Patterns of β-diversity in a highly diverse tropical dry forest tree community are described; the contribution of environmental heterogeneity and distance to β-diversity was assessed. Significant differences in elevation, insolation, slope and soil water holding capacity (p < 0.01), variables related to water availability, were found among 8 30 m × 100 m transects laid along contrasting slopes of a system of three parallel microbasins. A gradient in elevation and insolation was found within north-facing transects, among 10 m × 10 m sites; south-facing transects showed an elevation gradient while crest transects showed a gradient in water holding capacity. In total 119 species were registered, with 27 to 64 species per transect, and 4 to 16 species per site. A large β-diversity was found among and within transects; two indices of β-diversity consistently showed a higher β-diversity within transects than among them. Among transects, 64% of the variance in species composition could be attributed to the environmental variables; an additional 22% to the spatial distribution of sites. Within transects, 42% of the deviance in β-diversity values was explained by insolation, and 19% by distance. β-diversity increased with distance and with difference in insolation among sites; north-facing transects, those with most contrasting insolation conditions, had the steepest increase in β-diversity with distance. Such increase was clearly associated with changes in species composition, not with changes in species richness.

Nomenclature: Lott (1993; in press).

Abbreviations: AWHC = Available water holding capacity; TDF = Tropical dry forest.

We analysed the relationship between seed traits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same area. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Multiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small-seeded species without specialised dispersal structures are over-represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and small-seeded species were relatively more abundant in the upper than the lower slope zones. This pattern is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability.

Abbreviations: CSA = Cross species analysis; PIA = Phylogenetically independent analysis; PIC = Phylogenetically independent contrast.

Nomenclature: Castroviejo (1986–1999), except taxa yet to be covered which follow Tutin et al. (1964–1980).

The validation of plant functional type models across contrasting landscapes is seen as a step towards the claim that plant functional types should recur regionally or even globally. I sampled the vegetation of an urban landscape on a range of sites representing gradients of resource supply and disturbance intensity. A group of plants with similar attributes was considered a ‘functional type’, if the species significantly co-occurred in a certain segment of the gradient plane of resource supply and disturbance intensity. Vegetative and regeneration traits were considered. A similar study was performed in a nearby agricultural landscape (Kleyer 1999). The logistic regression models from the urban landscape were applied to the data set of the agricultural landscape and vice versa. Although the overall environment of the two landscapes was very different, recurrent patterns of several functional types were found. At high fertility and high disturbance levels, annual species predominated with a persistent seed bank, high seed output, and short vertical expansion. When disturbances changed from below-ground to above-ground, the sexual regeneration mode was replaced by the vegetative mode, while vertical expansion remained low. At medium disturbance intensities, the vertical expansion and vegetative regeneration increased with fertility, while the seed bank remained mostly transient to short-term persistent and lateral expansion and sexual regeneration was intermediate. At low disturbances and low resource supplies, seed bank longevity, and vertical and lateral expansion tended to be long. Diversity of groups of plants with similar attributes was highest at intermediate disturbance levels and low fertility. These results correspond with Grime’s humped-back model and Connell’s intermediate disturbance hypothesis.

Nomenclature: Ellenberg et al. (1991) for plants; Anon. (1988; modified) for soil types.

Fagus grandifolia var. mexicana seedling dynamics were studied in a relict forest inside the crater of a volcano where it is the only canopy tree species. The objectives of the study were (1) to determine changes in seedling density before and after a mast year, as well as to monitor seedling recruitment, mortality and growth rate in relation to the micro-environment, and (2) to discuss the impact of masting on seedling dynamics of the population. Before masting, seedling density was 8.8 seedlings m^–2, afterwards seedling density peaked at 51.5 seedlings m^–2, then decreased by 50% in 3 months and by 85% after 8 months, down to levels of before the mast year (9.9 seedlings m^–2) after 16 months. Seedlings in the forest floor before the mast-seeding event showed a low relative growth rate − 0.14 mm mm^–1 mo^–1 against 1.27 mm mm^–1 mo^–1 for seedlings that emerged immediately after masting. Seed germination was high (83%) and rapid (L₅₀ = 7 days) following masting. Seedling growth was positively correlated with soil water content but not with temperatures and relative humidity. Data suggested that seed production in mast years is important in maintaining the seedling bank, and thus the viability of relict Fagus populations.

Abbreviations: PAR = Photosynthetically active radiation; RGR = Relative growth rate.

