ALBERT

Description: Community assembly processes do not only influence community structure, but can also affect ecosystem processes. To understand the effect of initial community development on ecosystem processes, we studied natural fungal community dynamics during initial wood decay. We hypothesize that fungal community assembly dynamics are driven by strong priority effects of early-arriving species, which lead to predictable successional patterns and wood decay rates. Alternatively, equivalent colonization success of randomly arriving spores has the potential to drive stochastic community composition and wood decay rates over time. To test these competing hypotheses, we explored the changes in fungal community composition in logs of two tree species (one coniferous and one broadleaf) during the early stages of wood decomposition in a common garden approach. Initial communities were characterized by endophytic fungi, which were highly diverse and variable among logs. Over the first year of decomposition, there was little evidence for priority effects, as early colonizers displaced the endophytic species, and diversity fell as logs were dominated by a few fungal species. During this period, the composition of colonizing fungi was related to the decomposition rates of sapwood. During the second year of decomposition, fungal community composition shifted drastically and the successional dynamics varied considerably between tree species. Variation in fungal community composition among coniferous ( Larix kaempferi ) logs increased, and there remained no evidence for any priority effects as community composition became stochastic. In contrast, early colonizers still dominated many of the deciduous ( Quercus rubra ) logs, with a temporally consistent impact on community composition. For both tree species, wood decay rates levelled off and the relationship with fungal community composition disappeared. Our results indicate that priority effects are relatively minimal in naturally occurring fungal community assembly processes. Instead, fungal successional dynamics are governed predominantly by combative abilities of colonizing fungi, and factors that shape fungal communities over time can differ considerably between tree species. Our results indicate that an increased focus of competitive strength among species, rather than priority effects, may be key to predict community assembly and the ecosystem process they provide.

Description: The increase in cloud optical depth with warming at middle and high latitudes is a robust cloud feedback response found across all climate models. This study builds on results that suggest the optical depth response to temperature is timescale invariant for low-level clouds. The timescale invariance allows one to use satellite observations to constrain the models' optical depth feedbacks. Three passive-sensor satellite retrievals are compared against simulations from eight models from the Atmosphere Model Intercomparison Project (AMIP) of the 5th Coupled Model Intercomparison Project (CMIP5). This study confirms that the low-cloud optical depth response is timescale invariant in the AMIP simulations, generally at latitudes higher than 40°. Compared to satellite estimates, most models overestimate the increase in optical depth with warming at the monthly and interannual timescales. Many models also do not capture the increase in optical depth with estimated inversion strength that is found in all three satellite observations and in previous studies. The discrepancy between models and satellites exists in both hemispheres and in most months of the year. A simple replacement of the models' optical depth sensitivities with the satellites' sensitivities reduces the negative shortwave cloud feedback by at least 50 % in the 40° – 70°S latitude band and by at least 65 % in the 40° – 70°N latitude band. Based on this analysis of satellite observations, we conclude that the low-cloud optical depth feedback at middle and high latitudes is likely too negative in climate models.

Description: To assess marine boundary layer (MBL) cloud simulations in three versions of the Community Atmosphere Model (CAM), three sets of short-term global hindcasts are performed and compared to Atmospheric Radiation Measurement (ARM) program observations on Graciosa Island in the Azores from June 2009 to December 2010. The three versions consist of CAM5.3 with default schemes (CAM5.3), CAM5.3 with Cloud Layers Unified By Binormals (CLUBB-MG1), and CAM5.3 with CLUBB and updated microphysics scheme (CLUBB-MG2). Our results show that relative to CAM5.3 default schemes, simulations with CLUBB better represent MBL cloud-base height, the height of the major cloud layer, and the daily cloud cover variability. CLUBB also better simulates the relationship of cloud fraction to cloud liquid water path (LWP) most likely due to CLUBB's consistent treatment of these variables through a probability distribution function (PDF) approach. Sub-cloud evaporation of precipitation is substantially enhanced in simulations with CLUBB-MG2 and is more realistic based on the limited observational estimate. Despite these improvements, all model versions underestimate MBL cloud cover. CLUBB-MG2 reduces biases in in-cloud LWP (clouds are not too bright) but there are still too few of MBL clouds due to an underestimate in the frequency of overcast scenes. Thus combining CLUBB with MG2 scheme better simulates MBL cloud processes, but because biases remain in MBL cloud cover CLUBB-MG2 does not improve the simulation of the surface shortwave cloud radiative effect ( CRE SW ).

