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Current predictions on species responses to climate change strongly rely on projecting altered environmental conditions on species distributions. However, it is increasingly acknowledged that climate change also influences species interactions. We review and synthesize literature information on biotic interactions and use it to argue that the abundance of species and the direction of selection during climate change vary depending on how their trophic interactions become disrupted. Plant abundance can be controlled by aboveground and belowground multitrophic level interactions with herbivores, pathogens, symbionts and their enemies. We discuss how these interactions may alter during climate change and the resulting species range shifts. We suggest conceptual analogies between species responses to climate warming and exotic species introduced in new ranges. There are also important differences: the herbivores, pathogens and mutualistic symbionts of range-expanding species and their enemies may co-migrate, and the continuous gene flow under climate warming can make adaptation in the expansion zone of range expanders different from that of cross-continental exotic species. We conclude that under climate change, results of altered species interactions may vary, ranging from species becoming rare to disproportionately abundant. Taking these possibilities into account will provide a new perspective on predicting species distribution under climate change.

Anthropogenic changes in biodiversity and atmospheric temperature significantly influence ecosystem processes. However, little is known about potential interactive effects of plant diversity and warming on essential ecosystem properties, such as soil microbial functions and element cycling. We studied the effects of orthogonal manipulations of plant diversity (one, four, and 16 species) and warming (ambient, +1.5°C, and +3°C) on soil microbial biomass, respiration, growth after nutrient additions, and activities of extracellular enzymes in 2011 and 2012 in the BAC (biodiversity and climate) perennial grassland experiment site at Cedar Creek, Minnesota, USA. Focal enzymes are involved in essential biogeochemical processes of the carbon, nitrogen, and phosphorus cycles. Soil microbial biomass and some enzyme activities involved in the C and N cycle increased significantly with increasing plant diversity in both years. In addition, 16-species mixtures buffered warming induced reductions in topsoil water content. We found no interactive effects of plant diversity and warming on soil microbial biomass and growth rates. However, the activity of several enzymes (1,4-β-glucosidase, 1,4-β-N-acetylglucosaminidase, phosphatase, peroxidase) depended on interactions between plant diversity and warming with elevated activities of enzymes involved in the C, N, and P cycles at both high plant diversity and high warming levels. Increasing plant diversity consistently decreased microbial biomass-specific enzyme activities and altered soil microbial growth responses to nutrient additions, indicating that plant diversity changed nutrient limitations and/or microbial community composition. In contrast to our expectations, higher plant diversity only buffered temperature effects on soil water content, but not on microbial functions. Temperature effects on some soil enzymes were greatest at high plant diversity. In total, our results suggest that the fundamental temperature ranges of soil microbial communities may be sufficiently broad to buffer their functioning against changes in temperature and that plant diversity may be a dominant control of soil microbial processes in a changing world.

Variation in plant functional traits is the product of evolutionary and environmental drivers operating at different scales. Little is known about whether, or how, this variation is coordinated between aboveground and belowground organs across and within spatial scales. We address these questions using a hierarchically designed dataset of pairwise leaf and root traits related to carbon and nutrient economy of 64 species belonging to 14 plant communities in northern Chinese semi-arid and arid regions. While both root and leaf traits showed most of their variance among (individuals and) species within communities, leaf trait variance tended to be relatively higher at coarser spatial scales than root trait variance. While leaf nitrogen (N) per area to root N per length ratio increased and specific leaf area to specific root length and leaf [N] to root [N] ratios decreased from semi-arid to arid environments owing to climatic/edaphic shifts, the matching pairs showed a strong pattern of positive correlation that was upheld across spatial scales and geographic areas. Thus, trade-offs in plant resource investment across organs within individual vascular plants are constrained within a rather narrow range of variation. A new challenge will be to test whether and how such trait coordination is also seen within and across other biomes of the world.

