Leaf traits and herbivory in the mangrove fern Acrostichum aureum: a comparative study case with co-occurring angiosperms in Brazil

Despite their ecological relevance, ferns remain understudied in terms of herbivory and defense strategies, especially in mangrove ecosystems. Here we analyzed the leaf traits and herbivory damage in Acrostichum aureum, a notable fern species occurring in mangroves, and compared the data with two angiosperms occurring in the same area, Laguncularia racemosa and Rhizophora mangle. For this study case, we estimated herbivory damage in 100 leaves collected from 10 to 15 individuals of each species in a Brazilian mangrove using the classical visual method. We measured the leaf thickness, nutrient (N, P, K and Ca), heavy metals (Cd, Cr, Ni, and Pb), and phenol concentrations and compared them using ANOVA. Cd, Cr, and Pb were not detected in all studied species. Our results demonstrated that A. aureum had higher palatability associated with thinner leaves, absence of Ni, lower Ca and phenol content, and higher NPK concentrations, compared to the other mangrove species. However, A. aureum exhibited significantly lower herbivory damage (0.05 %) than L. racemosa (0.75 %) and R. mangle (2.86 %). Therefore, the palatability of A. aureum leaves did not lead to higher herbivory damage in this species. Our findings underscore the challenges in generalizing the effects of leaf defenses and nutritional composition on herbivory in mangrove species. We encourage global herbivory data collection in A. aureum to better understand its role in mangrove herbivory. We propose hypotheses to explain why the high palatability does not translate into high herbivory in A. aureum. Finally, we highlight the absence of bottom-up effects on herbivory patterns in our study area.