Simon Dufour

Riparian vegetation research in Mediterranean-climate regions: common patterns, ecological processes, and considerations for management

Riparian corridors in Mediterranean-climate regions (med-regions) are resource-rich habitats within water-limited, larger landscapes. However, little is known about how their plant communities compare functionally and compositionally across med-regions. In recent decades, research on these ecosystems has expanded in both geographic scope and disciplinary depth. We reviewed 286 riparian-vegetation studies across the five med-regions, and identified common themes, including: (1) high levels of plant biodiversity, structural complexity, and cross-region species introductions; (2) strong physical controls on plant demographics and community structure; and (3) intensive human impacts. European and Californian ecosystems were the most represented among the studies reviewed, but Australia, South Africa, and Chile had the greatest proportional increases in articles published since 2000. All med-regions support distinct riparian flora, although many genera have invaded across regions. Plant species in all regions are adapted to multiple abiotic stressors, including dynamic flooding and sediment regimes, seasonal water shortage, and fire. The most severe human impacts are from land-use conversion to agriculture, streamflow regulation, nutrient enrichment, and climate change. Current knowledge gaps and subjects for future research include cumulative impacts to small, ephemeral streams and large, regulated rivers, as well as understudied ecosystems in North Africa, the western Mediterranean basin, and Chile.

GEOMORPHOLOGICAL CONTROLS OF FRAXINUS EXCELSIOR GROWTH AND REGENERATION IN FLOODPLAIN FORESTS

Geomorphological changes can alter river hydrology and thus influence floodplain forest growth and regeneration. In this paper we quantify the effect of changes in channel elevation at the scale of four decades on hydrological conditions, overbank sediment deposits, water availability, and their impacts on common ash (Fraxinus excelsior) growth and recruitment in floodplain forests of the Ain River, France. Ash is a drought-sensitive species, and its regeneration is influenced by flood disturbance. We compared ash growth between 20 sample plots located in two contrasting geomorphological contexts using dendrochronological measurements: 11 along reaches degrading over the last 80 years and nine along stable or slightly aggrading reaches. In each context, half of the plots were located near the channel, and the remainder were within the floodplain. Ash regeneration was also quantified in each plot. This work showed that sites which undergo channel degradation are associated with less frequent overbank flows. Whereas there is no trend in either the climatic or stream flow timeseries over the last four decades, the growth patterns of ashes of these sites are significantly different as opposed to plots located in stable or aggraded reaches (bell shaped curve vs. constant increase in tree ring width). The variance through time due to climatic or stream flow control is masked by variance due to topographical changes. Currently plots located in degraded reaches show a lower ash growth (mean of 0.20 cm/yr vs. 0.34 cm/yr over the last decade) and less frequent ash seedling establishment. We have identified a hydrologic threshold for ash growth response when stand elevation values reach 1.5-2 m above the mean annual flow stage. Our results confirm that, at a many-decade timescale, changes in channel elevation must be considered in addition to other hydrological controls of hardwood species growth and recruitment.

Spatial and temporal variability in sedimentation rates associated with cutoff channel infill deposits: Ain River, France

Floodplain development is associated with lateral accretion along stable channel geometry. Along shifting rivers, the floodplain sedimentation is more complex because of changes in channel position but also cutoff channel presence, which exhibit specific overflow patterns. In this contribution, the spatial and temporal variability of sedimentation rates in cutoff channel infill deposits is related to channel changes of a shifting gravel bed river (Ain River, France). The sedimentation rates estimated from dendrogeomorphic analysis are compared between and within 14 cutoff channel infills. Detailed analyses along a single channel infill are performed to assess changes in the sedimentation rates through time by analyzing activity profiles of the fallout radionuclides 137Cs and unsupported 210Pb. Sedimentation rates are also compared within the channel infills with rates in other plots located in the adjacent floodplain. Sedimentation rates range between 0.65 and 2.4 cm a1 over a period of 10 to 40 years. The data provide additional information on the role of distance from the bank, overbank flow frequency, and channel geometry in controlling the sedimentation rate. Channel infills, lower than adjacent floodplains, exhibit higher sedimentation rates and convey overbank sediment farther away within the floodplain. Additionally, channel degradation, aggradation, and bank erosion, which reduce or increase the distance between the main channel and the cutoff channel aquatic zone, affect local overbank flow magnitude and frequency and therefore sedimentation rates, thereby creating a complex mosaic of sedimentation zones within the floodplain and along the cutoff channel infills. Last, the dendrogeomorphic and 137Cs approaches are cross validated for estimating the sedimentation rate within a channel infill

Regional variability of aquatic pattern in braided reaches (example of the French Rhône basin)

We analysed, at a regional scale, the braiding pattern and the structure of the water channel habitats of 53 braided reaches located in the Rhône hydrographical district (France), in relation to three control factors: (i) spatial location along the network (i.e. slope, altitude), (ii) hydrological conditions and (iii) morphological conditions. This research is based on aerial orthophotos belonging to the French National Geographic Institute (IGN) taken between 2002 and 2006. We defined a regional typology of braided reaches based on the distribution of the variables listed above, and identified five hydro-geographical types of braided reaches. Following this regional classification, we compared the pattern of braided reaches by testing several parameters, which describe the physical characters of water channel habitats (such as braided index, channel sinuosity, or aquatic habitat diversity). We found that discharge appears to be less relevant than sediment supply or groundwater level in structuring the braided geometry and the channel habitat pattern in natural braided rivers at a regional scale. We discussed the importance of local factors, such as the position of the groundwater table and the width of the active channel, for explaining the intensity of the braided pattern. There are numerous cases where the discharge is very low but the braided index high. These findings support management and conservation recommendations of braided rivers for implementing the European Water Framework Directive (2000).

