Understanding how hundreds of species of tree manage to coexist in tropical forests lies at the heart of efforts to understand what generates and maintains species diversity on the planet. Consensus appears to be mounting that biotic interactions with specialist pests and pathogens comprise a key mechanism that limits the local recruitment of conspecific individuals, and thereby promotes diversity. Host use of plant enemies is limited by the capacity for pests to tolerate a range of plant defenses, including noxious and toxic secondary metabolites. Plant species with similar defenses are likely to experience competition mediated by shared natural enemies. Hence, the diversity of woody plant species occurring locally in a tropical forest may be determined in large part by species niche differences defined by secondary chemistry.
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, an attractive utility in chemically diverse and understudied tropical forests.
Here, we assemble MS molecular networks for 185 tree species that represent 95% of all individuals in the Smithsonian Institution Global Earth Observatory (ForestGEO-CTFS) Forest Dynamics Plots in Maryland and at Barro Colorado Island (BCI), Panama. We ask i) does seedling recruitment depends on the chemical neighborhood in which the seedling occurs? and ii) does phylogenetic signal in secondary chemistry decline with phylogenetic scale (i.e. is there divergence in chemistry among close relatives within species-rich genera)?
Results/Conclusions
Our models indicate a substantial effect of chemical neighborhood on the recruitment of seedlings in both forests. We find a much stronger chemical neighborhood effect in tropical tree genera, and a breakdown of phylogenetic signal in chemistry in tropical but not temperate tree genera.
Our results suggest that species differences in secondary chemistry may comprise important axes of niche differentiation among coexisting species of tropical trees. Furthermore, chemical differences may distinguish members of species-rich woody plant genera in ways that facilitate coexistence among species which otherwise share many aspects of the niche. Finally, the contrast in results between Maryland and Panama hint that the importance of chemically-defined niche differences to species coexistence may vary importantly over latitude.