The degree of soil water saturation in the narrow-leaved ash (Fraxinus angustifolia Vahl.) floodplain forest

Research is aimed at analysing the dynamics of the soil water saturation degree (FDR method) during three characteristic developmental phases of ash stands. The studied stands are located in the lowland forest in Upper Posavina (a part of the Sava river valley) in Croatia. The experiment was established in natural stands of a community of narrow-leaved ash with autumn snowflake (Leucoio-Fraxinetum angustifoliae G 1 a v. 1959), which is distributed in the area dominated by the community of pedunculate oak and great greenweed (Genisto elatae-Quercetum roboris Rauš 1969). Three locations in a micro-depression of a flooded plain were chosen: 1) the lowest micro-relief position - the bottom of a bogged site - initial development phase; 2) the central micro-relief position - optimal development phase; 3) the marginal bogged area towards a fresh micro-depression - terminal development phase. The degree of water content in the soil is regarded a very practical indicator of ash stand conditions, in which the soil in the lower part of the rhizosphere (in the initial phase it is within a depth of 1 m) is completely saturated with water over most of the year or the whole year. Anaerobiosis or reduction conditions in the soil correspond to the soil saturation degree. In terms of the degree of soil water saturation, the terminal phase of ash forest is the most distinct, while in terms of total water quantity in the profile and its dynamics, the initial phase takes up the first place. Based on research we can conclude that developmental phases differ from one another in the dynamics of soil water saturation at almost all depths. At the beginning of a drier vegetation period there was no difference between the initial and the optimal phase in the bare regeneration area and between the terminal and the optimal phase in the old stand. Similarity between these pairs was evident throughout the vegetation period. In the year with a more humid vegetation period, this differentiation regularity was hardly evident in terms of saturation degree dynamics. In conditions of a drier vegetation period (2000), the most favourable distribution of water in the soil occurred in the terminal phase.