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During their long lifespan, pines must cope with simultaneous abiotic and biotic stresses such as drought and herbivory. Mediterranean pines are isohydric species that rapidly close their stomata in response to drought reducing carbon fixation. In such situations, the synthesis of chemical defences could be impaired. Here, we tested the hypothesis that drought stress may constrain the capability of Mediterranean pines to defend against herbivory and to induce chemical defences. For this purpose, we subjected three contrasting populations of Aleppo pine (Nuts halepensis Mill.) to three levels of drought stress, thereafter exposing the seedlings to the herbivore Hylobius abietis L. A suite of ecophysiological and defensive traits was measured to explore the interaction between both stresses.

Drought significantly affected the C-13 signature and reduced starch and fatty acids concentration. Damage caused by the insect was affected by drought stress, being 75% higher at the moderate stress level but returning under severe stress to similar values as control seedlings. Seedlings responded to herbivory by decreasing the concentration of total polyphenolics and condensed tannins, increasing the concentration of total diterpenes, and modifying the profile of major terpenes. Induced responses to herbivory were, as expected, altered by drought. Inducibility of polyphenolics decreased as drought stress increased while for diterpenes it was higher at moderate stress. Moreover, a significant drought x herbivory interaction was found on the multivariate terpene profile.

These results should be considered for predicting responses of pine forests to the forecasted increase of abiotic and biotic risks associated with global change.