The biogeomorphological life cycle of poplars during the fluvial biogeomorphological succession: a special focus on Populus nigra L.

Riverine ecosystems are recurrently rejuvenated during destructive flood events and vegetation succession starts again. Poplars (i.e. species from Populus genera) respond to hydrogeomorphological constraints, but, in turn, also influence these processes. Thus, poplar development on bare mineral substrates is not exclusively a one‐way vegetative process. Reciprocal interactions and adjustments between poplar species and sediment dynamics during their life cycle lead to the emergence of biogeomorphological entities within the fluvial corridor, such as vegetated islands, benches and floodplains. Based on a review of geomorphological, biological and ecological literature, we have identified and described the co‐constructing processes between riparian poplars and their fluvial environment. We have explored the possibility that the modification of the hydrogeomorphological environment exerted, in particular, by the European black poplar (Populus nigra L.), increases its fitness and thus results in positive niche construction. We focus on the fundamental phases of dispersal, recruitment and establishment until sexual maturity of P. nigra by describing the hierarchy of interactions and the pattern of feedbacks between biotic and abiotic components. We explicitly relate the biological life cycle of P. nigra to the fluvial biogeomorphic succession model by referring to the ‘biogeomorphological life cycle’ of P. nigra. Finally, we propose new research perspectives based on this theoretical framework. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismicity assessment using earthquake catalogues with uncertain and incomplete data: probabilistic formulation

A novel generalized probabilistic formulation is proposed to assess seismicity using earthquake catalogues with uncertain and incomplete data. The seismicity, described by the complete exceedance rate of magnitudes, is estimated starting from a consistent incomplete exceedance rate which is rationally linked to the catalogue data. Complete and incomplete exceedance rates are represented by similar functional forms and they are related by a completeness function, which expresses the probability that an event is included in a data set. Completeness is considered uncertain and it is defined by a suitable, continuous, analytical, magnitude dependent function. The importance of this work lies on its applicability because it can be useful in seismic zones where information about seismic activity is scarce or simply when the catalogue is incomplete in a range of magnitudes that can have a significant influence on the seismic hazard analysis and on the resulting seismic risk assessment. Moreover, it can also be applied in the common case when the catalogue is considered complete above a given magnitude threshold. Numerical examples are presented to illustrate the influence of catalogue incompleteness on the complete exceedance rate estimations. In companion papers, attention is focused on the estimation of completeness probabilities of available catalogues and on parameter estimation of the exceedance rate functions.     

Hydrological dynamics of snowmelt induced streamflow in a high mountain catchment of the Pyrenees under contrasting snow accumulation and duration years

Snowmelt drives a large portion of streamflow in many mountain areas of the world. However, the water paths from snowmelt to the arrival of the water in the streams are still largely unknown. This work analyzes for first time the influence of snowmelt on spring streamflow with different snow accumulation and duration, in an alpine catchment of the central Spanish Pyrenees. This study presents the water balance of the main melting months (May and June). Piezometric values, water temperature, electrical conductivity and isotope data (δ¹⁸O) allow a better understanding of the hydrological functioning of the basin during these months. Results of the water balance calculations showed that snow represented on average 73% of the water available for streamflow in May and June while precipitation during these months accounted for only 27%. However, rainfall during the melting period was important to determine the shape of the spring hydrographs. On average, 78% of the sum of both the snow water equivalent (SWE) accumulated at the beginning of May and the precipitation in May and June converted into runoff during the May–June melting period. The average evaporation-sublimation during the 2 months corresponded to 8.4% of the accumulated SWE and rainfall, so that only a small part of the water input was ultimately available for soil and groundwater storage. When snow cover disappeared from the catchment, soil water storage and streamflow showed a sharp decline. Consequently, streamflow electrical conductivity, temperature and δ¹⁸O showed a marked tipping point towards higher values. The fast hydrological response of the catchment to snow and meteorological fluctuations, as well as the marked diel fluctuations of streamflow δ¹⁸O during the melting period, strongly suggests short meltwater transit times. As a consequence of this hydrological behaviour, independently of the amount of snow accumulated and of melting date, summer streamflow remained always low, with only small runoff peaks driven by rainfall events.     

