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Riparian ecosystems are of utmost importance for many key ecosystem services, such as nutrient filtering or flood and erosion control. Restoring degraded riverbanks using soil bioengineering techniques initiates secondary succession improving both protection from erosion and riparian ecological functions. These techniques often rely on the settlement of Salicaceae and sometimes Tamaricaceae species that can be strongly impacted by beaver browsing. During the last decade European beaver (Castor fiber) populations have largely grown in France and can thus impact restored riverbanks, leading practitioners to adapt their management strategy. We conducted a study to assess the feeding preference of C. fiber and their browsing spatial pattern in semi natural riverbanks in the French Alps (Isère, France) by measuring browsing impacts along a transect. Our results indicate selective browsing occurrences at two nested spatial scales: firstly, beavers create large preferentially browsed patches (~ 8 m diameter) secondly, at a finer scale within these patches, they demonstrate a selective browsing between plant species. Among the species used in soil bioengineering, Myricaria germanica showed the lower browsing selectivity with only 2.5% of browsing probability while Salix eleagnos and Salix purpurea (10%) and Populus nigra, Salix daphnoides and Salix alba (20%) showed significant higher selectivity by beaver. Our results suggest that within the beaver presence area soil bioengineering techniques should consider adjusting both the composition and the spatial pattern of plant species settlement to mitigate the impacts of beaver on restored riverbanks. [ABSTRACT FROM AUTHOR]

The free-flowing Allier River in central France provides an ideal study system to analyze the colonization and succession of riparian woodlands dominated by the European black poplar (Populus nigra). As displayed in the foreground, dynamic alluvial processes along the meandering river provide extensive gravel bars suitable for seedling colonization, generative or sexual reproduction. Mature woodlands occur along the opposite bank and in those zones, vegetative, clonal or asexual reproduction occurs. Through field measurements, DNA genotyping, dendrochronology, and the extensive interpretations of aerial photographs, the spatiotemporal patterns of generative and vegetative poplar reproduction were analyzed for this study region. This knowledge will benefit the conservation and restoration of the biodiverse but threatened floodplain forests in Europe and elsewhere. • Reproductive ecology of riparian poplars was investigated along a free-flowing river. • Sapling excavation, stand uniformity and genotyping enabled assessments of clonality. • Colonization of barren sites involved generative (sexual, seedlings) reproduction. • Subsequently, vegetative (asexual, clonal) reproduction expanded and infilled stands. • The two reproductive strategies contributed to the woodland diversity and succession. The European black poplar (Populus nigra L.) is a riparian tree that dominates the early successional stages of alluvial floodplain forests throughout Continental Europe. River management and hybrid poplar plantations have contributed to the loss of this foundational species, and measures to conserve and restore the ecologically rich and diverse natural woodlands must sustain poplar reproduction. P. nigra is able to reproduce through either generative (sexual) or vegetative (asexual or clonal) processes and this enhances its chances for preservation in future forests. This research was undertaken to quantify the dominant reproductive strategy of natural black poplar stands, and to determine how this influenced woodland structure and successional processes within the floodplains along the lower Allier River. Two approaches were integrated: (1) analyses of the degree of clonality of stands based on DNA genotyping, sapling excavations, and the uniformity of poplar height and spatial distributions; and (2) retrospective analyses of successional processes to investigate spatiotemporal correspondences with the reproductive strategies. Structural stand patterns indicated the dominant reproductive strategy, with vertical heterogenity indicating clonality, while generative stands were more uniform. One half (50.2%, or 164 ha) of the floodplain study area supported woodlands with P. nigra stands or individuals. Of this area, 42% had apparently reproduced generatively with limited clonality; 43% had dominantly vegetative regeneration with high clonality; and 15% included generatively established, solitary old trees. The generatively and vegetatively reproduced stands displayed generally similar establishment rates but differed in their spatial patterns, with generative seedling bands or patches largely limited to barren streamside sites, while vegetative stands were more diversely distributed. The reproductive types also differed in successional dynamics, with generative stands providing more persistent successional phases with longer residence times. Thus, sexual reproduction enables woodland colonization and introduces genetic diversity, while clonal reproduction expands the population and structural diversity. Both reproductive strategies should be encouraged to provide healthy, diverse and resilient riparian woodlands. [ABSTRACT FROM AUTHOR]

