Biotic soil-plant interaction processes explain most of hysteretic soil CO2 efux response to temperature in cross-factorial mesocosm experiment

Ecosystem carbon fux partitioning is strongly infuenced by poorly constrained soil CO2 efux (Fsoil). Simple model applications (Arrhenius and Q10) do not account for observed diel hysteresis between Fsoil and soil temperature. How this hysteresis emerges and how it will respond to variation in vegetation or soil moisture remains unknown. We used an ecosystem-level experimental system to independently control potential abiotic and biotic drivers of the Fsoil-T hysteresis. We hypothesized a principally biological cause for the hysteresis. Alternatively, Fsoil hysteresis is primarily driven by thermal convection through the soil profle. We conducted experiments under normal, fuctuating diurnal soil temperatures and under conditions where we held soil temperature near constant. We found (i) signifcant and nearly equal amplitudes of hysteresis regardless of soil temperature regime, and (ii) the amplitude of hysteresis was most closely tied to baseline rates of Fsoil, which were mostly driven by photosynthetic rates. Together, these fndings suggest a more biologically-driven mechanism associated with photosynthate transport in yielding the observed patterns of soil CO2 efux being out of sync with soil temperature. These fndings should be considered on future partitioning models of ecosystem respiration.French governmentFrench National Research Agency (ANR) ANR-10-IDEX-0001-02 PSL ANR-11-INBS-0001ENSUniversity of Arizona (UofA)Philecology Foundation (Fort Worth, Texas, USA)Thomas R. Brown Family FoundationRegion Ile-de-France I-05-098/R 2011-11017735European Union (EU)National Science Foundation (NSF) 1417101 1331408European Union (EU) 625988UofA Office of Global InitiativesOffice of the Vice President of Research at the UofAUMI iGLOBES program at the Uof

Genotypic variability enhances the reproducibility of an ecological study

Many scientific disciplines are currently experiencing a “reproducibility crisis” because numerous scientific findings cannot be repeated consistently. A novel but controversial hypothesis postulates that stringent levels of environmental and biotic standardization in experimental studies reduces reproducibility by amplifying impacts of lab-specific environmental factors not accounted for in study designs. A corollary to this hypothesis is that a deliberate introduction of controlled systematic variability (CSV) in experimental designs may lead to increased reproducibility. We tested this hypothesis using a multi-laboratory microcosm study in which the same ecological experiment was repeated in 14 laboratories across Europe. Each laboratory introduced environmental and genotypic CSV within and among replicated microcosms established in either growth chambers (with stringent control of environmental conditions) or glasshouses (with more variable environmental conditions). The introduction of genotypic CSV led to lower among-laboratory variability in growth chambers, indicating increased reproducibility, but had no significant effect in glasshouses where reproducibility was generally lower. Environmental CSV had little effect on reproducibility. Although there are multiple causes for the “reproducibility crisis”, deliberately including genetic variation may be a simple solution for increasing the reproducibility of ecological studies performed in controlled environments

Modelling fungal sink competitiveness with grains for assimilates in wheat infected by a biotrophic pathogen

Experiments have shown that biotrophic fungi divert assimilates for their growth. However, no attempt has been made either to account for this additional sink or to predict to what extent it competes with both grain filling and plant reserve metabolism for carbon. Fungal sink competitiveness with grains was quantified by a mixed experimentalmodelling approach based on winter wheat infected by Puccinia triticina. One week after anthesis, plants grown under controlled conditions were inoculated with varying loads. Sporulation was recorded while plants underwent varying degrees of shading, ensuring a range of both fungal sink and host source levels. Inoculation load significantly increased both sporulating area and rate. Shading significantly affected net assimilation, reserve mobilization and sporulating area, but not grain filling or sporulation rates. An existing carbon partitioning (sourcesink) model for wheat during the grain filling period was then enhanced, in which two parameters characterize every sink: carriage capacity and substrate affinity. Fungal sink competitiveness with host sources and sinks was modelled by representing spore production as another sink in diseased wheat during grain filling. Data from the experiment were fitted to the model to provide the fungal sink parameters. Fungal carriage capacity was 056 001 g dry matter Cd-1 per lesion, much less than grain filling capacity, even in highly infected plants; however, fungal sporulation had a competitive priority for assimilates over grain filling. Simulation with virtual crops accounted for the importance of the relative contribution of photosynthesis loss, anticipated reserve depletion and spore production when light level and disease severity vary. The grain filling rate was less reduced than photosynthesis; however, over the long term, yield loss could double because the earlier reserve depletion observed here would shorten the duration of grain filling. Sourcesink modelling holds the promise of accounting for plantpathogen interactions over time under fluctuating climatic/lighting conditions in a robust way

