computer programs. I also wish to thank Fil lic Lennart Lindgren and Fil kand Tommy Wikberg for providing primary data, and Mrs Mimmi Varga for drawing the figures.     

Correlation between stand structure and ground vegetation: an analytical approach

The effect of stand structure on the diversity of the forest's ground vegetation was examined based on data on permanent sample plots collected in the northern parts of North-Carelia, eastern Finland. Different ordination methods (DCA, GNMDS, LNMDS, and HMDS) were used together with the TWINSPAN classification method. The aim was to construct a basis for classifying forests with respect to the biodiversity of the forest vegetation. Fertility and stand age showed the strongest correlation with the variation in ground vegetation. Other important factors were basal area, tree species composition, and crown cover. These variables were important in the division of the sample plots into different classes. According to the diversity indices, species diversity was at its highest in young stands on fertile forest sites. As a result, twenty-one different classes were formed based on the relative abundance of understorey species. The variables with the highest correlation were used to describe the stand structure in these classes.     

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。     

海北高寒草甸植物群落的数量分类和排序

本文采用数学分析的方法,对海北高寒草甸生态系统开放实验站地区的植物群落进行了数量分类和排序。结果表明：当相关系数R=0.85时,64个样地群落可以划分为23个群落类型,各群落类型的层次结构、种类组成、生态条件以及地理分布上均有明显特点。排序结果证明：群落类型的排序坐标分别与海拔、土壤温度和土壤湿度之间有密切的关系。     

Bird community composition in an actively managed savanna reserve, importance of vegetation structure and vegetation composition

The mosaic of trees, shrubs and open grassland in mesic African savannas is highly dynamic and strongly influenced by mammal herbivory and fire. We investigated the bird fauna in four different savanna habitats to help assess the impacts of vegetation change on this component of faunal diversity. Birds were censused, plant species were identified and vegetation structure was measured in four different vegetation types (Acacia nilotica woodland, Acacia nigrescens woodland, broadleaf thicket and open grassland) in the Hluhluwe-Umfolozi Park in northern KwaZulu Natal, South Africa. Multivariate ordination analyses were used to determine the relative importance of vegetation structure and floristic composition in defining bird assemblages. The bird communities of the grasslands, the acacia woodlands, and the broadleaf woodlands were clearly separated on the first axis of the detrended canonical correspondence analysis (DCCA). Canopy cover and foliage height diversity (FHD) were strongly correlated with the first axis of DCCA, possibly reflecting a secondary successional series from grassland to woodland, known as bush encroachment. Floristic composition (based on presence–absence data only) seemed to be less important for bird community composition than vegetation structure. The results indicate that changes in vegetation structure, caused by bush encroachment, could cause concomitant changes in bird community composition.     

山西芦芽山植物群落的数量分类

本文在群落样方调查基础上,采用双向指示种分析法（TWINSPAN）和除趋势对应分析（DCA）对山本芦芽山植物群落进行数量分类和排序。TWINSPAN等级分类将芦芽山82个样方分为28类,可归属于7个植被型,分类结果反映了植物群落类型与环境梯度之间的关系,并在DCA二维排序图上得到较好的验证,DCA排序轴对角线基本反映了海拔,温度和湿度的梯度变化,表明群落生境所在地的海拔,温度和湿度是决定群落分布的主要因素。     

Classification of vegetational diversity in managed boreal forests in eastern Finland

Diversity of vegetation in managed forests is studied. A classification based on forest stand structure, the abundances of vegetation species and variations in these abundances is developed and diversity indices are calculated for the classes to describe the diversity of the vegetation within the classes. The classes were formed using detrended correspondence analysis (DCA), global nonmetric multidimensional scaling (GNMDS) and TWINSPAN classification. Discriminant analysis was used to determine the environmental variables differentiating between the classes, and Duncan's multiple range test was used to examine the ability of the diversity measures to distinguish the classes. Beta diversity was estimated with Økland's method based on DCA ordination of the sample plots. The results point to fertility and the successional stage of the stand as the main factors affecting species diversity, in addition to which soil type, the number of tree species, crown cover, basal area and certain variables describing the management of the stand were relevant to the classification. The most distinct diversity indices were the reciprocal of Simpson, Pielou's J' and species richness, while the highest alpha diversity was found in young stands with a low crown cover on herb-rich or mesic forest sites. Beta diversity was quite high, its highest values of all for the whole data being recorded along the fertility gradient.