Plants are composed of modules, such as shoots and leaves. However, modules have been overlooked as potential abundance measures in describing plant communities. We sampled 8 communities, to examine whether module counts gave different conclusions from biomass on community structure, and better discriminated between communities. Different abundance measures – number of leaves, number of shoots and photosynthetic biomass – gave different results for between-site comparisons of evenness and rank consistency. Since evenness is intended to represent a feature of the whole community, it should vary more between communities than within; module-based abundance achieved this better than biomass. It is speculated that the restrictions on species co-occurrences when plant communities assemble may sometimes be based on the number of modules rather than on biomass.

Abbreviation: PSU = Photosynthetic unit.

Species pools are increasingly recognized as important controls of local plant community structure and diversity. While existing approaches to estimate their content and size either rely on phytosociological expert knowledge or on simple response models across environmental gradients, the proposed application of phytosociological smoothing according to Beals exploits the full information of plant co-occurrence patterns statistically. Where numerous representative compositional data are available, the new method yields robust estimates of the potential of sites to harbour plant species.

To test the new method, a large phytosociological databank covering the forested regions of Oregon (US) was subsampled randomly and evenly across strata defined by geographic regions and elevation belts. The resulting matrix of species presence/absence in 874 plots was smoothed by calculating Beals’ index of sociological favourability, which estimates the probability of encountering each species at each site from the actual plot composition and the pattern of species co-occurrence in the matrix. In a second step, the resulting lists of sociologically probable species were intersected with complete species lists for each of 14 geographical subregions. Species pools were compared to observed species composition and richness. Species pool size exhibited much clearer spatial trends than plot richness and could be modelled much better as a function of climatic factors. In this framework the goal of modelling species pools is not to test a hypothesis, but to bridge the gap between manageable scales of empirical observation and the spatio-temporal hierarchy of diversity patterns.

We studied small-scale turnover in a calcareous grassland community in 2 cm × 2 cm subplots within 46 10 cm × 10 cm plots from 1996 to 1999. In different years, 73–84% of the subplots were occupied by rooted plant individuals, but the location of empty subplots varied considerably between years. Seedlings were the most mobile components in subplots – during one year (from July to July), seedlings either appeared or disappeared in on average 85% of all subplots where seedlings had occurred during that year. Within a one-year period, vegetative ramets appeared or disappeared in an average of 30% of all subplots that had rooted plants at either the beginning or the end of that year. When only previously empty subplots were taken into account, 80% of appearances were due to clonal growth and 20% due to seedling establishment. Ca. 75% of the seedlings, however, established in subplots that already contained a rooted plant individual. There were significantly less generative appearances when a moist summer was followed by a dry one, and significantly more vegetative appearances when a dry summer was followed by a moist one. 24–33% of the established seedlings survived the first year; from the second year onwards the annual survival was greater than 50%. Seedling survival was higher among short-lived species. Generative turnover varied between species, but we found no correlation with life history characteristics or frequency. Vegetative turnover of species was dependent on the year and on species traits. Sedges had a significantly higher turnover than grasses. Species potentially able to move clonally > 10 cm per year had a higher turnover than slower species. Turnover was not dependent on species frequency in the community. Forbs with more seedlings also had a higher vegetative turnover. There were no evident links between spatial mobility and species coexistence; predominating species exhibited contrasting turnover rates, subordinate species had contrasting turnover rates as well. Turnover is important in more general terms as a process resulting in free gaps – arenas for recruitment of whatever species.

The decline of deciduous woodland populations of Anemone hepatica L. in southern Sweden is documented and possible causes are discussed. The study was based on (1) re-investigations of 6.25 km² grid-squares first studied in 1938–1970, (2) distribution of A. hepatica in woodland sites with well-known soil chemical properties and (3) a detailed study over 12 consecutive years into the relationships between biological characteristics of the species (number of individuals, vegetative development, flowering frequency) and environmental variables (temperature, precipitation), soil chemistry and time. There was a close relationship between soil acidity (pH, solubility of Al³ ) and both distribution and biological characteristics. The biological variables declined significantly over time but were not related to climatic variability. Increasing soil acidity and Al³ solubility are concluded to be the main factors responsible for the decline of A. hepatica in S. Swedish deciduous woodlands.

Abbreviations: CEC = Cation exchange capacity.

Nomenclature: Karlsson (1998).