Summary: • We examined the relationships among productivity, water use efficiency (WUE) and drought tolerance in 29 genotypes of Populus × euramericana (Populus deltoides×Populus nigra), and investigated whether some leaf traits could be used as predictors for productivity, WUE and drought tolerance.• At Orléans, France, drought was induced on one field plot by withholding water, while a second plot remained irrigated and was used as a control. Recorded variables included stem traits (e.g. biomass) and leaf structural (e.g. leaf area) and functional traits [e.g. intrinsic water use efficiency (Wi) and carbon isotope discrimination (Δ)].• Productivity and Δ displayed large genotypic variability and were not correlated. Δ scaled negatively with Wi and positively with stomatal conductance under moderate drought, suggesting that the diversity for Δ was mainly driven by stomatal conductance.• Most of the productive genotypes displayed a low level of drought tolerance (i.e. a large reduction of biomass), while the less productive genotypes presented a large range of drought tolerance. The ability to increase WUE in response to water deficit was necessary but not sufficient to explain the genotypic diversity of drought tolerance. New Phytologist (2006) 169 : 753–763 © The Authors (2005). Journal compilation © New Phytologist (2005) doi : 10.1111/j.1469-8137.2005.01630.x --> [ABSTRACT FROM AUTHOR]

Summary 1 We used genetic markers to study gene flow of the riparian pioneer tree species Populus nigra along the Drôme river (France). This dioecious species is supposed to have more efficient dispersal mechanisms for pollen (wind) and seeds (wind and water) than other trees. 2 Seedlings belonging to the same reproduction/migration event were sampled in 22 riparian forest fragments along the river and their genetic diversity assessed through six nuclear microsatellites. 3 We found a high level of diversity and significant differentiation among populations. The significant isolation by distance allowed us to reject the infinite island model of migration. 4 Gene flow parameters were higher in the upper, mountainous part than in the alluvial plain downstream. There was no accumulation of diversity downstream, indicating migration rates were symmetrical upstream and downstream. This was confirmed by computing individual migration parameters between adjacent populations. 5 The results are discussed with regard to the dispersal mechanisms of seeds and pollen. The discrepancy between potential gene flow and effective gene flow is interpreted as an effect of fragmentation, due to the alteration of the natural dynamics of the riparian ecosystem rather than to physical barriers. [ABSTRACT FROM AUTHOR]

Partial resistance of Populus nigra L. to three races of the foliar rust Melampsora larici-populina Kleb. was studied in a field trial and in laboratory tests, using a collection of P. nigra originating from different places throughout France. No total resistance was found. The partial resistance was split into epidemiological components, which proved to be under genetic control. Various patterns of association of epidemiological components values were found. Principal components analysis revealed their relationships. Only 24% of the variance of the field susceptibility could be explained by the variation of the epidemiological components of susceptibility. This variable was significantly correlated with susceptibility to the most ancient and widespread race of the pathogen, and with the variables related to the size of the lesions of the different races. Analysis of variance showed significant differences in susceptibility between regions and between stands within one region. Up to 20% of variation was between regions, and up to 22% between stands, so that these genetic factors appeared to be more differentiated than the neutral diversity (up to 3.5% Legionnet & Lefèvre, 1996). However, no clear pattern of geographical distribution of diversity was detected. [ABSTRACT FROM AUTHOR]

