Vegetationsstufen in perhumiden bis perariden Bereichen der tropischen Anden
[Vegetation zones in the perhumid to perarid areas of the tropical Andes]
This paper is condensed from a lecture (given in May 1973 at Montpellier) which was honored by the presence of Josias Braun-Blanquet. The Andean Mountains and the adjacent lowlands, from Southern Colombia to Northern Chile and Argentina, offer an unique opportunity to study the variation of vegetation with the altitude as well as with the degree of humidity of climate within the tropics. In a series of diagrams this variation is represented schematically, using the number of arid months as horizontal and the altitudinal belts as vertical axis. Primary productivity is highest in the humid, not in the perhumid lowlands. More realistic graphs may give an idea of the structure of the zonal plant formation in a perhumid, in a semihumid to semiarid and in an arid climate. Climatic diagrams of the daily movement of air temperature and relative air humidity have been developed which show the more or less wet (hatched) and dry hours (dotted). Where possible, climatic diagrams with monthly average temperature and rain fall (according to Walter) are added. The abundance of vascular and cryptogamic epiphytes depends mainly on the degree of humidity at midday in the growing season, may this be caused by fog drift or frequency of rain fall. "Drip tips" of the leaves are lacking in the wettest forest climate (i. e. in the upper montanous belt) but are quite frequent where the average temperature is above 15° C, the whole growing season is humid, and the dry season is not longer than 4 months. Under these conditions, most leaves normally have no or only a thin and individual bud protection, and their tip is differenciated already when the lamina begins to be enlarged quickly, thus causing a curved transition to the tip. Most natural forest and woodland types have been partially or nearly totally replaced by grassland which was regularly burned and grazed since the Spanish conquest or even much longer. The best future land use, according to the author's experience and taking into account the different danger of soil erosion, is sketched in fig. 8 (lower part).