High-resolution spectroscopy and broad-band imaging of the young planetary nebula K 3-35

We present high-resolution echelle and long-slit spectra and broad-band ( R , I ) images of the very young planetary nebula K 3-35. Several emission lines are identified, including the He  ii  4686 line and strong [N  ii ]6548, 6583 and [O  iii ]4959, 5007 emissions [ I ([N  ii ])/ I (H α )≃5.5, I ([O  iii ])/ I (H β )≃30]. A systemic velocity V _LSR≃10±2 km s⁻¹ for K 3-35 is obtained from the optical emission lines. Two different kinematic components are identified in the nebula. One of them is probably related to the elliptical envelope previously observed. The second component exhibits systematic changes of the radial velocity with position, and a relatively small velocity width. This component may be attributed to the precessing jet-like outflows previously identified. The R and I images and the deduced R − I colour map strongly support the existence of a dense, partially neutral disc-like region in the equatorial plane of the nebula, which probably represents an equatorial density enhancement in a previously ejected slow wind. Diagnostic diagrams for line intensity ratios in K 3-35 and collimated components of other planetary nebulae suggest that the emission spectrum of this kind of structure is a combination of radiative and shock excitation, in agreement with recent models of shocks in a strongly photoionized medium.     

Formation and dynamics of vegetated fluvial landforms follow the biogeomorphological succession model in a channelized river

Feedback between hydrogeomorphological processes and riparian plants drives landscape dynamics and vegetation succession in river corridors. We describe the consequences of biogeomorphological feedback on the formation and dynamics of vegetated fluvial landforms based on observations from the channelized Isère River in France. The channel was laterally confined with embankments and mostly straightened. From the beginning of the 1970s to the end of the 1990s, alternate bars were progressively but heavily colonized by vegetation. This context presented an exceptional opportunity to analyse temporal adjustments between fluvial landforms and vegetation succession from bare gravel bars to mature upland forest as the consequence of biogeomorphological interactions. Based on a GIS analysis of aerial photographs (between 1948 and 1996), we show that the spatiotemporal organization of vegetated bars within the river channel observed in 1996 resulted from a bioconstruction and biostabilization effect of vegetation and interactions between bars of varying age, size and mobility. Field measurements in 1996 reflected how a strong positive feedback between sedimentary dynamics and riparian vegetation succession resulted in the construction of the vegetated bars. A highly significant statistical association of geomorphological and vegetation variables (RV of co-inertia analysis = 0.41, p < 0.001) explained 95% of the variability in just one axis, supporting the existence of very strong feedback between geomorphological changes (i.e. the transformation of small bare alternate bars to fluvial landforms covered by mature upland forest, and vegetation succession). Such dynamics reflect the fluvial biogeomorphological successions model, as described by the authors earlier. © 2020 John Wiley & Sons, Ltd.     

Out-of-plane shaking table tests of full-scale historic adobe corner walls retrofitted with timber elements

Historic adobe structures pose a high seismic risk mainly because of the poor out-of-plane bending response of their walls that may produce fatalities and significant economic, cultural, and heritage losses. In this paper, we propose a retrofitting technique that increases the wall strength for both in-plane and out-of-plane directions. This technique consists of vertical and horizontal timber elements symmetrically installed on each face of the wall to form a confining wood frame, supplemented with vertical tensors that pre-compress the wall. This study evaluates the performance of this retrofitting technique with a two-set experimental program on full-scale historic adobe walls. On the first set, four specimens were subjected to a static overturning test with boundary conditions representing the confinement effect at both ends by orthogonal walls. On the second set, three full-scale specimens, one unretrofitted and two retrofitted, were subjected to four ground motion records on a shaking table to assess the out-of-plane dynamic behavior of typical corner walls. The unretrofitted specimen collapsed during the second motion (peak ground acceleration [PGA] = 0.39 g), while both retrofitted walls survived all four motions (maximum PGA of 0.75 g) proving the high effectiveness of the proposed retrofitting. The addition of base anchors as a variation of the retrofitting technique significantly reduced the rocking effects and the residual drifts of the system, thus improving its overall seismic performance. Further research is needed to develop guidelines for seismic retrofit of heritage buildings including multistory full-scale tests of specimens with various types of openings and retrofitting strategies that minimize their architectural impact.