Determinants of cryptogamic epiphyte diversity in a river valley (Flanders)

We studied cryptogamic epiphytes in a river valley in northern Belgium in order to investigate the effects of potential environmental and ecological determinants on epiphyte diversity. The study was done on ecotope level. In practice different habitat types, historical variables (forest history, tree age) and ecological indicator values of the epiphytes were used as explaining variables. The environmental variables were linked with ecological indicator values by means of canonical correlation analysis. Thereby correlations between indicator values and epiphyte diversity were calculated in a qualitative as well as in a quantitative sense. Classification and regression trees were applied to study the importance of the different environmental variables for epiphyte diversity. Cryptogamic epiphyte diversity of old forests proved to be low, contradicting the 'ecological continuity' hypothesis, while forests or shrubs with a high quantity of young trees were most suitable for epiphytes. These woodlands have a short forest history and are situated on alluvial soils in the centre of the valley, on former pastures. Main reasons for a high diversity in the young forests are elevated (air) humidity levels as well as the prominent presence of (several types of) wet broad-leaved forests, causing a high (micro)habitat diversity and diverse site conditions. As also structural aspects are important for the α-diversity of cryptogamic epiphytes, right actions should be implemented in management practices.