The Effect of Long-Term Periodic Fertilization on the Structure of the Stand and the Ground Cover in the Cowberry Pine Forests of Middle Taiga (Republic of Karelia)

The results of research on studying the influence of after-effects of periodic nitrogen (N) and complex fertilizer (NPK) treatments on the structure of the tree layer and ground cover are reported. The study objects were pine plantations growing on poor sandy soils in the cowberry pine forests (Karelian taiga district and middle taiga). At the time of survey, the plantations were 53 years old; fertilizers were last applied 30 years ago. A geobotanical relevé was carried out, stand valuation characteristics were determined, and microphytocoenoses were mapped at each sample plot. The NPK fertilization resulted in an increase in the average diameter and height of the pine plantations with more intensive thinning of the stand in comparison to the control and nitrogen fertilizer treatments. The fertilizer and control treatments are the same according to the total number of species in the ground cover. At the same time, significant differences were observed in the ratio of projective covers of mosses and lichens. In all fertilized plots, the abundance of mosses was twice or even more higher than the abundance of lichens, whereas the moss cover at the control site was almost the same as the lichen cover, and locally even lower. This pattern is probably associated with the response of true mosses to elevated carbon and nitrogen content in upper soil horizons. The only stand valuation50 characteristic reliably and unambiguously correlating with the ground cover structure is the average diameter: the larger the value of the average diameter, the larger the projective cover of true mosses. Generally speaking, four-time mineral fertilization with 5 year intervals of pine plantations growing on sandy soils continue to exert an effect on the structure of the tree layer and ground cover even 30 years after the last treatment. In the fertilized sample plots, elevated nitrogen and carbon levels are observed only in the forest floor and the soil horizon directly underlying the floor. This is due to the fact that trace amounts of nitrogen are fixed in the phytocenosis and released back with litter fall only into the topmost soil horizons; influencing the structure of the moss-and-lichen layer towards the prevalence of true mosses (Pleurozium schreberi, Hylocomium splendens) over fruticose lichens of the genus Cladonia in the ground cover.