bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic aspects of diversity
Authors: Margalef Ramon
Published: May 2006

Abstract. Ecological diversity is viewed as a measure of information, as the variety of messages compounded with the number of individuals belonging to different species. Two limit situations of diversity are indicated: a chemostat‐like system tending to become monospecific, and a Noah's ark or museum situation with an infinite number of species each represented by just one specimen. In a dynamic approach, two differential equations for S, the number of species, and N, the number of individuals, can be considered. A new index of diversity is proposed based on these two equations.

The concept of diversity spectrum is proposed for a series of diversity measurements in samples of increasing size and related to the concept of minimal area. Diversity can also be measured on non‐taxonomic categories, e.g. size classes.

The contrast between the two boundary types, limes convergens (ecotone) and limes divergens (ecocline), is emphasized. The nature of the difference between them is compared with that of tension zones between fluids in contact.

Differences between ecosystems can be characterized through differences in the ratio P/B (Production/Biomass) and a boundary situation is imagined including two systems with P/B different values and different diversity levels. Some examples are presented from communities in streams.
Current problems of environmental gradients and species response curves in relation to continuum theory
Authors: Austin M.P. , Gaywood M.J.
Published: May 2006

Abstract. Comparisons of the positions of species on Grimes'C‐S‐R triangular ordination model with their responses to individual environmental gradients indicates that the C‐S‐R model does not necessarily predict species ecological behaviour. The importance of the stress, productivity and disturbance gradients relative to other environmental gradients needs to be determined. In studies of species behaviour along a biomass/productivity gradient the collective vegetation property, biomass, has been confused with the environmental factor, fertility. Patterns of responses to biomass gradients e.g. Keddy's centrifugal model, should be examined in a two‐dimensional environmental space to avoid such confounding effects. Assumptions regarding the shapes of species responses to environmental gradients remain untested. A recent model of species response functions to environmental gradients suggested that skewed responses curves show a pattern in the direction of the skew, always with the tail towards the presumed most mesic position on the gradient. Further evidence is presented to support this model for a temperature gradient in eucalypt forest in south‐eastern Australia. 21 out of 24 species tested conform to the model.
Shrub architecture and occupation of space in a Mediterranean sclerophyllous shrubland
Authors: Guillén Dolores F. , De las Heras Paloma , Herrera Pedro , Pineda Francisco D.
Published: May 2006

Abstract. Shrubland communities in Central Spain were studied through classifying growth forms of woody species and determining the shared use of the ground in progressively smaller spaces. 516 plants belonging to the six most abundant species and taken from different sites were included in biometric measurements. Principal Component Analysis (PCA) was used to detect the trends of variation in the architecture of plants. The individuals were classified on the basis of the results of the PCA and different morphological types were detected, mainly ‘elliptical’, ‘spherical’or ‘variable’according to their shape. These morphological types were adopted by most plants depending on their location and community.

The horizontal occupation of space seems to be determined by whether or not the species rooted close to each other are able to occupy different strata. The co‐occurrence of two species in a reduced space is not facilitated when the two species have the same architecture. Then a spatial segregation tends to occur at a fine scale. The results can be interpreted as an optimization strategy of the shrubland ‘biomass/horizontal occupied area ratio’, which can be maximized in different environmental situations. It can also help to explain the ‘grain’ size of the pattern of horizontal spatial organization of the shrubland.
Evidence for lottery recruitment in Mediterranean old fields
Authors: Lavorel S. , Lebreton J. D.
Published: May 2006

Abstract. The hypothesis of lottery establishment (Sale 1977) explains coexistence of species with similar niches through processes of stochastic recruitment. This initial idea forms the basis for a variety of mathematical models, but has not been tested empirically.

This study is a field investigation of lottery establishment for plants with a seed bank, using Canonical Correspondence Analysis to compare the compositions of the vegetation and the seed bank according to different hypotheses on the mechanisms of establishment.

This method was used for a data set from old fields from southern France. The weighted lottery (i.e. a random draw from the seed pool, weighted by the frequencies of each species) appeared as the best suited hypothesis to explain the high degree of similarity between the vegetation and the seed bank and the relative spatial distributions of the species. Several mechanisms are probably interacting, depending on the life histories of the species. Modelling and experimental approaches are needed to further test the hypothesis of lottery recruitment.
Spatial patterns and dynamics of woody vegetation in an arid savanna
Authors: Skarpe Christina
Published: May 2006

The spatial distribution of woody plants was studied in an arid savanna in Botswana. The study included stands of mixed species and sizes as well as monospecific even‐sized stands of different size classes of the tree Acacia erioloba and the shrub Acacia mellifera. In the case of A. mellifera both dense stands on overgrazed land and more open stands were included. The analysis used all plant‐to‐plant distances, and individuals were represented with a realistic canopy extension. The mixed stands showed aggregated distribution of individuals, mainly caused by strong clumping of small shrubs. In A. erioloba saplings were aggregated, small trees were randomly or regularly distributed and large trees were randomly spaced. In open stands of A. mellifera aggregation increased with size of the shrubs, while in dense stands with overgrazing aggregation decreased with increasing size. The different patterns are discussed in relation to the relative importance of inter‐ and intraspecific competition for water and of disturbance by fire as regulatory mechanisms for total amount and spatial distribution of woody plants in this savanna.
Seed dispersal and vegetation development on a debris avalanche on the Ontake volcano, Central Japan
Authors: Nakashizuka T. , Iida S. , Suzuki W. , Tanimoto T.
Published: May 2006

