Elevated air temperature alters an old-field insect community in a multifactor climate change experiment

To address how multiple, interacting climate drivers may affect plant–insect community associations, we sampled insects that naturally colonized a constructed old‐field plant community grown for over 2 years under simultaneous CO₂, temperature, and water manipulation. Insects were sampled using a combination of sticky traps and vacuum sampling, identified to morphospecies and the insect community with respect to abundance, richness, and evenness quantified. Individuals were assigned to four broad feeding guilds in order to examine potential trophic level effects. Although there were occasional effects of CO₂ and water treatment, the effects of warming on the insect community were large and consistent. Warming significantly increased Order Thysanoptera abundance and reduced overall morphospecies richness and evenness. Nonmetric multidimensional scaling found that only temperature affected insect community composition, while a Sørensen similarity index showed less correspondence in the insect community between temperature treatments compared with CO₂ or soil water treatments. Within the herbivore guild, elevated temperature significantly reduced richness and evenness. Corresponding reductions of diversity measures at higher trophic levels (i.e. parasitoids), along with the finding that herbivore richness was a significant predictor of parasitoid richness, suggest trophic‐level effects within the insect community. When the most abundant species were considered in temperature treatments, a small number of species increased in abundance at elevated temperature, while others declined compared with ambient temperature. Effects of temperature in the dominant insects demonstrated that treatment effects were limited to a relatively small number of morphospecies. Observed effects of elevated CO₂ concentration on whole‐community foliar N concentration did not result in any effect on herbivores, which are probably the most susceptible guild to changes in plant nutritional quality. These results demonstrate that climatic warming may alter certain insect communities via effects on insect species most responsive to a higher temperature, contributing to a change in community structure.     

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.     

Effects of Warming, Summer Drought, and CO2 Enrichment on Aboveground Biomass Production, Flowering Phenology, and Community Structure in an Upland Grassland Ecosystem

Future climate scenarios predict simultaneous changes in environmental conditions, but the impacts of multiple climate change drivers on ecosystem structure and function remain unclear. We used a novel experimental approach to examine the responses of an upland grassland ecosystem to the 2080 climate scenario predicted for the study area (3.5°C temperature increase, 20% reduction in summer precipitation, atmospheric CO₂ levels of 600 ppm) over three growing seasons. We also assessed whether patterns of grassland response to a combination of climate change treatments could be forecast by ecosystem responses to single climate change drivers. Effects of climate change on aboveground production showed considerable seasonal and interannual variation; April biomass increased in response to both warming and the simultaneous application of warming, summer drought, and CO₂ enrichment, whereas October biomass responses were either non-significant or negative depending on the year. Negative impacts of summer drought on production were only observed in combination with a below-average rainfall regime, and showed lagged effects on spring biomass. Elevated CO₂ had no significant effect on aboveground biomass during this study. Both warming and the 2080 climate change scenario were associated with a significant advance in flowering time for the dominant grass species studied. However, flowering phenology showed no significant response to either summer drought or elevated CO₂. Species diversity and equitability showed no response to climate change treatments throughout this study. Overall, our data suggest that single-factor warming experiments may provide valuable information for projections of future ecosystem changes in cool temperate grasslands.     

Accumulation of soil carbon under elevated CO2 unaffected by warming and drought

Elevated atmospheric CO₂ concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO₂ (eCO₂) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO₂, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m² higher across all treatment combinations with eCO₂ compared to ambient CO₂ treatments (equal to an increase of 0.120 ± 0.043 kg C m^?2 year^?1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO₂ may be as high as 0.177 ± 0.070 kg C m^?2 year^?1. Furthermore, the response to eCO₂ was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO₂ observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO₂ concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO₂, warming, and changes in precipitation patterns, are, therefore, urgently needed.     

