bluuebunny/crossref_metadata_2025 huggingface dataset:
A new measure of fidelity and its application to defining species groups
Authors: Bruelheide Helge
Published: May 2006

Abstract. The first objective of this paper is to define a new measure of fidelity of a species to a vegetation unit, called u. The value of u is derived from the approximation of the binomial or the hypergeometric distribution by the normal distribution. It is shown that the properties of u meet the requirements for a fidelity measure in vegetation science, i.e. (1) to reflect differences of a species’relative frequency inside a certain vegetation unit and its relative frequency in the remainder of the data set; (2) to increase with increasing size of the data set. Additionally (3), u has the property to be dependent on the proportion of the vegetation unit's size to the size of the whole data set.

The second objective is to present a method of how to use the value of u for finding species groups in large data bases and for defining vegetation units. A species group is defined by possession of species that show the highest value of u among all species in the data set with regard to the vegetation unit defined by this species group. The vegetation unit is defined as comprising all relevés that include a minimum number of the species in the species group. This minimum number is derived statistically in such a way that fewer relevés always belong to a species group than would be expected if the differential species were distributed randomly among the relevés. An iterative algorithm is described for detecting species groups in data bases. Starting with an initial species group, species composition of this group and the vegetation unit defined by this group are mutually optimized. With this algorithm species groups are formed in a data set independently of each other. Subsequently, these species groups can be combined in such a way that they are suited to define commonly known syntaxa a posteriori.
Similarity between seed banks and above‐ground vegetation along a salinity gradient
Authors: Egan Todd P. , Ungar Irwin A.
Published: May 2006

Abstract. Zonation of above‐ground vegetation often occurs in salt marshes along salinity and moisture gradients. The above‐ground vegetation and seed bank in four physiognomically different vegetation zones in a salt marsh were compared to determine their level of similarity using percent similarity as a distance measure. 10‐m transects were established along a salinity gradient through four different vegetation zones; a Salicornia zone, a Salicornia‐Atriplex zone, an Atriplex zone and an Atriplex‐Hordeum zone. A UPGMA cluster analysis demonstrated that the above‐ground vegetation was not usually highly correlated with the seed bank composition of zonal communities. Since seeds of these annual salt marsh species occurred in all zones, the levels of salt stress may be the main factor determining which species were found in the above‐ground vegetation.
Variation in stand structure, light and seedling abundance across a tropical moist forest chronosequence, Panama
Authors: Denslow Julie S. , Guzman G. Sandra
Published: May 2006

Abstract. We asked whether forest structure and understory light environments across a tropical moist forest chronosequence followed predictions of a 4‐phase model of secondary succession (establishment, thinning, transition and steady‐state) and whether seedling density and diversity were functions of light availability as predicted by this model. Using aerial photographs, we identified eight second‐growth stands (two each aged ca. 20, 40, 70, and 100 yr) and two old‐growth stands within Barro Colorado Nature Monument, Panama. Trees and seedlings were sampled in nested, contiguous quadrats in 2 160‐m transects in each stand. Light was measured as percent transmittance of diffuse photosynthetically active radiation (TPAR) at each seedling quadrat and by estimation of percent total incident radiation during the growing season from hemispherical canopy photographs. Basal area, tree density, and canopy height followed predictions of the 4‐phase model. Percent total radiation, but not TPAR, declined with stand age as did seedling density. While seedlings were more likely to occur in quadrats at higher light levels, much variation in seedling density was not related to light availability. Seedling patch sizes were small irrespective of light patches, estimated as semivariance ranges. Seedling species richness was a function of seedling density; estimates of species diversity unbiased by density did not vary systematically as a function of stand age. Proximate seed sources, efficient dispersal mechanisms, and appropriate establishment conditions can promote establishment of species‐rich communities early in successions of heterogeneous tropical moist forest.
Landscape and species‐level distribution of morphological and life history traits in a temperate woodland flora
Authors: Mabry Cathy , Ackerly David , Gerhardt Fritz
Published: May 2006

Abstract.We developed a description of a central New England deciduous hardwood forest based on the distribution within the community of morphological and life history traits (N= 34) and environmental factors. Classification by TWINSPAN of 186 species based on morphological and life history traits identified six major functional groups of species largely corresponding to growth form. A data matrix of plots x traits was ordinated using PCA. Each of the resulting four PCA axes was associated with a major environmental gradient: drainage, site exposure, disturbance due to past land use and degree of disturbance in the 1938 New England Hurricane (24.9, 19.8, 11.7 and 8.4% of the variation respectively). Two patterns suggested that a suite of potentially functional traits, rather than a few key characters (e.g. vital attributes), govern the distribution of species in this community: (1) each of the four axes was largely associated with a different group of traits and (2) each axis was associated with several traits that appeared to sort independently (i.e. not to co‐occur within species). Evaluating one often‐examined trait, there was no evidence that dispersal ability limited the colonization of species into secondary woodlands. We also found that landscape‐scale abundance was associated with a small number of traits. Production of fleshy fruits and few diaspores per plant were positively associated with landscape‐level abundance. Our results suggest that attempts to understand the overall structure and function of this plant community based on a few key characters, such as dispersal ability, will meet with limited success. However, when focusing on one aspect of the community, such as frequency across the landscape, relatively few characters may be important.
BOOK REVIEWS
Authors: Keast Allen
Published: May 2006

