THE DYNAMICS OF LITTERFALL, THROUGHFALL AND STEMFLOW IN RADIOACTIVELY CONTAMINATED PINE STANDS

The authors present the data of the study of the downflow of a matter of small biological cycle in radioactively contaminated forest ecosystems of Scots pine (Pinus sylvestris L.) as a result of the Chornobyl accident. During the accident, a large number of radionuclides came into the surrounding environment. Currently, 90Sr and 137Cs are the most dangerous for forest phytocoenoses, so as they are included in the biogeochemical cycle through the trophic chains. In this research, the downfall of the matter of the tree stand was divided into two groups: solid parts (litterfall) and liquid fraction (rainwater). In turn, the structure of litterfall was classified as follows: needles, bark, branches and generative organs (cones, buds, seeds and their parts). Rainwater is separated as throughfall and stemflow. Using the obtained data from field observations for 2.5 years (2014-2016), we estimated the dynamics of the formation of litterfall components in calendar months. Experimental materials, collected in the interval from 01.10.2015 to 05.10.2016, are also used to represent the regularities of 137Cs activity concentration for downfall components. The analysis of field data confirmed the presence of a maximum values for litterfall formation at the end of the growing season (September-October) due to the drop of the needles from crown. For other components of litterfall, clearly expressed regularities are not revealed. We calculated summary annual parameters for the components of the downfall: the average 137Cs activity concentration, the accumulated weight and the total activity of 137Cs per 1 m2 of the investigated forest area. 137Cs activity concentration in litterfall samples (1287-6310 Bq·kg-1) is much bigger than in rainwater (1.28-11.2 Bg·kg-1). However, due to much larger amounts of throughfall per unit area of the surface of the site, it has a decisive role in the formation of the return of 137Cs from the elements of the pine stand to the soil (42.3 %). Other components of the downfall are characterized by a much lower percentage in the return of 137Cs to the forest soil: needles (25.7 %), generative organs (13.7 %), bark (12.2 %), branches (3.8 %), stemflow (2.2 %). Previous studies of 90Sr in components of downfall show that this radionuclide differs significantly from 137Cs: it is much more accumulated in the dead parts of pine stand and less washed away by rain than 137Cs. Therefore, the experimental data on the main depots and fluxes of 90Sr must be obtained very quickly for adequately predict the radiological condition in the forests of the Chоrnobyl exclusion zone.