Experimental warming increases herbivory by leaf‐chewing insects in an alpine plant community

Climate warming is predicted to affect species and trophic interactions worldwide, and alpine ecosystems are expected to be especially sensitive to changes. In this study, we used two ongoing climate warming (open‐top chambers) experiments at Finse, southern Norway, to examine whether warming had an effect on herbivory by leaf‐chewing insects in an alpine Dryas heath community. We recorded feeding marks on the most common vascular plant species in warmed and control plots at two experimental sites at different elevations and carried out a brief inventory of insect herbivores. Experimental warming increased herbivory on Dryas octopetala and Bistorta vivipara. Dryas octopetala also experienced increased herbivory at the lower and warmer site, indicating an overall positive effect of warming, whereas B. vivipara experienced an increased herbivory at the colder and higher site indicating a mixed effect of warming. The Lepidoptera Zygaena exulans and Sympistis nigrita were the two most common leaf‐chewing insects in the Dryas heath. Based on the observed patterns of herbivory, the insects life cycles and feeding preferences, we argue that Z. exulans is the most important herbivore on B. vivipara, and S. nigrita the most important herbivore on D. octopetala. We conclude that if the degree of insect herbivory increases in a warmer world, as suggested by this study and others, complex interactions between plants, insects, and site‐specific conditions make it hard to predict overall effects on plant communities.     

Herbivory in global climate change research: direct effects of rising temperature on insect herbivores

This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO₂ or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life‐history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.     

The forgotten season: the impact of autumn phenology on a specialist insect herbivore community on oak

1. Variation in spring phenology – like tree budburst – affects the structure of insect communities, but impacts of autumn phenology have been neglected. Many plant species have recently delayed their autumn phenology, and the timing of leaf senescence may be important for herbivorous insects. 2. This study explored how an insect herbivore community associated with Quercus robur is influenced by variation in autumn phenology. For this, schools were asked to record, across the range of oak in Sweden, the autumn phenology of oaks and to conduct a survey of the insect community. 3. To tease apart the relative impacts of climate from that of tree phenology, regional tree phenology was first modelled as a function of regional climate, and the tree‐specific deviation from this relationship was then used as the metric of relative tree‐specific phenology. 4. At the regional scale, a warmer climate postponed oak leaf senescence. This was also reflected in the insect herbivore community: six out of 15 taxa occurred at a higher incidence and five out of 18 taxa were more abundant, in locations with a warmer climate. Similarly, taxonomic richness and herbivory were higher in warmer locations. 5. Trees with a relatively late autumn phenology had higher abundances of leaf miners (Phyllonorycter spp.). This caused lower community diversity and evenness on trees with later autumn phenology. 6. The findings of the present study illustrate that both regional climate‐driven patterns and local variation in oak autumn phenology contribute to shaping the insect herbivore community. Community patterns may thus shift with a changing climate.     

Antagonism between elevated CO2, nighttime warming,and summer drought reduces the robustness of PSII performance to freezing events

