There is increasing evidence that geographic and climatic clines drive the patterns of plant defence allocation and defensive strategies. We quantified early growth rate and both constitutive and inducible chemical defences of 18 Pinaceae species in a common greenhouse environment and assessed their defensive allocation with respect to each species' range across climatic gradients spanning 31° latitude and 2300 m elevation. Constitutive defences traded-off with induced defences, and these defensive strategies were associated with growth rate such that slow-growing species invested more in constitutive defence, whereas fast-growing species invested more in inducible defence. The position of each pine species along this trade-off axis was in turn associated with geography; moving poleward and to higher elevations, growth rate and inducible defences decreased, while constitutive defence increased. These geographic patterns in plant defence were most strongly associated with variation in temperature. Climatic and geographical clines thus act as drivers of defence profiles by mediating the constraints imposed by trade-offs, and this dynamic underlays global patterns of defence allocation.

BackgroundDespite empirical support for an increase in ecosystem productivity with species diversity in synthetic systems, there is ample evidence that this relationship is dependent on environmental characteristics, especially in structurally more complex natural systems. Empirical support for this relationship in forests is urgently needed, as these ecosystems play an important role in carbon sequestration.Methodology/Principal FindingsWe tested whether tree wood production is positively related to tree species richness while controlling for climatic factors, by analyzing 55265 forest inventory plots in 11 forest types across five European countries. On average, wood production was 24% higher in mixed than in monospecific forests. Taken alone, wood production was enhanced with increasing tree species richness in almost all forest types. In some forests, wood production was also greater with increasing numbers of tree types. Structural Equation Modeling indicated that the increase in wood production with tree species richness was largely mediated by a positive association between stand basal area and tree species richness. Mean annual temperature and mean annual precipitation affected wood production and species richness directly. However, the direction and magnitude of the influence of climatic variables on wood production and species richness was not consistent, and vary dependent on forest type.ConclusionsOur analysis is the first to find a local scale positive relationship between tree species richness and tree wood production occurring across a continent. Our results strongly support incorporating the role of biodiversity in management and policy plans for forest carbon sequestration.

There is increasing evidence that plants can react to biotic aggressions with highly specific responses. However, few studies have attempted to jointly investigate whether the induction of plant defences is specific to a targeted plant tissue, plant species, herbivore identity, and defensive trait. Here we studied those factors contributing to the specificity of induced defensive responses in two economically important pine species against two chewing insect pest herbivores. Juvenile trees of Pinus pinaster and P. radiata were exposed to herbivory by two major pest threats, the large pine weevil Hylobius abietis (a bark-feeder) and the pine processionary caterpillar Thaumetopoea pityocampa (a folivore). We quantified in two tissues (stem and needles) the constitutive (control plants) and herbivore-induced concentrations of total polyphenolics, volatile and non-volatile resin, as well as the profile of mono- and sesquiterpenes. Stem chewing by the pine weevil increased concentrations of non-volatile resin, volatile monoterpenes, and (marginally) polyphenolics in stem tissues. Weevil feeding also increased the concentration of non-volatile resin and decreased polyphenolics in the needle tissues. Folivory by the caterpillar had no major effects on needle defensive chemistry, but a strong increase in the concentration of polyphenolics in the stem. Interestingly, we found similar patterns for all these above-reported effects in both pine species. These results offer convincing evidence that induced defences are highly specific and may vary depending on the targeted plant tissue, the insect herbivore causing the damage and the considered defensive compound.

Aim Using pine trees as a long-lived woody plant model, we analysed the extent to which constitutive and induced allocation to chemical defences were conserved across the phylogeny, and tested the hypothesis that patterns of defensive allocation and defensive strategies differ between Palaearctic and Nearctic pines.Location Common-environment study with Palaearctic and Nearctic pine species.Methods We grew 17 pine species (eight Palaearctic and nine Nearctic) in a common environment and measured the stem and needle concentrations of phenolics and non-volatile resin, two major quantitative chemical defences in pines. In particular, we studied the constitutive allocation to chemical defences (in control plants) and the potential inducibility of those traits in response to elicitation with hormonal analogues of the jasmonic acid (methyl jasmonate) and salicylic acid (benzothiadiazole) pathways of plant defence against herbivorous insects and pathogens.Results We found a strong phylogenetic signal in the constitutive concentration of phenolics in the needles and resin in the stem, but not elsewhere. We found that Nearctic pines showed 70% greater concentration of constitutive needle phenolics than Palaearctic species. In contrast, the inducibility potential of defences did not differ between Nearctic and Palaearctic pine species.Main conclusions Our findings suggest that macroevolutionary history (phylogeny and biogeographical origin combined) has played an important role in the evolution of needle constitutive defences among pine species, but not in that of induced defences. Specifically, constitutive allocation to pine chemical defences seems to be well preserved across the phylogeny. In contrast, the inducibility of chemical defensive traits was evolutionarily labile and may depend more on the particular climate, biotic interactions and resource availability in each species' range.

Summary1. The natural enemy hypothesis (NEH) predicts that alien plant species might receive less pressure from natural enemies than do related coexisting native plants. However, most studies to date are based on pairs of native and alien species, and the results remain inconclusive. The level of attack by native generalist herbivores can vary considerably between plant species, depending on defensive traits and strategies. Plant defences include preformed constitutive and induced defences that are activated as plastic responses to herbivore attack. However, the efficacy of induced defences could be altered when alien species entering an area are exposed to native enemies. 2. We tested the NEH for several closely related alien and native pines to Europe by examining early anti-herbivore resistance to damage by two generalist native insect herbivores (Hylobius abietis and Thaumetopoea pityocampa); the differences in constitutive and inducible chemical defences (i.e. non-volatile resin and total phenolics in the stem and needles); and whether consumption preferences shift after induced defences have been triggered by real herbivory. 3. We did not find alien pines to be less damaged by two generalist herbivores than native pines were. The constitutive concentration of chemical defences significantly differed among pine species. The concentration of constitutive total phenolics in the stem was greater in native than in alien pines. The opposite trend was found for constitutive total phenolics in the needles. The concentration of chemical defences (non-volatile resin and total phenolics) in the stem significantly increased after herbivory by H. abietis. Moreover, the induction of total phenolics by H. abietis damage was significantly greater in native pine species than in alien pines. On the other hand, only concentrations of non-volatile resin in needles significantly increased after herbivory by T. pityocampa, but without significant differences in inducibility between alien and native pines. In cafeteria bioassays, H. abietis consumed the twigs from alien more than those from native species irrespective of prior exposure to the insect. Meanwhile, no differences among range origin were found in the T. pityocampa cafeteria bioassays. 4. Overall, we found no support for the NEH in alien pines to Europe. This suggests that alien pines, in regions where they coexist with native congeners, may be controlled by native generalist herbivores, this being one reason that invasion by alien pines is not frequent in Europe.