Ordination and classification in the upland Rugege forest (Rwanda,Central Africa)

Summary Quantitative data on habitat and vegetation from 42 releves of the Rugege forest have been submitted to different ordination and classification methods.The habitat factors which have an important effect upon vegetation are: hydromorphy, abundance of tree layer and depth of soil.The different ordination (principal components analysis and reciprocal averaging) and numerical classification methods applied to the same set of data lead to consistent results. The numerical classification seems better than the Braun-Blanquet's method.Four groupings have been defined.The future developments of ecological studies in the Rugege forest are discussed in relation to the forest conservation.All data presented in this work were collected during my stay in the National Institute of Scientific Research of Rwanda, 1971 – 1972. Part of the computations (numerical analysis and principal components analysis) was carried out at the Nijmegen University's IBM 370/155; I am happy to thank Dr. E. van der Maarel and Dr. J.G.M. Janssen for their welcome and assistance. The reciprocal averaging ordinations and the principal components analysis on the basis of correlation between relevés were performed on the Louvain-La-Neuve University's IBM 370/158; I am very grateful to Professeur Berthet for this important assistance.This paper corresponds to part of a thesis presented at the Liège University on 7th November 1973.     

Comparisons of three ordination techniques

Summary Simulated coenoclines were used to test performance of several techniques for ordinating samples by species composition: Wisconsin polar or Bray-Curtis ordination with Euclidean distance (ED) and the complements of percentage similarity (PD) and coefficient of community (CD) as distance measures, Principal components analysis, and polar and non-polar or indirect use of Discriminant function analysis. In general the Bray-Curtis technique gave the best ordinations, and PD was the best distance measure. Euclidean distance gave greater distortion than PD in all tests; CD may be better than PD only for some sample sets of high alpha and beta diversity and high levels of noise or sample error. Principal components ordinations are increasingly distored as beta diversity increases, and are highly vulnerable to effects of both noise and sample clustering. Discriminant function analysis was found generally unsuitable for ordination of samples by species composition, but likely to be useful for sample classification.     

Performance of four ordination techniques assuming three different non-linear species response models

Summary The performances of four ordination techniques (reciprocal averaging, parametric mapping, gaussian ordination and non-metric multidimensional scaling) are evaluated using artificial data based on three vegetation models: the gaussian response curve, -function curves, and an ecological response model. No technique performs well with the artificial data not based on a gaussian model. Non-metric multidimensional scaling seemed marginally better than other techniques. Some of the implications of this failure are discussed.     

Structure of coniferous forest communities in Western Washington: Diversity and ecotope properties

Summary Vegetation patterns were examined in three regions of coniferous forest, arrayed along a gradient of decreasing maritime influence. The following responses to decreasing maritime influence were noted: all aspects of species diversity increase; growth-form complexity increases; habitat heterogeneity increases; species habitat breadths decrease; and habitat overlaps decrease. These observations result from studies of both individual plots and data aggregated into community types. The contrasts between the maritime western study region and the more continental eastern study region imply that ecotope differentiation increases from west to east.The enhanced structural complexity of the eastern region results from climatical limitation of the dominant tree species. Reduced canopy dominance produces greater within-habitat heterogeneity with respect to moisture, light, and soil properties. Furthermore, contrasts between the extremes of the topographic-moisture gradient increase toward the east. These factors combine to increase alpha, beta, and gamma diversities in the eastern study region above those of the western study region. Corresponding to these diversity trends are decreasing mean relative niche, habitat, and ecotope breadths of species toward the eastNomenclature is that of Hitchcock & Cronquist (1973).Funds for the study were provided in part by the Graduate School Research Fund, University of Washington. We thank Alan F. Watson, B.C. Cannon, S.G. Fleming, C. Brewer, K.E. Wade, K. Loughney, and M. Swanson for their able field assistance and Joan Canfield for assistance with data analysis. R.H. Whittaker, R.K. Peet, and an anonymous reviewer made valuable suggestions.     

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.     

On the interpretation of principal components analysis in ecological contexts

Summary Principal components analysis is well suited for many data analysis problems in ecology, particularly for data reduction and hypothesis generation; but the structure of PCA is poorly suited for indirect gradient analysis. Whatever the intended application of PCA, the user must exercise special care in selecting data transformations to prevent the analysis from being overwhelmed by the purely numerical effects in the variance structure of the data.I would like to thank R. H. Whittaker, H. G. Gauch, R. E. Moeller, and S. R. Searle for their guidance and assistance.     