Plant responses to warming, elevated CO₂, and changes in summer precipitation patterns involve complex interactions. In this study we aim to reveal the single factor responses and their interactive effects on photosystem II (PSII) performance during an autumn-to-winter period. The study was carried out in the CLIMAITE multifactor experiment, which includes the combined impact of elevated CO₂ (free air carbon enrichment; CO₂), warming (passive nighttime warming; T) and summer drought (rain-excluding curtains; D) in a temperate heath ecosystem. PSII performance was probed by the effective quantum yield in light, F_v′/F_m′, using the pulse amplitude methodology, and the total performance index, PI_total, which integrate changes of the chlorophyll-a fluorescence transient including the maximal quantum yield in darkness, F_v/F_m.Decreasing temperature during autumn linearly reduced PI_total, both in the wavy hair-grass, Deschampsia flexuosa, and in the evergreen dwarf shrub common heather, Calluna vulgaris, and following freezing events the PI_total and F_v′/F_m′ were reduced even more. Contrary to expected, indirect effects of the previous summer drought reduced PSII performance before freezing events, particularly in Calluna. In combinations with elevated CO₂ interactive effects with drought, D × CO₂ and warming, T × D × CO₂, were negatively skewed and caused the reduction of PSII performance in both species after occurrence of freezing events. Neither passive nighttime warming nor elevated CO₂ as single factors reduced PSII performance via incomplete cold hardening as hypothesized. Instead, the passive nighttime warming strongly increased PSII performance, especially after freezing events, and when combined with elevated CO₂ a strongly skewed positive T × CO₂ interactive effect was seen. This indicates that these plants take advantage of the longer growing season induced by the warming in elevated CO₂ until a winter frost period becomes permanent. However, if previously exposed to summer drought this positive effect reverses via interactive D × CO₂ and T × D × CO₂ effects immediately after freezing events, causing the full combination of TDCO₂ not to differ from the control.In a future warmer climate with high CO₂ and summer drought, the occurrence of freezing events thus seem highly decisive for reducing PSII performance in the autumn-to-winter period. Such a reduced robustness of PSII performance may be highly decisive for the magnitude of the late season photosynthetic carbon uptake and reduce the growing season length in these temperate heath plants.     

Consequences of elevated temperature and pCO2 on insect folivory at the ecosystem level: perspectives from the fossil record

Paleoecological studies document the net effects of atmospheric and climate change in a natural laboratory over timescales not accessible to laboratory or ecological studies. Insect feeding damage is visible on well‐preserved fossil leaves, and changes in leaf damage through time can be compared to environmental changes. We measured percent leaf area damaged on four fossil leaf assemblages from the Bighorn Basin, Wyoming, that range in age from 56.1 to 52.65 million years (Ma). We also include similar published data from three US sites 49.4 to ~45 Ma in our analyses. Regional climate was subtropical or warmer throughout this period, and the second oldest assemblage (56 Ma) was deposited during the Paleocene–Eocene Thermal Maximum (PETM), a geologically abrupt global warming event caused by massive release of carbon into the atmosphere. Total and leaf‐chewing damage are highest during the PETM, whether considering percent area damaged on the bulk flora, the average of individual host plants, or a single plant host that occurs at multiple sites. Another fossil assemblage in our study, the 52.65 Ma Fifteenmile Creek paleoflora, also lived during a period of globally high temperature and pCO₂, but does not have elevated herbivory. Comparison of these two sites, as well as regression analyses conducted on the entire dataset, demonstrates that, over long timescales, temperature and pCO₂ are uncorrelated with total insect consumption at the ecosystem level. Rather, the most important factor affecting herbivory is the relative abundance of plants with nitrogen‐fixing symbionts. Legumes dominate the PETM site; their prevalence would have decreased nitrogen limitation across the ecosystem, buffering generalist herbivore populations against decreased leaf nutritional quality that commonly occurs at high pCO₂. We hypothesize that nitrogen concentration regulates the opposing effects of elevated temperature and CO₂ on insect abundance and thereby total insect consumption, which has important implications for agricultural practices in today's world of steadily increasing pCO₂.     

Interactive responses of old‐field plant growth and composition to warming and precipitation

As Earth's atmosphere accumulates carbon dioxide (CO₂) and other greenhouse gases, Earth's climate is expected to warm and precipitation patterns will likely change. The manner in which terrestrial ecosystems respond to climatic changes will in turn affect the rate of climate change. Here we describe responses of an old‐field herbaceous community to a factorial combination of four levels of warming (up to 4 °C) and three precipitation regimes (drought, ambient and rain addition) over 2 years. Warming suppressed total production, shoot production, and species richness, but only in the drought treatment. Root production did not respond to warming, but drought stimulated the growth of deeper (> 10 cm) roots by 121% in 1 year. Warming and precipitation treatments both affected functional group composition, with C₄ grasses and other annual and biennial species entering the C₃ perennial‐dominated community in ambient rainfall and rain addition treatments as well as in warmed treatments. Our results suggest that, in this mesic system, expected changes in temperature or large changes in precipitation alone can alter functional composition, but they have little effect on total herbaceous plant growth. However, drought limits the capacity of the entire system to withstand warming. The relative insensitivity of our study system to climate suggests that the herbaceous component of old‐field communities will not dramatically increase production in response to warming or precipitation change, and so it is unlikely to provide either substantial increases in forage production or a meaningful negative feedback to climate change later this century.     

