Soil erosion and flooding degrade terrestrial ecosystems and affect vegetation development (Garcia-Ruiz et al., 2015). Considerable research has been conducted on the functioning and protection of soils from degradation, bringing the issues of soil conservation and the importance of soil ecosystem services to the forefront (Keesstra et al., 2012; Brevik et al., 2015). The protective role of vegetation against erosion processes is well known, as stated by Stokes et al. (2014). Other studies also showed the effectiveness of mulch and litter in reducing soil erodibility in a context of agricultural production (Keesstra et al., 2016; Prosdocimi et al., 2016). Given that a plant's functional traits determine the suitability of species to limit soil erosion (Stokes et al., 2009; Burylo et al., 2014), the choice of plant species in bioengineering structures is essential. Numerous methods commonly used for revegetation of degraded lands use living materials, which are able to resist hydrological and erosive forces on a sloping site. Three sources of propagules can be highlighted: (i) seeds that are possibly present on topsoil (soil seed bank) or brought by different techniques (seeding, hydroseeding), (ii) plants and (iii) cuttings. However, eroded areas often do not offer satisfactory conditions to support natural colonization or artificial revegetation due to soil loss as well as low water and nutrient availability. Therefore, the success of bioengineering works and revegetation operations can depend on previous stages of soil fertility improvement (Donn et al., 2014; Young et al., 2015).

One way to improve soil conditions is to apply an organic amendment to the soil surface. Numerous studies have assessed the value of organic amendments on vegetation establishment and soil fertility, as for contaminated areas (Mahmoud and Abd El-Kader, 2015), post-mine soils (Eldridge et al., 2012; Benigno et al., 2013), semi-arid conditions (Hueso-González et al., 2014; Jiménez et al., 2013; Tejada and Benítez, 2014) and eroded soils (Ojeda et al., 2003; Cohen-Fernández and Naeth, 2013; Prats et al., 2013; Hosseini Bai et al., 2014; Donn et al., 2014; Hueso-González et al., 2015). Others have also shown the direct effect of organic mulch in reducing surface runoff (Moreno-Ramón et al., 2014; Cerda et al., 2015; Sadeghi et al., 2015). These organic amendments can have different forms: manure, green waste compost, straw, wood chips, etc. They can be incorporated into the soil or surface applied. The type of amendment and the application requirements depend on site conditions (access, topography, soil). The effects, in particular when surface applied (mulch), mainly concern the soil water availability (van Donk et al., 2012). This practice conserves soil water by rainfall interception and reduction of soil evaporation. It also reduces surface runoff and moderates soil temperature (Scopel et al., 2004).

Wood chips of small branches (Lemieux, 1988) are a form of organic amendment. Their use is developing, in particular on certain crops, even if validation provided by the scientific literature is incomplete (Barthès et al., 2010). Likewise, considering natural processes, the role played by woody litter on humus formation in forest ecosystems has not been sufficiently studied (Berg and McClaugherty, 2008). These latter authors admitted furthermore that white-rot fungi play an important role in woody material decomposition processes. The capacity of wood chips to improve the nutrient status of the soil depends on a quick and efficient stage of decomposition of organic matter and therefore on white-rot fungi presence and action. Therefore, Lemieux (1988) advised using low-diameter branches (less than 7 cm), which have a low carbon-to-nitrogen ratio and limit nitrogen removal used for wood degradation. Moreover, considering lignin resistance to degradation, it is recommended to favor deciduous trees rather than coniferous trees (Stevanovic, 2007).

In marly catchments of the French Southern Alps, extensive areas are subjected to intense soil water erosion, resulting in climatic events such as torrential floods specific to this mountainous Mediterranean climate: wetting–drying cycles, frost in winter, high-intensity rainfalls in summer and consequently high sediment yields transported by floods at the exit of catchments (Yamakoshi et al., 2009). Experimental knowledge for ecological restoration of marly eroded lands is available through the voluminous research conducted in these areas (Rey, 2009; Erktan et al., 2016; Burylo et al., 2014). These studies have researched gully beds, where vegetation can counter erosive forces and trap sediment (Rey and Burylo, 2014; Rey and Labonne, 2015). Therefore it appeared that vegetation could also be profitable in gully slopes where soil fertility is very low (soil loss, low soil moisture and lack of an organic layer) and where natural vegetation is nearly nonexistent. The slopes are covered by a hard regolith layer, and vegetation cannot be established with the techniques currently used in gully beds, especially willow cuttings buried in soil. Planting and seedling methods for installing vegetation on slopes are therefore required. Such methods have already been tested on several other eroded lands (Reubens et al., 2009; Bochet et al., 2010; Fernández et al., 2012; Commander et al., 2013; Lee et al., 2013), but these studies did not apply to our climatic, topographic and pedological conditions, and the studied plant species are not appropriate or currently available. Therefore, the main issue for practitioners is to find the adequate solution to develop vegetation cover on gully slopes. The hydrological and erosive forces are generally less marked than on gully floors but occasionally they can be strong. As on slopes and floors, the vegetation must allow both withstanding hydrological forces and trapping sediment as quickly as possible. The low fertility of the soil, which is in the form of a regolith layer composed of coarse particles embedded in a matrix of finer material, is evident on gully slopes: export of nutrients, very low water and nutrient availability, with no accumulation or mineralization of organic matter. For this reason, soil fertility improvement appears necessary.

