Photosynthetic and defensive responses of two Mediterranean oaks to insect leaf herbivory

Abstract

Insect herbivory is a dominant interaction across virtually all ecosystems globally and has dramatic effects on plant function such as reduced photosynthesis activity and increased levels of defenses. However, most previous work assessing the link between insect herbivory, photosynthesis and plant defenses has been performed on cultivated model plant species, neglecting a full understanding of patterns in natural systems. In this study, we performed a field experiment to investigate the effects of herbivory by a generalist foliar feeding insect (Lymantria dispar) and leaf mechanical damage on multiple leaf traits associated with defense against herbivory and photosynthesis activity on two sympatric oak species with contrasting leaf habit (the evergreen Quercus coccifera L. and the deciduous Quercus pubescens Willd). Our results showed that, although herbivory treatments and oak species did not strongly affect photosynthesis and dark respiration, these two factors exerted interactive effects. Insect herbivory and mechanical damage (vs control) decreased photosynthesis activity for Q. coccifera but not for Q. pubescens. Insect herbivory and mechanical damage tended to increase chemical (increased flavonoid and lignin concentration) defenses, but these effects were stronger for Q. pubescens. Overall, this study shows that two congeneric oak species with contrasting leaf habit differ in their photosynthetic and defensive responses to insect herbivory. While the evergreen oak species followed a more conservative strategy (reduced photosynthesis and higher physical defenses), the deciduous oak species followed a more acquisitive strategy (maintained photosynthesis and higher chemical defenses).

Keywords: Lymantria dispar; Quercus coccifera; Quercus pubescens; chemical defenses; light response curves; physical defenses.

Figure 1.

Description of leaf area measurements made across the three induction treatments and the two study species. ‘Leaf area inside chamber’ was estimated in order to correct the CO₂ fluxes, as in some leaves, the whole chamber area (6 cm²) was not fully covered either due to smaller leaf size or inclusion of ‘damaged area’ from induction treatments.

Figure 2.

The mean light response curve (leaf-level dataset) per induction treatment (control, herbivory by L. dispar, mechanical damage) and oak species (Q. coccifera and Q. pubescens). Light response curves were developed by measuring net photosynthesis at different PAR levels and fitting a MM response curve afterward (see Materials and methods section).

Figure 3.

Effect of oak species (Q. coccifera and Q. pubescens) and oak species induction treatment (control, herbivory and mechanical damage) interaction on the light saturated photosynthesis (A_sat) (a,b), and dark respiration (R_dark) (c). Red colors indicate a negative random effect and blue colors indicate a positive random effect.

Figure 4.

Effect of oak species (Q. coccifera and Q. pubescens) and induction treatment (control, herbivory and mechanical damage) on leaf structural defensive traits, as inferred from the random effect model: Leaf Area (a,b), Leaf thickness (c) and Leaf dry Mass per Area (d). Red colors indicate a negative random effect and blue colors indicate a positive random effect.

Figure 5.

Effect of oak species (Q. coccifera and Q. pubescens), induction treatment (control, herbivory and mechanical damage) and their interaction on leaf chemical defensive traits, as inferred from the random effect model: Flavonoids (a,b), Lignins (c,d), Condensed Tannins (e) and Hydrolysable Tannins (f).