Effects of Climate Change Drivers on Nitrous Oxide Fluxes in an Upland Temperate Grassland

Despite increasing interest in the patterns of trace gas emissions in terrestrial ecosystems, little is known about the impacts of climate change on nitrous oxide (N₂O) fluxes. The aim of this study was to determine the importance of the three main drivers of climate change (warming, summer drought, and elevated CO₂ concentrations) on N₂O fluxes from an extensively managed, upland grassland. Over a 2-year period, we monitored N₂O fluxes in an in situ ecosystem manipulation experiment simulating the climate predicted for the study area in 2080 (3.5°C temperature increase, 20% reduction in summer rainfall and atmospheric CO₂ levels of 600 ppm). N₂O fluxes showed significant seasonal and interannual variation irrespective of climate treatment, and were higher in summer and autumn compared with winter and spring. Overall, N₂O emissions showed a positive correlation with soil temperature and rainfall. Elevated temperature had a positive impact on mean annual N₂O fluxes but effects were only significant in 2007. Contrary to expectations, neither combined summer drought and warming nor the simultaneous application of elevated atmospheric CO₂ concentrations, summer drought and warming had any significant effect on annual N₂O fluxes. However, the maximum N₂O flux rates observed during the study occurred when elevated CO₂ was combined with warming and drought, suggesting the potential for important, short-term N₂O–N losses in enriched CO₂ environments. Taken together, our results suggest that the N₂O responses of temperate, extensively managed grasslands to future climate change scenarios may be primarily driven by temperature effects.     

Effects of elevated CO₂, warming and drought episodes on plant carbon uptake in a temperate heath ecosystem are controlled by soil water status

The impact of elevated CO₂, periodic drought and warming on photosynthesis and leaf characteristics of the evergreen dwarf shrub Calluna vulgaris in a temperate heath ecosystem was investigated. Photosynthesis was reduced by drought in midsummer and increased by elevated CO₂ throughout the growing season, whereas warming only stimulated photosynthesis early in the year. At the beginning and end of the growing season, a T × CO₂ interaction synergistically stimulated plant carbon uptake in the combination of warming and elevated CO₂. At peak drought, the D × CO₂ interaction antagonistically down‐regulated photosynthesis, suggesting a limited ability of elevated CO₂ to counteract the negative effect of drought. The response of photosynthesis in the full factorial combination (TDCO₂) could be explained by the main effect of experimental treatments (T, D, CO₂) and the two‐factor interactions (D × CO₂, T × CO₂). The interactive responses in the experimental treatments including elevated CO₂ seemed to be linked to the realized range of treatment variability, for example with negative effects following experimental drought or positive effects following the relatively higher impact of night‐time warming during cold periods early and late in the year. Longer‐term experiments are needed to evaluate whether photosynthetic down‐regulation will dampen the stimulation of photosynthesis under prolonged exposure to elevated CO₂.     

