Stable isotopic composition of carbon and nitrogen and nitrogen content in vascular epiphytes along an altitudinal transect

The foliar content of nitrogen and the relative abundances of C-13 and N-15 were analysed in vascular epiphytes collected from six sites along an altitudinal gradient from tropical dry forests to humid montane forests in eastern Mexico. The proportion of epiphyte species showing crassulacean acid metabolism (CAM) (atmospheric bromeliads, thick-leaved orchids, Cactaceae, and Crassulaceae) decreased with increasing elevation and precipitation from 58 to 6%. Atmospheric bromeliads, almost all of which had delta(13)C values indicating CAM, were more depleted in N-15 (x = -10.9% +/- 2.11) than the C-3 bromeliads which form water-storing tanks (-6.05% +/- 2.26). As there was no difference in delta(15)N values between C-3 and CAM orchids, the difference in bromeliads was not related to photosynthetic pathways but to different nitrogen sources. While epiphytes with strong N-15 depletion appear to obtain their nitrogen mainly from direct atmospheric deposition, others have access to nitrogen in intercepted mater and from organic matter decomposing on branches and in their phytotelmata, Bromeliads and succulent orchids had a loner foliar nitrogen content than thin-leaved orchids, ferns and Piperaceae. Ground-rooted hemi-epiphytes exhibited the highest nitrogen contents and delta(15)N values.