Möglichkeiten zur anwendung von Vegetationskundlichen untersuchungen auf Dauerflächen

Summary

Besides describing and photographing there are two other methods of registrating vegetation changes: analysis of permanent quadrats (p.q.'s) and successive mapping. Small changes can only be detected by p.q.'s, but not by, successive mapping. A disadvantage of p.q.'s compared with successive mapping is their spatial limitation.

By studying p.q.'s valuable information about vegetation succession can be obtained, provided the following points are taken into account:

• - The relev\'es of the different years must be comparable and differences according to seasonal aspects must be excluded.

• - Since differences in time are mostly smaller than those in space, a fine scale is needed for p.q.-analysis. The decimal scale (Londo 1975a) for coverage and abundance is very suitable and has the advantage that calculations are easy. For very large quadrats determination of the coverage is less practical; in those cases the method of Tansley for global estimation of dominance and abundance is more suitable. For detailed studies of small quadrats the pointquadrat method may be useful as well.

• - When it is not possible to make a relev\'e of all p.q.'s every year, it is important to study at least some of them yearly, because extremes in environmental factors can cause changes in the vegetation which remain traceable many years later. These changes may be incomprehensible when the relev\'e of the extreme year is missing.

• - For determining the dimensions of new p.q.'s one has to take into account changes in the minimum area of the plant communities expected during the succession.

Research through permanent quadrats can be done for several purposes.

• - The purpose can be concerned with vegetation science itself, notably the syndynamics of plant communities. Most statements about vegetation succession are concluded from spatial studies and little is known which is really based on succession studies. It is clear that the most reliable information can be obtained from analysing p.q.'s and successive mapping.

When also environmental factors are studied, p.q.'s can contribute much to synecology as well. Research-in-time is especially needed and suitable in cases of communities which are bound to great changes in one or more environmental factors. Also information about community stability and diversity can be obtained in this way.

When space-time relationships are to be studied the spatial distribution of the p.q.'s is very important. It became clear that relatively few p.q.'s carefully located in one or some transects gave much more information than many p.q.'s distributed at random. Especially transects across the vegetation zonation gave much information about pattern and process. Thus from a study of duneslack vegetation a negative correlation could be observed between the species diversity and the relative vegetation changeability.

• - P.q.-studies can also be done with respect to autecology and population dynamics: how is the behaviour of plant species in space and time in different vegetation types and in connection with their environmental factors? Permanent transects as well as series of p.q.'s arranged in two dimensions can provide valuable information in this respect.

• - The last category is concerned with applied p.q.-studies. Firstly it is done on behalf of the management of nature reserves. It is necessary to study the influence of the management measures on the vegetation types as well as the influences from the environment on the reserve. Further applied p.q.-studies are done to obtain information about the ecological side-effects of human activities a.o. air pollution, infiltration of river water in dune areas, embankment of salt marshes, management of forests, etc.

The applied and the purely scientific studies by p.q.'s can be descriptive or experimental. Studies on p.q.'s are mostly not important for only one but for more of the above mentioned aspects together.