Size-dependent variation of carbon and nitrogen isotope abundances in epiphytic bromeliads

While atmospheric species of bromeliads have narrow leaves, densely covered with water-absorbing trichomes throughout their life cycles, many tank bromeliads with broad leaves, forming phytotelmata, go through an atmospheric juvenile phase. The effect of the different habits and the phase change in tank-forming bromeliads on water and nutrient relations was investigated by analysing the relationship between plant size, C/N ratios and the natural abundance of C-13 and N-15 in five epiphytic bromeliad species or morphospecies of a humid montane forest in Xalapa, Mexico. The atmospheric species Till-andsia juncea and T. butzii exhibited full crassulacean acid metabolism, with delta(13)C values (mean - 15.3parts per thousand and - 14.7parts per thousand, respectively) independent of size. In Tillandsia species with C-3 photosynthesis, delta(13)C decreased with increasing plant size, indicating stronger drought stress in juveniles. The increase of the C/N ratio with size suggests that, at least in heteroblastic bromeliads, the availability of water is more limiting during early growth, and that limitations of nitrogen supply become more important later on, when water stored in the tank helps to bridge dry periods, reducing water shortage. delta(15)N values of the two atmospheric species were very negative (- 12.6parts per thousand and - 12.2parts per thousand, respectively) and did not change with plant size. Tank-forming bromeliads had less negative delta(15)N values (c - 6parts per thousand), and, in species with atmospheric juveniles and tank-forming adults, delta(15)N values increased significantly with plant size. These differences do not appear to be an effect of the isotopic composition of N sources, but rather reflect N availability and limitation and stress-induced changes in N-15 discrimination.