Epiphytes and their contribution to canopy diversity

About ten percent of all vascular plant species world-wide are epiphytes an d they are almost exclusively found in tropical forests. Therefore, they co nstitute a large part of the global plant biodiversity (10% of all species) , and in tropical countries represent up to 25% of all vascular plant speci es. Focusing on the differences between epiphytes in the strict sense or 'h olo-epiphytes' (non-parasitic plants that use other plants - usually trees - as growing sites all through their life-cycle) and 'hemi-epiphytes' ('hal f' epiphytes which only spend part of their life as epiphytes until their a erial roots become connected to the ground), horizontal and vertical distri bution patterns of both in relation to some of their ecological requirement s are compared. Vertical ecological gradients (i.e., insolation and humidit y differences from the forest floor to the canopy surface) are relevant for the composition of the holo- epiphytic vegetation. For hemi-epiphytes, how ever, ecological differences between distinct forest habitats (i.e., horizo ntal gradients) are relevant, but not primarily the canopy structure, as th e individual host tree structure is more important. The scale-dependence of epiphyte diversity assessment (relatively small study areas for holo-epiph ytes, large study areas for hemi-epiphytes) is mainly due to the striking d ifferences in plant sizes and related mechanical and physiological requirem ents.