Rocky Mountain Biological Laboratory

Snowmelt triggers a soil microbial bloom and crash that affects nitrogen (N) export in high-elevation watersheds. The mechanisms underlying these microbial dynamics are uncertain, making soil nitrogen processes difficult to predict as snowpack declines globally. Here, integration of genome-resolved metagenomics, metatranscriptomics and metabolomics in a high-elevation watershed revealed ecologically distinct soil microorganisms linked across the snowmelt time-period by their unique nitrogen cycling capacities. The molecular properties and transformations of dissolved organic N suggested that degradation or recycling of microbial biomass provided N for biosynthesis during the microbial bloom. Winter-adapted Bradyrhizobia spp. oxidized amino acids anaerobically and had the highest gene expression for denitrification during the microbial bloom. A pulse of nitrate was driven by spring-adapted Nitrososphaerales after snowmelt, but dissimilatory nitrate reduction to ammonia (DNRA) gene expression indicated significant nitrate retention potential. These findings inform our understanding of nitrogen cycling in environments sensitive to snowpack decline due to global change.

Abstract Floral nectar plays a critical role in shaping the ecology and evolution of plant-pollinator interactions. Effective and efficient methods that allow for broad-scale sampling of nectar volume and sugar concentration across a diversity of taxa are needed to improve our understanding of many dimensions of mutualistic plant-pollinator interactions—including their basic ecology and evolution, their responses to environmental change, and their conservation and restoration. Despite the key importance of nectar for mediating plant-pollinator interactions, quantifying floral nectar in the field from many different plant species is challenging because there is often no one-size-fits-all sampling method that is effective across a diversity of floral structures and nectar traits. Different methods require different preparation, and sampling from many species involves a variety of logistical challenges. Here we provide a methodological roadmap for sampling floral nectar in the field from many different plant species. We describe our nectar collection methods in detail, including necessary equipment, calculations, and approaches appropriate for different floral morphologies. We also provide a troubleshooting guide for common problems encountered while collecting nectar in the field. To demonstrate the utility and effectiveness of our methods for collecting nectar from many different species, we present results on nectar trait variation from 53 species in an ecosystem. Our method illustrates that nectar traits vary considerably within and among plant species, indicating that large-scale nectar sampling projects are an important consideration for many basic and applied questions in pollination ecology and evolution. We hope that across many plant communities and ecosystems, our paper provides a practical roadmap for how to navigate the complexities of quantifying floral nectar traits.

Climate change poses a threat to organisms globally, but its effects may be particularly pronounced in multi-trophic systems. For example, pollinators are vulnerable to changes in climate, as are flowering plants, but the extent of direct versus floral-mediated indirect effects of climate on pollinators has not been studied for most solitary bee species. We use a long-term dataset (9 years) encompassing a diverse assemblage of bees (13 species) and their flowering plants from a region experiencing rapid effects of climate change (Rocky Mountains, USA). Utilizing piecewise structural equation models, we assess direct and flower-mediated indirect effects of climate variables (snow cover duration and growing degree day accumulation) on each bee species's abundance. For 62% of species, longer snow cover duration had a direct, positive effect on bee abundance the following year and on flower abundance and duration. However, flower abundance and duration in the preceding year did not affect bee abundance the following year. Snow cover provides temperature stabilization for energy-efficient diapausing among bees and frost protection for flowering plants. Given the shift towards earlier snowmelt in this system, our findings raise concern for regional bee community stability and underscore the importance of seasonal snow cover for bees in other systems.