Mueller‐Dombois, D. & Fosberg, F.R. 1998. Vegetation of the tropical Pacific islands. 733 pp. Springer‐Verlag. ISBN 0–387–98313–9 (softcover). Price USD 59.95.
Short reviews of Nordic publications
Authors: Lawesson Jonas E.
Published: May 2006

Rydin, H., Snoeijs, P. & Diekmann, M. (eds.) 1999. Swedish plant geography ‐ Dedicated to Eddy van der Maarel on his 65th birthday. 233 pp. Opulus Press, Uppsala. ISBN 91–7210–084–2 (paperback), 91–7210–484–8 (cloth). Price: USD 56.‐ (paperback); xx USD (cloth). Joost: graag even checken
Diversity of the understory vascular vegetation in 40 year‐old and old‐growth forest stands on Vancouver Island, British Columbia
Authors: Qian Hong , Klinka Karel , Sivak Bela
Published: May 2006

Abstract. We studied plant diversity of the understory vascular vegetation in 40 yr‐old plantations (immature stands) and old‐growth forest stands on southwestern Vancouver Island, British Columbia, Canada. Site‐specific comparisons using several indices of species diversity were made between: (1) immature stands segregated according to the canopy cover and dominant canopy tree species; and (2) immature and old‐growth stands. There were no significant differences (P < 0.05) among immature stands in species richness (S) and the Shannon‐Wiener index (H′), in relation to the canopy cover or in S, H′ and evenness (E) in relation to the dominant canopy tree species. Using the same indices, the plant diversity varied with edaphic conditions (represented by five site associations) and time (represented by two developmental stages). At both stand‐ and site levels, plant diversity increased with increasing soil moisture, from slightly dry to moist sites, and with increasing plant‐available soil nitrogen in both immature and old‐growth stands; and the plant diversity of immature stands across the sites studied was considerably lower than in old‐growth stands, regardless of site association. The indices of plant diversity, floristic similarity indices, and species turnover rates indicated that the immature stands had their plant diversity at a minimum, but a drastic loss of diversity expected in the stem exclusion stage had not materialized. We attributed decline in plant diversity to the absence of old‐growth structural features in immature stands. Several measures to foster the stand‐level diversity were proposed.
Competition by herbs as a limiting factor in shrub invasion in grassland: a test with different growth forms
Authors: Brown Joel R. , Scanlan J.C. , McIvor John G.
Published: May 2006

Abstract. We tested the hypothesis that seedling establishment, the critical stage in the invasion of grassland by shrubs, is limited by competition with perennial grasses in seasonally wet/dry savannas. We placed seeds of two invasive exotic shrubs – Cryptostegia grandiflora, a woody vine, and Acacia nilotica, an arborescent legume – into pots with a wide range of existing above‐ and below‐ground herbaceous biomass provided by either a tussock or a stoloniferous perennial grass. We also imposed different levels of watering frequency (5, 10 and 21 d), nutrient addition (+ and ‐) and grass clipping intensity (no clipping, clipped to 5 cm and clipped to 25 cm). There was no effect of any treatment on shrub seedling emergence or survival and all of the seedlings that emerged survived the 90‐d growing period. Herbaceous competition also failed to have an effect on biomass accumulation in shrub seedlings. More frequent watering significantly increased above‐ and below‐ground biomass accumulation for both shrub species and nutrient addition significantly increased Cryptostegia biomass accumulation. Based on these results, we question the proposition that reduction in competition by herbs via livestock grazing has been a significant factor in determining the rate or pattern of exotic shrub increase in the seasonally wet/dry tropics. We also question the suitability of the two‐layer soil moisture hypothesis as a basis for management practices to control the ingress of woody species into grasslands and open savannas.
A method to optimize precision and scale in grassland monitoring
Authors: Critchley C. Nigel R. , Poulton Simon M.C.
Published: May 2006

Abstract. Methods for monitoring and survey of plant species abundance which do not account for variation in scale are often insensitive and imprecise. In monitoring, repeated observations are usually made within a fixed unit. Counts of species' presence must be made in a range of subunit sizes to accommodate the range of scales at which different species occur. A method comprising a rectangular fixed unit containing an 8 × 4 grid of square subunits has been developed and tested. Each subunit comprises a series of nested cells of increasing size, within which species are recorded cumulatively. Using this method, the concept of optimum scale is introduced. The optimum scale for a species is that for which its frequency count is closest to the midpoint. Two characteristics, sensitivity (absolute change detected) and blindness (failure to detect change in a species) were calculated for 24 plots from lowland grazing marsh which had been partially flooded between the time of two surveys. Optimum scale had greater sensitivity and lower blindness than any single scale in the majority of plots. Combining sensitivity and blindness, optimum scale was always superior. For the whole sample, optimum scale was less likely to cause Type I or Type II errors. The method is recommended for monitoring grasslands and allied plant communities in large geographical areas.
Plant functional traits, ecosystem structure and land‐use history along a climatic gradient in central‐western Argentina
Authors: Díaz Sandra , Cabido Marcelo , Zak Marcelo , Martínez Carretero Eduardo , Araníbar Julieta
Published: May 2006