This paper compares the regeneration by seeds of heath and meadow and studies relationships between the floristic composition of phases in the regeneration pathway. Seed densities in the seed rain and seed bank as well as the densities of emerged seedlings in gaps and in closed vegetation were greater in the meadow than in the heath. In the heath, environmental constraints hindered seedling emergence almost completely so seeds accumulated in the seed bank. In the meadow, the decrease in the seed bank was due to high seedling emergence. Within both plant communities, seedling emergence in gaps and in closed vegetation was comparable. In the meadow, the seed rain and seedling emergence in gaps, as well as the seed bank and seedling emergence in gaps were positively correlated. Differences in seed and adult plant sizes were reasons for the low correlation between the standing vegetation and the other phases. In DCA ordination the first axis separated the phase of seedling emergence in closed vegetation and seed bank. The second axis separated the standing vegetation from the other phases. The structure of the seed rain was more heterogeneous than that of other phases. In the heath, the standing vegetation and the seed rain were positively correlated. The ordination of these phases reflected the patchiness of standing vegetation and the ability of the diaspores of Betula nana to disperse over long distances.

Nomenclature: Hämet-Ahti et al. (1998).

The only trees in most of the Negev desert are 3 native Acacia species. We tested the hypothesis that they act as keystone species as a result of the improved soil conditions under their canopies. Furthermore, because many Acacia populations suffer high levels of mortality due to water stress, we tested whether trees in high mortality populations had diminished effects on plant species and soil quality under their canopies. We show that plant species diversity beneath the tree canopies is higher than in the surrounding areas. There was also a clearly identifiable suite of species with higher occurrence under the trees. Plant species composition differed significantly between high and low mortality sites. However, there was higher species diversity in high mortality sites and under trees with higher water stress. Soil nutrient content was higher under the trees than in the open areas, especially under larger trees and trees with higher water status. The results indicate that there is a combination of positive and negative effects of Acacia trees on the under-canopy environment, which may include positive effects of higher soil nutrients and a negative influence of higher soil salinity.

Abbreviations: MANCOVA = Multiple Analysis of Covariance; PAR = Photosynthetically active radiation; PCA = Principal Components Analysis.

Nomenclature: Feinbrun-Dothan et al. (1991).

Clonal plants play important roles in maintaining wetland ecosystems in China. By analysing 108 wetland quadrats distributed throughout China, we evaluated (1) the importance of clonal growth forms in different Chinese wetlands, (2) how the abundance of clonal plants is related to climatic and geographical conditions, and (3) how plant species diversity is related to the abundance of clonal plants.

Significant differences in clonal plant importance values were found between different regions of China. Clonal plants were more important in wetland ecosystems located towards the West and North and at higher elevations and, accordingly, experiencing a colder and drier climate. Plant species diversity showed a significant inverse correlation with the importance value of ‘guerilla’-type plants in most of the wetland regions. However, we found no significant correlation between plant species diversity and importance values of ‘phalanx’-type plants. In most Chinese wetlands, plant species diversity decreased with increasing importance of guerilla plants and also with an increase of the entire guild of clonal plants. In wetlands with low species richness, however, plant species diversity increased with increasing importance of guerilla plants and of all clonal plants together, suggesting that in these disturbed habitats clonal growth may facilitate the establishment of other, non-clonal wetland plants.

Nomenclature: Anon. (1974); Anon. (1979–1999); Lang (1999).

We associated patterns of plant diversity with possible causal factors by considering 93 local regions in the Iberian Peninsula and Balearic Islands with respect to biogeography, environmental favourability, and environmental heterogeneity, and their relationship with measured species diversity at four different scales: mean local species richness standardized at a grain of 100 m², total species richness in a community type within a region (regional community richness), mean compositional similarity, and mosaic diversity.

Local regions in biogeographic transition zones to the North African and Atlantic floras had higher regional community richness and greater mosaic diversity than did non-transitional regions, whereas no differences existed in mean local species richness or mean compositional similarity. Mean local species richness was positively related to environmental favourability as measured by actual evapotranspiration, but negatively related to total precipitation and temporal heterogeneity in precipitation. Mean local species richness was greatest in annual grassland and dwarf shrubland communities, and on calcareous bedrock types. Regional community richness was similarly related to actual evapotranspiration and total precipitation, but in addition was positively related to spatial heterogeneity in topography and soil water holding capacity. Mean compositional similarity decreased with increasing spatial heterogeneity and temperature seasonality. Mosaic diversity, a measure of complexity, increased with increasing local and regional richness.