Oviposition acceptance and larval development of Chilo partellus stemborers in drought‐stressed wild and cultivated grasses of East Africa

Maternal host choices during oviposition by herbivorous insects determine the fitness of their offspring and may be influenced by environmental changes that can alter host‐plant quality. This is of particular relevance to ‘push‐pull’ cropping systems where host preferences are exploited to manage insect pest populations. We tested how drought stress in maize and companion plants that are used in these systems affect oviposition preference, larval feeding, and development of the spotted stemborer, Chilo partellus Swinhoe (Lepidoptera: Crambidae). Five host species were tested (all Poaceae): maize (Zea mays L.), Napier grass (Pennisetum purpureum Schumach), signal grass [Brachiaria brizantha (A. Rich) Stapf], Brachiaria cv. ‘Mulato’, and molasses grass [Melinis minutiflora (Beauv.)]. Under drought stress, maize experienced as much oviposition as control unstressed maize in choice and no‐choice experiments. Similarly, larval leaf damage was not significantly different in drought‐stressed vs. unstressed maize. In contrast, oviposition occurred less on drought‐stressed than on unstressed Napier and signal grass. Oviposition acceptance and leaf damage remained low in both drought‐stressed and unstressed molasses grass and Mulato. Larval survival and development remained high in drought‐stressed maize, but not in Napier, signal, and molasses grass and Mulato, where survival and development were low in both drought‐stressed and unstressed plants. Our results indicate that herbivore responses to drought‐stressed plants depend on the plant species and that drought stress can change host preference and acceptance rankings. In particular, trap‐crops such as Napier grass may not divert oviposition from the main maize crop under drought stress conditions.     

Influences of elevated CO2 and pest damage on the allocation of plant defense compounds in Bt‐transgenic cotton and enzymatic activity of cotton aphid

Abstract Plant allocation to defensive compounds by elevated CO₂‐grown non‐transgenic and transgenic Bt cotton in response to infestation by cotton aphid, Aphis gossypii (Glover) in open‐top chambers under elevated CO₂ were studied. The results showed that significantly lower foliar nitrogen concentration and Bt toxin protein occurred in transgenic Bt cotton with and without cotton aphid infestation under elevated CO_2. However, significantly higher carbon/nitrogen ratio, condensed tannin and gossypol were observed in transgenic Bt cotton “GK‐12” and non‐transgenic Bt cotton ‘Simian‐3’ under elevated CO_2. The CO₂ level and cotton variety significantly influenced the foliar nitrogen, condensed tannin and gossypol concentrations in the plant leaves after feeding by A. gossypii. The interaction between CO₂ level × infestation time (24 h, 48 h and 72 h) showed a significant increase in cotton condensed tannin concentrations, while the interaction between CO₂ level × cotton variety significantly decreased the true choline esterase (TChE) concentration in the body of A. gossypi. This study exemplified the complexities of predicting how transgenic and non‐transgenic plants will allocate defensive compounds in response to herbivorous insects under differing climatic conditions. Plant defensive compound allocation patterns and aphid enzyme changes observed in this study appear to be broadly applicable across a range of plant and herbivorous insect interactions as CO₂ atmosphere rises.     

Interactive direct and plant‐mediated effects of elevated atmospheric [CO2] and temperature on a eucalypt‐feeding insect herbivore

Understanding the direct and indirect effects of elevated [CO₂] and temperature on insect herbivores and how these factors interact are essential to predict ecosystem‐level responses to climate change scenarios. In three concurrent glasshouse experiments, we measured both the individual and interactive effects of elevated [CO₂] and temperature on foliar quality. We also assessed the interactions between their direct and plant‐mediated effects on the development of an insect herbivore of eucalypts. Eucalyptus tereticornis saplings were grown at ambient or elevated [CO₂] (400 and 650 μmol mol^?1 respectively) and ambient or elevated ( + 4 °C) temperature for 10 months. Doratifera quadriguttata (Lepidoptera: Limacodidae) larvae were feeding directly on these trees, on their excised leaves in a separate glasshouse, or on excised field‐grown leaves within the temperature and [CO₂] controlled glasshouse. To allow insect gender to be determined and to ensure that any sex‐specific developmental differences could be distinguished from treatment effects, insect development time and consumption were measured from egg hatch to pupation. No direct [CO₂] effects on insects were observed. Elevated temperature accelerated larval development, but did not affect leaf consumption. Elevated [CO₂] and temperature independently reduced foliar quality, slowing larval development and increasing consumption. Simultaneously increasing both [CO₂] and temperature reduced these shifts in foliar quality, and negative effects on larval performance were subsequently ameliorated. Negative nutritional effects of elevated [CO₂] and temperature were also independently outweighed by the direct positive effect of elevated temperature on larvae. Rising [CO₂] and temperature are thus predicted to have interactive effects on foliar quality that affect eucalypt‐feeding insects. However, the ecological consequences of these interactions will depend on the magnitude of concurrent temperature rise and its direct effects on insect physiology and feeding behaviour.     