Abstract. This paper deals with theoretical concepts, methodological steps, and case studies related to the use of plant functional traits in the assessment of vegetation responses to climate and land use. Trait‐environment links are considered, and special emphasis is put on the links between vegetation structure and ecosystem function, and on the role of disturbance history in determining vegetation responses to land use at present. As a basis for discussion, published and new case studies from central‐western Argentina are presented. Similar plant traits measured with different levels of precision are utilized in the description of ecosystem structure in different land‐use situations along a steep regional climatic gradient. The general protocol followed in the case studies represents a data‐driven, non‐hierarchical, low‐tech approach, that can be applied to a wide range of spatial scales, from plots to regions. Climatic factors (including extreme events and seasonality), disturbance frequency and intensity, and disturbance history are suggested as key factors to be considered in global comparisons of vegetation responses to land use and in predictive models of ecosystem dynamics.
Plant response to disturbance in a Mediterranean grassland: How many functional groups?
Authors: Lavorel Sandra , McIntyre S. , Grigulis Karl
Published: May 2006

Abstract. Data referring to changes in vegetation composition resulting from cattle exclosure and ploughing in a Portuguese pasture dominated by annuals were used to test hypotheses regarding the biology of species favoured or eliminated by disturbance in semi‐natural herbaceous communities. These hypotheses were tested in two ways. First we compared the distribution of six a priori groups – grasses, small rosettes, large rosettes, small species with leafy stems, large species with leafy stems, legumes – across grazed, ploughed and undisturbed plots. In a second set of analyses we examined changes in the frequencies of individual biological attributes in response to grazing and ploughing. These analyses were carried out separately for grasses and dicot forbs. Overall, the species composition showed little response to either grazing or ploughing, though species dominance changed. This lack of responsiveness of species composition was attributed to the long history of intensive land use which has resulted in the loss of disturbance‐intolerant species over entire landscapes.

When considering a priori groups, small rosettes were indifferent to disturbance. grazing and ploughing showed that dominated. Large rosettes, large species with leafy stems and legumes were generally intolerant to both grazing and ploughing, though individual species may increase in response to disturbance. Small species with leafy stems were the only group favoured by grazing whereas ploughing favoured grasses.

As to individual traits, grazing excluded large grass species with heavy seeds and promoted a flat rosette canopy structure and a small size, along with a moderate dormancy and protected inflorescences. In forbs, grazing favoured small species, as expected, while it excluded tall species, and, in contrast to earlier results, a rosette canopy. These attributes were consistent with responses of the a priori groups, though it would not have been possible to reconstruct groups directly from the attribute list. Ploughing had no effect on any of the forb traits. As to grass traits, flat‐ and short‐statured species increased and heavy‐seeded species decreased.

Our analysis revealed two advantages of establishing plant functional classifications within life forms. Subgroups within forbs had contrasting types of behaviour. For the same trait patterns could differ within the grass group from within the forb group. Finally, this analysis emphasizes the need for plant functional classifications aiming at the identification of syndromes of co‐occurring attributes rather than of lists of isolated traits of which actual combinations are not specified.
Patterns in species associations in plant communities: the importance of scale
Authors: Jonsson Bengt Gunnar , Moen Jon
Published: May 2006

Abstract. Present discussions on competitive interactions and the occurrence of predictable patterns in species composition – including assembly rules – are likely to benefit from appropriate analyses of the spatial structure in plant communities. We suggest such an analysis when we specifically want to detect scale regions where fine‐scale local processes may affect the spatial pattern of species composition. We combine indirect ordination in the form of Detrended Correspondence Analysis (DCA) and geostatistics in the form of variography. The species abundance data in the sampled quadrats are summarized as positions on the axes in the ordination. Each axis is used as a regionalized variable in the variography to obtain the spatial dependence of the quadrats. The spatial pattern found will suggest the relevant scale region in which to perform an analysis of species associations. A significant spatial dependence (the ‘range’ in geostatistical jargon) will define the size of a sampling plot that will minimize both the problem of being too small and thus having the risk of oversampling of e.g. clonal individuals and of being too large which will risk including individuals that do not interact. We also suggest that plots are spaced at least a ‘range’ apart to insure spatial and statistical independence. Comparisons of species compositions in such plots will reveal any positive or negative associations between species on a scale where these should reflect species‐species interactions. To illustrate the method it is applied to three different data sets from two different plant communities.
Dynamics of species richness and composition in a limestone grassland restored after tree cutting
Authors: Dzwonko Zbigniew , Loster Stefania
Published: May 2006

