N-15 NATURAL-ABUNDANCE OF VASCULAR RAIN-FOREST EPIPHYTES - IMPLICATIONS FOR NITROGEN-SOURCE AND ACQUISITION

The foliar natural abundance of N-15 was analysed to compare the poten tial nitrogen sources of vascular rainforest epiphytes and associated soil-rooted trees. Leaves of epiphytes collected from six rainforest c ommunities in Brazil, Australia and the Solomon Islands were depleted in N-15 relative to the trees at each site. Epiphyte delta(15)N was as low as -6.4 parts per thousand, while trees were generally enriched i n N-15 (0.7 to 3.5 parts per thousand). These results indicate either that epiphytes use nitrogen sources depleted in N-15 Or that discrimin ation against N-15 is an intrinsic function of epiphyte physiology, At three sites, epiphytes could be grouped into those having both low de lta(15)N and low leaf-nitrogen content and those possessing both high delta(15)N and high leaf-nitrogen content, The second group had delta( 15)N values in the range sometimes attributable to N-2 fixation (-2 to 0 parts per thousand), There was no correlation between growth form a nd delta(15)N. It is concluded that epiphytes may utilize N-15-deplete d nitrogen from atmospheric deposition and N-2 fixation.