Different responses of two carbon-based defences in Scots pine needles to nitrogen fertilization

Decomposition of Herbivore-Damaged Leaves of Understory Species Growing in Oak and Pine Stands

Leaves are the largest component of forest litter. Their decomposition rate depends mainly on plant species, leaf chemical composition, microorganism biodiversity, and habitat conditions. It is known that herbivory by insects can modify the chemical composition of leaves, such as through induction. The aim of this study was to determine whether the rate of leaf decomposition is related to the susceptibility of the plant species to insect feeding and how leaf damage affects this rate. For our research, we chose six species differing in leaf resistance to insect damage: Cornus sanguinea, Frangula alnus, and Sambucus nigra (herbivore resistant), and Corylus avellana, P. padus, and Prunus serotina (herbivore susceptible). The decomposition of these plant leaves was examined in two monoculture forest stands, deciduous (Quercus robur) and coniferous (Pinus sylvestris). Litter decay rate k and change of litter mass, content of defensive metabolites (total phenols (TPh) and condensed tannins), and substances beneficial for organisms decomposing litter (nitrogen (N) and nonstructural carbohydrates (TNC)) were determined. Contrary to our expectations, leaf litter of herbivore-resistant species decomposed faster than that of herbivore-susceptible species, and damaged leaves decayed faster than undamaged leaves. We found that faster decaying leaf litter had a lower content of defensive compounds and a higher content of TNC and N, regardless of the plant species or leaf damage. Leaf litter decomposition caused a large and rapid decrease in the content of defensive compounds and TNC, and an increase in N. In all species, the tannin content was lower in damaged than in undamaged leaves. This pattern was also observed for TPh, except in S. nigra. We interpret this as the main reason for faster decay of damaged leaves. Moreover, the loss of leaf mass was greater under oak than pine stands, indicating that the microorganisms in deciduous stands are more effective at decomposing litter, regardless of leaf damage.

Tree defence and bark beetles in a drying world: carbon partitioning, functioning and modelling

Drought has promoted large-scale, insect-induced tree mortality in recent years, with severe consequences for ecosystem function, atmospheric processes, sustainable resources and global biogeochemical cycles. However, the physiological linkages among drought, tree defences, and insect outbreaks are still uncertain, hindering our ability to accurately predict tree mortality under on-going climate change. Here we propose an interdisciplinary research agenda for addressing these crucial knowledge gaps. Our framework includes field manipulations, laboratory experiments, and modelling of insect and vegetation dynamics, and focuses on how drought affects interactions between conifer trees and bark beetles. We build upon existing theory and examine several key assumptions: (1) there is a trade-off in tree carbon investment between primary and secondary metabolites (e.g. growth vs defence); (2) secondary metabolites are one of the main component of tree defence against bark beetles and associated microbes; and (3) implementing conifer-bark beetle interactions in current models improves predictions of forest disturbance in a changing climate. Our framework provides guidance for addressing a major shortcoming in current implementations of large-scale vegetation models, the under-representation of insect-induced tree mortality.

Ecological conditions alter cooperative behaviour and its costs in a chemically defended sawfly

The evolution of cooperation and social behaviour is often studied in isolation from the ecology of organisms. Yet, the selective environment under which individuals evolve is much more complex in nature, consisting of ecological and abiotic interactions in addition to social ones. Here, we measured the life-history costs of cooperative chemical defence in a gregarious social herbivore, Diprion pini pine sawfly larvae, and how these costs vary under different ecological conditions. We ran a rearing experiment where we manipulated diet (resin content) and attack intensity by repeatedly harassing larvae to produce a chemical defence. We show that forcing individuals to allocate more to cooperative defence (high attack intensity) incurred a clear cost by decreasing individual survival and potency of chemical defence. Cooperative behaviour and the magnitude of its costs were further shaped by host plant quality. The number of individuals participating in group defence, immune responses and female growth decreased on a high resin diet under high attack intensity. We also found some benefits of cheating: non-defending males had higher growth rates across treatments. Taken together, these results suggest that ecological interactions can shape the adaptive value of cooperative behaviour and maintain variation in the frequency of cooperation and cheating.