Experimental climate warming alters aspen and birch phytochemistry and performance traits for an outbreak insect herbivore

Climate change and insect outbreaks are key factors contributing to regional and global patterns of increased tree mortality. While links between these environmental stressors have been established, our understanding of the mechanisms by which elevated temperature may affect tree–insect interactions is limited. Using a forest warming mesocosm, we investigated the influence of elevated temperature on phytochemistry, tree resistance traits, and insect performance. Specifically, we examined warming effects on forest tent caterpillar (Malacosoma disstria) and host trees aspen (Populus tremuloides) and birch (Betula papyrifera). Trees were grown under one of three temperature treatments (ambient, +1.7 °C, +3.4 °C) in a multiyear open‐air warming experiment. In the third and fourth years of warming (2011, 2012), we assessed foliar nutrients and defense chemistry. Elevated temperatures altered foliar nitrogen, carbohydrates, lignin, and condensed tannins, with differences in responses between species and years. In 2012, we performed bioassays using a common environment approach to evaluate plant‐mediated indirect warming effects on larval performance. Warming resulted in decreased food conversion efficiency and increased consumption, ultimately with minimal effect on larval development and biomass. These changes suggest that insects exhibited compensatory feeding due to reduced host quality. Within the context of observed phytochemical variation, primary metabolites were stronger predictors of insect performance than secondary metabolites. Between‐year differences in phytochemical shifts corresponded with substantially different weather conditions during these two years. By sampling across years within an ecologically realistic and environmentally open setting, our study demonstrates that plant and insect responses to warming can be temporally variable and context dependent. Results indicate that elevated temperatures can alter phytochemistry, tree resistance traits, and herbivore feeding, but that annual weather variability may modulate warming effects leading to uncertain consequences for plant–insect interactions with projected climate change.     

Do cities simulate climate change? A comparison of herbivore response to urban and global warming

Cities experience elevated temperature, CO₂, and nitrogen deposition decades ahead of the global average, such that biological response to urbanization may predict response to future climate change. This hypothesis remains untested due to a lack of complementary urban and long‐term observations. Here, we examine the response of an herbivore, the scale insect Melanaspis tenebricosa, to temperature in the context of an urban heat island, a series of historical temperature fluctuations, and recent climate warming. We survey M. tenebricosa on 55 urban street trees in Raleigh, NC, 342 herbarium specimens collected in the rural southeastern United States from 1895 to 2011, and at 20 rural forest sites represented by both modern (2013) and historical samples. We relate scale insect abundance to August temperatures and find that M. tenebricosa is most common in the hottest parts of the city, on historical specimens collected during warm time periods, and in present‐day rural forests compared to the same sites when they were cooler. Scale insects reached their highest densities in the city, but abundance peaked at similar temperatures in urban and historical datasets and tracked temperature on a decadal scale. Although urban habitats are highly modified, species response to a key abiotic factor, temperature, was consistent across urban and rural‐forest ecosystems. Cities may be an appropriate but underused system for developing and testing hypotheses about biological effects of climate change. Future work should test the applicability of this model to other groups of organisms.     

