Elevation Sites Research Articles

Estimating variability in downwelling surface shortwave radiation in a tropical highland environment.

Surface incoming shortwave (solar) radiation data are an important component of many scientific analyses, but direct measurements are not commonly available. Estimates can be obtained from gridded meteorological analysis or reanalysis systems, such as the Global Data Assimilation Systems (GDAS) and Modern Era Retrospective Reanalysis System (MERRA-2), or calculated using empirical models dependent on meteorological variables such as air temperature. The purpose of this analysis was to compare multiple methods for estimating daily shortwave radiation in a tropical highland environment in Ethiopia. Direct solar radiation outputs of GDAS and MERRA-2, topographically corrected outputs of the two analysis systems, and empirically estimated solar radiation values calculated with the systems’ air temperature data were compared to see which produced the most reliable radiation values. GDAS appeared to underestimate the seasonal variability, resulting in low correlation (R2) with in situ data and large mean bias error (MBE). In comparison, MERRA-2 did not underestimate variability, but produced larger bias than the empirical model estimates. There was an improvement in correlation and reduction in MBE when using the GDAS air temperature predictions in the empirical model, but the opposite was true for MERRA-2. The empirical model using station air temperature data (stationT) produced the highest correlation across all four stations, with best performance at the lower elevation sites. The direct shortwave radiation outputs of MERRA-2 produced comparable correlation values, with larger R2 at stations at higher elevation. Topography possibly influenced these results, as MERRA-2 performed comparably to stationT at the stations in moderate terrain, but not in steeper terrain. This work can serve as a starting point for analyses in other tropical highland regions, where continuous in situ solar radiation data are rarely available.

Termite diversity in Neotropical dry forests of Colombia and the potential role of rainfall in structuring termite diversity

AbstractTermites are ecosystem engineers that play an important role in the biotransformation and re‐distribution of nutrients in soil. The dry forests are endemic repositories, but at same time, they are most threatened by extensive livestock and crop farming, fires, and climate change. In Colombia, the best‐protected dry forests are located in the north. The termite fauna of dry forests are poorly known. The aim was to identify the termite species occurring in tropical dry forests of the Colombian Caribbean coast in relation to diet and precipitation, temperature, elevation, and soil properties. A total of 32 species in 1,103 occurrences were found. Termitidae accounted for 78% of the species richness with the Anoplotermes‐group, Microcerotermes, and Nasutitermes being the dominant genera. Differences in species composition and abundance were found across sites. These differences may be linked to anthropogenic disturbance and polygyny and polydomy. Strikingly, our highest elevation site (334 m) had the highest species richness much higher than the two lower elevation sites. This implies an inversion of the common elevation‐diversity gradient, also found for termites which can be explained by increasing precipitation with elevation in the dry forest. An analysis of termite species richness at the global scale confirms that termite species richness correlates positively with rainfall. Hence, rainfall seems to positively affect termite diversity. In line, the studied Colombian tropical dry forests had low diversity compared to rain forests. A decline of species‐rich soil‐feeding termites with increasing aridity may explain why the highest termite diversity occurs in humid tropical rain forests.Abstract in Spanish is available with online material.

Growth variability of two native pine species on Corsica as a function of elevation

Located in the western Mediterranean basin, the island of Corsica (France) is expected to be strongly affected by climate change. With its diverse topography and altitudinal gradients from sea level to 2700 m asl, it hosts two pine species, i.e. Pinus pinaster in lower areas and P. nigra ssp. laricio in higher regions up to the upper tree line. However, the growth performance of these two dominant forest tree species along different elevation belts is still unknown, so it is not clear how climate change will affect their distribution. The relationships between tree age, elevation, and growth performance were evaluated using growth trajectories derived from tree-ring analysis of pines in different age classes at 13 sites across the island, ranging from sea level to 1600 m asl. Results show that the growth performance of both species under the same environmental conditions is very similar. For both species, we found that growth rates change over time, whereby trees younger than 50 years grew faster than older trees. We also found that P. pinaster is becoming common at higher elevation sites, indicating a migration upslope.