<strong class="journal-contentHeaderColor">Abstract.</strong> Meteoric beryllium-10 (¹⁰Be_met) has a wide range of applications as a geochronometer and tracer of geological processes. ¹⁰Be_met is produced in the atmosphere by cosmic rays and delivered to Earth&rsquo;s surface primarily via precipitation. ¹⁰Be_met is particularly suitable for quantifying surface process rates where use of <em>in situ</em>-produced ¹⁰Be is challenging, such as landscapes with quartz-poor bedrock. However, using ¹⁰Be_met for dating and quantifying surface process rates requires constraining depositional fluxes across space and time. Although empirical and physical models for predicting fluxes exist, the predictions can deviate substantially from measured values. Here we quantify ¹⁰Be_met flux in the East River watershed in Colorado, USA where precipitation is dominated by snowfall. We measured the ¹⁰Be_met inventory in soils on five glacial moraines 13&ndash;18 ka in age that span 700 m of elevation and calculated ¹⁰Be_met fluxes by dividing each inventory by moraine age. Inheritance-corrected fluxes range from 1.12 x10⁶&ndash;3.79x10⁶ atoms cm^-2 yr^-1, and are well correlated with elevation, mean annual precipitation, mean snow depth, and snow water equivalent (R² = 0.84 to 0.99). Regression models based on elevation, precipitation, snow depth and snow water equivalent predict watershed-averaged fluxes of 1.23x10⁶&ndash;3.62x10⁶ atoms cm^-2 yr^-1. Predicted fluxes from a published empirical model that estimates fluxes as a function of precipitation were within a factor of 1.1&ndash;1.6 of measured values at each site. Fluxes predicted by physically-based general circulation models (GCM) are generally within a factor of three of our estimated watershed-averaged values, but the GCM predictions are too coarse to capture the intra-watershed spatial variability in fluxes. Our results highlight both the importance of factors that drive variability in ¹⁰Be_met delivery to soils and how local calibration can improve estimates of ¹⁰Be_met flux in mountain watersheds.

Plant functional traits typically show strong covariation, e.g. as in the worldwide leaf economics spectrum (LES). Covariation is thought to arise from selective forces and physical constraints. However, processes shaping covariation at interspecific scales may differ from those at intraspecific scales. Potential sources of intraspecific trait variation include variation in genetics, abiotic environment, and biotic environment; any of these factors may cause divergence from interspecific patterns. Quaking aspen, Populus tremuloides (Salicaceae), is a widespread tree species ideal for assessing these processes' role in the intraspecific LES, due to its clonal growth along environmental gradients, intraspecific variation in cytotype (ploidy level), and variation in biotic interactions (herbivory pressure). Here, we investigate genotype, cytotype, microclimate and herbivory as potential drivers in shaping LES trait–trait and trait–environment relationships at intra‐specific scales. We studied 15 quaking aspen genotypes that varied in cytotype (diploid or triploid) along an elevation gradient in southwestern Colorado. We show that LES tradeoffs substantially weaken at the intraspecific scale in this species. Among genotypes, trait–trait slopes ranged from positive, weak, strong, negative, or absent compared to the global LES. We also found that cytotypes varied in resource‐use tradeoffs, and that increasing insect herbivory pressure decreased the strength of several trait–trait relationships. Microenvironment was a weak predictor of intraspecific functional trait variation in quaking aspen. Synthesis. In quaking aspen, there are relaxed constraints on LES trait co‐variation. Variation in genotype, cytotype, and herbivory pressure each contribute to this pattern. Relaxation of the LES may enable more flexible responses to environmental stressors through both genetic and plastic mechanisms.

ABSTRACTDifferences in dominance are frequently invoked to explain the outcomes of competition. Yet, what it means to be dominant, and which traits underlie dominance, are poorly understood. Here, we sought to determine the relationships between multiple aspects of dominance, the potential for trade‐offs with discovery ability, and the traits associated with these patterns within a high elevation community of five ant taxa. We examined several common dominance metrics—behavioral dominance (winning aggressive encounters at both the individual and colony levels), numerical dominance (abundance and activity in baits and pitfall traps), and ecological dominance (high relative frequency in baits)—and found that individual‐ and colony‐level behavioral dominance was positively correlated, as were ecological and numerical dominance. However, colony‐level behavioral and numerical dominance were negatively correlated, and no other dominance metrics were associated. There was a dominance‐discovery trade‐off, as increased behavioral (but not numerical or ecological) dominance was associated with slower resource discovery. This trade‐off was likely driven by behaviorally dominant ants having larger body sizes and recruiting a greater biomass of workers to baits. In contrast, fast discoverers were more abundant in the environment (i.e., numerically dominant). Complementing our empirical study, a meta‐analysis of 54 responses from 21 studies showed that the association between dominance and discovery ability depended on the dominance metric. Whereas discovery ability was positively correlated with numerical dominance, its relationships with behavioral and ecological dominance were highly variable and not significantly different from zero. Overall, our empirical findings, in combination with the synthesis of past studies, demonstrate that different aspects of ant dominance are not equivalent. Yet, regardless of dominance type, there is little evidence that dominance‐discovery trade‐offs occur in most ant communities.