Seed dispersal and early revegetation processes were studied in an area devastated by a debris avalanche that occurred on volcano Ontake, Central Japan in 1984. The avalanche was initiated by a volcanic earthquake, and a 3.4 x 107 m3 land mass destroyed the vegetation over 700 ha. The revegetation of the first five years was slow in the area of higher elevation (subalpine area). Both the speed of revegetation and the species richness of trapped seeds varied with the elevation. The post‐disturbance vegetation on the deposit with surface soil or plant fragments was richer in species than that without any surface soil. Almost all of the seeds caught by sticky traps and most of the species with frequent occurrence in the vegetation after disturbance were wind dispersed. Wind dispersal was especially important on the deposit without surface soil or plant fragments.
Small‐scale plant species turnover in a limestone grassland: the carousel model and some comments on the niche concept
Authors: van der Maarel Eddy , Sykes Martin T.
Published: May 2006

Abstract. This study reports on small‐scale changes in the distribution of plant species in a 2.5 m2 plot of grazed, species‐rich Veronica spicata ‐ Avenula pratensis grassland on shallow, dry, nutrient‐poor soil in the Great Alvar area (Stora Alvaret) of southern Öland, southeastern Sweden. Multivari‐ate analysis of 0.001 m2 and 0.25 m2 quadrats within the plot showed that there is little floristic variation without any trend in the plot. Average species richness varied little throughout the study period from 1986 to 1991 with 1986 averages of 7.0 on 0.001m2, 16.3 on 0.01 m2, and 26.1 on 0.25 m2. On 0.001 m2the highest species number found was 12, on 0.01 m2, 27. However, cumulative species richness, i.e. species number in the first year plus new species appearing in later years (averaged over 40 quadrats) increased over the same period, on 0.001 m2 from 7.0 in 1986 to 14.9 in 1991, and on 0.01 m2 from 16.3 to 24.1. Cumulative frequency, i.e. number of quadrats out of 40 where a species occurred in the first year or/and in later years, increased as well; the number of cumulatively highly frequent (> 80%) species increased from 5 in 1986 to 18 in 1991.

Species mobility on the scale of the small quadrats used implies both appearance and disappearance from these quadrats. Using six examples, species mobility is shown to vary individualistically, both in rate and degree.

We postulate that in homogeneous, grazed, nutrient‐ and water‐deficient environments many species can reach virtually all microsites within the plot, which we express through the idea of the ‘carousel model’. We also question the usefulness of the niche concept and re‐interpret it by stating that all species of this plant community have the same habitat niche, while most of them are short‐lived and have the same regeneration niche. The essential variation amongst the species is their individual ability to establish or re‐establish by making use of favourable conditions appearing in microsites in an unknown, complex spatio‐temporal pattern.
Early primary succession on the volcano Mount St. Helens
Authors: del Moral Roger , Wood David M.
Published: May 2006

Abstract. Primary succession on Mount St. Helens, Washington State, USA, was studied using long‐term observational and experimental methods. Distance from potential colonists is a major factor that impedes early primary succession. Sites near undisturbed vegetation remain low in plant cover, but species richness is comparable to intact vegetation. Sites over 500 m from sources of potential colonists have as many species, but mean species richness is much lower than in undisturbed plots. Cover is barely measurable after 11 growing seasons. Highly vagrant species of Asteraceae and Epilobium dominate isolated sites. Sites contiguous to undisturbed communities are dominated by large‐seeded species.

For a new surface to offer suitable conditions to invading plants, weathering, erosion and nutrient inputs must first occur. The earliest colonists are usually confined to specific microsites that offer some physical protection and enhanced resources. Primary succession on Mount St. Helens has been very slow because most habitats are isolated and physically stressful. Well‐dispersed species lack the ability to establish until physical processes ameliorate the site. Species capable of establishment lack suitable dispersal abilities. Subsequently, facilitation may occur, for example through symbiotic nitrogen fixation, but these effects are thus far of only local importance. Lupinus lepidus usually facilitates colonization of other species only after it dies, leaving behind enriched soil lacking any competitors. Experiments and fine‐scale observations suggest that successional sequences on Mount St. Helens are not mechanistically necessary. Rather, they result from local circumstances, landscape effects and chance.
Structure and tree‐fall gap dynamics of old‐growth Nothofagus forests in Tierra del Fuego, Argentina
Authors: Rebertus Alan J. , Veblen Thomas T.
Published: May 2006