Here, we focused on crushed wood chips of small branches, considering that due to its size and form, this material may be better able to remain in place on slopes, compared to other forms of common amendment, finer and easily exported by runoff. This study aimed to test the effects of wood chip amendments on the revegetation performance of plant species on eroded marly terrains in a Mediterranean mountainous climate in the French Southern Alps. We hypothesized that in a context of water erosion and drought conditions of marly eroded land, the wood chip amendment, if able to remain in place, could facilitate plant establishment. The first point was subjected to empirical observation. The second point was based on precise measurements on plants (counting, height, etc.). This study was based on two experiments, representative of two methods for installing vegetation: planting of seedlings and seeding (cuttings were dismissed owing to compactness and scarcely penetrable soil). They were carried out over three growing seasons (2012–2014). Different plant species were tested: 13 species – 6 were used in the two experiments – among those currently used and available in local tree nurseries, and a priori adapted to the type of environment under study.

Although based on empirical observations, we noted that wood chips remained in place during the observation period. On the basis of the meta-analysis of García-Ruiz et al. (2015), who compared numerous studies on erosion rates around the world, we can reasonably consider our experimental plots under very steep slope gradients and moderate mean annual precipitation. Moreover, local and particular events with very intense rainfall are possibly frequent in summer and were observed during the 3 years. So, according to the climatic and topographic factors observed during the study period, wood chips seem able to remain in place under severe erosive constraints.

Wood chips offered the best results with seeds as well as young plants. The benefit was (i) evident as regards seedling emergence and plant survival and (ii) less efficient as regards plant growth. For the first point, these findings are in agreement with the reported performance of various mulches preserving soil water and consequently promoting seedling establishment (Woods et al., 2012; Benigno et al., 2013; Hosseini Bai et al., 2014).

The positive effect on seedling emergence was obvious, although it did not last over 2 years. For four species, it only occurred with wood chip amendment: Buxus sempervirens, Acer campestre, Juniperus communis and Lavandula officinalis. For the five others, the positive effect of wood chips was often significant. We must also consider the possibility of the soil seed bank effect on seedling emergence. In each plot, we did not identify other species than the one sown. We cannot dismiss natural emergence from the soil seed bank (instead of sowing); considering the very low spontaneous emergence rate in the vicinity of sown plots, this possibility appeared unlikely. These results are consistent with previous research on seedling emergence (Eldridge et al., 2012; Benigno et al., 2013) where soil treatment and tested plant species were different. Cerdà and García-Fayos (2002) and Bochet (2015) noted the influence of the size and shape of seeds in their removal by water erosion. Our results are partly consistent, considering that the heaviest seeds (especially Acer campestre) were completely removed on untreated plots, but we did not have sufficient measurements on the seeds of the tested species to continue analysis of these criteria. The same holds true for the effect of runoff and sediment yield on seed removal studied by Han et al. (2011). It is obvious that wood chip amendment improved seed emergence capacity. As we cannot determine whether the decisive role is to avoid removal of seeds by water or to maintain soil moisture conditions, both cumulative effects must be considered. Other studies investigated the influence of organic amendment on the moisture of eroded soil (in particular Ojeda et al., 2003; Hueso-González et al., 2015), but the context of soils, types of amendment and slope gradients were significantly different, and the comparison is not relevant. According to Bochet et al. (2007), soil water availability after rainfalls occurring during the germination period played a major role in seed germination. These authors and Crosaz (1995) also suggested that the species' ability to germinate under drought conditions could indicate a species' potential for success under semiarid conditions. In this way, considering the germinations during the strong drought period in 2012, we can assume some species' potentials, especially with Hippophae rhamnoïdes, Dorycnium pentaphyllum, Anthyllis vulneraria, Ononis natrix and finally Buxus sempervirens to a lesser extent, even if this ability did not last more than two seasons.

