Natural 15N abundance of epiphytes depends on the position within the forest canopy: Source signals and isotope fractionation

Abstract

The natural 15N abundance (δ15N) of epiphytes and its N sources were studied in the canopy of a lowland rainforest in Costa Rica. Vascular and non-vascular epiphytes and canopy soils were collected from four canopy zones and analysed for N contents and δ15N signals. In addition, the N concentrations and δ15N signatures of bulk precipitation, throughfall and stemflow were measured during the wet and the dry season. The δ15N values of epiphyte leaves decreased significantly from the lower zones (means of -3.9 and -4.3‰) to the upper zones (means of -5-4 and -6.1‰) of the canopy. In contrast, δ15N signatures of canopy soils (average -0.3‰) differed little between the zones. Bulk deposition was enriched in 15N (+4.3‰) compared to all other potential N sources and was higher than throughfall and stemflow (+0.5 to -1.3‰). δ15N values of atmospheric deposition were inversely related to those of the epiphyte leaves, whereas N isotopic composition of canopy soils did not vary significantly. Consequently, it is concluded that the variations in foliar N isotope composition of epiphytes were not simply caused by utilization of isotopically different N sources, but by different 15N discrimination during N acquisition.