Abstract Understanding the biology of floral traits underlying plant‐pollinator relationships in a community context is important to predict the occurrence and frequency of pairwise mutualistic interactions. Flower nectar is a sugar‐rich solution that is key in plant–pollinator interactions. Nectar sugar concentration directly determines viscosity, regulating nectar ingestion rates by pollinators and, together with nectar volume, shapes foraging decisions. Furthermore, the link between nectar ingestion efficiency and sugar concentration in bee pollinators is mediated by their proboscis length and the spatial positioning of nectar within flowers. We therefore hypothesize that nectar concentration can be related to the structure of bee–flower interactions in a community. We tested the hypothesis in a species‐rich alpine meadow in the Hengduan Mountains, China, a global biodiversity hotspot. The study meadow sustains an extremely diverse bumblebee community. We constructed the bee–flower network in the 2020 flowering season. Visitation frequency of the bumblebees to each flowering plant was recorded while nectar sugar concentration and initial volume of those plants were quantified in addition to other pollination‐related floral traits. We conducted phylogenetically comprehensive analyses to assess factors influencing the network structure, bees' visitation frequency and flower preference, and to detect the correlations of bees' proboscis length with nectar sugar concentration and corolla tube length and the relationships among the pollination‐related floral traits. Modularity analysis indicated that the community bee–flower network was structured by nectar sugar concentration and corolla tube length, which was associated with bees' proboscis length. However, the overall bees' visitation frequency to flowers was correlated to nectar sugar concentration but not to corolla tube length. Flowers visited by long‐proboscid bee species had comparatively dilute nectar. Moreover, the bumblebees' flower preferences were positively influenced by floral abundance. Additionally, variation in nectar sugar concentration was linked with pollination‐related floral traits and pollinator composition, suggesting a diversity of pollination strategies among the plants in the community. Synthesis: To our knowledge, this is the first field investigation uncovering how bee‐flower interactions and nectar sugar concentration are linked at the community level, which broadens the niche dimensions for resource partitioning among co‐occurring species and thus fosters biodiversity.

Wild bees play indispensable ecological roles in many ecosystems, yet declines in many species have been documented in recent years. These declines have been linked to numerous anthropogenic pressures, including climate change, which can influence bee populations directly (i.e., via physiological mechanisms) and indirectly (e.g., through changes to floral resource quantity and quality). Widespread changes in the distributions and abundances of many bee species have been linked to climate change at large spatial scales; yet, the local-scale processes that generate these broad-scale patterns are little known. Specifically, it is unclear how climate change influences the fitness of individual bees, despite the fact that broad-scale patterns of decline must be driven by individual-level processes. Our study examines the individual fitness responses (i.e., reproductive output) of five solitary bee species in the genus Osmia to variations in local climate variables in subalpine habitats. Over nine years, we tracked brood cell production in over 700 individual female bees, conducted repeated surveys of floral abundance (while taking into account the specific floral host taxa used by each bee species), and recorded local precipitation and temperature at seven sites. Our data revealed both direct and indirect (floral resource-mediated) negative effects of drought: the total number of brood cells produced by nesting females decreased significantly in summers characterized by greater drought severity and high temperatures-a relationship-driven partly by significant decreases in floral host-plant availability in hotter and drier years. Our study highlights how the negative consequences of climate change for bee populations can manifest at the level of individuals and the role of floral resources in mediating these relationships. Overall, our results suggest that the increasingly warm and dry conditions forecasted for our study area will have net negative effects on the demographic success of these solitary bees.

Climatic warming influences ecosystem-scale carbon fluxes directly via effects on photosynthesis and respiration, and indirectly via effects on the plant community. Here, we report on a 10-year factorial warming and dominant plant species removal experiment established in both a high- and in a low-elevation montane meadow to explore how dominant plants modify the effect of warming on the carbon cycle across space and over time. At low-elevation, warming increased peak-season net carbon uptake in most years, driven by higher primary productivity, but only in plots where the dominant was left intact. Here, net carbon uptake tended to be positive, but was more likely to be negative when the dominant plant was removed, and in dry years. Surprisingly, the high-elevation site was unaffected by the warming and plant removal treatments, suggesting these sites are resistant to these disturbances. Taken together, these results demonstrate that dominant plant species can modify the impacts of warming on carbon fluxes, but show how this influence can vary spatially and temporally. These findings provide insight into when and how abiotic and biotic factors influence ecosystem carbon source/sink dynamics.