Drought-induced decline of productivity in the dominant grassland species Lolium perenne L. depends on soil type and prevailing climatic conditions

Severe constraints on grasslands productivity, ecosystem functions, goods and services are expected to result from projected warming and drought scenarios under climate change. Negative effects on vegetation can be mediated via soil fertility and water holding capacity, though specific mechanisms are fairly complex to generalise. In field drought experiments, it can be difficult to disentangle a drought effect per se from potential confounding effects related to vegetation or soil type, both varying along with climate. Furthermore, there is the need to distinguish the long-term responses of vegetation and soil to gradual climate shift from responses to extreme and stochastic climatic events. Here we address these limitations by means of a factorial experiment using a single dominant grassland species (the perennial ryegrass Lolium perenne L.) grown as a phytometer on two soils types with contrasted physicochemical characteristics, placed at two elevation sites along a climatic gradient, and exposed to early or late-season drought during the plant growing season.Warmer site conditions and reduced precipitation along the elevational gradient affected biogeochemistry and plant productivity more than the drought treatments alone, despite the similar magnitude in volumetric soil moisture reduction. Soil type, as defined here by its organic matter content (SOM), modulated the drought response in relation to local site climatic conditions and, through changes in microbial biomass and activity, determined the seasonal above and belowground productivity of L. perenne. More specifically, our combined uni- and multivariate analyses demonstrate that microbes in a loamy soil with low SOM are strongly responsive to change in climate, as indicated by a simultaneous increase in their C,N,P pools at high elevation with cooler temperatures and wetter soils. Contrastingly, microbes in a clay-loam soil with high SOM are mainly sensitive to temperature, as indicated by a strong increase in microbial biomass under warmer temperatures at low elevation and a concomitant increase in C:N, C:P and N:P ratios. High SOM promoted a better annual yield of the phytometer grass under warmer climate and the effect of drought on productivity was transient. In contrast, low SOM reduced cumulative yield under warmer conditions and root production strongly decreased, enduring a lasting drought effect. Microbes in soils with high organic matter remained more active during warmer and drier conditions, ensuring soil fertility and stimulating a higher overall plant nutrient availability and productivity.Our study highlights the important role of soil type for grassland responses to both stochastic climatic extremes and long-term climate change. Management practices enhancing SOM accumulation via organic residue incorporation seem a promising way to mitigate the effects of increased temperature and drought on plants and soil microbes alike promoting thereby a sustainable ecosystem functioning.

Interactions between arbuscular mycorrhizal fungi and non-host Carex capillacea.

A topic of confusion over the interactions between arbuscular mycorrhizal (AM) fungi and plants is the mycorrhizal status of some plant families such as Cyperaceae, which is generally considered to be non-mycorrhizal. Here, we conducted experiments to explore how the abiotic environmental conditions and AM network influence the interactions between AM fungi and Carex capillacea. We grew Carex capillacea alone or together with a mycorrhizal host species Medicago sativa in the presence or absence of AM fungi (soil inoculum from Mount Segrila and Rhizophagus intraradices from the Chinese Bank of the Glomeromycota, BGC). Plants were grown in a growth chamber and at two elevational sites of Mount Segrila, respectively. The results indicate that mycorrhizal host plants ensured the presence of an active AM fungal network whether under growth chamber or alpine conditions. The AM fungal network significantly depressed the growth of C. capillacea, especially when native inocula were used and the plants grew under alpine site conditions, although root colonization of C. capillacea increased in most cases. Moreover, the colonization level of C. capillacea was much higher (≤ 30%) when growing under alpine conditions compared with growth chamber conditions (< 8.5%). Up to 20% root colonization by Rhizophagus intraradices was observed in monocultures under alpine conditions. A significant negative relationship was found between shoot phosphorus concentrations in M. sativa and shoot dry mass of C. capillacea. These results indicate that growing conditions, AM network, and inoculum source are all important factors affecting the susceptibility of C. capillacea to AM fungi, and growing conditions might be a key driver of the interactions between AM fungi and C. capillacea.

