Are vascular epiphytes nitrogen or phosphorus limited? A study of plant (15) N fractionation and foliar N : P stoichiometry with the tank bromeliad Vriesea sanguinolenta.

Although there is unambiguous evidence for vascular epiphytic plants to be limited by insufficient water and nutrient supply under natural conditions, it is an open debate whether they are primarily phosphorus (P) or nitrogen (N) limited. Plant (15) N fractionation and foliar N : P stoichiometry of a tank epiphyte (Vriesea sanguinolenta), and its response to combined N-P fertilization, were studied under semi-natural conditions over 334 d to clarify the type of nutrient limitation. Plants collected in the field and experimental plants with limited nutrient supply showed significant plant (15) N fractionation (mean 5‰) and plant N : P ratios of c. 13.5. Higher relative growth rates and declines in plant (15) N fractionation (0.5‰) and in foliar N : P ratios to 8.5 in the high N-P treatment indicated that these epiphytes were P limited in situ. The critical foliar N : P ratio was 10.4, as derived from the breakpoint in the relationship between plant (15) N fractionation and foliar N : P. We interpret the widespread (15) N depletion of vascular epiphytes relative to their host trees as deriving from (15) N fractionation of epiphytes as a result of P limitation. High foliar N : P ratios (> 12) corroborate widespread P limitation (or co-limitation by N and P) of epiphytic bromeliads and, possibly, other epiphyte species.