• Herbivore-induced systemic resistance occurs in many plants and is commonly assumed to be adaptive. The mechanisms triggered by leaf-herbivores that lead to systemic resistance are largely understood, but it remains unknown how and why root herbivory also increases resistance in leaves. • To resolve this, we investigated the mechanism by which the root herbivore Diabrotica virgifera induces resistance against lepidopteran herbivores in the leaves of Zea mays. • Diabrotica virgifera infested plants suffered less aboveground herbivory in the field and showed reduced growth of Spodoptera littoralis caterpillars in the laboratory. Root herbivory did not lead to a jasmonate-dependent response in the leaves, but specifically triggered water loss and abscisic acid (ABA) accumulation. The induction of ABA by itself was partly responsible for the induction of leaf defenses, but not for the resistance against S. littoralis. Root-herbivore induced hydraulic changes in the leaves, however, were crucial for the increase in insect resistance. • We conclude that the induced leaf resistance after root feeding is the result of hydraulic changes, which reduce the quality of the leaves for chewing herbivores. This finding calls into question whether root-herbivore induced leaf-resistance is an evolved response.

Trade-offs between plant growth and defense are central to theoretical frameworks used to study the ecology and evolution of plant defense against herbivores. However, these frameworks, as well as the experiments designed to test them, rarely include belowground herbivores. We experimentally challenged seedlings of the tropical shrub Solanum lycocarpum (Solanaceae) with either aboveground foliar herbivores (Spodoptera caterpillars) or belowground root herbivores (the nematode Meloidogyne incognita) and measured the resulting changes in plant growth rates, biomass allocation, and the concentration of defensive terpenoids in roots and leaves. We found that plants that suffered aboveground herbivory responded with aboveground growth but belowground defense. Similarly, belowground herbivory resulted in root growth but elevated defenses of leaves. These results underscore the importance of belowground plant–herbivore interactions, and suggest that, in contrast to theoretical predictions, plants can simultaneously invest in both growth and defense. Finally, they emphasize the need for a \"whole-plant\" perspective in theoretical and empirical evaluations of plant–herbivore interactions.

Plants suffer multiple, simultaneous assaults from above and below ground. In the laboratory, pests and/or pathogen attack are commonly studied on an individual basis. The molecular response of the plant to attack from multiple organisms and the interaction of different defense pathways is unclear. The inducible systemic responses of the potato ( L.) host plant were analyzed to characterize the plant-mediated indirect interactions between a sedentary, endoparasitic nematode ( ), and a phloem-sucking herbivore ( ). The reproductive success of was greater on potato plants pre-infected with compared to control plants. Salicylic acid (SA) increased systemically in the leaves of potato plants following nematode and aphid infection singly with a corresponding increase in expression of SA-mediated marker genes. An increase in jasmonic acid associated with aphid infection was suppressed when plants were co-infected with nematodes. Our data suggests a positive, asymmetric interaction between a sedentary endoparasitic nematode and a sap-sucking insect. The systemic response of the potato plant following infection with indirectly influences the performance of . This work reveals additional secondary benefits of controlling individual crop pests.

Frass deposition to soil is an important pathway by which herbivorous insects impact decomposition and soil nutrient availability. However, little is known about how frass quality influences ecosystem properties. Here, we examined the effects of frass quality on the decomposition process, soil nitrogen (N) availability, and plant growth, using frass of Mamestra brassicae (L.) that fed on fertilized or unfertilized Brassica rapa L. var. perviridis Bailey. The frass quality was largely dependent on the host plant quality. Frass excreted by larvae that fed on the fertilized plants had higher N than that of larvae that fed on the unfertilized plants. The decomposition rate of the frass did not differ between N-rich and N-poor frass, except during the early decomposition period. The inorganic N concentration decreased during decomposition in both frass types. However, difference in the initial inorganic N concentration led to different consequences regarding soil N availability. Furthermore, addition of frass to the soil differently influenced the growth of B. rapa plants depending on the frass quality: plant biomass was increased by N-rich frass addition but decreased by N-poor frass addition, compared to the biomass without frass addition. These results indicate that frass quality is an important factor in determining the impact of herbivorous insects on nutrient dynamics, and that frass positively or negatively influences soil N availability and plant growth, depending on its quality.