Abstract. After abandonment many limestone grasslands have been overgrown by trees and shrubs; as a result, species‐rich communities with many regionally rare and endangered species are vanishing. Some studies suggest that, in cases where grassland is being restored, the species composition and rate of change is strongly dependent on the initial conditions, i.e. the earlier presence of grassland species and the opportunity for colonization of new sites by grassland species. These hypotheses were tested in a five‐year restoration experiment after the clearing of a 35‐yr‐old secondary pine wood developed on abandoned grassland. Tree cutting induced rapid changes in the floristic composition and species cover. The number of grassland species from the class Festuco‐Brometea increased significantly in the restored grassland, but their cover was much lower than in the old grassland. Canonical Correspondence Analysis showed significant differences in species composition between the grassland restored in former wood gaps and that developed in former closed wood. In wood gap sites the cover of species from the class Molinio‐Arrhenatheretea and tufted perennials was much higher, whereas the cover of Festuco‐Brometea species was lower. Significantly more shrubs, woodland species, ruderal and nitrophilous species as well as annual and biennial species occurred in the former closed wood site. It was found that richness and composition of the restored grassland depended strongly on the community composition before tree cutting, as well as on the presence of grassland species in the neighbourhood. Periodical tree cutting enables the maintenance of a temporal‐spatial mosaic of scrub‐grassland communities in isolated habitats and the preservation of local species diversity.
Long‐term species change in a Uruguayan grassland
Authors: Altesor A. , Di Landro E. , May H. , Ezcurra E.
Published: May 2006

Abstract. This paper reports on species changes which occurred in plots in a Uruguayan grassland after 55 yr of continuous grazing. The study was based on the pioneering work of B. Rosengurtt who sampled 10 plots in 1935; in 1990 we re‐sampled five of them. Comparative studies were based on rank‐frequency, diversity, evenness and multivariate analyses. Diversity was measured as species richness S, Shannon's index H and its exponential form N = eH, evenness E = N/S and Gini's coefficient of hierarchy G. Species‐richness of the plots increased significantly. Species which entered the sites are mostly weedy species, while those that have decreased are mostly palatable forage species. Evenness, however, remained constant. Floristic change was chiefly associated with time, and secondarily with plot soil‐type. During the 55 yr that elapsed between both observation periods, a dramatic floristic change has occurred in this grassland site.
Patterns of invasion of an urban remnant of a species‐rich grassland in southeastern Australia by non‐native plant species
Authors: Morgan John W.
Published: May 2006

Abstract. The invasion by non‐native plant species of an urban remnant of a species‐rich Themeda triandra grassland in southeastern Australia was quantified and related to abiotic influences. Richness and cover of non‐native species were highest at the edges of the remnant and declined to relatively uniform levels within the remnant. Native species richness and cover were lowest at the edge adjoining a roadside but then showed little relation to distance from edge. Roadside edge quadrats were floristically distinct from most other quadrats when ordinated by Detrended Correspondence Analysis.

Soil phosphorus was significantly higher at the roadside edge but did not vary within the remnant itself. All other abiotic factors measured (NH4, NO3, S, pH and % organic carbon) showed little variation across the remnant. Non‐native species richness and cover were strongly correlated with soil phosphorus levels. Native species were negatively correlated with soil phosphorus levels. Canonical Correspondence Analysis identified the perennial non‐native grasses of high biomass as species most dependent on high soil nutrient levels. Such species may be resource‐limited in undisturbed soils.

Three classes of non‐native plants have invaded this species‐rich grassland: (1) generalist species (> 50 % frequency), mostly therophytes with non‐specialized habitat or germination requirements; (2) resource‐limited species comprising perennial species of high biomass that are dependent on nutrient increases and/or soil disturbances before they can invade the community and; (3) species of intermediate frequency (1–30 %), of low to high biomass potential, that appear to have non‐specialized habitat requirements but are currently limited by seed dispersal, seedling establishment or the current site management. Native species richness and cover are most negatively affected by increases in non‐native cover. Declines are largely evident once the non‐native cover exceeds 40 %.

Widespread, generalist non‐native species are numerous in intact sites and will have to be considered a permanent part of the flora of remnant grasslands. Management must aim to minimize increases in cover of any non‐native species or the disturbances that favour the establishment of competitive non‐native grasses if the native grassland flora is to be conserved in small, fragmented remnants.
Altitudinal zonation of evergreen broad‐leaved forest on Mount Lopei, Taiwan
Authors: Hsieh Chang‐Fu , Chen Zueng‐Sang , Hsu Yueh‐Mei , Yang Kuoh‐Chieng , Hsieh Tsung‐Hsin
Published: May 2006