Abstract. Tree size and age structure, tree‐fall and gap characteristics, and regeneration in gaps were studied in Nothofagus‐dominated old‐growth forests in Tierra del Fuego, Argentina. Gap‐phase regeneration has resulted in all‐aged populations for N. pumilio, N. betuloides, and Drimys winteri, and regeneration in gaps appears to be maintaining coexistence between species in mixed stands. N. betuloides fills many gaps via advance regeneration and some individuals persist for > 150 yr in the understory. Multiple periods of release and suppression indicate that N. betuloides may take advantage of several gap events to reach the main canopy. Likewise, Drimys grows well under closed canopy and can rapidly respond to gap formation, sometimes impeding the regeneration of N. betuloides. In contrast, N. pumilio regenerates in gaps mainly from seed or from advance regeneration of small, ephemeral seedlings. Gap turnover times in Fuegian forests were estimated at 300 ‐ 500 yr, although gap formation was highly episodic and possibly associated with regionally extensive windstorms, earthquakes, and stand‐level dieback. 92 % of gaps involved multiple tree‐falls, and at least 53 % involved secondary expansion. Gap and tree‐fall characteristics in Tierra del Fuego were similar to results from northern Patagonia, Chile, and New Zealand; however, we emphasize that regeneration of Nothofagus spp. and Drimys winteri in gaps depends on associated vegetation and varies along both local and regional environmental gradients.
Multiple species effects and spatial autocorrelation in detecting species associations
Authors: Dale M. R. T. , Blundon D. J. , MacIsaac D. A. , Thomas A. G.
Published: May 2006

Abstract.The traditional approach to the analysis of species association within a community, based upon co‐occurrence in sampling units such as quadrats, has been to test all pairs of species, using a 2 × 2 contingency table for each pair. It has long been recognised that all these tests are not independent of each other, but there is an additional problem in that the association between any particular pair may depend on the combination of the other species that are present or on the environmental factors that determine that combination. We use a 2kcontingency table to examine this problem and find that pairwise associations are not independent of the other species.

The second problem that we consider is the effect of spatial autocorrelation in the data which makes the statistical tests too liberal. In the absence of a derived solution for a deflation factor to correct the test statistic calculated from a 2ktable, we describe a Monte Carlo approach that provides an approximate solution to this problem. In our data the amount of deflation that is necessary for a 2ktable is small compared to the amount required for the 2 × 2 tables used to test pairwise association.
Plant communities of trampled habitats in North Korea
Authors: Mucina L. , Dostálek J. , Jarolímek I. , Kolbek J. , Ostrý I.
Published: May 2006

Abstract. A syntaxonomic study of trampled plant communities in North Korea is presented. Analytic and synthetic methods of the Braun‐Blanquet approach together with a numeric‐syntaxonomical analysis (cluster analysis and principal coordinate analysis ordination) were employed. With the exception of the Bryo‐Saginetum japonicae, all the associations, such as the Artemisio asiaticae‐Plantaginetum asiaticae, Plantagini depressae‐Polygonetum avicularis, Polygono avicularis‐Potentilletum costatae, Eragrostio multicaulis‐Plantaginetum depressae, Euphorbio maculatae‐Centipedetum minimae, Digitario pectiniformis‐Eleusinetum indicae and Setario viridis‐Chlorisetum virgatae, are described for the first time. Some communities also include a number of subassociations. Trampled communities are found along edges of paths and in cracks among paving stones. Soils vary from loamy to sandy and skeletal.

A phenomenon of seasonality in expression of ruderal communities was observed in North Korea. It is assumed that this seasonality is controlled by pattern of precipitation showing distinct climatic seasons (e.g. pre‐monsoon and post‐monsoon periods). The ruderal vegetation seasonality is supposed to become more pronounced towards tropical regions. Several mesophilous or slightly hygrophilous European species occur frequently in North Korean trampled communities including Chenopodium glaucum, C. ficifolium and Potentilla supina. Their occurrence in trampled habitats is hypothesized as being related to high air humidity and associated wet climate.
Floristic division of the Arctic
Authors: Yurtsev Boris A.
Published: May 2006

Abstract:The progress in the floristic study of the circumpolar Arctic since the 1940s is summarized and a new floristic division of this region is presented. The treeless areas of the North Atlantic and North Pacific with an oceanic climate, absence of permafrost and a very high proportion of boreal taxa are excluded from the Arctic proper. It is argued that the Arctic deserves the status of a floristic region. The tundra zone and some oceanic areas are divided into subzones according to their flora and vegetation. Two groups of subzones are recognized: the Arctic group (including the Arctic tundras proper and the High Arctic) and the Hypoarctic group.