We hypothesize that these relationships can be explained by four ecological and evolutionary classes of causal factors: numbers of individuals, intermediate environments, limits to adaptation, and niche variation. These factors operate at various scales and manifest themselves in various ways. For example, at the site level, apparently processes that increase the number of individuals increase mean local species richness, but at the level of the entire region no such effects were found.

Abbreviations: AET = Actual evapotranspiration; NDVI = Normalized difference vegetation index; PET = Potential evapotranspiration; WHC = Water holding capacity.

The Mediterranean Basin harbours paleo-endemic species with a highly restricted and fragmented distribution. Many of them might also be of the remnant type, for which the regional dynamics depends on the persistence of extant populations. Therefore, a key issue for the long-term persistence of these species is to assess the variability and effects of ecological factors determining plant performance. We investigated the spatio-temporal variability in plant traits and ecological factors of Ramonda myconi, a preglacial relict species with remnant dynamics, in 5 populations over 4–7 yr.

Ecological factors contributing to fecundity showed a high degree of between-year variability. Pre-dispersal fruit predation had a minor influence on total reproductive output, and most of the variability was found among individuals within populations and years. Spatio-temporal variability in growth and survival was rather low but significant, whereas recruitment showed important between-population variability. Among-year variability in fecundity and growth was related to climatic fluctuations on a regional scale, notably rainfall and temperature in a particular period, while the spatial variability in survival and recruitment was explained by within-population (patch) habitat quality. Although R. myconi is able to withstand repeated periods of drought, water availability seems to be the most important factor affecting plant performance in all the study populations. These findings suggest that the long-term persistence of species showing remnant population dynamics in habitats under the influence of Mediterranean climate might be threatened by increased aridity as a result of climate change.

Nomenclature: Bolòs et al. (1993).

Abbreviations: NLRA = Number of leaves × rosette area; RGRA = Relative growth rate regarding rosette area; RGRL = Relative growth rate regarding number of leaves.

The relationship between mean Ellenberg indicator values (IV) per vegetation relevé and environmental parameters measured in the field usually shows a large variation. We tested the hypothesis that this variation is caused by bias dependent on the phytosociological class. For this purpose we collected data containing vegetation relevés and measured soil pH (3631 records) or mean spring groundwater level (MSL, 1600 records). The relevés were assigned to vegetation types by an automated procedure. Regression of the mean indicator values for acidity on soil pH and the mean indicator values for moisture on MSL gave percentages explained variance similar to values that were reported earlier in literature. When the phytosociological class was added as an explanatory factor the explained variance increased considerably. Regression lines per vegetation type were estimated, many of which were significantly different from each other. In most cases the intercepts were different, but in some cases their slopes differed as well. The results show that Ellenberg indicator values for acidity and moisture appear to be biased towards the values that experts expect for the various phytosociological classes. On the basis of the results, we advise to use Ellenberg IVs only for comparison within the same vegetation type.

Abbreviations: IV = Indicator value; MIV = Mean indicator value; MSL = Mean spring groundwater level.

A main floristic coenocline is extracted from a Danish forest vegetation data set with Detrended Correspondence Analysis. Huisman-Olff-Fresco (HOF) modelling of 262 species occurrences relative to site scores along the coenocline is compared with Gaussian and Generalized Linear Modelling. HOF models allow for responses of variously skewed forms, which may result from interspecific interactions. Most species have symmetric (60%) or skewed (29%), and only few species have flat or monotone response curves. This provides evidence for the generality of unimodal plant species response curves, provided the gradient in question comprises a sufficient amount of compositional turnover. The Gaussian niche width is correlated with density of optima and probability of occurrence of the species. Detailed analysis of HOF estimates for 28 woody species indicates that early succession species such as Pinus sylvestris and Populus tremula have wider niches than middle to late successional canopy species such as Alnus glutinosa, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Quercus robur and Tilia cordata. A marked overlap in niche range along the coenocline is interpreted in terms of species competition. Rubus idaeus intrudes into the niches of all other shrub species. Several canopy tree species, such as Fagus sylvatica, Fraxinus excelsior and Quercus robur overlap more than 60% of the ranges of all other species, while their internal overlaps are fairly equal, presenting evidence for their likely co-existence given more natural forest conditions. The combined analysis of coenoclines with HOF modelling of species behaviour is the most robust among the compared methods for estimating model parameters and niche characteristics of plant species. It is recommended for future ecological studies.

Nomenclature: Lawesson et al. (1997).