Abstract. Evergreen broad‐leaved forest was studied in a transect on the northwestern slope of Mount Lopei in order to reveal altitudinal zonation in structure and floristic composition and the decisive environmental factors. 20 plots of 20 m × 20 m at altitudes from 540 m to 1320 m were analysed. 144 woody species were found. The results of a DCA ordination clearly pointed to a single dominant altitudinal gradient. Nevertheless, wind‐exposure associated topography was found to account for additional variation for a given altitudinal range. Along the altitudinal gradient, four dominance‐based forest types were recognized. Tree density, species diversity and evenness of the four types differed significantly but total basal area and tree volume were not significantly different. The 95 % turnover range for woody species as measured by the Community Coefficient was calculated as 1030 m, and the 50 % turnover range as 238 m. For the understorey, the change in species composition with altitude was less obvious. Species population structures of 57 sufficiently abundant species revealed four characteristic patterns, but most species showed a good fit to the negatively exponential or power function distribution and thus appeared to have good reproduction and regular recruitment. Both ANOVA and Redundancy Analysis (RDA) showed that significant differences among forest types were found for most soil variables. Organic C, exchangeable Na and K tended to increase with altitude, while pH and available N showed a reverse trend. There was little evidence that the differences in soil pH and available N were responsible for the variation in forest growth.
Relative abundance distributions in plant communities: effects of species richness and of spatial scale
Authors: Wilson J. Bastow , Gitay Habiba , Steel John B. , King Warren McG.
Published: May 2006

Abstract. Relative abundance distributions (RADs) are an important feature of community structure, but little is known of the factors affecting which type of RAD is observed in a particular community. We examined the influences of species richness and of spatial scale on the RAD of plant communities.

The effect of species richness was examined by analysing simulated communities generated under the Broken stick model, the Sequential breakage model, and a randomized version of Niche pre‐emption model. In all cases, when there were few species in the community the data only occasionally gave the best fit to the model that was used to generate it. With 40–65 species, a best fit was obtained for the correct model in about 75 % of cases, almost irrespective of the model.

Effects of spatial scale were examined in data from four dune slacks and from two semi‐arid grasslands, by analysing biomass values at a range of sample sizes. The model that best fitted the whole sample differed between the four slacks and between the slacks and the semi‐arid grasslands. The change in which model of RAD fitted best, as sample size was reduced, varied between sites and between habitat types. At the smallest sample sizes, the Zipf‐Mandelbrot model often fitted, and in the slack sites the Broken stick also, though neither fitted (in the vegetation examined) at larger spatial scales.

It is concluded the RAD is affected by species richness and by spatial scale, in ways that currently do not enable simple prediction. RADs can theoretically give information on the processes such as resource partitioning, immigration and competition that have structured the community, but they are a blunt tool for this purpose.
Small rodents as significant dispersers of tree seeds in a Neotropical forest
Authors: Brewer Steven W. , Rejmánek Marcel
Published: May 2006

Abstract. Through seed dispersal and predation, terrestrial mammals should be an important component of the mechanisms that determine patterns of tree recruitment in tropical forests. Despite their great abundance and ubiquity in Neotropical forests, small rodents as seed predators and dispersers remain largely forgotten. To investigate the fates of seeds in a hunted primary forest in Belize, we tagged seeds of Astrocaryum mexicanum (Palmae), Ampelocera hottlei (Ulmaceae), and Pouteria sapota (Sapotaceae) and placed them into open plots, exclosures accessible only to small mammals, and exclosures accessible to medium‐sized and small mammals. The exclosure experiments and fates of the seeds show that the spiny pocket mouse, Heteromys desmarestianus (Heteromyidae), was the dominant handler of seeds of the first two species and also removed a significant proportion of the very large‐seeded Pouteria. Most of the seeds were killed immediately upon removal, but many of the seeds (3–18 %) of the first two species were scatterhoarded (dispersed and buried in the soil) by Heteromys. Some of the scatterhoarded seeds (29%) remain buried and therefore protected from predation by other animals. Agoutis (Dasyprocta punctata), a caviomorph rodent, buried 13 % of the seeds of Pouteria, and Heteromys consumed and dispersed but did not bury Pouteria seeds. Results of this study support predictions by some researchers that small rodents are dominant terrestrial granivores in Neotropical forests. The role of small rodents as seed dispersers, however, has never been fully appreciated.
The resource balance hypothesis of plant species diversity in grassland
Authors: Braakhekke Willem G. , Hooftman Danny A.P.
Published: May 2006

Abstract: We hypothesize that plant species diversity is favoured when actual resource supply ratios are balanced according to the optimum resource supply ratios for the vegetation as a whole. This ‘resource balance hypothesis of plant species diversity’ (RBH) follows from two different mechanisms of plant species coexistence, namely: ‘differential resource limitation’, which allows species to coexist in a competitive equilibrium in a homogeneous environment and ‘micro‐habitat differentiation’, which builds on spatial heterogeneity. Both mechanisms require that resource supply ratios are intermediate between the optimum supply ratios of the species present in the species pool. Additional conditions, concerning the resource acquisition and requirement ratios of the species, are easier to meet for the second mechanism than for the first. To test the RBH we measured species diversity parameters in 74 grassland plots, as well as the N, P and K concentrations in the above‐ground biomass. We used a new ceiling detection algorithm to examine the relationship between maximum observed diversity and the N/P‐, P/K‐ and K/N‐ratios in the biomass. Most of these ceiling relationsips could be described by parabolic curves with significant quadratic terms. This indicates that high diversity does not occur at the extremes of the observed ranges of nutrient ratios. This supports the RBH.
Stream power influence on southern Californian riparian vegetation
Authors: Bendix Jacob
Published: May 2006

