Plant‐mediated effects of drought on aphid population structure and parasitoid attack

The effects of predicted climate change on aphid–natural enemy interactions have principally considered the effects of elevated carbon dioxide concentration and air temperature. However, increased incidence of summer droughts are also predicted in Northern Europe, which could affect aphid–plant interactions and aphid antagonists. We investigated how simulated summer drought affected the bird cherry–oat aphid, Rhopalosiphum padi L., and its natural enemy the parasitoid wasp Aphidius ervi. Drought and, to a greater extent, aphids reduced barley ( Hordeum vulgare) dry mass by 33% and 39%, respectively. Drought reduced leaf and root nitrogen concentrations by 13% and 28%, respectively, but foliar amino acid concentrations and composition remained similar. Aphid numbers were unaffected by drought, but population demography changed significantly; adults constituted 41% of the population on drought‐treated plants, but only 26% on those receiving ambient irrigation. Nymphs constituted 56% and 69% of the population on these plants, respectively, suggesting altered aphid development rates on drought‐stressed plants. Parasitism rates were significantly lower on drought‐stressed plants (9 attacks h^?1 compared with 35 attacks h^?1 on ambient‐irrigated plants), most likely because of lower incidence of nymphs and more adults, the latter being more difficult to parasitize. Any physiological changes in individual aphids did not affect parasitoid preferences, suggesting that attacks were postponed because of drought‐induced changes in aphid demography. This study demonstrates the potential for sporadic climate change events, such as summer drought, to be disruptive to herbivore–antagonist interactions.     

Drought changes plant chemistry and causes contrasting responses in lepidopteran herbivores

Drought events are predicted to increase due to climate change, yet consequences for plant–insect interactions are only partially understood. Drought‐mediated interactions between herbivores and their host plants are affected by a combination of factors, including characteristics of the affected plant, its associated herbivore and of the prevailing drought. Studying the effect of these factors in combination may provide important insight into plant and herbivore responses to drought. We studied drought effects on plant resistance to two leaf‐chewing herbivores by considering differing growth conditions, plant chemistry and insect responses in concert. We exposed Alliaria petiolata plants from several wild populations to different intensities of intermittent drought stress and quantified drought‐mediated changes in plant chemistry. Simultaneously, we assessed behavior (feeding preference) and performance of two lepidopteran herbivores: Pieris brassicae, a specialist, and Spodoptera littoralis, a generalist. Drought led to lowest concentrations of secondary defense compounds in severely stressed plants, without affecting total nitrogen content. Additionally, drought evoked opposite patterns in feeding preferences (plant palatability) between the herbivore species. Pieris brassicae consumed most of well‐watered plants, while S. littoralis preferred severely drought‐stressed plants. Hence, feeding preferences of S. littoralis reflected changes in plant secondary chemistry. Contrary to their feeding preference, P. brassicae performed better on drought‐stressed than on well‐watered plants, with faster development and higher attained pupal mass (plant suitability). Spodoptera littoralis showed retarded development in all treatments. In conclusion, drought caused plant secondary defense compounds to decrease consistently across all studied plant populations, which evoked contrasting feeding preferences of two herbivore species of the same feeding guild. These results suggest herbivore specificity as a possible explanation for herbivore responses to drought and emphasize the importance of herbivore characteristics such as feeding specialization in understanding and predicting consequences of future drought events.     

Multi‐factor climate change effects on insect herbivore performance

The impact of climate change on herbivorous insects can have far‐reaching consequences for ecosystem processes. However, experiments investigating the combined effects of multiple climate change drivers on herbivorous insects are scarce. We independently manipulated three climate change drivers (CO₂, warming, drought) in a Danish heathland ecosystem. The experiment was established in 2005 as a full factorial split‐plot with 6 blocks × 2 levels of CO₂ × 2 levels of warming × 2 levels of drought = 48 plots. In 2008, we exposed 432 larvae (n = 9 per plot) of the heather beetle (Lochmaea suturalis Thomson ), an important herbivore on heather, to ambient versus elevated drought, temperature, and CO₂ (plus all combinations) for 5 weeks. Larval weight and survival were highest under ambient conditions and decreased significantly with the number of climate change drivers. Weight was lowest under the drought treatment, and there was a three‐way interaction between time, CO₂, and drought. Survival was lowest when drought, warming, and elevated CO₂ were combined. Effects of climate change drivers depended on other co‐acting factors and were mediated by changes in plant secondary compounds, nitrogen, and water content. Overall, drought was the most important factor for this insect herbivore. Our study shows that weight and survival of insect herbivores may decline under future climate. The complexity of insect herbivore responses increases with the number of combined climate change drivers.     