A Native Parasitic Plant Systemically Induces Resistance in Jack Pine to a Fungal Symbiont of Invasive Mountain Pine Beetle

Conifer trees resist pest and pathogen attacks by complex defense responses involving different classes of defense compounds. However, it is unknown whether prior infection by biotrophic pathogens can lead to subsequent resistance to necrotrophic pathogens in conifers. We used the infection of jack pine, Pinus banksiana, by a common biotrophic pathogen dwarf mistletoe, Arceuthobium americanum, to investigate induced resistance to a necrotrophic fungus, Grosmannia clavigera, associated with the mountain pine beetle, Dendroctonus ponderosae. Dwarf mistletoe infection had a non-linear, systemic effect on monoterpene production, with increasing concentrations at moderate infection levels and decreasing concentrations at high infection levels. Inoculation with G. clavigera resulted in 33 times higher monoterpene concentrations and half the level of phenolics in the necrotic lesions compared to uninoculated control trees. Monoterpene production following dwarf mistletoe infection seemed to result in systemic induced resistance, as trees with moderate disease severity were most resistant to G. clavigera, as evident from shorter lesion lengths. Furthermore, trees with moderate disease severity had the highest systemic but lowest local induction of α-pinene after G. clavigera inoculation, suggesting a possible tradeoff between systemically- and locally-induced defenses. The opposing effects to inoculation by G. clavigera on monoterpene and phenolic levels may indicate the potential for biosynthetic tradeoffs by the tree between these two major defense classes. Our results demonstrate that interactions between a biotrophic parasitic plant and a necrotrophic fungus may impact mountain pine beetle establishment in novel jack pine forests through systemic effects mediated by the coordination of jack pine defense chemicals.

Eco-evolutionary factors drive induced plant volatiles: a meta-analysis

Herbivore-induced plant volatiles (HIPVs) mediate critical ecological functions, but no studies have quantitatively synthesized data published on HIPVs to evaluate broad patterns. We tested three hypotheses that use eco-evolutionary theory to predict volatile induction: feeding guild (chewing arthropods > sap feeders), diet breadth (specialist herbivores > generalists), and selection history (domesticated plants < wild species). To test these hypotheses, we extracted data from 236 experiments that report volatiles produced by herbivore-damaged and undamaged plants. These data were subjected to meta-analysis, including effects on total volatiles and major biochemical classes. Overall, we found that chewers induced more volatiles than sap feeders, for both total volatiles and most volatile classes (e.g. green leaf volatiles, monoterpenes). Although specialist herbivores induced more total volatiles than generalists, this was inconsistent across chemical classes. Contrary to our expectation, domesticated species induced stronger volatile responses than wild species, even when controlling for plant taxonomy. Surprisingly, this is the first quantitative synthesis of published studies on HIPVs. Our analysis provides support for perceptions in the published literature (chewers > sap feeders), while challenging other commonly held notions (wild > crop). Despite the large number of experiments, we identified several gaps in the existing literature that should guide future investigations.

A review of the phytochemical support for the shifting defence hypothesis

Several theories have been developed to explain why invasive species are very successful and develop into pest species in their new area. The shifting defence hypothesis (SDH) argues that invasive plant species quickly evolve towards new defence levels in the invaded area because they lack their specialist herbivores but are still under attack by local (new) generalist herbivores. The SDH predicts that plants should increase their cheap, toxic defence compounds and lower their expensive digestibility reducing compounds. As a net result resources are saved that can be allocated to growth and reproduction giving these plants a competitive edge over the local plant species. We conducted a literature study to test whether toxic defence compounds in general are increased in the invaded area and if digestibility reducing compounds are lowered. We specifically studied the levels of pyrrolizidine alkaloids, a toxin which is known for its beneficial and detrimental impact against specialists and generalists, respectively. Digestibility reducers did not show a clear trend which might be due to the small number of studies and traits measured. The meta analysis showed that toxic compounds in general and pyrrolizidine alkaloid levels specifically, increased significantly in the invaded area, supporting the predictions of the SDH that a fast evolution takes place in the allocation towards defence.