Optimization of Labour Productivity in Elevated Corridor Metro Site Construction

Construction sector affects the economy of our nation to a greater extent. Hence, it is very important to take into consideration the various aspects influencing the smooth execution of construction projects all over the country. Labour productivity is one such important parameter that greatly affects the progress of a particular construction project as it accounts for almost 30-50% of the project cost. Optimization is the action of making the best use of a situation or resource. Questionnaire survey was conducted to obtain data regarding labour productivity in elevated corridor metro construction site. Various factors affecting labour productivity were taken into consideration in the questionnaire. Relative Importance Index (RII) was used to analyse the data obtained from questionnaire survey. Based on results of the analysis, the various factors affecting labour productivity were ranked according to priority. The research outcome aims to contribute towards the labour productivity improvement in the Indian elevated corridor metro construction projects for faster delivery of projects with minimum time and maximum quality.

Using MaxEnt modeling to predict the potential distribution of the endemic plant Rosa arabica Crép. in Egypt

Climate change poses negative impacts on plant species, particularly for those of restricted ecology and distribution range. Rosa arabica Crép., an exclusive endemic species to Saint Catherine Protectorate in Egypt, has severely declined and become critically endangered in the last years. In this paper, we applied the maximum-entropy algorithm (MaxEnt) to predict the current and future potential distribution of this species in order to provide a basis for its protection and conservation. In total, 32 field-based occurrence points and 22 environmental variables (19 bioclimatic and three topographic) were used to model the potential distribution area under current and two future representative concentration pathways (RCP2.6 and RCP8.5) for the years 2050 and 2070. Annual temperature, annual precipitation and elevation were the key factors for the distribution of R. arabica. The response curves showed that this species prefers habitats with an annual temperature of 8.05–15.4 °C, annual precipitation of 36 to 120 mm and elevation range of 1571 to 2273 m a.s.l. Most of the potential current suitable conditions were located at the middle northern region of Saint Catherine. Prediction models under two future climate change scenarios displayed habitat range shifts through the disappearance of R. arabica in sites below 1500 m a.s.l., an altitudinal range contraction at 1500–2000 m and possible expansions towards higher elevation sites (2000–2500 m a.s.l.). Our findings can be used to define the high priority areas for reintroduction or for protection against the expected climate change impacts and future modifications.

Structure and above ground biomass along an elevation small-scale gradient: case study in an Evergreen Andean Amazon forest, Ecuador

The purpose of this study was to examine how tree diversity, richness, and structural characteristics as well as above-ground biomass varies along a small-scale elevation gradient from 601 to 1000 m above sea level (m.a.s.l.) in an Evergreen Andean Amazon forest and their implications in terms of carbon storage. Trees with diameter at breast height greater than 10 cm were surveyed in 20 permanent 0.1 ha plots, five at each elevation site. We determined species richness, density, basal area, aerial biomass and calculated a biomass importance value (BIV). The 1378 trees surveyed were mainly contained in the families Moraceae (17 species) Fabaceae (16) and Meliaceae (10). Species richness significantly increased (P 0.579). At the highest tree density, the highest BIV of Iriartea deltoidea was found at 601–900 m.a.s.l. The disproportionate contribution of a few species, some being the least abundant but with high AGB in our surveys (e.g., Sterculia sp., Nectandra sp., Ficus sp., and Inga sp.) to carbon stocks is important to consider in furture research on carbon sequestration. As the production of above-ground biomass was concentrated in a few species, some uncommon, decision making in reforestation programs and how species should be selected may have implications when measuring and promoting carbon storage.

Air pollution monitoring with mosses in Western Rhodopes, Bulgaria

For the first time a complex study of the bioaccumulation of 22 macro- and microelements, as well as of 18 rare earth and radioactive elements in mosses was applied in the territory of Western Rhodopes. The research is a part of the international program (ICP Vegetation, UNECE) that has been carried out in most of the European countries since 1987. Certain local emitters like old and open mines, serpentines, as well as cross-border transmission were identified. In addition, highest concentrations of radioactive elements were registered in the area of an old uranium mine and most elevated sites, probably due to increased wet deposition. Both heavy metals and toxic elements, as well as radioactive elements maxima were lower than measured highest levels for Bulgaria and Europe. In such studies, inductively coupled plasma spectrometry (ICP-OES, ICP-MS) could be recommended.