The Arctic phytochorion is floristically divided into sectors: 6 provinces and 20 subprovinces reflecting the regional features of each sector in connection with flora history, physiography and continentality‐oceanity of the climate. Each sector is described and differentiated by a set of differential and co‐differential species. The peculiarities of the Arctic flora are manifest in different ways in the various sectors, and endemism is not the universal criterion for subdivision.
The evaluation of multi‐species pattern
Authors: Dale M.R.T. , Zbigniewicz M.W.
Published: May 2006

Abstract. Using artificial data, techniques that have been proposed for analyzing multiple species pattern in vegetation, are compared. No single method was capable of detecting, clearly and unambiguously, all scales of pattern in all cases, and the effectiveness of the different methods was found to depend on the scales of pattern of the component species and on how the patterns of individual species are combined. Some improvements for the application of the popular multi‐scale ordination method are suggested. Several sets of field data are analyzed and the results used to illustrate a discussion of the existence and nature of multi‐species pattern in vegetation and how it is to be evaluated.
Evaluating classification strategies
Authors: Dale M.B.
Published: May 2006

Abstract.This contribution is a comment on the paper by Belbin & McDonald (1993) on the comparison of three classification strategies for use in ecology. There are two problems in evaluating clustering methods: does the sample adequately reflect the population structure, and what is the nature of the clusters sought. First, one has to decide on the number of clusters to be obtained. Possibly the best approach of all is the Bayesian coding theory for inductive inference. This may depend on the objectives of the clustering, which can be manifold. Phytosociologists do not agree on the nature of the clusters they seek, and are reticent in providing a formal definition of their clusters. As a method for identifying gradients Correspondence Analysis has had some success, so that a classification method largely based on it, notably TWINSPAN, may better reflect what phytosociologists are intuitively seeking than alternative variance minimisation methods. Additionally, TWINSPAN incorporates the characterisation through indicator species. Maybe we are more interested in these differentiating species than in the existence of clustersper se.
Blurry artifacts: a retraction
Authors: Palmer Michael W.
Published: May 2006

Abstract. This note is to apologize for an error in the computer program used to evaluate the random data used in Fuzzy Set Ordination according to Zhang & Oxley. After correction of this error no artifacts could be detected any longer. However, the basic conclusion of the earlier critical note still stands: if one selects environmental variables after analyzing the results of a multivariate gradient analysis, and then uses these variables as input into a multiple univariate gradient analysis, the results are expected to be comparable.
Spatial and temporal variability in California annual grassland: results from a long‐term study
Authors: Hobbs R.J. , Mooney H.A.
Published: May 2006

Abstract. We present data from the first 11 years of a longterm study of the dynamics of an annual grassland on serpentine soil in Jasper Ridge Biological Preserve, Northern California. Annual rainfall amounts and distributions varied greatly over the period 1982‐1993, as did the amount and distribution of gopher disturbance. Temporal variation in gopher disturbance showed no relationship with rainfall, but spatial variation in disturbance frequency was related to soil depth. The disturbance regime experienced by the grassland is complex, both spatially and temporally, and most of the area is disturbed at least once every 3‐5 years. Plant species abundances showed a variety of responses to climate variation and disturbance. Abundances of individual species in any given year could not be linked directly to rainfall amount due to hysteresis effects and other interactions. The grassland composition changed markedly over the study. Exclusion of gophers suggested that changing abundances of several species were linked to gopher disturbance. In particular, perennial species' abundances increased greatly in the years following exclosure, but then subsequently declined. Data on plant densities on gopher mounds disturbed at different times of year and in different years indicate that the local species composition remains distinct for a number of years following disturbance. Disturbance history is hence a major factor controlling local community variation. Changing species importances, a complex disturbance regime and the importance of disturbance history make prediction and modelling of this system difficult. It is suggested that the same is probably true for many plant communities, and that long‐term studies must be an essential part of ecological research programs. This study illustrates the practical problems inherent in maintaining long‐term field experiments and in analyzing complex time series data which suffer from inadvertent deviations from the original experimental design.
Spatial relations between shrubs andProsopis glandulosacanopies
Authors: Franco‐Pizaña Jesus , Fulbright Timothy E. , Gardiner Duane T.
Published: May 2006

Abstract.Prosopis glandulosa, an arborescent legume, may act as a nurse plant that facilitates the establishment of other woody species. We hypothesized that attenuation of radiant energy and increased soil nutrients beneathP. glandulosacanopies facilitate establishment of subordinate shrubs and shrub cluster development. We determined the spatial distribution pattern of shrubs underP. glandulosaat three locations in southern Texas. Density ofCeltis pallida,Zanthoxylum fagara, and total woody plants were comparable among the four cardinal directions at each location, which countered the prediction that shrub density would be greater on the north side ofP. glandulosacanopies if attenuation of solar energy was a factor in cluster development. Total woody plant density increased with increasingP. glandulosabasal diameter, canopy radius, and height only at one location. Total woody plant density decreased with increasing total N in the upper 15 cm of soil at two of the three locations. Late in shrub cluster development, extraction of N from the soil and incorporation of N into plant tissue in dense shrub clusters may operate to inhibit further increases in subordinate shrub density.
A functional analysis of a limestone grassland community
Authors: Thompson K. , Hillier S.H. , Grime J.P. , Bossard C.C. , Band S.R.
Published: May 2006

