The vast diversity of plant chemical defenses has traditionally precluded community-level studies of chemical ecology. We take advantage of novel methods for acquiring and assembling mass spectra (MS) into molecular networks in which similarities in MS fragmentation patterns, and hence chemical structure, are indicated by proximity of compounds within the network. Molecular networks permit the quantification of chemical similarities between samples for which few compounds have been unambiguously structurally elucidated, making possible the study of the ecological function of diverse chemical traits even in species-rich and understudied tropical forests.
Here, we assembled MS molecular networks for 203 tree species that represent 89% of all individuals in the Smithsonian Institution Global Earth Observatory (ForestGEO-CTFS) Forest Dynamics Plots (FDPs) at the Smithsonian Environmental Research Center (SERC) in Maryland and at Barro Colorado Island (BCI), Panama. We compared interspecific chemical similarity and phylogenetic signal for 65 species in the 16-hectare (ha) Maryland FDP and 138 species in the 50-ha FDP in Panama. In addition, we modeled seedling survival at BCI and sapling growth in both forests as a function of metabolomic similarity to neighboring heterospecific trees, accounting for light environment and conspecific density.
Results/Conclusions
We found a dramatic difference in interspecific chemical similarity and phylogenetic signal between the temperate forest in Maryland and the tropical forest in Panama. The temperate trees are more chemically similar and exhibit phylogenetic signal. Among tropical trees, chemical similarity is low overall and does not exhibit a relationship with phylogenetic distance. This is driven by strong chemical divergence among congeneric tree species in a small number of hyperdiverse tree genera.
Despite this fundamental difference in the chemical diversity between the tropical and temperate forest, sapling growth was greater for saplings that occurred near chemically dissimilar neighboring trees at both SERC and BCI. In addition, seedling survival in the tropical forest at BCI was greater in the presence of chemically dissimilar trees. These results suggest that pests and pathogens, for which plant secondary metabolites shape host use patterns, influence fitness in plant communities in both temperate and tropical forests.