Drought responses,phenotypic plasticity and survival of Mediterranean species in two different microclimatic sites

• Climate models predict a further drying of the Mediterranean summer. One way for plant species to persist during such climate changes is through acclimation. Here, we determine the extent to which trait plasticity in response to drought differs between species and between sites, and address the question whether there is a trade‐off between drought survival and phenotypic plasticity.
• Throughout the summer we measured physiological traits (photosynthesis – A_max, stomatal conductance – g_s, transpiration – E, leaf water potential – ψl) and structural traits (specific leaf area – SLA, leaf density – LD, leaf dry matter content – LDMC, leaf relative water content – LRWC) of leaves of eight woody species in two sites with slightly different microclimate (north‐ versus south‐facing slopes) in southern Spain. Plant recovery and survival was estimated after the summer drought period.
• We found high trait variability between species. In most variables, phenotypic plasticity was lower in the drier site. Phenotypic plasticity of SLA and LDMC correlated negatively with drought survival, which suggests a trade‐off between them. On the other hand, high phenotypic plasticity of SLA and LDMC was positively related to traits associated with rapid recovery and growth after the drought period.
• Although phenotypic plasticity is generally seen as favourable during stress conditions, here it seemed beneficial for favourable conditions. We propose that in environments with fluctuating drought periods there can be a trade‐off between drought survival and growth during favourable conditions. When climate become drier, species with high drought survival but low phenotypic plasticity might be selected for.

     

Influence of drought on plant performance through changes in belowground tritrophic interactions

Climate change is predicted to increase the risk of drought in many temperate agroecosystems. While the impact of drought on aboveground plant‐herbivore‐natural enemy interactions has been studied, little is known about its effects on belowground tritrophic interactions and root defense chemistry. We investigated the effects of low soil moisture on the interaction between maize, the western corn rootworm (WCR, Diabrotica virgifera), and soil‐borne natural enemies of WCR. In a manipulative field experiment, reduced soil moisture and WCR attack reduced plant performance and increased benzoxazinoid levels. The negative effects of WCR on cob dry weight and silk emergence were strongest at low moisture levels. Inoculation with entomopathogenic nematodes (EPNs, Heterorhabditis bacteriophora) was ineffective in controlling WCR, and the EPNs died rapidly in the warm and dry soil. However, ants of the species Solenopsis molesta invaded the experiment, were more abundant in WCR‐infested pots and predated WCR independently of soil moisture. Ant presence increased root and shoot biomass and was associated with attenuated moisture‐dependent effects of WCR on maize cob weight. Our study suggests that apart from directly reducing plant performance, drought can also increase the negative effects of root herbivores such as WCR. It furthermore identifies S. molesta as a natural enemy of WCR that can protect maize plants from the negative impact of herbivory under drought stress. Robust herbivore natural enemies may play an important role in buffering the impact of climate change on plant‐herbivore interactions.     

How do herbivorous insects respond to drought stress in trees?

Increased frequency and severity of drought, as a result of climate change, is expected to drive critical changes in plant–insect interactions that may elevate rates of tree mortality. The mechanisms that link water stress in plants to insect performance are not well understood. Here, we build on previous reviews and develop a framework that incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought. Using this framework, we investigate in greater depth how insect performance responds to increasing drought severity for: (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. We outline how intermittent and moderate drought can result in increases of carbon‐based and nitrogen‐based chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. We predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap‐sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. We argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. Current evidence suggests that insect outbreaks are shorter and more spatially restricted in tropical than in temperate forests. We suggest that future research on the impact of drought on insect communities should include (i) assessing how drought‐induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores; (ii) food web implications for other insects and those that feed on them; and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO₂ levels. There is a need for larger, temperate and tropical forest‐scale drought experiments to look at herbivorous insect responses and their role in tree death.     

