Nitrogen isotope ratios shift with plant size in tropical bromeliads.

We describe an ontogenetic shift in nitrogen (N) isotopic values in two rosette-forming epiphytic bromeliads. Leaf tissue N isotope values of small individuals of two bromeliad species (mean -6.2 per thousand ) differed from those of large individuals within each species (mean -0.5 per thousand ). Using references for potential N sources, we calculated the relative contribution of autochthonous (soil-derived through leaf litter) and allochthonous (atmospheric deposition) N with a two-member mixing model. Atmospheric sources contributed as much as 77-80% of the N in small individuals, whereas soil-derived N contributed 64-72% (conservative reference value) to 100% (less conservative reference value) of leaf tissue N in large plants. Shifts in N source with increasing plant size may be important aspects of rainforest complexity, an understudied aspect of ecosystem diversity.