Peculiarities of phytocenosis formation on former arable lands destroyed by fire in the Leningrad region

The article deals with the restoration of phytocenosis on the lands withdrawn from agricultural use and subjected to pyrogenic impact. A large number of lands previously used in agriculture have been withdrawn from active economic turnover, and on these areas the process of restoration of native forest vegetation is underway. In this regard, these areas of postagrogenic lands are of interest for studying the patterns of subsequent succession, as these processes have a renewal specificity different from natural forest phytocenoses. The objects of the study were plots previously used briefly for arable land, overgrown with coniferous-deciduous young forests, which were destroyed by an intensive lowland fire. Currently, separate groups of forests, different in predominance of coniferous and deciduous species, have formed on the site. Three permanent sample plots were established at the experimental sites, one dominated by aspen and birch, and the other two dominated by pine and spruce. A complete taxation of plantations, geobotanical description of the living ground cover, soil half pits and soil sampling were performed. The main agrochemical indicators of soil fertility were determined in laboratory conditions: humus content, nitrate and total nitrogen, exchangeable soil acidity, mobile forms of phosphorus and potassium. Humus content distinguished the soil as extremely poor. The degree of acidity distinguished the soil as strongly acidic. The degree of soil supply with mobile nitrogen was very low, as it was actively used both by the stand and vegetation cover. Differences in the development, composition and structure of the undergrowth of the main forest forming species were revealed. The absence of non-viable spruce undergrowth was noted on all sample plots, and pine undergrowth was absent. It is concluded that there is no correlation between the number of undergrowth specimens and the projective cover of the living ground cover. The authors characterized floristic composition and calculated Shannon and Pielu indices, Jaccard's diversity coefficient, average indices of species richness and projective cover of species. The study sites were assessed according to the Tsyganov and Ramensky ecological scales. The cenotic positions of floristic composition were analyzed. With the increase of fallow period the soil complex fertility indicators returned to the state close to undisturbed soils of a wooded area. The results obtained allow us to conclude that even small differences in the values of soil agrochemical indicators have a noticeable impact on the structure of phytocenosis. With increasing soil fertility, changes in the structure of plantations can be observed: an increase in the proportion of birch and aspen in the composition of the stand, an increase in the amount of undergrowth and species diversity of living ground cover. These processes require further close study in order to create the most rational management regime in post-pyrogenic phytocenoses growing on lands withdrawn from agricultural turnover.