Restoring Floodplain Forests

Floodplain forests are unique ecosystems that are located alongside rivers and streams. These systems derive their characteristics from periodic inundations. The extent, structure, and diversity of floodplain forests have been strongly modified by human pressures acting at the catchment, reach, and local scales. Even though many floodplains in Europe are characterised by natural forestation that began after the Second World War due to widespread changes in land-use practices, most European floodplain forests have disappeared. Since the 1970s, the scientific community and land managers have recognised the ecological, economic, and social values of floodplain forest. These forests are very valuable because of their high potential in terms of wood production, protection of water quality, flood control, recreation, and improvement of the landscape. In addition, they are natural areas with a high biological diversity and ecological specificity due to the influence of water on habitat conditions. Riparian areas are important for fish, amphibians, and mammals (e.g., beavers). Additionally, the forests provide breeding habitat for birds, and are navigational aids and stopover sites for migrating species (e.g., the songbirds in the North Platte River). The need to preserve and restore them is now widely recognised. Forest ecosystems that are under hydrological control evolved their original ecological processes in response to their proximity to and the dynamics of the river. Thus, the periodic water supply is a key process characterising floodplain forests. The land–water interfaces are important areas for biological exchanges, water supply and content, soil moisture, organic matter evolution, seed dispersal, and nutrient cycling. Floodplain forests are part of 44 Restoring Floodplain Forests

Green and blue belt networks: Mapping connectivity areas at a regional scale using medium and high spatial resolution satellite images

The distribution of ecosystem sites, their connectivity and their evolution are key links between ecology and society, as these landscape patterns determine the regional sustainability for biodiversity and are subject to human interventions. While little attention has been paid to procedures for mapping connectivity among numerous “natural” landscape elements over large areas, the principal objective of this study was to map ecological corridors at a regional scale using remotely sensed data. To achieve this goal, we developed an approach including three stages: first, permanent “natural” landscape elements were identified from Landsat TM images; second, permanent grassland connected to these elements was extracted from a multitemporal series of MODIS images and aggregated to them; third, connectivity was depicted by least-cost modeling. The regional connectivity map can be analyzed at a regional scale to highlight large corridors, but also at a local scale to determine areas for conservation intervention.

Mapping Ecosystem Services: From Biophysical Processes to (Mis)Uses

Critical Physical Geography has two fundamental ambitions: to link deeply biophysical processes and social relationships and to increase reflexivity in scientific practice. Thus, it provides a powerful framework to analyze how general concepts such as ecosystem services are used in natural science and environmental management. Over the last decade, this concept has become a crucial component for justifying and implementing environmental policies. In this chapter, we aim to demonstrate the relevance of a Critical Physical Geography for improving identification of potentials and limitations of the concept of ecosystem services. We demonstrate that a thorough understanding of the biophysical processes that generate services and indicator choices has a strong influence on the ability to quantify and to map them.

Mapping ecosystem services at the regional scale: the validity of an upscaling approach

ABSTRACT Mapping ecosystem services (ES) over large scales is important for environmental monitoring but is often prohibitively expensive and difficult. We test a hybrid, low-cost method of mapping ES indicators over large scales in Pará State, Brazil. Four ES indicators (vegetation carbon stocks, biodiversity index, soil chemical quality index and rates of water infiltration into soil) were measured in the field and then summarized spatially for regional land-cover classes derived from satellite imagery. The regionally mapped ES values correlated strongly with independent and local measures of ES. For example, regional estimates of the vegetation carbon stocks are strongly correlated with actual measures derived from field samples and validation data (significant anova test – p-value = 4.51e−9) and differed on average by only 20 Mg/ha from the field data. Our spatially-nested approach provides reliable and accurate maps of ES at both local and regional scales. Local maps account for the specificities of an area while regional maps provide an accurate generalization of an ES’ state. Such up-scaling methods infuse large-scale ES maps with localized data and enable the estimation of uncertainty of at regional scales. Our approach is first step towards the spatial characterization of ES at large and potentially global scales.

Comparison of leaf-on and leaf-off ALS data for mapping riparian tree species

Forest species composition is a fundamental indicator of forest study and management. However, describing forest species composition at large scales and of highly diverse populations remains an issue for which remote sensing can provide significant contribution, in particular, Airborne Laser Scanning (ALS) data. Riparian corridors are good examples of highly valuable ecosystems, with high species richness and large surface areas that can be time consuming and expensive to monitor with in situ measurements. Remote sensing could be useful to study them, but few studies have focused on monitoring riparian tree species using ALS data. This study aimed to determine which metrics derived from ALS data are best suited to identify and map riparian tree species. We acquired very high density leaf-on and leaf-off ALS data along the Sélune River (France). In addition, we inventoried eight main riparian deciduous tree species along the study site. After manual segmentation of the inventoried trees, we extracted 68 morphological and structural metrics from both leaf-on and leaf-off ALS point clouds. Some of these metrics were then selected using Sequential Forward Selection (SFS) algorithm. Support Vector Machine (SVM) classification results showed good accuracy with 7 metrics (0.77). Both leaf-on and leafoff metrics were kept as important metrics for distinguishing tree species. Results demonstrate the ability of 3D information derived from high density ALS data to identify riparian tree species using external and internal structural metrics. They also highlight the complementarity of leaf-on and leaf-off Lidar data for distinguishing riparian tree species.

Forêts riveraines des cours d'eau et ripisylves : spécificités, fonctions et gestion

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