Competition with wind-pollinated plant species alters floral traits of insect-pollinated plant species

Plant traits related to attractiveness to pollinators (e.g. flowers and nectar) can be sensitive to abiotic or biotic conditions. Soil nutrient availability, as well as interactions among insect-pollinated plants species, can induce changes in flower and nectar production. However, further investigations are needed to determine the impact of interactions between insect-pollinated species and abiotically pollinated species on such floral traits, especially floral rewards. We carried out a pot experiment in which three insect-pollinated plant species were grown in binary mixtures with four wind-pollinated plant species, differing in their competitive ability. Along the flowering period, we measured floral traits of the insect-pollinated species involved in attractiveness to pollinators (i.e. floral display size, flower size, daily and total 1) flower production, 2) nectar volume, 3) amount of sucrose allocated to nectar). Final plant biomass was measured to quantify competitive interactions. For two out of three insect-pollinated species, we found that the presence of a wind-pollinated species can negatively impact floral traits involved in attractiveness to pollinators. This effect was stronger with wind-pollinated species that induced stronger competitive interactions. These results stress the importance of studying the whole plant community (and not just the insect-pollinated plant community) when working on plant-pollinator interactions

Seasonal Variations of Pollinator Assemblages among Urban and Rural Habitats: A Comparative Approach Using a Standardized Plant Community

International audienceEven though urban green spaces may host a relatively high diversity of wild bees, urban environments impact the pollinator taxonomic and functional diversity in a way that is still misunderstood. Here, we provide an assessment of the taxonomic and functional composition of pollinator assemblages and their response to urbanization in the Paris region (France). We performed a spring-to-fall survey of insect pollinators in green spaces embedded in a dense urban matrix and in rural grasslands, using a plant setup standardized across sites and throughout the seasons. We compared pollinator species composition and the occurrence of bee functional traits over the two habitats. There was no difference in species richness between habitats, though urban assemblages were dominated by very abundant generalist species and displayed a lower evenness. They also included fewer brood parasitic, solitary or ground-nesting bees. Overall, bees tended to be larger in the city than in the semi-natural grasslands, and this trait exhibited seasonal variations. The urban environment filters out some life history traits of insect pollinators and alters their seasonal patterns, likely as a result of the fragmentation and scarcity of feeding and nesting resources. This could have repercussions on pollination networks and the efficiency of the pollination function

Below-ground competition alters attractiveness of an insect-pollinated plant to pollinators

International audienceCompetitive interactions between plants can affect patterns of allocation to reproductive structures through modulation of resource availability. As floral traits involved in plant attractiveness to pollinators can be sensitive to these resources, competition with any neighbouring species may influence the attractiveness of insect-pollinated plants. While pollination research has primarily focused on above-ground interactions, this study aims at investigating if the presence of a competitor plant can modulate neighbouring insect-pollinated plant attractiveness to pollinators and resulting fecundity, especially through below-ground competitive interactions for soil resources. We set up a plot experiment in which we grew an insect-pollinated plant, Sinapis alba (Brassicaceae), in a mixture dominated by a wind-pollinated plant, Holcus lanatus (Poaceae). Individuals of S. alba were either subjected to or isolated from (with buried tubes in the soil) below-ground competition. Across the flowering season, floral traits involved in attractiveness of S. alba and pollinator visitation were followed at the plot and plant level to investigate different scales of attractiveness. At the end of the experiment, seeds were harvested to assess plant fecundity. Competition had a significant negative effect on plot and plant floral display size as well as flower size while nectar traits were not affected. When plants of S. alba were in competition, the time to first visit was altered: the proportion of plots that received a visit was smaller for a given time; in other words, it took more time for a given proportion of plots to be visited and some plots were even never visited. Moreover, pollinators made fewer visits per plots. The proportion of viable seeds produced by S. alba in competition was lower and probably linked to the competition itself rather than changes in pollinator visitation. This study suggests that competitive interactions between plants can modulate pollination interactions even when competing plant species are not insect-pollinated

Does competition with wind-pollinated species alter Echium plantagineum's attractiveness to a common pollinator Bombus terrestris?