Abstract. Mechanical damage by floodwaters is frequently invoked to explain the distribution of riparian plant species, but data have been lacking to relate vegetation to specific estimates of flood damage potential. This research uses detailed estimates of unit stream power (an appropriate measure of the potential for mechanical damage) in conjunction with vegetation cover data to test this relationship at 37 valley‐bottom sites in the Transverse Ranges of Southern California. A computer program, HEC‐2, was used to model the slope and the variation in flow depth and velocity of the 20‐yr flood across the sites. Regression models tested the influence of stream power (and of height above the water table) on the woody species composition of 393 4‐m cross‐section segments of the valley‐bottom sites. Results indicate that unit stream power does have a significant effect on the riparian vegetation, but that the amount of that influence and its importance relative to the influence of height above the water table varies between watersheds. Some species are found primarily in locations of high stream power, while others are limited to portions of the valley bottom that experience only low stream power.
Occurrence of pollen and spores in relation to present‐day vegetation in a Dutch heathland area
Authors: Mulder Christian , Janssen C. Roel
Published: May 2006

Abstract. Pollen, spores and fungal remains in moss cushions along a transect, traversing a Betula‐Quercus forest and a surrounding heathland, were analysed in order to study the relation between present‐day vegetation and recent pollen deposition. Pollen and spores are divided into local types from plant species encountered along the transect and regional types, not present along the transect. Relative percentages and absolute concentration of the palynomorphs were estimated. Radiocaesium activity in the moss was measured to assess the minimal duration of palynomorph accumulation (i.e. the age of the moss samples).

The absence of simultaneous trends in the regional pollen types indicates that the samples do not have major differences in age. The moss represents the later stages in the heath mosaic cycle. This is supported by the radiocaesium content of the moss. Thus, in this case the concentration values of regional pollen approach deposition values. The values of the local pollen types calculated on such a rather constant regional pollen flux is considered to represent real differences in the local deposition.

The local pollen types can be arranged topographically into five groups characteristic of the local vegetation types and their ecotones. The arrangement of the curves of types from phyco‐, zoo‐, and myco‐coenoses suggests their possible origin, either locally from the heath area or from the Betula‐Quercus forest, or regionally from elsewhere. Analyses and topographical arrangement of the curves of unknown types contribute in this way to their identification and their possible source of origin.
Burrowing activities of kangaroo rats and patterns in plant species dominance at a shortgrass steppe‐desert grassland ecotone
Authors: Fields Mark J. , Coffin Debra P. , Gosz James R.
Published: May 2006

Abstract. Our objective was to evaluate the effects of burrowing activities by banner‐tail kangaroo rats (Dipodomys spectabilis Merriam) on plant community structure and species dominance for two patch types at the ecotone between shortgrass steppe and desert grassland in New Mexico, USA. 10 mounds produced by kangaroo rats were selected in patches dominated by Bouteloua gracilis (the dominant in shortgrass steppe communities) and 10 mounds were selected in patches dominated by B. eriopoda (the dominant in Chihuahuan desert grasslands). Plant cover and density by species were sampled from three locations associated with each mound: the mound proper, the edge of the mound in the transition area, and the off‐mound vegetation. Similar cover of B. eriopoda for the edges of mounds in both patch types indicates the ability of this species to respond to animal disturbances regardless of the amount of cover in the surrounding undisturbed vegetation. By contrast, cover of B. gracilis was low for all mounds and mound edges in patches dominated by this species. Much higher cover of B. eriopoda on mound edges compared to the undisturbed vegetation in B. gracilis‐dominated patches indicates that kangaroo rats have important positive effects on this species. Lower cover of perennial grasses and higher cover of forbs, shrubs, and succulents on the edges of mounds in B. eriopoda‐dominated patches compared to patches dominated by B. gracilis indicate the importance of surrounding vegetation to plant responses on disturbed areas.

