Eddy van der Maarel

Small‐scale plant species turnover in a limestone grassland: the carousel model and some comments on the niche concept

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 shal- low, dry, nutrient-poor soil in the Great Alvar area (Stora Alvaret) of southern Oland, 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 m2 the 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 (aver- aged 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

The species pool and its relation to species richness: Evidence from Estonian plant communities

Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (1) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HO 1 postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO 2 postulates that any value of species richness per unit area (1 m 2 ) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P = 0.041 for HO 1 and P = 0.002 for HO 2 ). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per 1 m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.

Species pool: the concept, its determination and significance for community restoration

. The concept of species pool is reviewed. It is suggested to maintain the terms regional pool and local pool but replace actual pool by community pool. The regional and local pool are considered as selections from the regional and local flora based on ecological similarity. It is also suggested to include in the community pool a selection of species present only as diaspores in the diaspore bank (including diaspores from the seed rain), the selection being based on the same ecological criteria. Four approaches to determine the species pool are discussed: ecological, functional and phytosociological similarity, and an experimental approach. The phytosociological approach appears to be promising. The species pool is elaborated as a fuzzy set in the sense that each species of a community or a local or regional flora is a member of any community, local or regional species pool with different degrees of membership. This membership is defined as a probability of a species to become part of the community pool of a target community which is a function of the ecological (environmen-tal/functional/phytosociological) similarity of the species with the target community; the shortness of the distance of its nearest populations, the frequency/abundance, the dispersal capacity, the actual presence of dispersal mechanisms, the germinability of newly arrived diaspores, and the longevity of seeds (viability) in the diaspore bank. The information on species pools is needed for designing experiments where the number of species in a community is to be manipulated, for instance in restoration management.

The role of remnant forest trees in tropical secondary succession

This paper emphasizes the contribution of remnant trees to the establishment of woody species during succession on abandoned fields and pastures in the Mexican rain forest area, Los Tuxtlas, Veracruz. Remnant trees, original large forest trees left in the clearings by traditional farmers will become natural perching sites for both passing and resident birds. Frugivorous birds drop or regurgitate seeds and fruits which fall under the canopies of remnant trees during their stay, thus contributing to an accumulation of species, which make these remnant trees into ‘regeneration nuclei’. The species transported into these sites belong chiefly to older stages of successional development and reach these otherwise isolated areas, counteracting the depauperization of tropical land, brought about by both intensive and extensive clearing. In a study of seven remnant trees, 29 woody species and two climbers were found, 86% of which are bird dispersed. The total number of species per tree varied from 6 to 15 and was higher under remnant trees with fruits attractive to birds. Floristic variations of the understorey as detected by detrended correspondence analysis was correlated with the relative amount of shade-tolerant primary and late secondary trees versus light dependent pioneers and early successional trees.

Some remarks on disturbance and its relations to diversity and stability

. The concept of disturbance is related to the concepts of stability and diversity. Some definitions of disturbance are reviewed and three dimensions of disturbance are recommended to be included in any treatment of disturbance: spatial extent, time involved, and magnitude. The distinction between patch disturbance and community disturbance is emphasised and it is recommended that the study of regeneration after disturbance be studied through patch dynamics. The small-scale turnover of species is discussed and the concept of species turnaround time introduced by means of the carousel model. As an answer to a possible multiple choice question on the relation between the three concepts, ‘Disturbance for Stability’ and ‘Disturbance for Diversity’ are suggested.

Vegetation dynamics: patterns in time and space

This paper introduces the collection of contributions in this special volume on temporal and spatial patterns of vegetation dynamics. First, it is pointed out that the dynamics of any piece of vegetation, large or small, is always dependent on the degree of isolation of that piece towards its environment. Then ten types of island situation are treated ranging from very much to very little isolated: remote species-rich oceanic islands, remote species-poor islands, young big islands near a continent, small off-shore islands, emerging islands, isolated hills, landscape islands, isolated patches of vegetation, and gaps in stands of vegetation.

