15N natural abundance of vascular rainforest epiphytes: implications for nitrogen source and acquisition.

The foliar natural abundance of 15N was analysed to compare the potential nitrogen sources of vascular rainforest epiphytes and associated soil?rooted trees. Leaves of epiphytes collected from six rainforest communities in Brazil, Australia and the Solomon Islands were depleted in 15N relative to the trees at each site. Epiphyte ?15N was as low as ?6.4 percento, while trees were generally enriched in 15N (0.7 to 3.5 percento). These results indicate either that epiphytes use nitrogen sources depleted in 15N or that discrimination against 15N is an intrinsic function of epiphyte physiology. At three sites, epiphytes could be grouped into those having both low ?15N and low leaf?nitrogen content and those possessing both high ?15N and high leaf?nitrogen content. The second group had ?15N values in the range sometimes attributable to N2 fixation (?2 to 0 percento). There was no correlation between growth form and ?15N. It is concluded that epiphytes may utilize 15N?depleted nitrogen from atmospheric deposition and N2 fixation.