For the trees growing within, the vertical strata of a closed-canopy tropical broadleaved forest represent a heterogeneous combination of abiotic stresses and herbivore pressure overlaid upon a massive gradient of light availability. Leaf chemistry plays a large role in mediating both abiotic stress and herbivore pressure, and is likely a crucial aspect of these trees’ adaptations to a stratified environment. It has been shown that foliar secondary metabolites, particularly phenolics, can be upregulated in response to increased solar radiation. However, it is not clear whether these phenolics are functioning as photoprotectants, defenses against herbivores, or both. To parse out the ecological functions of phenolics which are upregulated in response to solar radiation, we conducted intraspecific comparisons of the content and functional properties of phenolics in two tropical tree species representing two plant families.
We sampled naturally growing trees across the forest floor - canopy gradient, as well as potted seedlings in an experiment crossing a range of PAR and UV exposures, analyzing both expanding and mature leaves from each stratum or treatment. Phenolic content was quantified as the mass of fractionated extract per dry mass of leaf tissue. We found that phenolic content varied less than twofold across light treatments but up to 7.5-fold across leaf and plant developmental stage. Using a tropical fungal pathogen and brine shrimp, we conducted bioassays to evaluate the relationships between changes in phenolic content and anti-consumer function of the phenolic extract. In both tree species, the growth inhibition/toxicity of the phenolics varied significantly, with the largest changes occurring across plant ontogeny rather than light environment. In these cases, growth inhibition and toxicity increased up to 6.4-fold in expanding leaves of adult trees as compared to expanding leaves of seedlings. However, all other significant trends in phenolic anti-consumer function were not shared across the two species and in some cases were divergent. This suggests that the functional roles of phenolics may differ across plant phylogeny, which would represent a novel axis on which to evaluate chemical defense niche space on the plant community level.