Vegetation succession on the dunes near Oostvoorne, The Netherlands; a comparison of the vegetation in 1959 and 1980

The vegetation of a 150 ha coastal dune area in the SW Netherlands was carefully analyzed and mapped at a scale of 1:2500 in 1959 and 1980. About 600 relevés were treated numerically. 127 vegetation types were distinguished, the overlap in community composition between the two years was very small. An intermediate level between this community level and the level of the structural-physiognomic formation was adopted. 56 so-called subformations, in which floristic and structural characters are combined, have been recognized and the overlap was then considerably larger, at least in number of types. Ordination results suggest that in each subset: grasslands, dune slacks, woodlands + scrubs, the first axis reflects floristic differentiation along with progression. The ordination of the entire material shows moisture as the main factor underlying the first axis, zonation as the second and nutrient status as the third factor. Change in vegetation was detected through a network overlay with 2 300 points for each of which the transition between 1959 and 1980 was scored. The pattern of change between 1959 and 1980 is apparently multiple pathway in type. This would be one of the few examples of such a pattern to be established on the medium term level of change, which is considered as succession sensu strictu. On an areal basis the change in vegetation is extremely large; even on the subformation level only 15% of the area was found to be unchanged, and on the community level only 4%. The overall trend is progression with a strong increase in the area of tall scrubs and woodlands. Retrogression also occurs, partly as a result of disturbances such as fire, partly due to local death of a dominant woody species. This latter phenomenon is interpreted against the background of the starting point of the succession; a sudden release of the inner dune area from overgrazing by cattle and a subsequent rapid dune development (outer dunes) on the beach in front of the inner dunes. Both species and community diversity increased, which is related to both differentiation under progression and to retrogression.

Pattern and process in the plant community: Fifty years after A.S. Watt

. This paper is a tribute to A.S. Watt who published his ‘Pattern and process in the plant community’ almost 50 years ago. Watts interpretation of the plant community “as a working mechanism, which maintains and regenerates itself” is still highly relevant, although the keywords have changed. ‘Process’ in Watts view involves both upgrade and downgrade aspects, whereas ‘Pattern’ was not specified, neither quantified. Nowadays, process is mainly approached as ‘disturbance’, that is natural disturbance and ‘pattern’ as patch structure. Together they make up the ‘patch dynamics’ of the community. Some implications of patch dynamics for phytosociology are discussed. A ‘Wattian’ concept of the plant community combines the Gleasonian idea of individualistic behaviour of species with the Clementsian (or rather Braun-Blanquetian) notion of community dynamics. Later work by Harper (demography), Grubb (regeneration niche) and earlier work of Sernander (forest gap dynamics) is significant for the understanding of the patch-dynamic nature of the community. Recent interest in plant species mobility can easily be linked to the concept of patch dynamics. Examples of mobility in a limestone grassland are given and a system of mobility types is proposed. Some perspectives for the study of patch dynamics are mentioned. Numerical pattern analysis should have a more prominent place in this type of study; the significance of the study of small permanent plots in a stand is emphasized, and unprejudiced demographic studies, as well as experimental studies of small-scale species replacement are recommended.

High species mobility in species-rich plant communities: An intercontinental comparison

Using results from a long-term study of fine-scale dynamics in grasslands in four widely separated study areas from two continents, we provide further evidence to support the idea of the carousel model as an aid to describe the high fine-scale temporal and spatial species mobility found in grassland communities. Cumulative species numbers on small subplots in plots situated in stable plant communities, determined as the sum of species appearing in these subplots in one or more years over a period of time, are very high. In floristically different species-rich grasslands, varying from moist pine savannas in North Carolina and Mississippi, to humid chalk grassland in the Netherlands and seasonally dry limestone grassland in Sweden, average species numbers on subplots of 0.01 m2 in plots of 2.5 m2 over the period 1985–1989 were similar, most plots falling in the range 10.8–13.2. The total cumulative species numbers were similar as well, most plots falling in the range 17.4 and 20.9. Yearly average species numbers remained relatively constant. Considerable species turnover is occurring in all these communities; on average three species appear and three disappear each year in each 0.01 m2 subplot. Total species accumulation on 0.01 m2 subplots over the period 1985–1989 varied considerably, from 4.1 to 11.6, and is correlated with the cumulative species total on the plot, the latter figure being considered as correlated with the size of the species pool.

Twenty years of numerical syntaxonomy

The development of numerical syntaxonomy during its first 20 yr is reviewed. The use of methods of numerical classification and ordination is the dominating feature of the development. National and local phytosociological data banks were established, large data sets handled and many important vegetation monographs were methodically based on multivariate data analysis. Particularly the development in Italy, the Netherlands, Czechoslovakia, and Sweden contributed to new theoretical elements of numerical syntaxonomy. Ordination became a common tool of searching for reticulate synsystematic relations between community types. The most popular ordination techniques have been Principal Components Analysis and Detrended Correspondence Analysis. Hierarchical agglomerative techniques of clustering still prevail in classification, although the divisive strategy of TWINSPAN has also become an effective tool for phytosociological clustering and table sorting. Extensive program packages, also for personal computers have now become standard equipment for many vegetation scientists.