The nutrient status of epiphytes and their host trees along an elevational gradient in Costa Rica

Abstract

Vascular epiphytes are a conspicuous and highly diverse group in tropical wet forests; yet, we understand little about their mineral nutrition across sites. In this study, we examined the mineral nutrition of three dominant vascular epiphyte groups: ferns, orchids, and bromeliads, and their host trees from samples collected along a 2600 m elevational gradient in the tropical wet forests of Costa Rica. We predicted that the mineral nutrition of ferns, orchids, and bromeliads would differ because of their putative differences in nutrient acquisition mechanisms and nutrient sources-atmospherically dependent, foliar feeding bromeliads would have lower nitrogen (N) and phosphorous (P) concentrations and more depleted d 15 N values than those in canopy soil-rooted ferns because canopy soil is higher in available N, and more enriched in d 15 N than the atmospheric sources of precipitation and throughfall. We also predicted that epiphyte foliar chemistry would mirror that of host trees because of the likely contribution of host trees to the nutrient cycle of epiphytes via foliar leaching and litter contributions to canopy soil. In the same vein, we predicted that epiphyte and host tree foliar chemistry would vary with elevation reflecting ecosystem-level nutrients-soil N availability increases and P availability decreases with increasing elevation. Our results confirmed that canopy soil-rooted epiphytes had higher N concentrations than atmospheric epiphytes; however, our predictions were not confirmed with respect to P which did not vary among groups indicating fixed P availability within sites. In addition, foliar d 15 N values did not match our prediction in that canopy soil-rooted as well as atmospheric epiphytes had variable signatures. Discriminant function analysis (DFA) on foliar measurements determined that ferns, orchids, and bromeliads are statistically distinct in mineral nutrition. We also found that P concentrations of ferns and orchids, but not bromeliads, were significantly correlated with those of host trees indicating a possible link in their mineral nutrition's via canopy soil. Interestingly, we did not find any patterns of epiphyte foliar chemistry with elevation. These data indicate that the mineral nutrition of the studied epiphyte groups are distinct and highly variable within sites and the diverse uptake mechanisms of these epiphyte groups enhance resource partitioning which may be a mechanism for species richness maintenance in tropical forest canopies.

Effects of simulated nitrogen deposition and a stable isotopic assessment for the neotropical epiphytic orchid Laelia speciosa

“…Here, the leaf nitrogen for Laelia speciosa was 1.2% of dry weight, similar to the content found for leaves PrePrints of several species of epiphytic orchids from environments with low nitrogen deposition, indicating that the two sites considered had low atmospheric nitrogen deposition rates (Hietz et al, 1999;Cardelús and Mack, 2010;Mardegan et al, 2011).…”

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

“…In wetlands, critical N : P ratios were found to range between 14 and 16, with N-limited plants having lower and P-limited plants having higher N : P ratios than this critical value (Koerselman & Meuleman, 1996;Güsewell, 2004). In vascular epiphytes, foliar N : P ratios were found to be 12.1 ± 10.5 (n = 41) (Zotz & Hietz, 2001), 16.1 ± 5.8 (n = 20) (Zotz, 2004) and 19.0 ± 5.3 (n = 195) (Cardelus & Mack, 2010), which implies a continuum between N-limited and P-limited vascular epiphytes. However, critical N : P ratios vary between plant species, and between developmental stages, and therefore the range of 14-16 cannot be applied universally (Güsewell, 2004).…”

Adaptive radiation, correlated and contingent evolution, and net species diversification in Bromeliaceae

“…Depletion of 15 N and higher N:P ratios in response to N + P fertilization of the tank epiphyte Werauhia (formerly Vriesea) sanguinolenta in Panama indicate that its growth is often P-limited, with a breakpoint at a foliar N:P ratio of 10.4 (Wanek and Zotz, 2011). A strong correlation of foliar P content -but not N content -between epiphytes and their hosts along an extensive elevational gradient in Costa Rica also suggests a close link of epiphytes to hosts in terms of P (but not N) nutrition (Cardelus and Mack, 2010). Widespread natural depletion of 15 N of foliar tissue in epiphytes vs. hosts, as well as epiphytic foliar N:P ratios often greater than 12, strongly suggest that P limitation or co-limitation is widespread in bromeliads and other epiphytes (Cardelus and Mack, 2010;Wanek and Zotz, 2011).…”

“…A strong correlation of foliar P content -but not N content -between epiphytes and their hosts along an extensive elevational gradient in Costa Rica also suggests a close link of epiphytes to hosts in terms of P (but not N) nutrition (Cardelus and Mack, 2010). Widespread natural depletion of 15 N of foliar tissue in epiphytes vs. hosts, as well as epiphytic foliar N:P ratios often greater than 12, strongly suggest that P limitation or co-limitation is widespread in bromeliads and other epiphytes (Cardelus and Mack, 2010;Wanek and Zotz, 2011). Tree litter in tropical rain forests exhibit the highest N:P ratios on Earth (McGroddy et al, 2004), and canopy soils -derived from litterfall, with both feeding rooted as well as tank epiphytes -have much higher N:P ratios than ordinary soils, in rain forests and especially cloud forests (Nadkarni and Solano, 2002;Cardelus et al, 2009;Wanek and Zotz, 2011).…”