Alteration of volcanic rocks and changes in physical-mechanical properties on the South-Kambalny thermal field (South Kamchatka)

The aim of this research project was to consider hydrothermal alteration of volcanic rocks and accompanying changes in their physical and mechanical properties on the thermal field of South Kamchatka. Under the influence of thermal water and steam original andesites and basaltic andesites gradually transform to clay-rich soils and then to opalites and secondary quartzites. The changes in the mineral composition, microstructure, and physical-mechanical properties of rocks and soils in the thermal field were studied. Argillic hydrothermal alteration has gradually decomposed andesites to weak clay-rich soils which form a cover above the field. Elevated sites on the thermal field are composed of opalites and monoquartzites. Progressive hydrothermal alteration of andesites and changes in their properties promotes a broad range of geological phenomena including initiation of landslides, migration of thermal manifestations, changes in relief, and surface deformation.

Stable isotope ratios and speleothem chronology from a high-elevation alpine cave, southern San Juan Mountains, Colorado (USA): Evidence for substantial deglaciation as early as 13.5 ka

Deglaciation ages for valley glaciers from the last glacial maximum are difficult to obtain with precision. The unequal or non-uniform loss of glacial ice cover (spatial heterogeneity) during glacial retreat results in exposure of considerable land surface area prior to complete deglaciation. New U-series dates from an 11cm long stalagmite in a shallow, high elevation (3242 m) cave in the southern San Juan Mountains of southwestern Colorado (USA), indicate that substantial, high-elevation glacial ice ablation occurred prior to 13.5 ka. The stalagmite dates compare well with other deglaciation studies in central and southern Colorado that show deglaciation occurred between ca. 13 ka to 15 ka. Low δ¹³C values from 13.5 ka speleothem growth bands, relative to the δ¹³C value of the host carbonate, suggest that alpine plant and soil cover was established at this location, aspect, and elevation prior to 13.5 ka. Calcite luminescence in the 13.5 ka growth bands indicates the presence of humic and fulvic acids released by the roots of living plants and decomposition of vegetative matter. The carbon isotope ratios, calcite luminescence, and U-series dates suggest that the southern San Juan Mountains, at this high elevation site and southern aspect were substantially ice-free prior to 13.5 ka.

Evidence for population differentiation among Jeffrey and Ponderosa pines in survival, growth and phenology

Ecological restoration projects that include reforestation require that land managers select appropriate source of seeds for long-term persistence. In California, the standard approach for making this choice is based on seed zone and elevational band, both geographically-based measures. However, given the pace of contemporary climate change, populations previously adapted to local conditions may become increasingly mismatched to the changes in climate. If there is a lag in adaptation, current seed zones which assume local is best, would be less useful for reforestation guidelines. Here we use a historic provenance test to evaluate genetic differences among provenances of two species of pine, Pinus ponderosa and P. jeffreyi, and assess performance following seedling transfer across an elevational gradient. Growth in Ponderosa pine shows evidence of a lag in adaptation: trees transferred from lower elevations had consistently increased growth when compared to those trees from higher elevations. In contrast, Jeffrey pine showed no evidence of a lag in adaptation for height. However, survival of Jeffrey pine provenances showed a significant quadratic relationship with transfer distance, consistent with local adaptation. In particular, Jeffrey pine trees from cooler, higher elevation sites had increased survival at high elevation. Jeffrey pine trees from higher elevation also exhibited earlier bud burst than trees from lower elevation grown in the same site, consistent with counter-gradient adaptation in phenology. Together, our results show that genetic variation within species is important for tree survival, growth and phenology in different climates. However, species-specific responses to elevational transfer indicates generalizing seed transfer guidelines across conifer species may be challenging and additional information is necessary to inform managed relocation in a changing climate.

