Seedling performance trade-offs influencing habitat filtering along a soil nutrient gradient in a tropical forest

Recent studies have provided evidence that trade-offs in biomass allocation, resource-use efficiency, and anti-herbivore defense influence seedling performance and distribution across soil types. Our previous research suggested that soil nutrient availability acted as a filter for understory palm community assembly and species coexistence in a lower montane forest in western Panama. Here, we experimentally tested three potential mechanisms that could be driving this filtering of palm communities along a soil nutrient gradient. We examined seedling performance of 13 understory palm taxa with differing distribution patterns transplanted to five sites, where they were both exposed to and protected from herbivores. We found the strongest evidence for habitat filtering at the lowest-nutrient site, where seedlings of locally occurring species had significant performance advantages over those of locally absent species, regardless of protection from herbivores. When transplanted to the lowest-nutrient site, seedlings of naturally occurring species maintained high above-to belowground biomass ratios, maximized photosynthetic nitrogen-use efficiency, and minimized leaf area loss to herbivores and pathogens. Species from low-fertility sites were therefore able to maintain higher survival and relative growth rates (RGR) at the lowest-nutrient site than species from more fertile sites. Similar patterns were found for a high-nutrient site, where naturally occurring species had higher RGR compared to species from lower-fertility sites. However, the functional traits associated with the differences in performance at high-nutrient sites were less clear, perhaps due to increased light limitation in the understory of these sites. At higher-nutrient sites, protection from herbivores significantly reduced leaf area damage and mortality rates in seedlings, particularly for naturally occurring species. Overall, morphological and physiological traits were more strongly coupled with soil nutrient availability than were leaf damage or seedling performance across the sites. Nonetheless, the coordination of all three trade-offs was related to the strongest performance advantage and, therefore, ecological filtering along the soil nutrient gradient in this lower montane forest.