Nocivité des boisements en Epicéas (Picea excelsa Link.) pour certains cours d'eau de l'Ardenne Belge

Abstract

SummaryI. The general effect of even-aged stands of Spruce.Stands of Spruce are injurious to streams for two reasons: a) They result in the poisoning of the water, andb) they reduce the light considerably, which, in turn, suppresses the aquatic vegetation.1. Poisoning of the water.Toxic substances are found in the humus formed by a Spruce stand. They have not been isolated, but their action is similar to that of Thuya occidentalis, which is known.The poisoning of the water, resulting from the action of these poisons, causes a reduction in the aquatic fauna by the disappearance of the most delicate species, and particularly of fish (Trout, Bull-heads and Minnows). When the pollution is strong, this reduction in population results in almost complete sterilisation.The action of this pollution on streams differs according to the origin, whether rheocrene or helocrene springs, both types of which are represented in the Ardenne. Water of helocrene source is always polluted. That of rheocrene source, however, even if situated in the middle of conifer plantations, is not polluted. If the stream resulting from such a spring is well lighted, its fisheries value is normal.2. Action of shade.The dense permanent cover, resulting from stands of Spruce, results in the complete disappearance of aquatic vegetation.This disappearance, which is not compensated for in any way, in turn results in the considerable reduction of the nutritive fauna.In addition, the elimination of the shrubs and vegetation on the river banks of the streams, causes erosion and falling in of the banks, and the consequent disappearanoe of the shelter sought by Trout.II. Action of large stands of conifers.The wide extent of Spruce afforestation is considerable in the High Ardenne (altitude exceeding 500 m), where vast areas, originally covered with decidious forests or by heath, have been planted to conifers (Hautes Fagnes, fagnes of St. Hubert).As a result of this extensive afforestation, the streams of these regions have progressively lost all fisheries value (e. g. La Basseille). Formerly these streams were fed from peat bogs, equally toxic, but this effect disappeared before long.It is impossible to envisage the complete restoration of these streams. One must endeavour to limit the ill effects by rapid neutralisation of the poisons. With this object, it would probably be interesting to observe the effects of increased light, by maintaining an open belt of five meters on either side of every stream.This measure will be adopted progressively, for new afforestations and limited to the most important streams.III. The effects of conifer stands on the valleys of the Ardenne.The majority of valleys in the Ardenne were formerly meadows, carefully maintained and irrigated. Many have now been abandoned, especially those which are too far from the villages. Often these afforested with conifers, almost always with Spruce.The water courses (often streamlets or streams) of a width less than 2 meters, play an important role in fisheries, and are spawning grounds for Trout. Natural spawning beds are numerous and the nutritive fauna abundant.In addition their value for artificial cultivation of Trout is high, e. g. stocking with alevins and the production of fingerlings for restocking in numerous diversions.The afforestation with conifers reduces almost all value of a water course, as the biogenic capacity is considerably diminished and it does not produce any shelter. Finally the maintenance of the trout population in the rivers and streams of the Ardenne is compromised.The majority of these streams are fed by rheocrene sources. Consequently, the ill effects of afforestation result from the excessive shading of the stream, and not from pollution of the water.To obviate this, it is necessary to insure sufficient light over the streams and streamlets, by providing a belt on each stream not afforested with conifers.