High Morphological Differentiation in Crown Architecture Contrasts with Low Population Genetic Structure of German Norway Spruce Stands

High elevation sites in the low mountain ranges in Germany are naturally covered by Norway spruce (Picea abies (Karst.) L.) stands. Historically, large scale anthropogenic range expansion starting in the mid to late 18th century had a huge impact on the forest composition throughout Germany. Utilisation and exploitation often led to artificial regeneration, mostly carried out using seeds from allochthonous provenances. Usually, autochthonous (natural) high elevation Norway spruce trees have narrow crown phenotypes, whereas lowland trees have broader crowns. Narrow crown phenotypes are likely the result of adaptation to heavy snow loads combined with high wind speeds. In the present study, neighbouring stand pairs of putative autochthonous and allochthonous origin with contrasting phenotypes in high elevation sites were investigated with 200 samples each. These stands are located in the Ore Mountains, the Thuringian Forest, and the Harz Mountains. Additionally, a relict population with the typical narrow high elevation phenotypes was sampled in Thuringia, known as “Schlossbergfichte”. The objective of the study was to quantify supposedly adaptive phenotypic differences in crown architecture and the genetic differentiation of 11 putatively neutral nuclear microsatellite markers (i.e., simple sequence repeats (nSSRs)). The high differentiation of morphological traits (PST = 0.952–0.989) between the neighbouring autochthonous and allochthonous stands of similar age contrasts with the very low neutral genetic differentiation (FST = 0.002–0.007; G″ST = 0.002–0.030), suggesting that directional selection at adaptive gene loci was involved in phenotypic differentiation. Comparing the regions, a small isolation by distance effect for the Harz Mountains was detected, suggesting landscape resistance restricting gene flow. Finally, the differentiation of the very old autochthonous (up to 250 years) stand “Schlossbergfichte” with typical high elevation phenotypes could cohere with the sampling of a relict genepool.

Implications of a shrinking Great Salt Lake for dust on snow deposition in the Wasatch Mountains, UT, as informed by a source to sink case study from the 13–14 April 2017 dust event

The deposition of dust on snow accelerates melt by perturbing snow albedo, directly by darkening the snow surface and indirectly by enhancing snow grain growth. The snow darkening process impacts hydrology by shifting runoff timing and magnitude. Dust on snow deposition has been documented in the Wasatch Mountains, snowmelt from which accounts for up to 80% of surface water supply for Salt Lake City, UT, but the impact on snow melt has not yet been investigated. Here, we present a case study of a dust event observed in the Wasatch (13–14th April, 2017), sampled coincidentally in the air and at the snow surface at an instrumented high elevation site (Atwater Study Plot, Alta, UT). Atmospheric backtrajectory modeling, the results of which were supported by measurements, showed that dust originated predominantly from the west: the Great Salt Lake Desert and the Great Salt Lake (GSL) dry lake bed. The deposited dust mass accounted for ∼50% of the season total dust loading in snow, and daily mean radiative forcing of 20–50 W m−2 accelerated snow melt by approximately 25%. This has important implications for The Greatest Snow on Earth®, and snow water resources; the water level of the GSL has been declining, exposing dry lake beds, and there are no legal water rights or protections to maintain lake levels or mitigate dust emission.

Impact of regulated water levels on willows (Salix subfragilis) at a flood-control dam, and the use of hydrogen peroxide as an indicator of environmental stress