Description: Janzen–Connell effects are negative effects on the survival of a plant's progeny at high conspecific densities or close to its conspecifics. Although the role of Janzen–Connell effects on the maintenance of plant diversity was frequently studied, only few studies targeted Janzen–Connell effects via postdispersal seed predation in temperate grassland systems. We examined effects of conspecific density (abundance of conspecific adult plants) on postdispersal seed predation by invertebrates of three grassland species ( Centaurea jacea, Geranium pratense, and Knautia arvensis ) in experimental plant communities. Additionally, we examined the impact of plant species richness and different seed predator communities on total and relative seed predation (= seed predation of one plant species relative to others). We offered seeds in an exclusion experiment, where treatments allowed access for (1) arthropods and slugs, (2) arthropods only, (3) small arthropods only, and (4) slugs only. Treatments were placed in plots covering a gradient of abundance of conspecific adults at different levels of plant species richness (1, 2, 3, 4, 8 species). Two of the plant species ( C. jacea and K. arvensis ) experienced higher rates of seed predation and relative predation with increasing abundance of conspecific adults. For C. jacea , this effect was mitigated with increasing plant species richness. Differences in seed predator communities shifted seed predation between the plant species and changed the magnitude of seed predation of one plant species relative to the others. We exemplify density-dependent increase in seed predation via invertebrates in grassland communities shaping both the total magnitude of species-specific seed predation and seed predation of one species relative to others. Further differences in seed predator groups shift the magnitude of seed predation between different plant species. This highlights the importance of invertebrate seed predation to structure grasslands via density-dependent effects and differing preferences of consumer groups. We show that species-specific seed predation by invertebrates increases with increasing abundance of conspecific adult plants in grassland communities. Our results provide evidence for Janzen–Connell effects in this system and suggest that plant community characteristics and consumer identity impact the reproductive success of one plant species relative to other species in a community. Overall, this highlights the importance of invertebrate seed predators to structure grasslands via plant-density dependent effects.

Description: Interactions between motions on different timescales are investigated in the SnoHATS dataset of near-surface stable boundary layer turbulence. In an earlier study, the authors applied a data clustering methodology based on a bounded variation, finite element, vector autoregressive factor method (FEM-BV-VARX) to characterize the influence of non-turbulent, submesoscale motions on the turbulence in the SnoHATS dataset. Regimes were identified, two of them weakly stable and two very stable turbulence states. In each identified regime, the variability of turbulent momentum fluxes is characterized here using an extended multiresolution flux decomposition methodology. The transport properties in each regime of near-surface SBL turbulence are thereby assessed. The same methodology is used to investigate the scales of motion responsible for shear generation of turbulence.

Description: Ongoing biodiversity decline impairs ecosystem processes, including pollination. Flower visitation, an important indicator of pollination services, is influenced by plant species richness. However, the spatio-temporal responses of different pollinator groups to plant species richness have not yet been analyzed experimentally. Here, we used an experimental plant species richness gradient to analyze plant–pollinator interactions with an unprecedented spatio-temporal resolution. We observed four pollinator functional groups (honeybees, bumblebees, solitary bees, and hoverflies) in experimental plots at three different vegetation strata between sunrise and sunset. Visits were modified by plant species richness interacting with time and space. Furthermore, the complementarity of pollinator functional groups in space and time was stronger in species-rich mixtures. We conclude that high plant diversity should ensure stable pollination services, mediated via spatio-temporal niche complementarity in flower visitation. Declining plant diversity has been shown to affect ecosystem processes such as plant–pollinator interactions, but it is currently not known if or how spatio-temporal niche partitioning of pollinators is affected by plant biodiversity. In a grassland biodiversity experiment, where we manipulated plant species richness and studied spatio-temporal resource use of pollinators, we found that complementarity of pollinator groups in space and time was stronger in species-rich mixtures. Our study shows that high plant diversity may ensure stable pollination services due to increased pollinator complementarity.

