The effects of insect herbivory on the photosynthetic capacity of oak leaves

Background/Question/Methods

In almost all terrestrial ecosystems, plants are eaten by insect herbivores. By consuming plant tissue, insects can trigger substantial changes in the metabolism and chemical composition of their host plant. These changes are likely to affect important plant functions, like photosynthesis. So far, the effects of herbivory on the photosynthetic rate have mostly been studied in few model species, or under greenhouse conditions. Hence, the extent of these effects in natural settings remains largely unknown. We studied how a common insect herbivore, the winter moth (Operophtera brumata), affects the photosynthetic rate of one of its most common host plants, the pedunculate oak (Quercus robur). We created an experimental set-up, in which leaves from a set of randomly selected oak shoots were either damaged by moth caterpillars, damaged mechanically by cutting and punching holes, or left completely intact. We then measured the photosynthetic rate of damaged leaves, neighbouring intact leaves on the same shoots, and leaves on intact shoots. We measured photosynthesis with an infrared gas analyser (CIRAS-2, PPSystems) in saturating light (A_sat) and as A/C_i curves.

Results/Conclusions

The photosynthetic rate per unit leaf area in saturating light (A_sat) and the electron transport rate as measured from the A/C_i curves (J_max) were significantly lower in leaves damaged by caterpillars (1.4 and 1.8 times, respectively), compared to intact leaves on shoots that had not received any damage. Surprisingly, intact leaves on shoots with leaves damaged by herbivores experienced as large of a reduction in their photosynthetic rate as their damaged neighbours. Mechanical damage had no significant effect on photosynthetic rate, suggesting that the reduction in photosynthetic rate is due to an herbivore-specific reaction, rather than simply a result of reduced leaf area. Ongoing work assesses the wider, ecosystem-level impacts of these results. Including information on the effects of herbivory in models of the forest carbon cycle is important in the changing climate, which is predicted to lead to altered levels of herbivory in some systems.