Eight polymorphic isozyme loci were used to describe the level and organization of the genetic diversity in Populus nigra L. over 111 sites in France and among 60 additional individuals from throughout the species range. Best descriptors and estimators of gene diversity and differentiation were sought. Overall diversity level was found to be comparable to other widespread species. Differentiation at three geographical scales was low, although local allele fixations indicated limitation of gene flow. Inferences were made on the effect of the natural history of the species on the organization of neutral diversity, and recommendations for collection of genetic resources were derived. Keywords: genetic diversity; genetic resources; isozymes; population structure; Populus nigra. [ABSTRACT FROM AUTHOR]

• We investigated the fine-scale spatial genetic structure (at a metre scale) in three cohorts in a natural population of Populus nigra and assessed the links with the intra-specific interactions in early-life stages. • We found a significant fine-scale spatial genetic structure in patches of young P. nigra but not in older cohorts. This indicates a shift in intra-specific interactions from positive to negative with time. • Facilitation processes are enhanced by the presence of significantly more clones of P.nigra in the edges of recruitment bands and in more disturbed portions of the alluvial bars. Competition occurs later through self-thinning. • Studying FSGS for different age cohorts is thus a valuable tool to study the intra-specific interactions and how they change through different life stages. Spatial genetic structure (SGS) studies contribute to our understanding of gene flow and species dispersal. Only a few studies have linked the spatio-temporal pattern of SGS and intra-specific interactions. Black poplar (Populus nigra L.) is a threatened pioneer riparian tree species along many rivers across Europe. We studied its SGS in cohorts of varying ages at a fine-scale (i.e. at distances including the zone of influence between individuals) to better understand local patterns of dispersal and intra-specific interactions during early life-stages. We genotyped 349 P. nigra individuals in three gravel bars along a 1.6 km reach of the Allier River in the Réserve Naturelle du Val d'Allier, central France. We found high genetic diversity values (H e = 0.860) thus identifying this site as important for the conservation of P. nigra genetic diversity. We also found significantly more clones in locations exposed to the river flow compared with less exposed locations. These clones offer mechanical protection and habitat improvement (through facilitation) to individuals in close proximity. A significant fine-scale SGS was observed in the youngest cohort at the gravel bar scale while none was observed in the older cohorts or at the river reach scale. This pattern was confirmed by a reanalysis of published data on the Garonne River. A shift in intra-specific interactions, from facilitation to competition through self-thinning, could explain the loss of SGS in older cohorts. This highlights the importance of shifts in intraspecific interactions through life and their consequence on population genetic structure. [ABSTRACT FROM AUTHOR]

Within riparian corridors, vegetation establishment is controlled by hydrogeomorphology which in turn is modulated by plant species acting as ecosystem engineers. The potential of different plant species to affect geomorphology according to their exposure to different level of mechanical stress was poorly considered. The comprehension of the variation of morphological and biomechanical response trait attributes of plants is crucial to better understand how pioneer riparian plants succeed in establishing on alluvial bars and affect fluvial geomorphology. We investigated the response of vegetation to hydrogeomorphic constraints and its engineering effect at two spatial scales within the dynamic Allier River, France. At the broader reach scale, we studied on sixteen alluvial bars the aptitude of three dominant pioneer riparian Salicaceae species (Populus nigra L., Salix purpurea L. and Salix alba L.) to establish and to act as ecosystem engineers by trapping fine sediment. At the finer bar scale, we quantified the relation between response trait attributes of young (1-2yrs) P. nigra plants and their exposure to three different levels of mechanical stress. Our results show (i) that the capacity of riparian plants to establish and act as ecosystem engineers depends of species ecology, the age of individuals and on their locations and (ii) that P. nigra plants develop different response traits depending on their exposure to mechanical stress. From a functional trait perspective, our results suggest that interspecific functional trait diversity and intraspecific plasticity of riparian engineer species plays an important role in vegetation resilience and resistance. Trait diversity and plasticity also control the overall capacity of plants to enhance fluvial landform construction at the scale of the alluvial bar. Keywords: riparian Salicaceae species; fluvial landforms; ecosystem engineer; response and effect plant traits; multi-scale study [ABSTRACT FROM AUTHOR]