How do extreme drought and plant community composition affect host plant metabolites and herbivore performance?

Water availability and plant community composition alter plant nutrient availability and the accumulation of plant defence compounds therefore having an impact on herbivore performance. Combined effects of drought stress and plant community composition on leaf chemicals and herbivore performance are largely unexplored. The objective of our study was, therefore, to find out the impact of extreme drought and of plant community composition on plant–herbivore interactions. Larvae of the generalist butterfly Spodoptera littoralis were reared on leaves of the grass Holcus lanatus which was grown in experimental communities, differing in species- and functional group richness. These communities were either subjected to extreme drought or remained under ambient climatic conditions. Drought decreased relative water content, soluble protein content, nitrogen and total phenol content and increased the content of carbohydrates in the grass. As a consequence, the larvae feeding on drought-exposed plants revealed a longer larval stage, increased pupal weight and higher adult eclosion rates. Plant community composition mainly caused changes to the defensive compounds of the grass, but also marginally affected protein and carbohydrate content. Larvae feeding on species-richest communities without legumes showed the highest mortality. Our findings imply that climate change that is projected to increase the frequency of severe droughts, as well as alter plant community compositions, is likely to affect arthropod–plant interactions through an alteration of leaf chemicals.     

Warming affects different components of plant–herbivore interaction in a simplified community but not net interaction strength

Global warming impacts natural communities through effects on performance of individual species and through changes in the strength of interactions between them. While there is a body of evidence of the former, we lack experimental evidence on potential changes in interaction strengths. Knowledge about multispecies interactions is fundamental to understand the regulation of biodiversity and the impact of climate change on communities. This study investigated the effect of warming on a simplified community consisting of three species: rosy apple aphid Dysaphis plantaginea feeding on plantain, Plantago lanceolata, and a heterospecific neighbouring plant species, perennial ryegrass, Lolium perenne. The aphid does not feed on L. perenne. The experimental design consisted of monocultures and mixtures of L. perenne and P. lanceolata at three temperature levels. We did not find indication for indirect temperature effects on D. plantaginea through changes in leaf nitrogen or relative water content. However, experimental warming affected the life history traits of the aphid directly, in a non‐linear manner. Aphids performed best at moderate warming, where they grew faster and had a shorter generation time. In spite of the increased population growth of the aphids under warming, the herbivory rates were not changed and consequently the plant–herbivore interaction was not altered under warming. This suggests reduced consumption rates at higher temperature. Also plant competition affected the aphids but through an interaction with temperature. We provide proof‐of‐concept that net interactions between plants and herbivores should not change under warming despite direct effects of warming on herbivores when plant–plant interaction are considered. Our study stresses the importance of indirect non–trophic interactions as an additional layer of complexity to improve our understanding of how trophic interactions will alter under climate change.     

Climate‐related genetic variation in drought‐resistance of Douglas‐fir (Pseudotsuga menziesii)