Abstract. We examine the potential of standardised data, mainly collected in the laboratory, to elucidate mechanisms of species coexistence, using a south‐facing, infertile, species‐ rich limestone grassland at Miller's Dale, Derbyshire, England as a model system. Standardised autecological data for 15 of the more important Miller's Dale species were obtained from the Integrated Screening Programme (ISP), and their functional attributes were reviewed in the context of a larger database, broadly representative of the British flora. Ordination of adult plant attributes demonstrated that the Miller's Dale species are strategically relatively uniform; most are slow‐growing, evergreen and tolerant of mineral nutrient deficiency. Ordination of seed attributes revealed more variety among the Miller's Dale species; the main axis of variation was from species with large, transient seeds (mostly grasses) to species with small, persistent seeds (mostly dicots). From the ordination we derive the predictions that germination of the latter species will be more temporally opportunistic and more inhibited by the presence of vegetation than the former. These predictions are consistent with data collected in experimental studies of seed persistence and germination at the site. These data suggest that spatial and temporal variation in opportunities for regeneration is an important mechanism contributing to the high species richness of dry limestone grasslands.
Competition between three Chihuahuan desert species: evidence from plant size‐distance relations and root distribution
Authors: Briones Oscar , Montaña Carlos , Ezcurra Exequiel
Published: May 2006

Abstract. Using size‐distance data we tested the intensity and importance of competition between Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a succulent) in the Chihuahuan desert. We also compared the vertical and horizontal distribution of roots to assess the potential degree of overlap in the use of soil resources. The relationships between sizes and distances of nearest‐neighbour plants suggested that intraspecific competition is generally more important than interspecific competition. However, evidence of stronger inter than intraspecific competition was found in some cases. Species combinations showing significant interspecific competition involved always Opuntia, whereas Larrea and Hilaria did not influence each other. The analysis of the symmetry of competition showed that Opuntia was adversely affected by the presence of Hilaria or Larrea.

Although differences were found in the distribution of roots, the results of the size‐distance study support the idea that, (particularly) Opuntia, below‐ground niche differentiation is not sufficiently important to totally avoid the negative effects of plant competition.
Stand dynamics of the shade‐tolerant conifers Podocarpus nubigena and Saxegothaea conspicua in Chilean temperate rain forest
Authors: Lusk Christopher H.
Published: May 2006

Abstract. Population structures were used to infer regeneration patterns of the conifers Podocarpus nubigena and Saxegothaea conspicua and associated angiosperm tree species in six stands in the temperate rainforests of the coastal ranges of southern Chile. On poorly‐drained, nutrient‐poor and high altitude sites, where most of the associated species were fairly shade‐intolerant and light‐crowned, dense all‐aged populations and the presence of numerous saplings beneath the canopy suggested continuous regeneration of both P. nubigena and S. conspicua. On more favourable sites, where several of the associated angiosperms were highly shade‐tolerant and densecrowned, both conifers, particularly P. nubigena, were less abundant, and their regeneration from seed appeared to be sporadic. S. conspicua's great longevity (≥ 750 yr) and frequent vegetative reproduction are probably crucial to its persistence in competition with shade‐tolerant broadleaved species in undisturbed stands on favourable sites. Results of this study, and a review of related literature on southern temperate forests are not consistent with the hypothesis that heavily‐shaded, infrequently‐disturbed habitats are an evolutionary refuge for conifers. Sites likely to have high leaf area indices and infrequent disturbance are better exploited by shade‐tolerant angiosperms.
Rate of compositional turnover along gradients and total gradient length
Authors: Oksanen Jari , Tonteri Tiina
Published: May 2006

Abstract. A new measure of compositional turnover rate at any gradient point is proposed. The absolute values of the derivatives of estimated species response curves are used to estimate the instantaneous rate of change of the community. The total gradient length can be found by integrating the instantaneous turnover rates over the gradient span. Both the instantaneous rate and total gradient length have been known under the name of beta diversity. Our method is elaborated for the Gaussian response function. It is compared to Hill's SD measure, much used for estimating the gradient length in connection of Detrended Correspondence Analysis with nonlinear rescaling for axes. The turnover rate along a canopy cover gradient in forest vegetation in Finland was used as an example. Forest site type and stand age were used as subsidiary gradients. The two ways to compute Hill's scaling gave very different results and were not comparable with the new method. The turnover rate was dependent on the gradient position. However, the turnover rate along the gradient of interest and its total length were dependent on the subsidiary gradients.
The relationship between the soil seed bank and above‐ground vegetation of a coastal barrier island
Authors: Looney Paul B. , Gibson David J.
Published: May 2006

