Étude des relations sol-végétation en conditions d'hydromorphie dans une forêt de la plaine Lorraine

Abstract

The chief object of this work is the study of soil-vegetation relationships under hydromorphic conditions. Contrary of most similar works, which are static studies, we particularly paid attention to the dynamic aspect of the question: the simultaneous evolution of soil and vegetation under the influence of human degradation. The applications of the main results to sylviculture are called up. In a previous phyto-ecological inventory of the studied forest (AUSSENAC, BECKER, 1968), had been made evident a mean relation between vegetation and depth of appearing of the Bg horizon, which ought to reflect the degree of soil hydromorphy. Then we doubted that these Bg horizons were really " functional ". The present study is effected in five kinds of stands, variously damaged, and characterized by the five most important herbaceous species: Molinia coerulea, Carex brizoides, Deschampsia coespitosa, Luzula albida, Poa chaixi (see table 1). The water regime of the corresponding soils is particularly studied: water table (fig. 1) and moisture during summer (fig. 2). The figure 1 shows that, through the depth of Bg is everywhere the same (about -30 cm) and its aspect very uniform, water table fluctuations are very different according to the kind of stand. Besides, the soils which are the most choked during winter, are still the moistest during summer drought. A systematical study of the ecological factors which could be responsible for these differences, shows that the degree of damage of the forest is the primary reason (through the reduction of rainfall interception and of potential evapotranspiration). Secondarily, the soil sinking decreases macroporosity and increases the phenomenon. Drainage is practically the same in all the stands. An accurate examination of the soils shows that, if the deep horizons are very comparable (glossic horizons or fragipans; a micromorphological study proves they are fossit), the superficial horizons, on the contrary, underwent different "actual" evolutions. Humus kind, structure, A2g (or A 2(g) colour, iron movement (see table 6) are the most interesting tests. The general name "sol lessivé à pseudogley ", which was used till now in our study, is replaced, according to the different cases, by "sol brun acide sur fragipan", "pseudogley mésotrophe sur fragipan" and "pseudogley podzolique sur fragipan" (see fig. 3). The results of the static study allow to reconstitute the dynamic aspect of the question, i.e. the degradation processes of forest and soils, and the worked up mechanisms; they are schemed in figure 4. The possibility of inverse evolution - of regradation - is debated. A device of cultivation in half-controlled conditions is imagined in order to confirm experimentally the conclusions which are drawn about the autecology of the five herbaceous species. In this experimental part, "light " and "interspecific competition" also are studied (see fig. 6). More strictly applied forest measurements are made. They allow to class the different kinds of stands according to technological wood quality (gravity test) and productivity. They also show the high superiority of Beech on Oak, in all the stands where it is present. Measures are preconized in order to mend the forest: to forsake coppice with standards, which is chiefly ill-adapted to this kind of environment, and to convert it in a high forest; to stop aIl fellings in the most damaged areas, so as to aid their "regradation"; to drain the same areas, with chemical treatments against Molinia coerulea and Carex brizoides; to give priority to Beech, wherever it is present; to choose Sessile Oak rather than Pedunculate to be mixed with Beech; to keep Pedunculate Oak in the Deschampsia coespitosa stands.