Vegetatio

Questions of successional pattern and causality have been central concerns in vegetation ecology. In this paper we address the limits of the overextended models of Connell and Slatyer by discussing problems encountered in field tests. To help prevent such problems, we define the essential concepts needed to understand succession: pathway, cause, mechanism, and model. We then suggest a more complete enumeration of successional causes, and place them in a three-level hierarchy. The highest level in the hierarchy defines the general and universal conditions under which succession occurs: (1) availability of open sites, (2) differential availability of species, and (3) differential performance of species at the site. To provide a more detailed under-standing of succession, each of these causes is decomposed into ecological processes. A further decomposition results in the third level of the hierarchy, which is required to elucidate the mechanisms of succession at particular sites and to make detailed predictions. The hierarchy allows the appropriate causes to be chosen to answer questions about succession at the desired level of generality or level of organization. Recognizing the appropriate level(s) in the hierarchy is critical for the successful explanation of succession, design of experiments, statement of predictions, construction of models and development of general theory.

Lanthanides (rare-earth elements) were quantitatively determined by atomic emission spectrometry/inductively coupled plasma (AES/ICP) in various plants (Picea abies, Vaccinium vitis-idaea, Vaccinium myrtillus, Polytrichum commune, Sphagnum spec., and Hypogymna physodes) collected in the Forest Biosphere Reserve 350 km northwest of Moscow (USSR). Compared with previously established background values for lanthanide elements in central Europe, the rare elements determined in the USSR samples appear in lower concentrations. The lichen Hypogymna physodes can be characterized as an extreme accumulator of lanthanide elements up to a factor of ten compared to the other plant species collected in the Forest Biosphere Reserve. With regard to the lanthanide contents in plants collected in a German reference forest ecosystem, it can be seen, that the German samples represent without exception higher lanthanide values. Leaves of Vaccinium vitis-idaea display contents 3–4 times higher, leaves of Vaccinium myrtillus show concentrations higher by a factor of about 0.3, approximately twice the contents were determined in the German samples of Polytrichum formosum and P. commune, and the values in the German samples of the Sphagnum species are about 3 times that of the Soviet samples. As pointed out for pollution by heavy metals in part I of this series, the Forest Biosphere Reserve is generally characterized by lower contents of lanthanides in the vegetation cover than a comparable forest ecosystem (Grasmoor near Osnabrück, F.R.G.) in central Europe.

Phytoplankton data consisting of 145 species from a limnological study of lakes from relatively undisturbed areas throughout Sweden were analysed in relation to 11 physical and chemical environmental variables. Three multivariate methods were applied: WPGMA clustering and TWINSPAN for classification, and detrended canonical correspondence analysis (DCCA), a recent technique which extracts ordination axes that can be related directly to variation in the environment. Three types of lakes were recognized consistently: acid humic lakes with Gonyostomum semen as the dominant species, very acid impoverished lakes with rather few, stress-tolerant species, and subarctic lakes with low total biomass but with a varied phytoplankton flora. DCCA allowed a straightforward display of the locations of lakes and species along environmental gradients (including the acidification gradient) reflected in phytoplankton composition. It is suggested that such analyses may be a useful tool for the early detection of environmental change.

Along ria shorelines mangrove assemblages are closely related to habitat types and there is a recurring pattern in their distribution that is linked to the origin and history of the geomorphic units. The coastal geomorphology is related to ancestral landforms developed prior to the post-glacial transgression, as well as to tidal levels, aspect, modern shore-forming processes, and types of hinterland/tidal flat contacts. These historical and process patterns develop distinct geomorphic units which dictate the distribution of habitats and their mangrove assemblages. Mangrove assemblages are classified according to their habitat: main tidal flat, hinterland fringe, alluvial fan, spit/chenier, tidal creek bank, tidal creek shoal and rocky shore. Within a given habitat there are various physico-chemical gradients which are instrumental in developing zonation within the assemblages. The variety of internal zones within an assemblage is related to the richness of species within the regional species pool which in turn is related to climate.

The values of nitrogen phosphorus ratios measured in green parts of evergreen sclerophylls, seasonal dimorphic, deciduous and herbaceous plants found in Mediterranean-type ecosystems of Greece, form a homogeneous group, which is comparable only with those of analogous ecosystems of California but not with Mediterranean systems of the southern hemisphere.

Association of species abundance with a continuous environmental variable is frequently tested with regression or correlation analyses. However, because these methods ignore the range and frequency distribution of levels of the variable occurring in the study area, they may generate misleading results. We give examples to illustrate the argument. A better approach to test the association between species abundance and a continuous variable should compare levels of the variable in the study area to levels of the variable occurring in sites occupied by the species. If a particular species abundance is not associated with a given continuous variable, then the frequency distribution of levels of this variable measured where individuals of the species occur should mirror the frequency distribution of levels of the variable measured over the study area. We explain how to use the one- and two-sample Kolmogorov-Smirnov statistics to compare the cumulative relative frequencies of levels of the variable where individuals are present with points in the study area. We discuss the statistics, assumptions, limitations, and advantages of these tests.