There is a general assumption that intraspecific populations originating from relatively arid climates will be better adapted to cope with the expected increase in drought from climate change. For ecologically and economically important species, more comprehensive, genecological studies that utilize large distributions of populations and direct measures of traits associated with drought‐resistance are needed to empirically support this assumption because of the implications for the natural or assisted regeneration of species. We conducted a space‐for‐time substitution, common garden experiment with 35 populations of coast Douglas‐fir (Pseudotsuga menziesii var. menziesii) growing at three test sites with distinct summer temperature and precipitation (referred to as ‘cool/moist’, ‘moderate’, or ‘warm/dry’) to test the hypotheses that (i) there is large genetic variation among populations and regions in traits associated with drought‐resistance, (ii) the patterns of genetic variation are related to the native source‐climate of each population, in particular with summer temperature and precipitation, (iii) the differences among populations and relationships with climate are stronger at the warm/dry test site owing to greater expression of drought‐resistance traits (i.e., a genotype × environment interaction). During midsummer 2012, we measured the rate of water loss after stomatal closure (transpiration_min), water deficit (% below turgid saturation), and specific leaf area (SLA, cm² g^?1) on new growth of sapling branches. There was significant genetic variation in all plant traits, with populations originating from warmer and drier climates having greater drought‐resistance (i.e., lower transpiration_min, water deficit and SLA), but these trends were most clearly expressed only at the warm/dry test site. Contrary to expectations, populations from cooler climates also had greater drought‐resistance across all test sites. Multiple regression analysis indicated that Douglas‐fir populations from regions with relatively cool winters and arid summers may be most adapted to cope with drought conditions that are expected in the future.     

Links between soil microbial communities and plant traits in a species‐rich grassland under long‐term climate change

Climate change can influence soil microorganisms directly by altering their growth and activity but also indirectly via effects on the vegetation, which modifies the availability of resources. Direct impacts of climate change on soil microorganisms can occur rapidly, whereas indirect effects mediated by shifts in plant community composition are not immediately apparent and likely to increase over time. We used molecular fingerprinting of bacterial and fungal communities in the soil to investigate the effects of 17 years of temperature and rainfall manipulations in a species‐rich grassland near Buxton, UK. We compared shifts in microbial community structure to changes in plant species composition and key plant traits across 78 microsites within plots subjected to winter heating, rainfall supplementation, or summer drought. We observed marked shifts in soil fungal and bacterial community structure in response to chronic summer drought. Importantly, although dominant microbial taxa were largely unaffected by drought, there were substantial changes in the abundances of subordinate fungal and bacterial taxa. In contrast to short‐term studies that report high resistance of soil fungi to drought, we observed substantial losses of fungal taxa in the summer drought treatments. There was moderate concordance between soil microbial communities and plant species composition within microsites. Vector fitting of community‐weighted mean plant traits to ordinations of soil bacterial and fungal communities showed that shifts in soil microbial community structure were related to plant traits representing the quality of resources available to soil microorganisms: the construction cost of leaf material, foliar carbon‐to‐nitrogen ratios, and leaf dry matter content. Thus, our study provides evidence that climate change could affect soil microbial communities indirectly via changes in plant inputs and highlights the importance of considering long‐term climate change effects, especially in nutrient‐poor systems with slow‐growing vegetation.     

Does cross‐acclimation between drought and freezing stress persist over ecologically relevant time spans? A test using the grass Poa pratensis

• Despite evidence that prior exposure to drought can increase subsequent plant freezing tolerance, few studies have explored such interactions over ecologically relevant time spans. We examined the combined effects of drought and subsequent freezing on tiller growth and leaf sugar concentrations in the grass, Poa pratensis .
• We exposed tillers to no drought (?0.04 MP a), moderate drought (?0.19 MP a) or severe drought (?0.42 MP a) for 3 weeks in summer. Tillers were then frozen in autumn or spring at ?5 °C (frost damage) or at 0 °C (control) for 3 days and harvested after a re‐growth period.
• For shoot growth, there was a significant interaction between drought and autumn freezing, whereby the relative effect of freezing on growth was least for the plants previously exposed to severe drought; however, there was no significant interaction between drought and spring freezing. For root growth, there were no significant interactions between drought and freezing in either season. Leaf sugar concentrations increased significantly with drought intensity, but these effects dissipated within a month, prior to the onset of the autumn freezing treatment.
• Overall, our results suggest that interactions between prior drought and subsequent freezing in P. pratensis may be most relevant in the context of autumn freezing, and despite the important role of soluble sugars in increasing both drought and freezing tolerance in this species, the retention of these compounds after drought stress does not appear to explain the occurrence of drought–frost interactions at ecologically relevant time scales.

     

Environmental and genetic control of phenotypic adaptation to drought in Potentilla glandulosa Lindl.