Plants are subjected to various environmental stressors, both natural and artificial. Environmental stressors that lead to an excessive generation of reactive oxygen species (ROS) may adversely affect physiology and well-being of organisms. The amount of ROS, therefore, could potentially be used to quantify environmental stresses in plants. In this study, the effect of changes in the water levels and elevation at which plants are located was examined in the drawdown zone of a flood control dam (Miharu dam), located in Fukushima in northern Japan. The Salix subfragilis willows are an important plant species in the reservoir as these plants are resistant to prolonged submerged conditions and provide the most suitable habitat for the growth and reproduction of many aquatic organisms. S. subfragilis growing in the drawdown zone exhibited high mortality rates from 2008 to 2016. Leaf samples of S. subfragilis were collected in the drawdown zone at different elevations shortly after drawdown (June), and several months after drawdown (August and October). The samples were taken under light exposure; additionally, dark-adapted (30 min) samples were collected in order to evaluate photosystem-induced ROS generation. The mortality rate of S. subfragilis trees was abnormally high over the previous years, particularly at higher elevated sites. Foliar H2O2 concentrations were significantly increased in samples collected at August and October samples, compared to June samples. Catalase, ascorbate peroxidase, and guaiacol peroxidase activities were also increased, corresponding with the H2O2 content. No significant difference in H2O2 concentrations was found between samples from the lower edge of the drawdown zone and those collected at constantly inundated sites. The H2O2 concentration of light- and dark-treated samples indicated that there was no additional ROS accumulation due to photosynthesis. The results indicated that the high mortality of the trees was associated with long-term exposure to drought conditions due to the drawdown, although this was not confirmed by conventional monitoring. These findings support the use of H2O2 as an environmental stress indicator in vegetation management.

Limits to biocontrol: the effects of urbanization and elevation on Bruchidius villosus and Exapion fuscirostre—two biological control agents of Cytisus scoparius

Both invasive species and their biological control agents face barriers to expansion, which provide opportunities to limit invasions or may enable target invasive species to exist in enemy-free space. A better understanding of the various barriers to the spread of insects introduced to control invasive plants will allow for more targeted release programs and potentially shorter lag times from introduction to management. In the Pacific Northwest of the United States, two seed eating beetles (Exapion fuscirostre and Bruchidius villosus) have been introduced to control the invasive plant Cytisus scoparius. These biological controls are predicted to be effective only at high rates of seed destruction, so any factors that limit their colonization or population sizes may allow C. scoparius populations to grow, leading to ecological and economic harm. In this study, we investigate relative impacts of biological control agents in relation to two barriers to insect movement: urbanization and elevation. We find that the impacts of B. villosus are not different between urban and rural sites, but that relative impacts of both biological control agents decrease with increasing elevation, a pattern consistent across 2 years of measurements. Cytisus scoparius populations experience substantial seed destruction in urban settings, strongly suggesting successful population control. The low seed destruction at high elevation sites could indicate that biological control agents are ineffective there, and that C. scoparius may exist in enemy-reduced space.

Climatic Niche Model for Overwintering Monarch Butterflies in a Topographically Complex Region of California.

We use climatic conditions that are associated with known monarch butterfly overwintering groves in California to build a Maxent model, and focus on the fine scale probability of overwintering grove occurrence in a topographically complex region of the state (Santa Barbara County). Grove locations are known from recent and historical surveys and a long-term citizen science database. The climatic niche model performs well, predicting that overwintering habitat is most likely to occur along the coast and at low elevations, as shown by empirical data. We then use climatic variables in conjunction with climate change scenarios to model the future location of overwintering habitat, and find a substantial shift in the predicted distribution. Under a plausible scenario, the probability of occurrence of overwintering habitat directly reflects elevation, with coastal regions having a reduced probability relative to today, and higher elevation sites increasing in probability. Under a more extreme scenario, high probability sites are only located along ridgelines and in mountaintop regions of the county. This predicted shift in distribution is likely to have management implications, as sites that currently lack monarchs may become critical to conservation in the future. Our results suggest that estimating the size of the western overwintering population in the future will be problematic, unless annual counts compensate for a shift in the distribution and a potential change in the number and location of occupied sites.

