L'accessibilité, terme nouveau en Phytogéographie

Accessibility a new term in plant ecology (Summary)

For a better understanding of the causal relations between a certain unit of vegetation and its station the proposal is put forward to designate as accessibility factors the totality of conditions prevailing at a certain locality, that may influence the possibility of diaspores to reach that spot and settle there.

The accessibility factors must be considered as a separate group of ecological conditions beside and comparable to, but specifically differentiated from the climatical, edaphical and biotical groups of factors.

Exactly in the same way as plants become distributed according to their specific faculties to meet the local conditions of climate and soil, they are in a similar way selected on account of the interaction between their migratory faculties and the accessibility factors occurring at a certain locality.

The habitat of a certain plant association must therefore be defined on the basis of its climatic, edaphic, biotic and accessibility factors.

Objections raised against this proposal may be refuted by compairing the significance of accessibility conditions with the effect exercised by conditions of climate and soil with reference to settling.

A mountain range e.g. may influence the accessibility together with the climate of the leeside country by acting as a screen to the winds that bring over rain as well as plant seeds.

Accessibility conditions are of great importance with respect to pioneer vegetations on bare soil and in those plant groupings within which phytosociological succession is in rapid progress. However there are many other cases where they surely must be taken into consideration.

An illustration of the aforesaid considerations is furnished by vegetations of microscopical algae in small ponds. The astonishing richness in species of one such pond as compaired to some others, may in some cases be explained by exceptionally favorable accessibility, caused for example by the regular visit of migratory water birds.

The study of accessibility conditions is quite necessary for the selection and conservation of small nature reserves in highly cultivated regions.