Over the course of their journey from the forest floor to the canopy, shade-tolerant trees in lowland tropical forests experience unparalleled shifts in both their capacity to grow and defend themselves and the identity and density of their pests. Specifically, light availability increases by approximately 40-fold, and among leaf-chewing insects, communities change by over 50% in species composition and increase by up to three fold in total population density. Yet little is known of the extent to which these factors are associated with shifts in allocation and/or composition of chemical defenses among tropical trees. We investigated and characterized these chemical shifts in five shade-tolerant tree species from Panama representing four families: Alseis blackiana (Rubiaceae), Brosimum utile (Moraceae), Inga multijuga (Fabaceae), Inga pezizifera (Fabaceae), and Virola multiflora (Myristicaceae).
To quantify total allocation to secondary metabolites, we measured the dry mass of secondary metabolite extract per dry mass of leaf material. To compare the chemical composition of suites of secondary metabolites, we conducted targeted metabolomics analyses on saplings and adults of each species, collecting data using ultra-high-pressure liquid chromatography and tandem mass spectrometry (UPLC-MS2). We then used molecular network analysis to quantify chemical similarity and to classify compounds based on molecular structure.
Results/Conclusions
Ontogenetic shifts in chemical defenses varied in pattern and magnitude across the five study species. For both expanding and mature leaves, total allocation to defense was either stable (A. blackiana, I. multijuga, I. pezizifera) or increased (B. utile, 4.4/4.6-fold in expanding/mature; V. multiflora, 6.2/4.5-fold in expanding/mature) with plant age. All species exhibited shifts across ontogeny in the chemical composition of their secondary metabolite suites, both based on principal components (PLS-DA) and on relative abundances of 10 or more unranked components (pairwise t-tests with Bonferroni correction).
Our results indicate that reduced risk of pest-related mortality with increasing plant age does not lead to a reduction in chemical defense among tropical trees. Further, together with previous studies documenting high species turnover across forest floor and canopy communities of insect herbivores, including a study of the herbivores associated with A. blackiana, our results point to a possible evolutionary association between ontogenetic differentiation in chemical defense and vertical stratification of herbivore communities. For trees with generation times that are orders of magnitude longer than those of their pests, an ecological niche shift across ontogeny may serve as a mechanism for slowing or diverting the adaptation of specialized pests to the trees’ chemical defenses.