Compost may affect volatile and semi-volatile plant emissions through nitrogen supply and chlorophyll fluorescence

The use of composted biosolids as an amendment for forest regeneration in degraded ecosystems is growing since sewage-sludge dumping has been banned in the European Community. Its consequences on plant terpenes are however unknown. Terpene emissions of both Rosmarinus officinalis (a terpene-storing species) and Quercus coccifera (a non-storing species) and terpene content of the former, were studied after a middle-term exposure to compost at intermediate (50tha(-1): D50) and high (100tha(-1): D100) compost rates, in a seven-year-old post-fire shrubland ecosystem. Some chlorophyll fluorescence parameters (Fv/Fm, ETR, Phi(PSII)), soil and plant enrichment in phosphorus (P) and nitrogen (N) were monitored simultaneously in amended and non-amended plots in order to establish what factors were responsible for possible compost effect on terpenes. Compost affected all studied parameters with the exception of Fv/Fm and terpene content. For both species, mono- and sesquiterpene basal emissions were intensified solely under D50 plots. On the contrary leaf P, leaf N levels reached in D50 were partly responsible of terpene changes, suggesting that optimal N conditions occurred therein. N also affected ETR and Phi(PSII) which were, in turn, robustly correlated to terpene emissions. These results imply that emissions of terpene-storing and non-storing species were under nitrogen and chlorophyll fluorescence control, and that a correct management of compost rates applied on soil may modify terpene emission rate of plants, which in turn has consequences in air quality and plant defense mechanisms.

Growth and defence in young pine and spruce and the expression of resistance to a stem-feeding weevil

Defence in young trees has been much less studied than defence in older ones. In conifers, resin within ducts in bark is an important quantitative defence, but its expression in young trees may be influenced by developmental or physical constraints on the absolute size of the resin ducts as well as by differential allocation of resources to growth and resin synthesis. To examine these relationships, we used nitrogen fertilisation of 1- and 2-year-old pine and spruce to produce trees of different sizes and measured the effect on the number and size of resin ducts and the amount of resin they contained. All of these variables tended to increase with stem diameter, indicating a positive relationship between resin-based defence and growth of 1- and 2-year-old trees. In pine, however, the mass of resin flowing from severed ducts was much lower relative to duct area in 1- than in 2-year-old trees, suggesting that the older trees allocated a higher proportion of the carbon budget to resin synthesis. Resin-based defence in 1-year-old pines appears to be both positively related to growth and resource limited. In spruce, resin production was generally lower, and age-related differences were not observed, suggesting that resin-based defence is less important in this species. Bio-assays of 2-year-old trees with the pine weevil, Hylobius abietis, emphasised the importance of resin as a defence against this bark feeding insect. Nitrogen fertilisation had a limited influence on resistance expression. One-year-old trees remained susceptible because of their small size, low resin production and limited response to fertilisation. The strong growth response of 2-year-old trees to fertilisation increased resin-based defence, but most spruce trees remained susceptible, while most pines were resistant at all levels of fertilisation.

Effects of nutrient variability on the genetic-based resistance of Eucalyptus globulus to a mammalian herbivore and on plant defensive chemistry

Plant resistance to herbivores can be influenced not only by the independent effects of plant genotype and environmental variation, but by interactions between the two. The main aim of this study was to assess the effects of environmental variability (nutrient treatment) on the known genetic-based expression of resistance and defensive chemistry of Eucalyptus globulus to browsing by the generalist mammalian herbivore Trichosurus vulpecula. In a captive feeding trial, we measured intake of seedlings from one relatively resistant (Blue Gum Hill) and one relatively susceptible (St Helens) population of E. globulus grown under two nutrient treatments (no fertiliser, plus fertiliser). There was a significant genotypexfertiliser interaction effect on intake of E. globulus foliage by T. vulpecula, and the predicted genetic-based resistance of the two populations was expressed only for the non-fertilised treatment. Expression of resistance largely reflected the combined and inverse effects of nitrogen and condensed tannin concentrations. The expression of plant secondary metabolite concentration differed between compounds, but in all cases the effects of plant genotype and fertiliser treatment were independent. The formylated phloroglucinol compounds differed significantly between genotypes but not between fertiliser treatments. In contrast, the effect of plant genotype on the expression of condensed tannins was weak but they were significantly reduced by fertiliser. Essential oils were influenced by both plant genotype and fertiliser treatment and were significantly higher in the fertilised seedlings than in the non-fertilised seedlings. This study highlights interactive effects of plant genotype and environment in influencing the phenotypic expression of resistance in a eucalypt species to a mammalian browser. It also demonstrates that this interactive effect is the net result of independent effects of genotype and environment on plant chemistry and finally, that different groups of compounds within a plant can respond very differently to variation in environmental conditions.