Plant diversity and insect herbivores: effects of environmental change in contrasting model systems

There is increasing concern over the potential impact of anthropogenic factors (e.g. increasing nutrient inputs, global climate change) on the rate of loss of diversity in ecosystems. Such losses may affect ecosystem processes. In addition, a change in diversity of one group of organisms may influence the diversity of species of the next trophic level. We examined the extent to which plant species richness influences that of insect herbivores in two systems: a long‐term field experiment on heather moorland and a model community in the Ecotron controlled environment facility. We examined the response of these two plant communities to environmental change, specifically increased levels of nutrients, grazing and atmospheric CO₂. We measured the indirect effects of changes in these factors on insect herbivores, both above‐ and below‐ground. In the moorland system, grazing was the largest influence on plant community structure. The community was dominated by one species, Calluna vulgaris, and loss of cover under heavy grazing allowed competing species to invade. However, grazing regime was not a major influence on the species richness of the insect herbivore community. Site was more important: there were a greater number of Hemiptera species on sites with more mineral soils than on peat sites, possibly because a greater variety of grass and herb species was present on the former sites. In the Ecotron, below‐ground factors were also important drivers of community change: elevated CO₂ increased carbon availability in the soil and there were simultaneous changes in the community composition of soil biota. Above‐ground, some plant species increased in abundance and others decreased, leading to interaction‐specific effects on the insect herbivores. In two very different studies of the effects of environmental change on the interactions between plants and their herbivores, several similar conclusions can be drawn: (1) effects are likely to be site‐ and interaction‐specific; (2) outcomes are likely to be strongly dependent on the initial state and the dominant species of the plant community; and (3) indirect effects, often mediated by below‐ground factors, may have a bigger influence on insect‐plant interactions than more direct effects of above‐ground factors.     

Large herbivores limit CO2 uptake and suppress carbon cycle responses to warming in West Greenland

Changes in the terrestrial carbon cycle may ameliorate or exacerbate future climatic warming. Research on this topic has focused almost exclusively on abiotic drivers, whereas biotic factors, including trophic interactions, have received comparatively little attention. We quantified the singular and interactive effects of herbivore exclusion and simulated warming on ecosystem CO₂ exchange over two consecutive growing seasons in West Greenland. Exclusion of caribou and muskoxen over the past 8 years has led to dramatic increases in shrub cover, leaf area, ecosystem photosynthesis, and a nearly threefold increase in net C uptake. These responses were accentuated by warming, but only in the absence of herbivores. Carbon cycle responses to herbivore exclusion alone and combined with warming were driven by changes in gross ecosystem photosynthesis, as limited differences in ecosystem respiration were observed. Our results show that large herbivores can be of critical importance as mediators of arctic ecosystem responses to climate change.     

Increase in CO2 concentration could alter the response of Hedera helix to climate change

Increasing CO₂ concentration ([CO₂]) is likely to affect future species distributions, in interaction with other climate change drivers. However, current modeling approaches still seldom consider interactions between climatic factors and the importance of these interactions therefore remains mostly unexplored. Here, we combined dendrochronological and modeling approaches to study the interactive effects of increasing [CO₂] and temperature on the distribution of one of the main European liana species, Hedera helix. We combined a classical continent‐wide species distribution modeling approach with a case study using H. helix and Quercus cerris tree rings, where we explored the long‐term influence of a variety of climate drivers, including increasing [CO₂], and their interactions, on secondary growth. Finally, we explored how our findings could influence the model predictions. Climate‐only model predictions showed a small decrease in habitat suitability for H. helix in Europe; however, this was accompanied by a strong shift in the distribution toward the north and east. Our growth ring data suggested that H. helix can benefit from high [CO₂] under warm conditions, more than its tree hosts, which showed a weaker response to [CO₂] coupled with higher cavitation risk under high temperature. Increasing [CO₂] might therefore offset the negative effects of high temperatures on H. helix, and we illustrate how this might translate into maintenance of H. helix in warmer areas. Our results highlight the need to consider carbon fertilization and interactions between climate variables in ecological modeling. Combining dendrochronological analyses with spatial distribution modeling may provide opportunities to refine predictions of how climate change will affect species distributions.