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Plants have evolved the ability to communicate with neighboring plants for alleviating stresses within communities by transmitting volatile compounds aboveground or a variety of organic and inorganic compounds belowground. Mycorrhizal networks, comprised of mycorrhizal fungi connecting the roots of multiple plants, are potentially direct pathways for belowground transmittance of these biochemical messages between plants1. There is increasing evidence that mycorrhizal networks can transmit, for example, herbivore- or pathogen-induced defense signaling compounds to warn neighbors of pest infestations2,3,4, kin recognition signaling compounds involving micronutrients to communicate genetic relationships of neighbors5,6, toxins such as allelochemicals to convey negative interactions to competing neighbors7 and essential resources such as carbon, nitrogen, phosphorus or water for altering physiology, survival or growth of conspecific or heterospecific neighbors8. Mycorrhizal networks have also been shown to rapidly transmit phosphorus and nitrogen from dying plants to healthy conspecific neighbors9, providing a conduit for legacy transference across generations. Similarly, clipping has prompted transport of labile carbon from stressed to healthy heterospecific neighbours through arbuscular mycorrhizal networks10. The neighbors receiving these messages could potentially then modify their behavior through altered morphology, physiology or biochemistry, thus reducing their stress and improving fitness. Although there is increasing evidence for interplant communication through mycorrhizal networks, the majority of studies conducted so far have focused on herbaceous or grass species forming arbuscular mycorrhizal networks. Belowground communication between trees linked by ectomycorrhizal networks in forests, however, has received little attention.
Trees have coevolved with native insects and pathogens so that they can respond to infection by producing an array of defense compounds that mediate their interactions with the invader24. Trees have also coevolved with ectomycorrhizal fungi that are responsible for nutrient and water uptake in exchange for carbon20. Previous research with arbuscular mycorrhizal tomatoes also shows that, when plants are connected by a mycorrhizal network, stress signals can transfer from infected plants to conspecific neighbors through this network, thus increasing activities of defense compounds, inducing defense-related genes, activating the jasmonate pathway and increasing pest resistance in receiver plants2,3. Additionally, in other previous research, we have shown that interior Douglas-fir can transfer carbon, nitrogen and water through ectomycorrhizal networks to conspecific or heterospecific neighbors and that this has been associated with increased survival, growth and foliar nutrition of recipient neighbors23,25,26,27. Taken together, these studies suggest that signal and resource transfer from interior Douglas-fir to ponderosa pine through ectomycorrhizal networks could play a role in facilitating and shaping the predicted forest vegetation shifts in this region as climate changes.
The objective of this study was to determine whether injury to interior Douglas-fir by insect or manual defoliation would induce interspecific transfer of carbon and stress signals to neighboring healthy ponderosa pine seedlings through a mycorrhizal network. Our first hypothesis was that manual and insect defoliation would cause interior Douglas-fir to export labile carbon directly to neighboring ponderosa pine through mycorrhizal networks. We expected increasing levels of carbon export with increasing degree of injury. We also expected the presence of root competition to reduce amount of transfer. Our second hypothesis was that manual and insect defoliation would cause interior Douglas-fir to communicate via organic stress signals with ponderosa pine to increase its defense response. We expected greater defense response with insect than manual defoliation because of coevolution between tree and insect species. We also expected greater stress signal transfer directly through mycorrhizal networks than indirectly through soil pathways.
In damaged trees, belowground communication with and transfer of carbon legacies to other encroaching tree species may facilitate forest vegetation shifts with climate change. We demonstrate for the first time in ectomycorrhizal conifers that injury to one tree species induces substantial belowground transfer of photosynthetic carbon and elicits a rapid defense response of a different tree species, likely through transfer of stress signals. Moreover, this interspecific communication occurs directly through mycorrhizal networks, bypassing microbial transformations that can occur along soil pathways. The ectomycorrhizal network of the 4-month old interior Douglas-fir and ponderosa pine was comprised of the single taxon, ________________ (Ascomycota, Pezizales order), an E-strain fungal species36 well known as an early colonizer of interior Douglas-fir and ponderosa pine seedlings in recently disturbed forest soils36,37,38.
