Wybrane aspekty badania długoterminowych cykli zmian zapasu wody glebowej w drzewostanach jednogeneracyjnych

This article describes cyclic changes in the soil water storage that take place within developmental stages of the stands managed by clear−cutting system. The aim of the paper is to present possibilities of studying these changes and their significance for the dynamics of waterlogging of forest ecosystems and water outflow from the forest. Soil water storage (SWS) in forest soils undergoes cyclic changes that can be divided into short−term, seasonal and long−term. Short−term changes, calculated on a daily basis, and seasonal, covering the entire growing season, can be identified and analyzed based on stock values measured over relatively short periods. Studying long−term cycles in this way is practically impossible, as there are no long−term measuring sequences. However, it is possible to use water−balance equilibrium in atmosphere−stand−soil (A−D−G) for this purpose. The dependent variables include increments of soil water storage, while independent ones are represented by the biometric features of stands and hydro−climatic factors. The balance equations should be built separately for SWS supply and losses. In the recharge phase the increase of the SWS after single rainfall, the amount of potential interception proportional to the plant surface and the weight of the forest litter as well as rainfall intensity and duration, which determine the level of its use are taken into account. SWS losses are explained by the rates of plant transpiration, linearly dependent on the amount of biomass produced and evaporation from the soil surface, which is a function of the amount of solar energy and wind velocity within the stand. They are modulated in relation to the conditions above the canopy that in turn depend on biometric characteristics of the stand. Taking into account the hydro−climatic conditions above the canopy ensures that the ceteris paribus condition is met. Recognition of long−term SWS cycles is possible due to the knowledge of stand dynamics manifested by changes in the value of its biometric features. The identification of equations according to the above−mentioned indications was carried out many times with very high compliance in the conditions of even−aged low−energy stands. The occurrence of long−term SWS cycles raises a number of questions regarding the forest ecosystem functioning, description of forest sites, silvicultural systems applied as well as the ability to control the water balance in forest ecosystems. The hydrological conditions in the forest ecosystem are shaped mostly upon selecting silvicultural system to be implemented. It results in specific changes in biometric features in the subsequent stand development stages modulating the components of the A−D−G balance and, as a consequence, soil water relations. The impact of ongoing stand tending is limited. However, forest drainage, tree felling uncovering soil surface and sudden changes in the forest structure triggered by abiotic and biotic factors, may produce significant effects.