Ordination of vegetation change in Guadalupe Mountains,New Mexico,USA

Vegetation change over a nine-year period was studied in Guadalupe Mountains, New Mexico. Permanent transects in desert shrub vegetation were sampled in 1972 and 1980. Emphasis was given to shrubs because of their importance to big game diets. Univariate paired t-tests and reciprocal averaging ordination were used to detect and display coordinated changes in species composition over time. Despite apparently less browsing pressure in desert shrub vegetation in 1980 there were few significant changes in species composition. In addition, preferred forage species showed reduced reproduction while species of intermediate and poor forage value dis-played increased reproduction during this time. These data do not support traditional rangeland succession theory which states that enhanced reproduction of preferred species should follow grazing or browsing pressure reduction.     

Correlations among vegetation strata and environmental variables in subalpine spruce-fir forests,southeastern British Columbia

A set of forest vegetation-environment data consisting of 110 plots from subalpine spruce-fir forests in southeastern British Columbia was analyzed with multivariate methods. The aims were to describe and compare relationships among plots using species composition data from four strata of the forest (trees, shrubs, herbs, bryophytes), and to compare patterns in the separate strata with patterns in the environmental data. Weak but statistically significant correlations were detected among PCA axes from all four strata, with a tendency for correlations to be stronger between axes of adjacent strata. Relationships between stratal patterns and environmental variation were examined by correlating PCA axes with environmental variables separately, and jointly with canonical correlation analysis. Linear composites of the environmental data identified by the first canonical axis were similar for shrub, herb, and bryophyte strata implying similar responses to environmental variation in understorey strata; however, < 16% of the total variance was explained. No clear relationships emerged between tree stratum and environmental variation. Probable determinants of pattern in the tree stratum include historical events such as fire and winter snowpack, as well as possible interactions between species. Collectively, these events bear little resemblance to existing environmental conditions in the forest.Nomenclature used by BCMF follows Crum, Steere & Anderson (1973), Hale & Culberson (1970), and Stottler & Crandall-Stottler (1977) for non-vascular plants, and Taylor & MacBryde (1977) for vascular plants.The British Columbia Ministry of Forests provided the data on which this paper is based, as well as a grant to pay for the computer time used. We are grateful to G. Utzig for initially suggesting that a project of this nature be done, and to S. Phelps for helping with the retrieval of computer data files from tape. This paper has greatly benefited from discussions with P. Comeau and R. K. Scagel, and the comments of two anonymous reviewers.     

Dynamics of herbaceous vegetation recovery on Mount St. Helens,Washington, USA,after a volcanic eruption

Recovery of herbaceous vegetation on Mount St. Helens was studied annually after the massive lateral eruption of May 18, 1980. Measures such as species richness, cover, and diversity were combined with detrended correspondence analysis to describe vegetation recovery rates under different combinations of initial impact intensity and degree of isolation from recolonization sources. A major key to recovery is whether any plants survived the devastation. Survival of even a few individuals markedly accelerated recovery. Where no plants survived, the degree of isolation becomes paramount. New, barren substrates, a few meters from undisturbed sites, have begun to develop some vegetation, while more isolated sites have scarcely any subalpine plants present. On any site, plant-mediated processes that improve conditions for growth and the invasion of other species predominate in the early stages, but as vegetation develops, biotic inhibition and establishment of seedlings from adults already in the habitat gain importance. The rate at which this conversion occurs is a function of the size and intensity of the initial impact.Abbreviations DCA Detrended correspondence analysis     

Topographical pattern of the forest vegetation on a river basin in a warm-temperate hilly region,central Japan

The relationship between tree distribution and topography was examined in a small river basin (3.4 ha) comprising a complex mosaic of topographical units at 10² to 10³ m² order, each of which had a shallow valley bordered by small ridges or breaks of slopes. Twenty-five major woody species were divided into two groups (groups A and B) based on a cluster analysis using the distribution data in the basin. Group A, which mainly consisted of early-successional species, was distributed around the valley sites of the topographical units, while group B, which mainly consisted of late-successional species, was distributed around the ridge sites of the topographical units. This vegetation pattern coincided with erosional condition in the basin. That is, the valley sites were eroded more actively than the ridge sites, as soil depth tended to be thin in the valley sites and thick in the ridge sites, and because large (canopy) trees were restricted in the ridge sites. There was no tendency that group B was replacing group A, and hence it was suggested that repeated disturbance by slope failures or small-scale shallow landslides have prevented compositional change from the early-successional (group A) to the late-successional (group B) species by preventing the invasion of the latter into valley sites.     

