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Linking sediment trapping efficiency with morphological traits of Salix tiller barriers on marly gully floors under ecological rehabilitation

Dans le document en fr (Page 123-126)

Amandine Erktan, Freddy Rey

Article publié dans Ecological Engineering

Abstract

Soil erosion is considered to be a major threat for soil quality worldwide. Overgrazing and deforestation are the main causes of a recent increase in soil erosion. When devoid of vegetation, marly terrains are particularly prone to severe hydric erosion with gully formation. It is commonly acknowledged that the presence of vegetation interacts with the sediment yield, especially in the gully floor. In the Southern Alps, there is a promising ecological engineering approach used to decrease the sediment yield at marly gullies, consisting in implanting bioengineering works to trap sediment in gully floors.

Here, vegetation barriers are made of a line of Salix purpurea cuttings (non-native species but widely used in bioengineering), arranged perpendicularly to the flow, on a dead-wood sill, resprouting and forming tillers. Our objective was to better understand how morphological traits of Salix tiller barriers can explain their sediment trapping efficiency. The history of sediment trapping has been recorded on a sample of 77 Salix barriers since their implantation, from 2002 to 2008. Then, focusing on still visible and homogeneous segments of tiller barriers only (49 segments), the basal diameter, basal stem density and basal shoot branching of the tillers, as well as the sediment deposit heights were measured in spring 2010. The results showed that Salix tillers barriers are able to trap sediment in marly gully floors under ecological rehabilitation. The sediment trapping records showed that there is a structural efficient threshold beyond which Salix barriers can trap sediment. We suggest that this threshold is explained by the fact that the basal stem diameter also reaches a threshold value of about 6 mm, usually during the 3rd year after Salix cuttings are implanted. Beyond this threshold, the results suggest that the main tiller morphological trait which explains the efficiency of sediment-trapping barriers is the basal shoot branching. The present study is innovative in the sense that the sediment flow conditions in gully beds are much stronger than those

found in previous studies. The results found here provide practitioners with useful data for designing gullies restoration with Salix cuttings.

Keywords

Salix barriers, Gully erosion, Plant morphological traits, Sediment trapping, Efficiency threshold, Eroded terrain restoration

Abbreviations

hbarrier : Sediment deposit height upstream whole Salix barrier (cuttings and tillers) hsegment : Sediment deposit height upstream Salix tiller barrier segments

Introduction

Soil erosion shows numerous negative environmental and socio-economic consequences worldwide (Pimentel, 1993). The presence of vegetation is well known to have a strong influence on soil erosion and sedimentation from plot to watershed scale (Branson 1975; Morgan and Rickson, 1995; Andreu, et al. 1998; Hairsine et al., 1999; Gurtz et al., 1999; Casermeiro et al., 2004; Bautista et al., 2007). In particular, the use of vegetation to trap sediment has been widely developed at various scales (vegetated filter strips or grass barriers, for example) (Magette et al., 1989; Croke et al., 1999; Abu-Zreig 2001). To assess the effectiveness of these management practices, a great deal of previous work has been devoted to studying the performance of plants to trap sediment. For example, the ability of plants to trap sediment and nutrients has been investigated at landscape scale (Anderson and Potts 1987; Croke et al., 1999; Descheemaeker et al., 2006) and at agricultural plot scale through the study of grass filter strips (Magette et al., 1989;

Barling and Moore 1994; VanDijk et al., 1996; Lalonde 1998; Abu-Zreig 2001; Yuan et al., 2009; Noij et al., 2012). A fair amount of literature also focuses on sediment trapping in the rehabilitation field, in a riparian context (Sand-Jensen, 1998; Liébault et al., 2005;

Gurnell et al., 2006; Heppell et al., 2009), or in the context of mediterranean eroded hillslope restoration (Bochet et al., 2000; De Baets et al., 2009; Molina et al., 2009;

Burylo et al., 2012).

In this field, a growing amount of literature concentrates on the relationships between the morphological traits of vegetative barriers and their potential to trap sediment. This interest in investigation of these links is in line with the development of the trait-based approach in functional ecology, based on the identification of plant traits to explain and predict ecosystem functions without referring to the species organization level (Messier et al., 2010).

Without wishing to produce a comprehensive literature review, a few relevant studies linking plant traits and the effectiveness of sediment retention are presented below.

Several plant traits have been shown to influence sediment retention, such as the intercepted bio-volume (vegetation coverage (m²) × mean plant height (m)) in a riparian context, as described by Corenblit et al. (2009), and the sediment obstruction potential (SOP) defined by De Baets et al. (2009) by SOP = Ʃ (basal stem diameter / barrier length), while working on gullies. Focusing on isolated plants in the context of Mediterranean slope rehabilitation, the canopy cover of perennial bushes (Quinton et al., 1997), plant roundness (length of the standing canopy developed perpendicular to the slope divided by the length of the canopy along the slope) and stem density (Bochet et al., 2000) were also shown to influence sediment retention. In the context of restored alpine ski-slopes, Isselin-Nondedeu and Bedecarrats (2007) showed that the same traits as those pinpointed by Bochet et al. (2000) were key for sediment retention by individual plants. Finally, canopy density, foliage surface, canopy shape (Burylo et al., 2012) and stem density (Erktan et al., 2012) were identified as key functional traits to explain the sediment trapping efficiency of individual plants and tiller barriers respectively, in flume experiments.

Although a large amount of literature is available on this topic of sediment retention by plants, only a few studies concentrated on this question applied to gully restoration, even though it is known that vegetation in a gully bed can shut down gully activity through its sediment trapping activity (Rey, 2003). In general, research on soil erosion has indeed been highly active for a long time, but gully erosion seems to have been neglected until recently despite its significant share in global erosion activity (Poesen et al., 2003). In the French Southern Alps, the brittleness of the Jurassic black marl terrains renders them prone to intense gully erosion leading to erosion rates of up to 100 m3 ha-1

year-1 at the exit of a catchment devoid of vegetation (Mathys et al., 2003). In this part of France, a minimal promising approach used to decrease sediment yield at the gully exit consists of implanted bioengineering works leading to the development of vegetation barriers which can trap sediment on gully floors (Rey, 2009). Here, vegetation barriers are made of a line of Salix purpurea cuttings, arranged perpendicularly to the flow on a dead-wood sill, resprouting and forming tillers. S. cuttings, widely used by land managers in a riparian restoration context, are chosen here for their potential to withstand harsh climatic and hydraulic constraints and for their resprouting potential, even though they are not native. For now, no alternative to Salix purpurea has been found in terms of the cuttings’ ability to resprout. After ten years of experimentation, this non-native plant does not show any risk of plant invasion or introduction of pathogens. Moreover, sediment trapping by these cuttings has been demonstrated from the first one to two years onwards (Rey and Burylo, in press). However, there was a need to better assess the sustainability of this process, as no study has so far investigated the potential of tillers, resprouted from these cuttings, to trap sediment.

Consequently, we first intended to improve understanding of the mechanism of sediment trapping by whole Salix barriers (cuttings and tillers) in the context of gully rehabilitation. We then investigated the links between the morphology of Salix tiller barrier segments and sediment trapping efficiency on gully floors. The results found here provide practitioners with useful data for designing gullies and also potentially river restoration with Salix cuttings.

Dans le document en fr (Page 123-126)