PARENT SESSION
Oral Session #33: Biodiversity and Ecosystem Function: Nutrients, soils, microbes.
Presiding: K. Brown
Tuesday, August 6. 8:00 AM to 11:30 AM. Grand Ballroom Central, Radisson.

Nitrogen sources and functional roles shift with plant size in tropical bromeliads.

Reich, Alexandra^*,1, Ewel, John², Nadkarni, Nalini³, Evans, R. Dave⁴, Dawson, Todd⁵, ¹ La Selva Biological Station, Costa Rica² Institute of Pacific Islands Forestry, Honolulu, Hawaii³ The Evergreen State College, Olympia, Washington⁴ University of California, Berkeley⁵ University of Arkansas, Fayetteville

ABSTRACT- Epiphytic bromeliads exhibit a shift in the relative importance of different nitrogen (N) sources as their tank size increases: small bromeliads are primarily dependent upon atmospheric N inputs, whereas large bromeliads depend primarily on detrital N. The youngest fully expanded leaf was collected from developing and mature individuals of Guzmania monostachya (n=47) and Vriesea gladioliflora (n=46); from mature individuals of two atmospheric bromeliad species, Tillandsia anceps and T. festucoides (n=5; used as the allochthonous reference); and from epiphytic ferns that utilize canopy soil derived from decomposed litter (used as an autochthonous reference). Fresh leaf litter was used as a more conservative autochthonous reference. Samples were analyzed for 15N values and percent N content by mass. Leaf tissue N isotope signatures of small individuals (mean = −6.2‰) differ from those of large individuals within each species (mean = −0.5‰). The relative contribution of potential sources was calculated with a two-member mixing model. Atmospheric sources contribute 77 to 79.9% of the N in small individuals of both species. A conservative estimate predicts that 64.3 to 72.1% of leaf tissue N is soil-derived; a less conservative estimate (based on signatures from epiphytes that use canopy soil) predicts that 100% of leaf tissue N is soil-derived. Our study supports the notion that shifts in ecological function with increasing plant size is an important component of rainforest complexity.

KEY WORDS: stable isotope, epiphyte, diversity