A hypothetical framework for testing phytodiversity in mountainous regions: the influence of airstreams and hygrothermic conditions
High mountain ranges are considered "hot spots" of phytodiversity due to their geomorphological, petrographical and climatic complexity in relation to the surrounding climate and vegetation zones. The degree of species numbers in high mountains depends on different factors: mass elevation, orographic heterogeneity, location within a floristic region and climate zone, frequency and amount of natural disturbances, as well as the position of the mountain chain against advective airstreams. While these factors are considered as the main triggers driving y-diversity at a general macro-level, additional stand conditions and processes play an important role for the degree of ß-turnover, considering the change of species number in altitude at a meso-level and of a-diversity at a micro-level. After analysing different effects caused by mass elevation, the second part of the paper focuses attention on the importance of different directions and types of airstreams. A main distinction is presented by the comparison of a species-poor type, given by convective currents and a species-rich type given by obvious climatic differences between windward and leeward exposures. Intermediate types result in transitional numbers of species. It must be emphasized that the extent of advective airstreams form an important, but only one of the decisive triggers concerning the degree of y-diversity of a high mountain range.