Summary Two populations of Potentilla glandulosa (Rosaceae) have strongly contrasting abilities to seasonally modify their phenotypic sensitivity to drought by low temperature-induced changes in leaf morphology. One population is native to an inland continental climate with unpredictable droughts occurring at any time during the year. The second population is native to a coastal mediterranean climate with a highly predictable annual cycle of winter rain and summer drought. In response to low temperatures in the autumn, the inland plants produce a compact rosette of small leaves and shed their large summer leaves, thereby reducing the total plant leaf area. The inland plants begin growth in the spring in the compact rosette phenotype and are much less sensitive to drought, maintaining higher values of stomatal conductance and leaf water potential, than when in the largeleaved summer phenotype. The coastal plants do not exhibit the low temperature-induced change in leaf morphology and are relatively sensitive to drought when grown at high or low temperatures. The F₁ hybrids of a coastal x inland cross exhibit an intermediate response to low temperature and drought. In the F₂ generation the inland parental class of individuals fully responding to low temperature segregates in a 1:63 ratio.     

Bottom‐up and top‐down effects of tree species diversity on leaf insect herbivory

The diversity of plant neighbors commonly results in direct, bottom‐up effects on herbivore ability to locate their host, and in indirect effects on herbivores involving changes in plant traits and a top‐down control by their enemies. Yet, the relative contribution of bottom‐up and top‐down forces remains poorly understood. We also lack knowledge on the effect of abiotic constraints such as summer drought on the strength and direction of these effects. We measured leaf damage on pedunculate oak (Quercus robur), alone or associated with birch, pine or both in a long‐term tree diversity experiment (ORPHEE), where half of the plots were irrigated while the other half remained without irrigation and received only rainfall. We tested three mechanisms likely to explain the effects of oak neighbors on herbivory: (1) Direct bottom‐up effects of heterospecific neighbors on oak accessibility to herbivores, (2) indirect bottom‐up effects of neighbors on the expression of leaf traits, and (3) top‐down control of herbivores by predators. Insect herbivory increased during the growth season but was independent of neighbor identity and irrigation. Specific leaf area, leaf toughness, and thickness varied with neighbor identity while leaf dry matter content or C:N ratio did not. When summarized in a principal component analysis (PCA), neighbor identity explained 87% of variability in leaf traits. PCA axes partially predicted herbivory. Despite greater rates of attack on dummy caterpillars in irrigated plots, avian predation, and insect herbivory remained unrelated. Our study suggests that neighbor identity can indirectly influence insect herbivory in mixed forests by modifying leaf traits. However, we found only partial evidence for these trait‐mediated effects and suggest that more attention should be paid to some unmeasured plant traits such as secondary metabolites, including volatile organic compounds, to better anticipate the effects of climate change on plant‐insect interactions in the future.     

Do photosynthetic limitations of evergreen Quercus ilex leaves change with long‐term increased drought severity?

Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long‐term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light‐saturated net assimilation rate (A_max), stomatal conductance (g_s), maximum carboxylation rate (V_cmax), maximum rate of electron transport (J_max), mesophyll conductance to CO₂ (g_m) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf photosynthetic parameters and leaf water potential remained identical in the two treatments. Leaf and canopy acclimation to progressive, long‐term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.     

Species trait syndrome drives the leaves’ functional variations of dominant grasses to modifications in summer water supply

Climate change models predict a strong reduction of average precipitation, especially of the summer rainfall, and an increase in intensity and frequency of drought events in the Mediterranean region. The research aim was to understand how four dominant grass species (Arrhenatherum elatius, Cynosurus cristatus, Elymus repens, and Lolium perenne) in sub-Mediterranean meadows (central Apennines, Italy) modulate their resource acquisition and conservation strategies to short-term variation of the pattern of summer water supply. During summer 2016, using a randomized block design, we tested the effect of three patterns of summer water supply, differing in water amount and watering frequency, on leaf area, leaf dry mass, specific leaf area (SLA), leaf senescence, and plant height. Our results showed that dominant grass species can modulate their strategies to variation of the pattern of summer water supply, but the response of leaf traits and plant height is mediated by the set of functional characteristics of the species. E. repens and A. elatius, with summer green leaves, lower SLA, later flowering period, and deeper roots, were less influenced by changes in water amount. C. cristatus and L. perenne, which display acquisitive strategies (persistent leaves, higher SLA values), earlier flowering, and shallower roots were more influenced by changes in the pattern of summer water supply. Our results suggest that a short-term decrease in water availability might affect primarily species with trait syndromes less adapted to face summer drought.