Pluralism and diversity: trends in the use and application of ordination methods 1990‐2007

Question: What are the trends and patterns in the application of ordination techniques in vegetation science since 1990? Location: Worldwide literature analysis. Methods: Evaluation of five major journals of vegetation science; search of all ISI‐listed ecological journals. Data were analysed with ANCOVAs, Spearman rank correlations, GLMs, biodiversity indices and simple graphs. Results: The ISI search retrieved fewer papers that used ordinations than the manual evaluation of five selected journals. Both retrieval methods revealed a clear trend in increasing frequency of ordination applications from 1990 to the present. Canonical Correspondence Analysis was far more frequently detected by the ISI search than any other method. Applications such as Correspondence Analysis/Reciprocal Averaging and Detrended Correspondence Analysis have increasingly been used in studies published in “applied” journals, while Canonical Correspondence Analysis, Redundancy Analysis and Non‐Metric Multidimensional Scaling were more frequently used in journals focusing on more “basic” research. Overall, Detrended Correspondence Analysis was the most commonly applied method within the five major journals, although the number of publications slightly decreased over time. Use of Non‐Metric Multidimensional Scaling has increased over the last 10 years. Conclusion: The availability of suitable software packages has facilitated the application of certain techniques such as Non‐Metric Multidimensional Scaling. However, choices of ordination techniques are currently less driven by the constraints imposed by the software; there is also limited evidence that the choice of methods follows social considerations such as the need to use fashionable methods. Methodological diversity has been maintained or has even increased over time and reflects the researcher's need for diverse analytical tools suitable to address a wide range of questions.     

Ordination of epiphytic bryophyte communities in a wet-temperate coniferous forest,South-Coastal British Columbia

Three coniferous tree species (Pseudotsuga menziesii, Tsuga heterophylla, and Thuja plicata) at Capilano Canyon, British Columbia, were studied for their epiphytic communities. Quantitative data were obtained for fourteen bryophyte species by sampling at heights of 0.5 m, 1 m and 2 m above ground level. Scapania bolanderi is an abundant and widely distributed species; Dicranum fuscescens, Bazzania denudata, Lepidozia reptans and Hypnum circinale are also common. An environmentally uniform study area was chosen to minimize the effects of factors other than bark and elevation on the distribution of epiphytes. In general, tree species are similar in terms of their epiphytic assemblages, but ordination methods using quantitative data exposed compositional variation that may be explained by differences in microclimate and bark-type. In this respect, the results point the way to further studies to examine epiphyte associations in closer detail, and to relate these associations to specific factors in the microenvironment.Nomenclature for vascular plants follows Hitchcock & Cronquist (1973); bryophyte nomenclature follows Schofield (1976) for mosses and Stotler & Crandall-Stotler (1977) for hepatics.We thank Dr. W. B. Schofield for aiding in bryophyte identification.     

Zonation of submerged macrophyte vegetation in Lake Kariba,Zimbabwe and its ecological interpretation

The submerged vegetation of Lake Kariba is described in relation to degree of slope (lake morphometry), depth and light transparency. The direct gradient analysis technique — canonical correspondence analysis and the TWINSPAN classification programs were used to analyse the data set. The western end of the lake with low transparency has a low species diversity (with Vallisneria aethiopica dominating). Species diversity increases with increased transparency in the other parts of the lake. The classification revealed monospecific communities for all species as well as mixed communities with Lagarosiphon as the associate species with the broadest distribution. The ordination revealed a first axis strongly related to depth and transparency gradients and the second axis related to slope. Vallisneria aethiopica has a growth form adapted to grow in shallow areas subjected to wave action. Potamogeton octandrus also growing in shallow areas is restricted to sheltered areas. Lagarosiphon ilicifolius precluded from the shallow exposed areas grows at medium depth but has a growth form adapted to tap light at the surface. Najas pectinata and Ceratophyllum demersum are adapted to grow in deep water.Abbreviations CCA Canonical correspondence analysis     