Abstract. The germinable soil seed bank is described from a coastal barrier island off the northwest coast of Florida, USA. Soil samples collected from seven vegetation types, recently deposited dredge spoil and unvegetated areas in autumn 1990 and spring 1991 were placed out in greenhouse trays. 110 taxa germinated from the samples with the largest number (41) being C3 perennial dicots. The largest number of taxa germinated from dry (57) and wet (54) swales, the fewest (one species: Heterotheca subaxillaris) from strand. Similarity of seed bank densities to above‐ground species cover was low (Jaccard's Index = 0.36), not different between vegetation types, but higher in the autumn than in the following spring. Compositional gradients in the seed bank and above‐ground vegetation determined using DCA ordination were highly correlated and related to distance from mean high water, and plot elevation. At the landscape scale, the seed bank provided an equally clear delineation of vegetation types to that based upon the above‐ground vegetation. The seed bank of low disturbance, late‐succession vegetation types (wooded dunes, swales, marshes) was well developed (high species richness, emergent density, and percentage annual species) with the exception that the large‐seeded woody species (i.e. Quercus spp.) were absent from the wooded dune seed bank. By contrast, a poorly developed and transient seed bank occurred in more frequently disturbed (extensive sand movement, salt spray), early successional dredge spoil, unvegetated areas and strand. These contrasts support a general pattern of increasing seed bank development and a persistent rather than transient seed bank with decreasing disturbance frequency, increasing time since disturbance and successional maturity.
Can soil organic carbon isotopes be used to describe grass‐tree dynamics at a savanna‐grassland ecotone and within the savanna?
Authors: McClaran Mitchel P. , McPherson Guy R.
Published: May 2006

Abstract. We evaluated the use of soil organic carbon (SOC) isotopes to describe grass‐tree dynamics at locations at the savanna‐C4 grassland ecotone and within a temperate semiarid Quercus savanna in southeastern Arizona, USA. SOC will not describe grass‐tree dynamics at locations within the savanna because isotope composition near the soil surface does not correspond with the overlying vegetation and recent C3 carbon has been deposited at deep soil depths with no C4analog. In contrast, SOC can describe grass‐tree dynamics at the savanna‐grassland ecotone because isotope composition near the soil surface corresponds with overlying vegetation and significant deep soil deposition of C3 carbon was not apparent. At the ecotone, trees became established in the last 700–1700 years. There is no evidence to suggest an unstable grass‐tree mixture at the ecotone since that time.
Seed rain, seedling establishment and clonal growth strategies on a glacier foreland
Authors: Stöcklin Jürg , Bäumler Esther
Published: May 2006

Abstract. Seed production, composition of the seed rain, germination, and seedling mortality, as well as vegetative growth characteristics of common pioneer plant species were studied on the foreland of the retreating Morteratsch glacier in the Swiss Alps. The frequency of diaspores trapped in different successional stages was related to their dispersal mode and was highly skewed towards a few species. Plenty of diaspores well adapted for dispersal by wind are a precondition for the most important pioneer species. Seeds from all pioneer species investigated had a good germination success, provided that the moisture content of the soil was high enough. However, requirement for seedling establishment differed among sites of increasing terrain age and among species. Only specialized pioneers such as Cerastium pedunulatum, Linaria alpina, Oxyria digyna and Saxifraga aizoides tolerate the cold and moist conditions near the glacier. However, these species are restricted to early successional stages. Seedlings of Epilobium fleischeri are affected not only by the cold and moist conditions near the glacier but also by moderately dry conditions on older sites. Availability of safe sites becomes crucial for most species with increasing age of sites and with drier conditions. Most species playing a dominant role during early succession and persisting during later successional stages have a distinctive ability to spread clonally and have a growth form with more or less widely spaced ramets: Achillea moschata. Cerastium pedunculatum. Epilobium fleischeri and Hieraium staticifolium. The growth strategy and demography of the clonal E. fleischeri is presented as an example. The life cycle of this species is characterized in succession by (1) the colonization of safe sites by small seeds adapted for wind dispersal, (2) horizontal spread by clonal growth, and (3) the persistence through phenotypic morphological plasticity in later successional stages. Seedling establishment and clonal growth are thus complementary mechanisms in plant succession on recently deglaciated terrain.
Impact of different sheep grazing intensities on salt marsh vegetation in northern Germany
Authors: Kiehl K. , Eischeid I. , Gettner S. , Walter J.
Published: May 2006

Abstract. Mainland salt marshes in Schleswig‐Holstein (northern Germany) have been grazed intensively by sheep for several decades. In 1988 experimental sites were established in the lower and middle salt marsh of Sönke‐Nissen‐Koog and subjected to different grazing intensities. From 1989 to 1993 the impact of sheep grazing on the composition and structure of the salt marsh vegetation was studied through the yearly analysis of permanent plots, vegetation mapping and measurements of the vegetation height.

The intensively grazed site (10 sheep/ha) is covered by a short monotonous Puccinellia maritima sward with Salicornia europaea and Suaeda maritima. Halimione portulacoides and Aster tripolium, especially flowering plants, are rare.

On the sites with 1.5 and 3 sheep/ha Puccinellia maritima remained dominant. The population density of Salicornia europaea decreases after reduction of the grazing intensity, whereas Suaeda maritima finds optimal growing conditions. Stands of Halimione portulacoides and flowering Aster tripolium plants are rare near the sea dike but their cover and size increases further away from the dike. In the plot with 1.5 sheep/ha the height of the vegetation increases along the gradient from the dike towards the tidal flats, due to local differences in actual grazing intensity.

When grazing is stopped, Puccinellia maritima is successively replaced by Festuca rubra, Halimione portulacoides and Aster tripolium. High variability of vegetation height indicates structural diversity. Patches of higher and lower vegetation correspond with the distribution pattern of different plant species.