Manual but not insect defoliation caused interior Douglas-fir to export labile carbon directly to neighboring ponderosa pine through mycorrhizal networks, partially supporting our first hypothesis. The lack of insect treatment response may be explained by the unexpected minimal levels of defoliation by western spruce budworm compared to manual excision, as evidenced by donor isotope contents in the 35 m mesh treatment, most likely because the two insects were insufficient or too immature at the third-instar stage for vigorous feeding. We also found that carbon transfer to ponderosa pine shoots and roots increased with declining donor Douglas-fir isotope content, suggesting that increasing severity of defoliation (causing lowered donor isotope uptake) stimulated the belowground flush to networked pine. With manual defoliation, the interior Douglas-fir exported carbon compounds to roots, a behavioral strategy known for helping trees survive subsequent defoliations39. The belowground pulses of labile C to roots were then transported to the extramatrical mycorrhizal network, as indicated by the significant C transfer to ponderosa pine receivers. Taken together, the difference in severity between the two defoliation methods and negative relationship between donor and receiver isotope content, supported our expectation that carbon export would increase with defoliation injury. Although our insect treatment was insufficient to elicit a response, we predict that severe sustained insect defoliation would elicit C transfer of similar magnitude as manual defoliation found in this study, but future research is still needed to quantify these effects in the greenhouse and in situ.
Carbon transfer from interior Douglas-fir to ponderosa pine through the mycorrhizal network may have occurred along a carbon or foliar nutrient source-sink gradient43. It is possible that C source-sink strength alone played a role in regulating C transfer. Defoliation likely stimulated interior Douglas-fir to rapidly export labile C from enriched roots to the mycorrhizal network39, thus increasing the source strength, while the rapid growth rate of ponderosa pine would at the same time have created a large sink strength. In this case, the high amount of C transferred to receiver shoots would have moved via the xylem or transpiration stream as carbohydrates, drawn by high transpiration rates of the ponderosa pine shoots44. The preferential movement of labelled C we found to receiver shoots has also been found by others23,25, including cases where sink strength shifts over growing seasons8,31,45, suggesting that sink strength of ponderosa pine was an important driver of C transfer through the mycorrhizal network. Carbon may have alternatively transferred along a foliar nutrient gradient, where C was translocated with nutrient elements as free amino acids across the Hartig net of donors and receivers. Rapid transfer of these nutrient elements in amino acids would meet an urgent nutritional demand of the fast-growing pine because they are essential for enzyme complexes involved in photosynthesis and protein synthesis.
The communication we observed between interior Douglas-fir and ponderosa pine in response to mechanical and insect defoliation of interior Douglas-fir suggests that the damage elicited a general response. The networking fungus may have acted to protect its net carbon source, by allocating carbon and signals to the healthy, more reliable ponderosa pine. In unstable environments, such as ecosystems under stress and experiencing species turnover as a result of climate change, the mycorrhizal network may therefore benefit from transferring carbon and defense signals interspecifically, favoring hosts that can supply more carbon54. It is possible, therefore, that mycorrhizal network-based transfers and signals may evolve to be more generic in stressful environments. The response of ponderosa pine to a stress signal from interior Douglas-fir may have a large cost and little benefit if the damaging agent is host-specific, but be worth investing in constitutive defense enzymes if the damage, as in the manual defoliation treatment, is non-specific. Western spruce budworm is a herbivore of interior Douglas-fir and to a lesser degree 1____________ and 2_____ species, so it is intriguing that ponderosa pine mounted a defense response to the attack on its interspecific neighbor. That the defense response occurred in response to host-specific and host-generalist damage suggests that the defense signal itself was a generic signal (e.g., jasmonate). It is also possible that ponderosa pine responds particularly well to abiotic damage and broad herbivore taxa55. The decoupling of carbon and defense signal transfer in this study, evident in the differential manual versus insect defoliation effect, suggests that interspecific carbon and defense signal transfer occurred with host-generalist damage (i.e., mechanical damage that can occur in response to abiotic stresses such as wind or drought), but that interspecific signal transduction was possible even with host-specific herbivore damage. Because of these defoliator treatment differences, carbon that was transferred was therefore unlikely a constituent of the defense signal. Further research is needed to understand the compounds, mechanisms, specificity and fitness consequences of communication through mycorrhizal networks. 5376163bf9
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