Spatial analysis of density dependent pattern in coniferous forest stands

An investigation of spatial pattern in relatively sparse Pinus ponderosa-P. Jeffreyi stands showed that a simple Poisson model of random distribution described the pattern at 5 to 50 m scales in the denser stands examined when allowance is made for inhibition between nearest neighbors. There is evidence for a clumped distribution in large quadrats for the sparsest stands, which concurs with prior work where a mixed Poisson model was fit to the data. The technique used was innovative in that it involved digitally recording tree locations from high resolution aerial photos, which allowed for the automatic application of several statistical techniques in order to determine how pattern varies with plot density and scale. Point locations were recorded for six 11.3 ha plots in three density regions of a 340 ha study area in northeastern California, USA. The inter-event distance distribution, and one- and two-dimensional power spectra were calculated, and variable quadrat analysis was performed for the data sets. The second order and spectral analyses showed no evidence of a distinctive clumped pattern at any scale, and all analyses showed that the pattern was regular at the scale of the average inter-plant distance in the denser stands. For the sparser stands, the counts in large quadrats did not fit a Poisson distribution, but were better fit by a mixed Poisson model describing aggregated pattern.     

Schoenus vegetation and environmental conditions in South and Southeast Sweden

Summary The relations between the Schoenus phytocoena and their site conditions are elucidated by stand and species ordinations (ORDINA, RA) and by comparisons of ordinations of environmental variables alone and ordination of the combined environmental and species variables (RA). Correlations between the environmental variables show that they may be gathered into two contrasting groups, the carbon and the carbonate groupings, respectively. This first direction of variation is related to hydrological conditions causing the differences in thickness of organic soils, content of organic carbon and dry weight of intact soil per unit volume. The second direction of variation is associated with a nutrient factor complex, in this study represented by available phosphate.The carbon grouping and a constantly high level of mainly topogeneous soil water is typical of sites of the Oxycoccus-Schoenus association. The Sesleria group of the Primula-Schoenus association is related to sites with a periodically low table of topogeneous or soligeneous soil water and mineral or mucky soils with high values of carbonate content, pH and CEC and a fairly low nutrient state, while the sites of the Bartsia-Ophrys and Valeriana groups have a fairly high level of mainly soligeneous soil water and mucky soils with variable carbonate content, intermediate pH and CEC and somewhat higher nutrient state. Periodically flooded sites with topogeneous soil water and mineral soils are typical of the Cladium-Schoenus nigricans, Schoenus intermedius-Schoenus ferrugineus and Glaux phytocoena.Nomenclature is the same as in Tyler (1980).This study is part of my doctor's thesis. I am most grateful to Professor Nils Malmer, Head of the Department, for variable advice and discussions. My thanks are also rendered to Dr. E. van der Maarel, University of Nijmegen, The Netherlands, Dr. R.S. Clymo, Westfield College, University of London, Great Britain, Fil kand. Stefan Persson, Dr. Germund Tyler, Mrs Maj-Britt Larsson, Mrs Anita Balogh, Mrs Mimmi Varga, Mrs Kerstin Richter, and Mr Robert Dewsnap.     

The structure and status of forest fragments outside protected areas in central Uganda

Given the extent of tropical forest deforestation and as a number of conservation programmes and local communities rely on forest fragments, it has become important to understand how fragment exploitation by local communities affects forest structure and function. The effects of forest exploitation on forest structure and status of forest fragments were investigated in 20 nonreserved forest fragments in central Uganda. Enumeration of plots showed that tree species composition of the forest fragments was 60.0%, 23.7% and 6.3% for under‐storey, middle and top canopy trees respectively. The major activity was fuel wood extraction (65%), followed by brick making (10%), cultivation and livestock paddocks (10%), charcoal burning (5%), local brew distillation and others (5%). These extractive processes caused drastic structural changes, habitat degradation and destruction. Tree stumps enumeration indicated that under storey trees formed the highest proportion for wood extraction. There was no significant difference in the level of forest exploitation (basal area loss) among forest patches of varying sizes.