. Effects of defoliation and site quality on growth and defenses of Pinus pinaster and P. radiata

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The susceptibility to pests and pathogens is frequently of only minor consideration by foresters when selecting tree species to plant, but growth reductions from pests and pathogens are commonly large relative to productivity differences among tree species. Because the selection of tree species and sites when establishing plantation forests has potential effects on subsequent damage by pests, there is need for consideration of joint effects of tree species, site quality, and risk of pests. We tested here how site quality influences the response of two pines species that differ in their evolutionary history with a primary pest in the system. Our specific research objectives were to (1) measure the growth response of two pine species (the native Pinus pinaster and the introduced P. radiata) to defoliation by the pine processionary moth, across a gradient of nutrient availability and (2) compare the constitutive and delayed defoliation induced chemical defenses of these two pine species on sites of low and high quality. We analyzed variation in foliar nitrogen, terpenes, tannins, total phenols and four defensive proteins. The impact on tree growth from experimentally applied defoliation by PPM was surprisingly low. Even with complete defoliation, large sample sizes, and baseline measurements that accounted for much of the variation among trees, we observed only modest effects on diameter growth and no effects on height growth; volume growth was initially reduced by only 6–8% initially, with full recovery within 3 years of defoliation. Pinus radiata, the faster growing species, had higher concentrations than P. pinaster of foliar N while P. pinaster had up to 12 times higher concentrations of terpenes, these results match with predictions of the resource availability hypothesis (RAH). Pinus radiata foliage was relatively rich in tannins and total phenolics. The pine species also differed in defensive proteins. Pinuspinaster had higher concentrations of peroxidase (POD), polyphenol oxidase (PPO) and chitinase (CHI), while P. radiata had higher concentrations of trypsin inhibitor (TI). Finally there was no evidence for delayed inducible defenses in either pine species following PPM defoliation; both species seem to use other defensive strategies to counter predictable attacks by the herbivore. In our study region, defoliation by PPM may not be a primary consideration in the selection of pine species for production forests, but this conclusion need not hold in regions where pines grow more slowly or are exposed to multiple years of defoliation by PPM.