In terms of nature conservation cessation of grazing is recommended.
Genetic variation in small, isolated fern populations
Authors: Schneller Johann Jakob , Holderegger Rolf
Published: May 2006

Abstract.Differences in genetic variability of several small, isolated populations of four fern species in a restricted area in the Swiss lowlands reflect differences in breeding system, population size, the degree of population fragmentation, and ecological requirements. The investigated populations ofAsplenium septentrionaleshow only little genetic variability (isozyme variation) without gene flow among populations (based on the banding pattern of multi‐locus phenotypes), and they persist for long periods despite the small population sizes. InAsplenium ruta‐muraria, genetic variability is correlated with age. Young populations show no genetic variation, while old populations show some. All individuals ofPolypodium vulgareinvestigated, either epiphytic or epilithic, share exactly the same enzyme phenotype. The results for these three species can be related to predominance of inbreeding, lack of inbreeding depression, polyploidy, long‐distance dispersal, production of large amounts of diaspores, single‐spore colonization, and perennial life cycles. Genetic variability in these three species does not seem to be absolutely necessary for the maintenance of their populations. Ecological and demographic factors are considered to be more important. An isolated, glacial relict population of diploidAsplenium virideshows high variability in two out of eight enzyme systems, which may be due to prevailing outbreeding. We discuss aspects of the importance of genetics and life history for conservation biology.
Structure and ecology of epiphyte communities of a cloud forest in central Veracruz, Mexico
Authors: Hietz Peter , Hietz‐Seifert Ursula
Published: May 2006

Abstract. 39 species of vascular epiphytes were found in a 625‐m2 plot in a cloud forest in Veracruz, Mexico. Epiphyte biomass was estimated for all species in six zones distinguished on each tree > 10 cm in DBH: stem base, lower stem, upper stem, branches > 20 cm, branches from 5 — 20 cm and branches < 5 cm diameter. Branches were additionally separated according to their position in the upper, middle or lower canopy. Total dry matter of green organs was 318 kg/ha. Dominance/diversity curves fitted best to the lognormal model. Principal Coordinate Analysis showed that canopy height and branch thickness are both important factors and also that the stem base was the most distinct zone. Microhabitat preferences and niche overlap of the frequent species proved some degree of resource partitioning within the canopy. Ecophysiological characters possibly responsible for these preferences and implications for conservation are discussed.
Ecological responses of dominant grasses along two climatic gradients in the Great Plains of the United States
Authors: Epstein H.E. , Lauenroth W.K. , Burke I.C. , Coffin D.P.
Published: May 2006

Abstract. Few empirical data exist to examine the influence of regional scale environmental gradients on productivity patterns of plant species. In this paper we analyzed the productivity of several dominant grass species along two climatic gradients, mean annual precipitation (MAP) and mean annual temperature (MAT), in the Great Plains of the United States. We used climatic data from 296 weather stations, species production data from Natural Resource Conservation Service rangeland surveys and a geographic information system to spatially integrate the data.

Both MAP and MAT were significantly related to annual above‐ground net primary production (ANPP). MAP explained 54% to 89% of the variation in ANPP of two C4 short‐grasses, Bouteloua gracilis and Buchloë dactyloides, and two C4 tall‐grasses, Andropogon gerardii and Schizachyrium scoparium (= Andropogon scoparius). MAT explained 19% to 41% of the variation in ANPP of two C4 grasses, B. gracilis and B. dactyloides, and 41% to 66% of the variation in ANPP of two C3 grasses, Agropyron smithii and Stipa comata. ANPP patterns for species along both gradients were described by either linear, negative exponential, logistic, normal or skewed curves. Patterns of absolute ANPP (g/m2) for species differed from those of relative ANPP (%) along the MAP gradient. Responses were similar for species with common functional characteristics (e.g. short‐grasses, tall‐grasses, C3, C4).

Our empirical results support asymmetric responses of species to environmental gradients. Results demonstrate the importance of species attributes, type of environmental gradient and measure of species importance (relative or absolute productivity) in evaluating ecological response patterns.
Patterns of species diversity on an oceanic versus a continental island mountain: a hypothesis on species diversification1
Authors: Kitayama Kanehiro
Published: May 2006

Abstract. The vegetation on a wet mountain slope on Haleakala (an oceanic island) is compared with that on Kinabalu (a continental island) to examine relationships between regional floristic richness and α‐ and β‐diversities. The two mountains are similar in their constant tropical climate, generic and family‐level floristic elements and geological age of the summit regions, but different in regional floristic richness (rich on Kinabalu vs. poor on Haleakala). α‐diversity of canopy and subcanopy tree species was much higher on Kinabalu than in comparable zones on Haleakala. Average turnover rate of species (as logarithmic community similarity) on the slope was one order of magnitude greater on Kinabalu than on Haleakala (0.127 vs. 0.017 per 100 m alt.). While there were genera with wide altitudinal ranges on both mountains, a large proportion of the genera was differentiated into parapatric altitudinal congeners on Kinabalu. By contrast, most genera are altitudinally monotypic on Haleakala. The number of sympatric congeners per genus, and the frequency of multi‐specific genera per plot were high on lower slopes but decreased with increasing altitude on Kinabalu, whereas the values were low across all altitudes on Haleakala. These patterns suggest that sympatric and parapatric species radiation was less on Haleakala than on Kinabalu. This may be related to Haleakala's initially poor and disharmonic flora.
Post‐fire understorey regeneration in boreal Pinus sylvestris forest sites with different fire histories
Authors: Kuuluvainen Timo , Rouvinen Seppo
Published: May 2006