Our results show that kangaroo rats have important effects on both species dominance and composition for different patch types, and may provide a mechanism for small‐scale dominance patterns at an ecotone; thus providing further support for their role as keystone species in desert grasslands.
Alvar grasslands in Estonia: variation in species composition and community structure
Authors: Pärtel Meelis , Kalamees Rein , Zobel Martin , Rosén Ejvind
Published: May 2006

Abstract.In order to understand the variation of Estonian calcareous thin‐soil grasslands on Ordovician or Silurian limestone (alvars), 58 stands, distributed over the whole alvar region of Estonia, were described and clustered numerically using TABORD. Alvars are characterized by a high species richness. These mainly semi‐natural communities have mostly developed after clear‐cutting of forests. Grazing by domestic animals and removal of woody plants is needed for their maintenance. Primary (natural) alvar grasslands are found in a few places in coastal regions or in areas with thin‐soil on monolithic bedrock. In data processing a whole stand, described by several small relevés, was used in the classification as one description, including species frequencies as a quantitative measure. Seven clusters were separated and their configuration checked in a PCA‐ordination. The resulting classification agreed with the previous ‘rough’ classification. Both geographical and ecological factors were related with the clustering and the ordination. The broad classification units, suggested for Estonian alvar grasslands, were quite similar to the ones described earlier for alvar vegetation on Öland, Sweden. Clusters differed in their species richness, environmental conditions and life‐form spectra. Differences in species richness were defined by regional species pools. Compared to earlier surveys, only a small fraction of alvars still remains in Estonia. Active conservation, i.e. suitable management is needed to protect the still surviving valuable plant communities. The present study can provide guidelines on how to protect and manage different types of alvars in Estonia.
Neighbour diversity in lichen‐dominated communities
Authors: Dale M.R.T. , John E.A.
Published: May 2006

Abstract. In this study, we examined two sets of contact neighbour data from lichen‐dominated communities (one saxicolous, the other epiphytic) to determine whether the diversity of contact neighbours is significantly less than expected, indicating segregation of species. We also wanted to test whether the observed neighbour diversity changed with the species' own frequency. Using two measures of diversity, we found that in both saxicolous and epiphytic communities, there was a significant tendency for neighbour diversity to be lower than expected. The results are consistent with Pacala's spatial segregation hypothesis but may indicate that the plants’responses to environmental variables act at very small scales in these communities.
Invasion and persistence of bird‐dispersed, subtropical thicket and forest species in fire‐prone coastal fynbos
Authors: Cowling R.M , Kirkwood D , Midgley J.J , Pierce S.M.
Published: May 2006

Abstract. Seedling abundance at four microsites (open fynbos, beneath emergent fynbos shrubs, beneath thicket, and beneath forest) was determined at three coastal dune landscapes, located along a gradient of increasing summer rainfall and where fire‐dependent fynbos was the predominant vegetation. At all sites thicket seedlings were most common beneath emergent fynbos shrubs and under thicket clumps; seedlings of forest species were most abundant at forest microsites although some individuals were recorded beneath thicket. Very few thicket seedlings were observed in open fynbos. Birds play a keystone role in facilitating establishment of the fleshy fruit‐bearing thicket flora. Seedling abundance at microsites of different thicket and forest species was generally unrelated to fruit abundance. Germination success of most species was highest under shaded conditions; soil organic content had no effect on germination. Removal of pulp and birdingestion enhanced the germination, relative to untreated controls, of two out of three species tested. A simple Markov model predicted a gradual increase in cover of the thicket and forest component and a gradual decline in fynbos under a ‘normal’ (20‐yr interval) fire regime simulated over 10 cycles. Although inter‐fire seedling establishment under emergent fynbos shrubs is important in the initial colonisation of fynbos by obligate resprouting thicket shrubs, these species persist and expand by vegetative recruitment after and between fires, respectively. In the prolonged absence of fire, the endemic‐rich and fire‐dependent fynbos flora would be replaced by species‐poor forest and thicket.
Ecosystem development on Hawaiian lava flows: biomass and species composition
Authors: Aplet Gregory H. , Hughes R. Flint , Vitousek Peter M.
Published: May 2006

Abstract. The strong environmental gradients and ‘natural experimental design’ of Mauna Loa volcano, Hawaii, provide an outstanding opportunity to study controls on ecosystem development. We measured above‐ground vascular plant biomass and species composition on 42 sites on which precipitation, temperature, substrate texture, and substrate age varied substantially and largely independently. Biomass and species richness of live plants were strongly correlated with precipitation and lava flow age, but not with temperature or lava flow texture. Species composition, as measured by correspondence analysis, was likewise correlated with precipitation and flow age, but composition was also strongly influenced by temperature. Lava texture had a complex effect on vegetation, with ‘a’ a lava favoring vegetation development on wet sites and pāhoehoe favoring development on dry sites. Many locations remain virtually free of invasion by alien species; aliens appear where disturbance has facilitated invasion, either from stand‐level dieback in rainforest or a grass‐fire cycle on the dry, leeward side of the mountain. All four of the environmental factors studied here (precipitation, temperature, substrate texture, and substrate age) exert significant and independent control over vegetation biomass and/or species composition on Mauna Loa.
Predicting the potential distribution of plant species in an alpine environment
Authors: Guisan Antoine , Theurillat Jean‐Paul , Kienast Felix
Published: May 2006

Abstract. The relationships between the distribution of alpine species and selected environmental variables are investigated by using two types of generalized linear models (GLMs) in a limited study area in the Valais region (Switzerland). The empirical relationships are used in a predictive sense to mimic the potential abundances of alpine species over a regular grid. Here, we present the results for the alpine sedge Carex curvula ssp. curvula. The modelling approach consists of (1) a binomial GLM, including only the mean annual temperature as explanatory variable, which is adjusted to species presence/absence data in the entire study area; (2) a logistic model restricted to stands occurring within the a priori defined temperature range for the species ‐ which allows ordinal abundance data to be adjusted; (3) the two species‐response functions combined in a GIS to generate a map of the species' potential abundance in the study area; (4) model predictions filtered by the classes of the qualitative variables under which the species never occur.

