Die Vegetation von Polylepis-Wäldern der Ostkordillere Boliviens
This paper describes the floristic combination of Polylepis stands in the eastern Cordilleres of Bolivia and analyses habitat factors that are likely to be essential to the development of the species combination. The area investigated comprises the spacious surroundings of the Cochabamba valley basin system. It has a western longitude of 65°00' to 66°33', a southern latitude of 17°00' to 17°45' and an altitude of between 3050 and 4050 m above sea level. In the area investigated five Polylepis species or subspecies were found. Their distribution depends upon varying site characteristics. Water supply is the vital factor for the development of these plant communities. Above all, however, differing types and extents of anthropogenous influence affect the species combination. The "partially altered" Polylepis woods ("bedingt naturnahe" Polylepis-Wälder), which are used by man mainly as a source of wood and as pastureland to a moderate to extreme degree, probably represent in the area investigated those plant communities whose structural combination most resembles that of the potential natural vegetation. Along a humidity gradient they can be subdivided into the following vegetation units: I. Humid Polylepis besseri alpha-forests, II. Fresh to dry Polylepis besseri beta-forest, III. Dry Polylepis tomentella alpha-forests, IV. Very dry Polylepis tomentella beta-forests. The "strongly altered" Polylepis stands ("bedingt naturferne" Polylepis-Bestände), on the other hand, result from a cultivation and pasturing of the fresh Polylepis besseri beta-forests that differ in intensity. They are described in terms of successive phases. As a general result of the investigations, it must be pointed out that in the region of Cochabamba human influence affected both the vegetation structure and the pattern of distribution of present-day Polylepis stands. In terms of their species combination the forests still existing correspond to open bushes, grass formations and pasture land communities, only a few plants can be characterized as "shadow species". Large-scale forest destruction is due to the fact that, over many centuries, fire was utilized in order to gain agricultural land. This method is still being employed. Nearly all stands are used as pasture land in a moderate to extreme way, substantially affecting their structure, species combination and rejuvenation. The major part of the area investigated from about 3000 m up to at least 4100 m above sea level was most probably once covered with forests. However, it is not possible to reconstruct the original structure of the Polylepis forest stand anywhere. On humid to fresh, hardly damaged soils one would expect to see forests densely covered with Polylepis sericea, P. besseri alpha or P. besseri beta as the most likely vegetation. In drier regions and at extremely degraded localities, if anthropogenous influence could be stopped, most probably open woods rich in species including P. besseribeta, P. tomentellaalpha or P. tomentella beta would grow.