Vegetation dynamics and sex structure of the populations of pioneer dioecious woody plants

Summary

In connection with vegetation succession in old farmland and meadows, changes in the sex structure were analysed, particularly the sex ratio in populations, of Juniperus communis, Populus tremula, Salix aurita, S. caprea, S. rosmarinifolia, S. pentandra ete. Particular attention was given to the changes in the characteristics of Juniperus communis populations, associated with succession in old fields, leading from the segetal community Teesdaleo-Arnoseridetum, through the therophyte communities with Corynephorus canescens, lichens Cornicularlo-Cladonietum mitis, sand sward with the alliance Koelerion glaucae to Juniperus-Populus brushwood and Peucedano-Pinetum typicum forest. The particular succession phases over 70 yr and the phases of development of the pioneer populations of tree species were dated by the dondro-chronological methods.

It was found that in the course of the succession the sex structure, and in particular the sex ratio, changes in the populations of the above-mentioned species. Among the factors that affect the natural populations, the increase in density of the Juniperus communis population itself together with changes in the structure of the whole associated community played the most important role. This factor inhibits growth of juniper individuals very strongly and retards the beginning of the generative reproduction phase of a given population by 10–15 yr.

A hypothesis is advanced concerning the role of dioecism in pioneer tree species in the process of colonisation.

In view of the changes in sex ratio in the population in connection with vegetation dynamics, at least two groups can be distinguished among the pioneer woody species:

• (1) Juniperus communis and Salix pentandra in which the sex ratio changes from prevalence of male individuals in the early phases of population (and vegetation) development to an equilibrium in the phase of stabilisation and to a greater or smaller numerical dominance of female individuals/in the latest phases;

• (2) Salix caprea, S. aurita, S. einerea and Salix rosmarinifolia and their hybrids, the populations of which are characterised by a constant prevalence of female individuals, but the sex ratio in the early phases of population (and vegetation) development reveals a higher contribution of male individuals.