Description: Wide-swath imaging has become a standard acquisition mode for radar missions aiming at applying SAR interferometry at global scale with enhanced revisit frequency. Increased swath width, compared to classical Stripmap imaging mode, is achieved at the expense of azimuthal resolution. This makes along-track displacements, and subsequently north-south displacements, difficult to measure using conventional split-beam (multiple-aperture) InSAR or cross-correlation techniques. Alternatively, we show here that the along-track component of ground motion can be deduced from the double-difference between backward- and forward-looking interferograms within regions of burst overlap. “Burst overlap interferometry” takes advantage of the large squint angle diversity of Sentinel-1 (∼1°) to achieve sub-decimetric accuracy on the along-track component of ground motion. We demonstrate the efficiency of this method using Sentinel-1 data covering the 2015 M w 8.3 Illapel earthquake (Chile) for which we retrieve the full 3D displacement field and validate it against observations from a dense network of GPS sensors.

Description: A method for solvent screening and searching for additives in the field of physical absorption processes is presented. A model based on the concept of virtual components and on the concept of a simplified abstract flow sheet is created. An estimation procedure for thermodynamically consistent property sets for the virtual component is developed. The model is implemented into MATLAB ® . Its application is shown in two studies including a study for a solvent additive. Potential candidates can be characterized and ranges for properties of the solvent or additive are obtained by the means of the model. Conclusions obtained from the model support the screening process by a reduction of the number of further investigated components.

Description: It has recently been estimated that serpentinization within continental lithosphere produces H 2 at rates comparable to oceanic lithosphere (both are ~10 11 mole H 2 /y) [ Sherwood Lollar et al., 2014]. Here we present a simple model that suggests H 2 production rates along the mid-oceanic ridge alone (i.e. excluding other marine settings) may exceed continental production by an order of magnitude (~10 12 moles H 2 /y). In our model, H 2 production rates increase with spreading rate and the net thickness of serpentinizing peridotite (S-P) in a column of lithosphere. Lithosphere with a faster spreading rate therefore requires a relatively smaller net thickness of S-P to produce H 2 at the same rate as lithosphere with a slower rate and greater thickness of S-P. We apply our model globally, incorporating an inverse relationship between spreading rate and net thickness of S-P to be consistent with observations that serpentinization is more common within lithosphere spreading at slower rates.

Description: The genetic consequences of range expansions have generally been investigated at wide geographical and temporal scales, long after the colonization event. A unique ecological system enabled us to both monitor the colonization dynamics and decipher the genetic footprints of expansion over a very short time period. Each year an epidemic of the poplar rust ( Melampsora larici-populina ) expands clonally and linearly along the Durance River, in the Alps. The colonization dynamics observed in 2004 showed two phases with different genetic outcomes. Upstream, fast colonization maintained high genetic diversity. Downstream, the colonization wave progressively faltered, diversity eroded, and differentiation increased, as expected under recurrent founder events. In line with the high dispersal abilities of rust pathogens, we provide evidence for leapfrog dispersal of clones. Our results thus emphasize the importance of colonization dynamics in shaping spatial genetic structure in the face of high gene flow. The Durance River valley is annually invaded by a plant pathogen, the poplar rust fungus. We monitored both disease dynamics and genetic structure resulting from a colonization event. We found that genetic differentiation went hand in hand with a decrease in genotypic diversity. Moreover, we observed a patchy distribution of clonal lineages which pointed to leapfrog dispersal.