     

Differences in xylem and leaf hydraulic traits explain differences in drought tolerance among mature Amazon rainforest trees

Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large‐scale ecosystem drought experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem‐P₅₀), leaf turgor loss point (TLP), cellular osmotic potential (π_o), and cellular bulk modulus of elasticity (ε), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought‐tolerant versus drought‐intolerant based on observed mortality rates, and subdivided into early‐ versus late‐successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem‐P₅₀, TLP, and π_o, but not ε, occurred at significantly higher water potentials for the drought‐intolerant PFT compared to the drought‐tolerant PFT; however, there were no significant differences between the early‐ and late‐successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density—a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought‐tolerant and drought‐intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry‐season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co‐occuring drought‐tolerant and drought‐intolerant tropical tree species promises to facilitate a much‐needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests.     

Reproduction of an arboreal aphid pest,Elatobium abietinum,is altered under drought stress

Climate change in the UK is predicted to increase both winter temperatures and the frequency of summer drought events. Elatobium abietinum, the green spruce aphid, is the most important defoliating pest of Sitka spruce, Picea sitchensis, a conifer very widely used in British forest industry. This aphid is expected to respond strongly to altered climate, with changes to population densities leading to more frequent serious outbreaks and defoliation. The impact of simulated spring–summer drought on the reproductive performance of E. abietinum was investigated under laboratory conditions. Rates were assessed under five drought treatments of differing frequencies and intensities to characterize the direction of responses under different drought scenarios, and in time‐staggered trials to explore seasonal variation. Variation in the response of reproduction to water deficit was mediated by drought frequency and magnitude. Low‐amplitude, moderate intermittent stress improved reproductive rates, while severe stress, both continuous and high‐amplitude intermittent, had a detrimental impact when compared with observations made on well‐watered controls. Season was also found to modify the response, with improvements to plant nutritional quality under high‐amplitude stress reflected by improving reproduction. Despite this, no differences in rates were found during the autumn, suggesting no advancement in spruce dormancy under drought. Drought stress therefore has the potential to alter E. abietinum population densities, structure and phenology in Sitka spruce plantations, with implications for forest management, damage levels and natural control of the aphid under future altered climate.     

Plant-mediated indirect effects of climate change on an insect herbivore

Anthropogenic climate change is a substantial challenge to biodiversity conservation, exerting direct effects on plants and animals alike. Herbivores may be additionally affected by indirect effects, mediated through, for instance, climate change-induced alterations in host-plant quality. Thus, climate change may pronouncedly impact long-evolved plant-animal interactions, but our knowledge is still in its infancy, particularly with regard to the combined effects of temperature and water availability. We here investigate the effects of simulated climate change, considering variation in both temperature and water availability, on (1) host-plant chemistry, (2) herbivore oviposition and larval feeding preference, and (3) larval and adult performance. As study system, we used the butterfly Pieris napi (Lepidoptera: Pieridae) and its host plant Sinapis alba (Brassicacae). Host-plant chemistry was affected by simulated climate change, with higher temperatures increasing the carbon-nitrogen ratio and concentrations of glucosinolates, while drought stress led to reduced glucosinolate concentrations. Both egg-laying females and larvae preferred plants with the highest concentrations of the glucosinolate glucosinalbin, potentially acting as oviposition and feeding stimulus. Herbivore performance was positively affected by plants grown at control temperatures or under drought stress and thus reduced glucosinolate concentrations. Hence, P. napi was not able to select the most profitable host. Our study indicates that (1) climate-induced changes in plant chemistry may exert indirect effects on herbivores, (2) effects of climate change will depend on the magnitude of change in specific abiotic parameters and their interactions, whereby positive (e.g. drought) and negative (e.g. temperature) effects may even cancel out each other, and (3) changes in critical chemical cues may diminish host-plant detectability and thereby result in reduced realised fecundity. We thus highlight the important role of temperature and water availability on plant chemistry, which may change interactions between insects and plants.     

