Although the negative effects of root herbivores on plant fitness are expected to be similar to those of aboveground herbivores, the study of belowground plant defenses is limited compared to the rich literature on aboveground defenses. Current theory predicts that concentrations of defensive chemicals above and belowground should be correlated, as the evolutionary drivers that shape plant defense are similar across the whole plant. To verify this theory more empirical data characterizing root and shoot defensive traits in individual plants is needed. Condensed tannins have been identified as an important defensive chemical and are found in both leaves and roots, making them an ideal target to test this theory.
In a recent study published in AoBP, Dettlaff et al. investigate the relationship between leaf and root tannin levels in mature, naturally occurring aspen (Populus tremuloides). They addressed three main research questions: (1) Within a single individual, are root tannin levels correlated with leaf tannin levels? (2) How do root and leaf tannins vary with time, stand position (edge vs interior), and stem diameter? (3) Is the ratio of root to leaf tannins within a single stem consistent over time?
Overall, root tannin concentrations were substantially lower than leaf tannin concentrations. At individual sampling points, root and leaf tannin concentrations were uncorrelated with each other, however across the growing season root and leaf tannin concentrations did show similar trends (highest in the early summer and declining through mid-summer and fall). This suggests that the factors influencing leaf and root tannin levels in aspen are independent to individual stems, possibly due to different evolutionary pressures experienced by the different tissue types or in response to localized (roots vs. foliage) stressors. The similar patterns in chemical defense over time indicate that larger scale processes can have consistent effects across individuals within a population, such as the relative investment in defense of tissues in the spring versus the fall. The results of this study suggest that using theories based on aboveground defense to predict belowground defenses may not be a valid approach until further studies examining belowground defense have been conducted.
Margarete Dettlaff obtained a BSc in Forest Science (Hons) from the University of British Columbia in 2014. Margarete is currently working on her PhD in Ecology under the supervision of James F Cahill Jr. and Nadir Erbilgin at the University of Alberta.
Margarete is a forest ecologist interested in how interactions between plants and herbivores shape forest structure and function. Her PhD work focuses on how secondary chemical defenses in trembling aspen vary due to stressors, time and geography and how this variation impacts the dynamics of aspen forests. You can follow Margarete and her research on Twitter (@MADaboutforests).