Der Wasser- und Photosynthese-Haushalt von Wüstenpflanzen der mauretanischen Sahara : I. Regengrüne und immergrüne Bäume

Abstract

Summary In the Mauritanien thorn-savannah (Sahel) and desert (Sahara) the water- and photosynthesis relations of Acacia raddiana (fig. 1, 2), as a representative of the raingreen (droughtdeciduous) tree-type, and of Salvadora persica (fig. 3) and Boscia senegalensis (fig. 4), as representings the evergreen tree-type, have been examined. Experimental bases were moment-measurings of the daily variations of transpiration and photosynthesis (fig. 5, 6, 8, 11), at the natural stands. In the Sahel, Acacia shows a very expensive and excellently stabilized water turn-over, and a very productive photosynthesis during the rainy season (fig. 5, 7, table 4). At the end of the dry season, the young leaves, which appear already before the first rains, suffer from the un sufficient water-supply; in consequence of that, transpiration and photosynthesis are lovered and unstable (table 4, fig. 6, 7). The two evergreen types demonstrate a different behaviour: in the culminating dry season, in the Sahel Salvadora (table 4, fig. 8, 9) is forced to reduce transpiration and photosynthesis in a similar manner as Acacia, whilst in the Sahara Boscia is able to maintain a high rate and stability of both functions (table 4, fig. 11, 12). This surprising phenomenon is due to locally higher amounts of soil-water, effected by the accumulation of the runoff of heavy precipitations in wadis and their chatchment areas. On this basis, in the Southern Sahara a contracted vegetation originates, composed of tree-types, which the tropical savanna offers in a rich assortment. To the Northern Sahara, on contrary, the northern steppe, inimical to tree-growth, can not supply tree-forms. Provided that the soil contains a sufficent amount of water, the evergreen type remains vital also on extreme arid conditions.