Abstract. We studied the characteristics of understorey regeneration on two sites with different fire history in a mature Pinus sylvestris forest in eastern Finland. The study area was a 4‐ha plot, which was divided into two parts based on fire history analysis. In one part the last fire event was a stand‐replacing fire in the early 19th century, after which the whole stand regenerated, while the other part of the study plot was subsequently burnt by a surface fire in 1906. Understorey P. sylvestris individuals were much more abundant in the area of the 1906 burn compared to the old burn. In both areas the size frequency distribution of living trees was bimodal, with frequency peaks at the < 5 cm and 30–150 cm height classes. In the old burn small understorey trees were mainly associated with microsites created by treefall disturbances while in the 1906 burn most small understorey trees occurred on vegetation‐covered microsites. This indicates that with increasing time since last fire establishment of new understorey trees becomes more restricted by the availability of microsites created by treefall disturbances. In both areas the proportion of vigorous small understorey trees was highest on decayed wood. In the older burn uprooted pits and mounds also had a significant proportion of healthy small understorey trees, while the majority of trees classified as seriously weakened or dying were growing on microhabitats characterized by undisturbed vegetation. Ripley's K‐function analyses showed that spatial distribution of understorey trees was clustered in both areas in all microsite types and clustering at small scales was most pronounced in understorey trees growing in uprooted spots or in association with decayed wood. The bivariate analysis showed a significant repulsion effect between large trees and understorey trees at intermediate spatial scales, indicating that competition had an effect on understorey tree distribution and this effect was more pronounced in the younger burn. The analysis suggests that in Pinus sylvestris forests the abundance, quality and spatial pattern of understorey tree population may vary considerably as a function of disturbance history.
Recovery of a Betula pubescens forest in northern Sweden after severe defoliation by Epirrita autumnata
Authors: Tenow Olle , Bylund Helena
Published: May 2006

Abstract. Mountain birch (Betula pubescens ssp. czerepanovii) forest in the Abisko valley of northern Sweden was completely defoliated by Epirrita autumnata caterpillars during an outbreak in 1954–1955. The defoliation resulted in an 80–90% mortality of the leaf‐carrying shoots of birches in 1956 and triggered a rejuvenation of stands. The subsequent regrowth of foliage was studied in two damaged birch stands and in one unattacked stand. The number of leaves approximately doubled in the damaged stands between 1961 and 1987, while the number on the reference plot fluctuated without significant increase. Regrowth started with increased production of long shoots from surviving shoots and basal sprouts. Basal sprouts were a substantial source of new shoots in the recovery of the foliage, especially on the most damaged plot. Trees of seed origin constituted a minor fraction of the regrowth. Initial rapid growth of foliage reduced gradually and the annual leaf production in 1986/1987 was 75% of that of the reference plot. Comparison between the recovery curve and data from the reference plot indicates that the shoot population of the damaged forest will, after more than 30 years, need many more years to reach the assumed size of a mature forest. The degree of rejuvenation varied between stands, with different consequences for future dynamics of E. autumnata populations.
Regeneration of a dominant Mediterranean dwarf‐shrub after fire
Authors: Seligman No'am G. , Henkin Zalmen
Published: May 2006

Abstract. The regeneration after fire of the dominant east Mediterranean dwarf‐shrub Sarcopoterium spinosum was studied under various habitat and geographic conditions in northern Israel, northern Crete and the Aegean coast of Turkey. Age structures of regenerating stands were determined 3–9 yr after fire on 13 1 mx 1 m sample stands by analysis of growth rings in the taproot crown of 874 plants. S. spinosum regenerated either by massive seedling recruitment in some habitats or by resprouting from surviving meristems in others. This versatility facilitates adaptation of the species to widely different environmental conditions and is a factor in the maintenance of its dominance after fire at occupied sites.

In the years after fire, the growth of younger plants within the stand at all sites was severely suppressed by the older shrubs. Consequently, the age structure of the stands was dominated by one or two older cohorts that became established soon after fire. In addition, on most habitats, older shrubs (usually older than 6 yr) were often composed of clusters of plants that suggested clonal proliferation of ramets. The strongly peaked age structure, together with the appearance of clonal regeneration on the older plants, suggests that subsequent maintenance of the stand was not dependent on recurrent seedling recruitment. This can explain the persistence of S. spinosum in stands of herbaceous vegetation despite the extreme sensitivity of the seedlings to competition under such conditions.