1. Mycorrhizal symbiosis is thought to affect interactions between plants and herbivores by its influence on plant growth, nutrition and the plants defence system. Moreover, arbuscular mycorrhizal fungi (AMF) may enhance the inducibility of resistance responses. Until now, induction of plant resistance has not been considered to be a mechanism affecting the outcome of mycorrhization for plant-herbivore interactions. 2. Here, we test the hypothesis that the resistance of plants against herbivores depends on the induction of plant resistance by previous herbivory and mycorrhization. With a full factorial experiment in a greenhouse, we examined responses in growth of seven herbaceous plant species to AMF and the induction of resistance. To evaluate whether induced resistance is higher in plants with AMF we analyzed the combined effects of AMF and induction on herbivory, using bioassay caterpillars (Spodoptera littoralis). 3. Across all species, mycorrhization increased growth of plants and performance of the bioassay herbivore feeding on them. If, however, we induced resistance by allowing a caterpillar to feed for a short period on the plants, mycorrhization did not increase plant growth and performance of a subsequent herbivore that fed on the plant. This suggests that the increased plant resistance after induction was dependent on the symbioses with AMF. 4. Our results indicate that induction interacts with the allocation of the additional resources provided by mycorrhization towards plant growth and plant resistance. Therefore, mycorrhiza may play an important but hitherto overlooked role in the induction of plant resistance against herbivores.

Plant competition may mediate the impacts of herbivory on invasive plant species through effects on plant growth and defense. This may predictably depend on whether herbivory occurs above or below ground and on relative plant competitive ability. We simulated the potential impact of above- or belowground damage by biocontrol agents on the growth of a woody invader (Chinese tallow tree, Triadica sebifera) through artificial herbivory, with or without competition with a native grass, little bluestem (Schizachyrium scoparium). We measured two defense responses of Triadica through quantifying constitutive and induced extrafloral nectar production and tolerance of above- and belowground damage (root and shoot biomass regrowth). We examined genetic variation in plant growth and defense across native (China) and invasive (United States) Triadica populations. Without competition, aboveground damage had a greater impact than belowground damage on Triadica performance, whereas with competition and above- and belowground damage impacted Triadica similarly. Whole plant tolerance to damage below ground was negatively associated with tolerance to grass competitors indicating tradeoffs in the ability to tolerate herbivory vs. compete. Competition reduced investment in defensive extrafloral nectar (EFN) production. Aboveground damage inhibited rather than induced EFN production while belowground plant damage did not impact aboveground nectar production. We found some support for the evolution of increased competitive ability hypothesis for invasive plants as United States plants were larger than native China plants and were more plastic in their response to biotic stressors than China plants (they altered their root to shoot ratios dependent on herbivory and competition treatments). Our results indicate that habitat type and the presence of competitors may be a larger determinant of herbivory impact than feeding mode and suggest that integrated pest management strategies including competitive dynamics of recipient communities should be incorporated into biological control agent evaluation at earlier stages.

Root- and shoot-feeding herbivores have the capacity to influence one another by modifying the chemistry of the shared host plant. This can alter rates of nutrient mineralization and uptake by neighboring plants and influence plant-plant competition, particularly in mixtures combining grasses and legumes. Root herbivory-induced exudation of nitrogen (N) from legume roots, for example, may increase N acquisition by co-occurring grasses, with knock-on effects on grassland community composition. Little is known about how climate change may affect these interactions, but an important and timely question is how will grass-legume mixtures respond in a future with an increasing reliance on legume N mineralization in terrestrial ecosystems. Using a model grass-legume mixture, this study investigated how simultaneous attack on lucerne (Medicago sativa) by belowground weevils (Sitona discoideus) and aboveground aphids (Acyrthosiphon pisum) affected a neighboring grass (Phalaris aquatica) when subjected to drought, ambient, and elevated precipitation. Feeding on rhizobial nodules by weevil larvae enhanced soil water retention under ambient and elevated precipitation, but only when aphids were absent. While drought decreased nodulation and root N content in lucerne, grass root and shoot chemistry were unaffected by changes in precipitation. However, plant communities containing weevils but not aphids showed increased grass height and N concentrations, most likely associated with the transfer of N from weevil-attacked lucerne plants containing more nodules and higher root N concentrations compared with insect-free plants. Drought decreased aphid abundance by 54% but increased total and some specific amino acid concentrations (glycine, lysine, methionine, tyrosine, cysteine, histidine, arginine, aspartate, and glutamate), suggesting that aphid declines were being driven by other facets of drought (e.g., reduced phloem hydraulics). The presence of weevil larvae belowground decreased aphid numbers by 30%, likely associated with a significant reduction in proline in weevil-treated lucerne plants. This study demonstrates how predicted changes to precipitation patterns and indirect interactions between herbivores can alter the outcome of competition between N-fixing legumes and non-N-fixing grasses, with important implications for plant community structure and productivity.