Such a stratified approach used to better fit the variability within the optimal altitudinal zone for the species. Removing stand descriptions from altitudes too high or too low, where the species is unlikely to occur, enhances the global modelling performance by allowing the identification of important environmental variables only retained in the second model.

The model evaluation is finally carried out with the γ‐measure of association in an ordinal contingency table. It shows that abundance is satisfactorily predicted for C. curvula.
Invasion of alien woody plants on the islands of Mahé and Silhouette, Seychelles
Authors: Fleischmann Karl
Published: May 2006

Abstract. A method is presented for a quick and easily repeatable evaluation of the state of invasion of alien woody plants and the ecological status (prominence value, diversity, singularity and rejuvenation) of potentially important biodiversity sites in the Seychelles. The usefulness of this method has been tested in a survey on the islands of Mahé and Silhouette. The survey revealed 34 species as invasive alien species, the most prominent being Cinnamomum zeylanicum, Psidium littorale, Adenanthera pavonina and Tabebuia pallida. The greatest intensity of invasion was recorded in Intermediate Forests on moderate or gentle slopes and in stream ravines protected from desiccating winds and insolation. ‘Mountain Mist Forests’, ‘Palm Forests’ and ‘Glacis’ are the least invaded vegetation types. The difference in prominence values of alien and native plant species between leeward and windward sides is significant. Glacis habitats do not show a decline in protection values with decreasing altitude. Prominence values and percentage frequency of invaders are exceeded by the endemic palm Phoenicophorium borsigianum which is able to maintain itself in areas suffering from forest destruction and topsoil erosion.
Forest species in an agricultural landscape in the Netherlands: Effects of habitat fragmentation
Authors: Grashof‐Bokdam Carla
Published: May 2006

Abstract. For 312 forest patches on sandy soils in the Netherlands, effects of fragmentation are studied of forest habitat in the past on the present occurrence of forest plant species. Using regression techniques, the numbers of forest edge, interior, zoochorous and anemochorous species, as well as occurrence of 24 individual species were related to patch area and connectivity measures. Connectivity was defined as the amount of forest habitat around patches within three zones up to 1000 m. Plant categories were distinguished by habitat type and dispersal mechanism. The results showed that number of total species and number of species of all habitat and dispersal categories increased with area. The occurrence of ten individually studied species were also positively related to area. Most of them were interior species. The number of zoochorous species increased with increasing connectivity. Also occurrence of ten individually studied species were affected by connectivity. Interior zoochorous species showed the highest percentage of affected species. The relationship of interior, animal‐dispersed plants to connectivity can be explained by the limited distances covered by their dispersal agents (forest birds and ants) in a non‐forest habitat. Also, some anemochorous plants appeared to be affected by connectivity, especially those with heavy seeds and potentially short distance dispersal. As not all species within a certain dispersal or habitat category react similar to area or isolation, it is suggested that differences in underlying processes of fragmentation such as local extinction and colonization need more focus.
Fire and late‐Holocene expansion of Quercus ilex and Pinus pinaster on Corsica
Authors: Carcaillet Christopher , Barakat Hala N. , Panaïotis Christophe , Loisel Roger
Published: May 2006

Abstract. The natural origin of old Quercus ilex (holm oak) forests on the west coast of Corsica is a matter of dispute. This paper discusses the use of pedoanthracology, especially adapted to approach topics in palaeoecology requiring a high spatial precision. It also shows the importance of fire in vegetation change during the Holocene. Pedoanthracology relies on botanical identification and AMS 14C dating of micro‐charcoal found in soil. Three test pits were dug in a forest currently protected within the ‘Man and Biosphere’ program. These pits yielded large quantities of charcoal. There is evidence that Pinus nigra ssp. laricio (laricio pine) and understorey heather species (Ericaceae) played an important role in the vegetation at the beginning of the Subatlantic (ca. 2500 BP). The importance of Pinus pinaster (maritime pine), Quercus ilex and a few mesophilic species increased between 2000 and 1000 BP. Charcoal fragments found in the soil show the role of fire in the vegetation changes recorded. The late expansion of P. pinaster results from a high fire frequency. The hypothesis proposed in the paper suggests that fires destroyed the remaining toxic humus that had accumulated during the millennia which preceded the arrival of Q. ilex to our study sites. The late development of holm oak forest is a combination of the migration of the species and the increase of fire frequency since 2500 BP, most probably resulting from slash‐and‐burn agriculture.