Secondary succession and post-fire regeneration of vegetation on abandoned arable land

Secondary succession, aside of mechanical destruction of the plant cover, is nowadays the most common process responsible for the transformation of plant cover and landscape. Land use: agriculture, irrigation and drainage, mowing, grazing, urbanization, tamping by vehicles and trampling have prevented the succession over the centuries, i.e. the return of spontaneous (though not always primary) vegetation of forests, steppes, moors, etc. to the land once converted. The liberation of land and vegetation from the long-term anthropogenic pressure triggered the succession and gave an opportunity to explain this process, its initiation, course and mechanisms, as well as the role played in it by plants and animals.

The Jelonka Reserve (in the south-western foreland of the Białowieża Primeval Forest) was established in 1989 on the area of 227 ha in order to secure the realization of the program of research on secondary succession and the role of pioneer tree species in this process, which began in 1971. The reserve includes a fragment of an extensive complex (about 12 thousand ha) of fields gradually abandoned since the First World War. In the nearby Czechy Orlańskie reserve (established in 1995; 75 ha) there are older forms of pine coniferous forest spontaneously formed on the former agricultural land, which served as a control study site.

The size of the area, homogeneity of habitat conditions and high diversity of vegetation in terms of the degree of advancement of succession made it possible to establish a very extensive system of 67 permanent plots, each with a size of 1000 m2. Until 2002, annual observations were made on the development of populations of pioneer woody species (Juniperus communis, Populus tremula) and Polytrichum piliferum moss. Large-scale mapping and repetitive photographic recording were applied and changes in the structure and species composition of the communities were observed (every 3 years). Diversification and integration of communities’ structure was mapped every 9 years.

During the 30 years of the research, the conditions for the initiation of secondary succession and the formation of the structure of the population of dioecious species were studied, including in particular the direction of changes in the sex ratio of individuals in the population, the course of flowering and fruiting and the size of production of seed cones in juniper populations, and the initiation of flowering and development of sexual dimorphism of aspen. The phenomenon of differentiation of the spatial structure of the grassland community under the influence of growing juniper shrubs and the integration of the structure of the forest under the influence of growing pine crowns and the accumulation of its needles were described. The durability of earlier differentiation of the community structure determines the later spatial differentiation in the course of fire regeneration of burnt grasslands and shrublands. This was revealed by detailed research undertaken after the local fire (in 1992) on 15 specially established permanent plots. Thirty years of direct observations on the permanent plots and the dendrochronological reconstruction of the juniper colonisation during the 55 preceding years, confirmed the earlier assumptions that the process of coniferous forest formation on the former farmland, as a result of secondary succession, takes about 140 years.