International audienceIn insect-pollinated plants, pollinator attraction is influenced by flowers (e.g. number, size) and their associated rewards (e.g. pollen, nectar). These traits can depend on plant interactions. Indeed, below-ground competition between plants can lead to a decrease in flower or reward production in insect-pollinated species. Wind-pollinated plants, in particular, which are almost never studied in plant pollinator networks, can alter insect-pollinated plants' attractiveness through competition for nutrients. The response of pollinators to such changes has never been investigated.A pot experiment was carried out in which an insect-pollinated species, Echium plantagineum, was grown in binary mixture with three wind-pollinated species selected to exert a panel of competitive interactions. Below-ground competition was controlled using dividers limiting interspecific root competition. Floral traits of E. plantugineum (i.e. flower production, floral display size, flower size and nectar production) were measured. For each species mixture, the visits (i.e. first visit, number of visits, 10-min sequences) of Bombus terrestris individuals released in a flight cage containing two pots were followed, one with and one without below-ground competition.Below-ground competition significantly affected nectar's sucrose concentration but did not influence flower and nectar production. Likewise, pollinator visits were not influenced by below-ground competition. Competitor identity significantly influenced flower and reward production of E. plantagineum, with a decrease in the presence of the most competitive wind-pollinated species. A tendency for faster flower visitation events was also detected in the presence of the least competitive competitor. This study raises new questions regarding the influence of wind-pollinated plants on plant pollinator interactions

Broader phenology of pollinator activity and higher plant reproductive success in an urban habitat compared to a rural one

International audienceUrban habitat characteristics create environmental filtering of pollinator communities. They also impact pollinating insect phenology through the presence of an urban heat island and the year-round availability of floral resources provided by ornamental plants. Here, we monitored the phenology and composition of pollinating insect communities visiting replicates of an experimental plant assemblage comprising two species, with contrasting floral traits: Sinapis alba and Lotus corniculatus, whose flowering periods were artificially extended. Plant assemblage replicates were set up over two consecutive years in two different habitats: rural and densely urbanized , within the same biogeographical region (Ile-de-France region, France). The phenology of pollination activity, recorded from the beginning (early March) to the end (early November) of the season, differed between these two habitats. Several pollinator morphogroups (small wild bees, bumblebees, honeybees) were significantly more active on our plant sets in the urban habitat compared to the rural one, especially in early spring and autumn. This resulted in different overall reproductive success of the plant assemblage between the two habitats. Over the course of the season, reproductive success of S. alba was always significantly higher in the urban habitat, while reproductive success of L. corniculatus was significantly higher in the urban habitat only during early flowering. These findings suggest different phenological adaptations to the urban habitat for different groups of pollinators. Overall, results indicate that the broadened activity period of pollinating insects recorded in the urban environment could enhance the pollination function and the reproductive success of plant communities in cities

Modeling the inter-individual variability of single-stemmed plant development

International audienceModeling the development of single-stemmed plants is classically done by expressing the number of phytomers in the main axis as a function of the thermal time. In many cases, a strong inter-individual variability is encountered, that cannot be straightforwardly associated to any environmental or micro-environmental factors. We propose a methodological framework to describe the heterogeneity of individual responses in a population by hierarchical or mixed-effect models : some regression parameters are random variables identically distributed in the population. The method is illustrated using experimental data collected on young Acacia erioloba plants, grown in CEREEP greenhouse, for which two piecewise-linear models of organogene-sis are identified using Expectation-Maximization (EM) algorithm and compared according to their adequacy to the data. The E-step and the M-step of the algorithm are analytic as long as the dependence is linear with respect to the individual parameters, i.e. the latent variables of the mixed-effect model. In the general case, the posterior distribution of the individual parameters cannot be analytically determined. Monte-Carlo Markov Chains are generated with Metropolis-Hasting during the E-step to estimate the distribution. Alternatively, a Laplace approximation of the posterior distribution appeared to be quite relevant in our case. Such framework will be adapted to the identification of a complete FSPM of Acacia growth combining organogenesis and functioning