Regardless of soil treatment (wood chips or no wood chips), with an overall survival rate close to 50 % (70 % for some species) after three growing seasons, the planting of seedlings appeared to be an efficient revegetation technique for gully slopes. In comparison, the advantage of seeding was not proved. Reserves in ligneous tissues seem to ensure the best chances against dry summer conditions during the critical establishment period. Most plant mortalities occurred during the first summer, which was the driest of the 3 years, and occurred very little during the following summer droughts. Another approach could be to associate seeding with drilling holes, as was experimented by Lee et al. (2013).

We also noted that differences in performance were generally greatest between species than between treatments (wood chips or control). The choice of species that were well adapted to drought conditions is essential and this study has provided additional data on that point. Several species showed the capacity to resist and remain in our experimental conditions (slopes, drought, flow erosion): Robinia pseudo-acacia, Pinus nigra and Buxus sempervirens hold both survival and growth capacities; Hippophae rhamnoïdes, Acer campestre and Quercus pubescens maintained a satisfactory survival rate. To meet the objective of revegetation, these species must also show performance for erosion control. Thanks to the synthesis work of Burylo et al. (2014), we have information on the response and effect traits of these species related to erosion dynamics in the same ecological conditions. The combined analysis of the two information sources allows us to improve the choice of appropriate plant species. Lastly, for the three species not tested in the young plant experiment (Dorycnium pentaphyllum, Anthyllis vulneraria, Ononis natrix), the field capacity on gully slopes was not known and will require additional experimentation.

Numerous studies have emphasized the value of organic amendment for revegetation and erosion control (see the introduction). The novelty value of our results concerns the positive effect of wood chips for revegetation in a steep slope context. Bochet and García-Fayos (2004) underscored “the difficulty of revegetating slopes with angles greater than 45∘ (100 %), where the probability of seeds moving downhill is high.” Other applications have to be examined in larger contexts of restoration and bioengineering works. The question of wood chip supply must be also considered. Considering the requirements for wood chips in revegetation works (deciduous trees, low-diameter branches), the supply of such material can be difficult. The commercial competition with other wood uses (the wood-energy industry in particular) and the low production margins that cannot support transport costs add further to these difficulties. As far as possible, solutions must be found in local production. This study has been carried out on small surface areas and with small quantities of mulch. Therefore it does not allow us to make a reliable analysis of the costs. In the experimental site, the supply of the wood chips cost between EUR 30 and 50 m-3 (between EUR 1.5 and 2.5 m-2). With large quantities, it is obvious that those prices must significantly decrease. Considering the total cost of the bioengineering project, the additional cost of wood chips (supply and application) represented a slight extent: 8–10 % for the plant experiment. As the seeds were free of charge, this rate cannot be estimated for the seed experiment.

Finally, we must also investigate the contribution of these results for the bioengineering works in the context of eroded marly areas in the Southern French Alps. We already knew the possibilities and the limits of revegetation on gully beds (Rey and Burylo, 2014). The main objectives of our two experiments were to assess revegetation techniques on gully slopes. The outcomes allow us to establish the requirements in those conditions such as planting seedlings of certain plant species (especially Pinus nigra, Buxus sempervirens and Robinia pseudo-acacia) that can be improved by a wood chip layer, in addition to willow cuttings and wooden sills on gully beds. Such operations will require new experiments to test the two bioengineering techniques on slopes and beds at the same time. Revegetation by seeding cannot be recommended on the basis of our results, even though the failure can be explained by very dry climatic conditions during the first year. Further experiments should be considered, in particular to determine whether seeding in autumn can limit the impact of a possible drought during the first growing season.

In the two experiments, the wood chip supply was distributed on steep slopes and in a context of water erosion. We showed that wood chips succeeded in remaining in place despite these erosive pressures and, at least during the first 2 years, have a positive effect on revegetation, mainly on plant survival and seed emergence capacities. This result is clearly confirmed in the seed experiment where, for several species, germination occurred only in the presence of wood chips during the second growing season, after intense rainfall events. The two experiments allowed us to test two methods of revegetation: planting of seedlings and seeding. We showed that revegetation on gully slopes is possible with the first method. The supply of wood chips can improve the establishment of young plants, limiting the mortality rate. The results with the seeding experiment were not as convincing. However, even if this study was limited in time, we noted seedling emergence capacity in association with wood chip amendment. All the possible applications of wood chips were not yet known, but we can consider that the gain in terms of plant survival and growth can be possibly decisive in revegetation work contexts, especially when extreme climatic events occur.