Complex carbon cycle responses to multi‐level warming and supplemental summer rain in the high Arctic

The Arctic has experienced rapid warming and, although there are uncertainties, increases in precipitation are projected to accompany future warming. Climate changes are expected to affect magnitudes of gross ecosystem photosynthesis (GEP), ecosystem respiration (ER) and the net ecosystem exchange of CO₂ (NEE). Furthermore, ecosystem responses to climate change are likely to be characterized by nonlinearities, thresholds and interactions among system components and the driving variables. These complex interactions increase the difficulty of predicting responses to climate change and necessitate the use of manipulative experiments. In 2003, we established a long‐term, multi‐level and multi‐factor climate change experiment in a polar semidesert in northwest Greenland. Two levels of heating (30 and 60 W m^?2) were applied and the higher level was combined with supplemental summer rain. We made plot‐level measurements of CO₂ exchange, plant community composition, foliar nitrogen concentrations, leaf δ¹³C and NDVI to examine responses to our treatments at ecosystem‐ and leaf‐levels. We confronted simple models of GEP and ER with our data to test hypotheses regarding key drivers of CO₂ exchange and to estimate growing season CO₂‐C budgets. Low‐level warming increased the magnitude of the ecosystem C sink. Meanwhile, high‐level warming made the ecosystem a source of C to the atmosphere. When high‐level warming was combined with increased summer rain, the ecosystem became a C sink of magnitude similar to that observed under low‐level warming. Competition among our ER models revealed the importance of soil moisture as a driving variable, likely through its effects on microbial activity and nutrient cycling. Measurements of community composition and proxies for leaf‐level physiology suggest GEP responses largely reflect changes in leaf area of Salix arctica, rather than changes in leaf‐level physiology. Our findings indicate that the sign and magnitude of the future High Arctic C budget may depend upon changes in summer rain.     

Interactive effects of elevated temperature and drought on plant carbon metabolism: A meta-analysis

Elevated temperature (T_e) and drought often co-occur and interactively affect plant carbon (C) metabolism and thus the ecosystem C cycling; however, the magnitude of their interaction is unclear, making the projection of global change impacts challenging. Here, we compiled 107 journal articles in which temperature and water availability were jointly manipulated, and we performed a meta-analysis of interactive effects of T_e and drought on leaf photosynthesis (A_growth) and respiration (R_growth) at growth temperature, nonstructural carbohydrates and biomass of plants, and their dependencies on experimental and biological moderators (e.g., treatment intensity, plant functional type). Our results showed that, overall, there was no significant interaction of T_e and drought on A_growth. T_e accelerated R_growth under well-watered conditions rather than under drought conditions. The T_e × drought interaction on leaf soluble sugar and starch concentrations were neutral and negative, respectively. The effect of T_e and drought on plant biomass displayed a negative interaction, with T_e deteriorating the drought impacts. Drought induced an increase in root to shoot ratio at ambient temperature but not at T_e. The magnitudes of T_e and drought negatively modulated the T_e × drought interactions on A_growth. Root biomass of woody plants was more vulnerable to drought than that of herbaceous plants at ambient temperature, but this difference diminished at T_e. Perennial herbs exhibited a stronger amplifying effect of T_e on plant biomass in response to drought than did annual herbs. T_e exacerbated the responses of A_growth and stomatal conductance to drought for evergreen broadleaf trees rather than for deciduous broadleaf and evergreen coniferous trees. A negative T_e × drought interaction on plant biomass was observed on species-level rather than on community-level. Collectively, our findings provide a mechanistic understanding of the interactive effects of T_e and drought on plant C metabolism, which would improve the prediction of climate change impacts.