Disentangling the role of climate and soil on tree growth and its interaction with seed origin

When considering options for adapting forests under climate change, climate is treated as the dominant driver of forest growth, while soil properties are often ignored mainly due to shortage of accurate data. The effects of climate and soil on forest growth may vary due to local adaptation to both climate and soil, and these local adaptations might need to be considered when transferring seed provenances under climate change.Data from 29 provenance trials of Norway spruce (Picea abies (L.) Karst.) across a wide gradient of planting conditions in Austria was used to develop Structural Equation Models (SEMs) to quantified the role of climatic and soil drivers and their interactions on juvenile growth performance and to test if provenance origin affects the relative importance of these drivers.Climate and soil of the planting site location were found to have similar direct effects on juvenile tree growth, however, climate was found to be more important because of additional indirect effects via interactions with soil parameters. Notably, the relative effects of climate and soil vary among different provenance groups. Climate constraints are dominant for seed sources originating from colder and/or high altitude locations, while test site climate and soil are equally important contributors of growth for provenances originating from warmer origin and lower elevation sites. Together with the better growth performance of the latter provenance group their plasticity allows them to utilize a wide range of soil conditions.

The topological differences between visitation and pollen transport networks: a comparison in species rich communities of the Himalaya–Hengduan Mountains

Pollination networks are usually constructed and assessed by direct field observations which commonly assume that all flower visitors are true pollinators. However, this assumption is often invalid and the use of data based on mere visitors to flowers may lead to a misunderstanding of intrinsic pollination networks. Here, using a large dataset by both sampling floral visitors and analyzing their pollen loads, we constructed 32 networks pairs (visitation versus pollen transport) across one flowering season at four elevation sites in the Himalaya–Hengduan Mountains region. Pollen analysis was conducted to determine which flower visitors acted as potential pollinators (pollen vectors) or as cheaters (those not carrying pollen of the visited plants). We tested whether there were topological differences between visitation and pollen transport networks and whether different taxonomic groups of insect visitors differed in their ability to carry pollen of the visited plants. Our results indicated that there was a significantly higher degree of specialization at both the network and species levels in the pollen transport networks in contrast to the visitation networks. Modularity was lower but nestedness was higher in the visitation networks compared to the pollen transport networks. All the cheaters were identified as peripheral species and most of them contributed positively to the nested structure. This may explain in part the differences in modularity and nestedness between the two network types. Bees carried the highest proportion of pollen of the visited plants. This was followed by Coleoptera, other Hymenoptera and Diptera. Lepidoptera carried the lowest proportion of pollen of the visited plants. Our study shows that the construction of pollen transport networks could provide a more in‐depth understanding of plant–pollinator interactions. Moreover, it suggests that detecting and removing cheater interactions when studying the topology of other mutualistic networks might be also important.

Progressive sensitivity of trophic levels to warming underlies an elevational gradient in ant–aphid mutualism strength

Although species interactions are often proposed to be stronger at lower latitudes and elevations, few studies have evaluated the mechanisms driving such patterns. In this study, we assessed whether, and by which mechanisms, abiotic changes associated with elevation altered the outcome of an ant–aphid protection mutualism. To do so, we characterized the multi‐trophic interactions among the ant Formica podzolica, the aphid Aphis varians, and aphid natural enemies occurring on the plant Chamerion angustifolium within replicate high and low elevation valleys. Low (versus high) elevation sites had longer summers (snowmelt 13 days earlier) and were on average 1.1°C warmer and 41% drier throughout the year. At low elevations, individual ant colonies consumed approximately double the volume of carbohydrate baits, likely due to a higher foraging tempo, and possibly due to a greater demand for sugar‐ versus protein‐rich resources (as indicated by stable isotope analysis). Wild aphid colonies at low elevations were visited by 1.4‐fold more natural enemies (controlling for variation in aphid abundance), while experimental aphid colonies on potted plants were tended 52% more frequently by ants. As a result, ants increased aphid colony survival by 66% at low elevations but had no detectable effect at high elevations; at low (versus high) elevations aphid colonies without ants had lower survival, demonstrating stronger predator effects, while aphid colonies with ants had higher survival, demonstrating even stronger ant benefits. Analyses for the effects of mean summer temperature yielded qualitatively identical results to those based on elevation. Collectively, these findings support predictions for a greater sensitivity of higher trophic levels to warming and demonstrate how species interactions can vary across environmental gradients due to simultaneous changes in species traits and abundances across multiple trophic levels.