How plants defend themselves from herbivorous insects is a topic that is of interest to many people, ranging from gardeners to mass crop growers. Related to this is the striking variability that is seen in plant edibility by herbivorous insects, which is often suggested to be underpinned by genetic differences between or within plant species. Trees are particularly under-represented in studies on the basis of this variability, despite being both commercially, environmentally and culturally important in many countries. In light of this Valdés-Correcher and colleagues from France and Spain in their paper in Annals of Botany investigate the relationships between the genetic make-up, presence of anti-herbivory plant chemicals and levels of herbivory in natural populations of common oak (Quercus robur) trees. Lead author Elena Valdés-Correcher told Botany One: ‘The main motivation of this study was to investigate the importance of tree genetics, insect herbivory and leaf chemistry in natural environments, as there are few studies that have investigated them simultaneously in the real world’.
The significance of herbivory as a challenge faced by natural tree populations is highlighted by Valdés-Correcher and colleagues, who find that the 235 oak trees they surveyed in detail lost between 7 and 22% of their foliage to insect herbivory. Unlike many other previous studies, in this study the authors investigated the differences between leaves at different heights on the trees, ranging from canopy leaves to lower-level leaves. From this, the authors identify that genetic makeup correlates to presence of anti-herbivory chemicals in leaves of the canopy. Herbivory levels in canopy leaves were significantly negatively correlated with the amount of anti-herbivory chemicals, a relationship that was also present at the whole-tree level (albeit weakly). However, this relationship is not present when only considering lower-level leaves.
As Valdés-Correcher and colleagues, the reason for this is likely multi-faceted. Phenolic compounds involved in repelling herbivorous insects are also known to be involved protection against UV radiation, which canopy leaves obviously receive more of than lower-level shaded leaves. Moreover, leaves of the canopy are often the most productive of trees and so it is unsurprising that plants may invest relatively more in protecting them against multiple threats compared to less productive lower-level leaves.
When analysing data at the whole-tree level, correlations between tree genetics , leaf chemical content and leaf herbivory were weak. In the absence of the within-individual comparisons that the authors also do, this may lead to the conclusion that leaf herbivory is mostly driven by environmental factors such as tree positioning.
In a previous study, Valdés-Correcher and colleagues do indeed show that such environmental factors do play a role, but the within-individual comparisons done by the authors in this study indicate that this is not the sole factor governing variation in herbivory that trees experience. As Valdés-Correcher and colleagues highlight, this underlines the importance of considering within-individual variation in such studies.
‘This study makes some important advances over past work. Most of previous studies were done in greenhouse or common garden experiments, limiting the ability to quantify the importance of plant genetics, insect herbivory and leaf chemistry in natural environments. Thus, this study investigates for the first time the relative importance of these factors in nature. Furthermore, most of previous studies focused on averages of chemistry and herbivory at the whole-plant scale, regardless of the importance of variability within plant individuals,’ Valdés-Correcher told Botany One.
The authors also highlight that production of anti-herbivory chemicals are but one of a range of defences trees may employ against herbivorous insects, and point out that future studies would benefit from also assessing occurrence and variation of such traits. While this study certainly raises key points for future studies to consider, the full story is yet to come to light!