SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-7-599-2016Effects of wood chip amendments on the revegetation performance of plant
species on eroded marly terrains in a Mediterranean mountainous climate
(Southern Alps, France)BretonVincentvincent.breton@irstea.frCrosazYvesReyFreddyUniv. Grenoble Alpes, Irstea, UR Ecosystèmes montagnards, BP 76,
38402 Saint-Martin-d'Hères, FranceGéophyte, 64 rue des Ecrins, 38530 Pontcharra, FranceVincent Breton (vincent.breton@irstea.fr)15April20167259961018January201621January201625March201629March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/articles/7/599/2016/se-7-599-2016.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/7/599/2016/se-7-599-2016.pdf
The establishment of plant species can limit soil erosion dynamics
in degraded lands. In marly areas in the Southern French Alps, both harsh
water erosion and drought conditions in summer due to the Mediterranean
mountainous climate prevent the natural implementation and regeneration of
vegetation. Soil fertility improvement is sometimes necessary. With the
purpose of revegetating such areas, we aimed to evaluate the effects of wood
chip amendments on the revegetation performance of different native or
sub-spontaneous plant species. We conducted two experiments on steep slopes
over three growing seasons (2012–2014). The first consisted of planting
seedlings (10 species), and the second consisted of seeding (nine species
including six used in the first experiment). First we noted that wood chips
were able to remain in place even in steep slope conditions. The planting of
seedlings showed both an impact of wood chip amendment and differences
between species. A positive effect of wood chips was shown with overall
improvement of plant survival (increasing by 11 % on average, by up to
50 % for some species). In the seeding experiment, no plants survived
after three growing seasons. However, intermediate results for the first
and second years showed a positive effect of wood chips on seedling
emergence: seeds of four species only sprouted on wood chips, and for the
five other species the average emergence rate increased by 50 %.
Introduction
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.
Location of the experimental site.
MethodsSite description
The site is located in the French Southern Alps (44∘9′ N,
6∘21′ E) near Digne in Alpes-de-Haute-Provence (Fig. 1),
in a badland area composed of gullies. The climate is mountainous and
sub-Mediterranean, showing summer droughts with often intense rainstorms.
Over the three growing seasons of the experiments (2012–2014), the
description of climatic parameters was based on data from the
Sévigné meteorological station, 800–1000 m far from both
experimental sites. During this period, the mean annual total rainfall was
920 mm and the average annual temperature was 10.2 ∘C. High
climatic variations were observed both on the season scale (20 ∘C
differences in monthly temperatures between winter and summer) and on the
year scale (long drought period in 2012). The 2012 summer drought was
particularly severe: June and July showed a rainfall deficit of 67 and
83 %, respectively, compared to 30-year averages. Even more fluctuating
are the intense rainfall events: rainstorms generally occur randomly and can
be very heavy, especially in summer. Some rainfall intensities can reach 70 mm h-1 lasting 1 h (Rey, 2009). Over the 3-year period, all
the most intense events (more than 50 mm h-1 lasting at least 15 min)
occurred from the last 2 weeks of June to the end of August (once in 2012,
three times in 2013, twice in 2014).
The soil composition depends on topographic conditions and alternates
between a loose regolith layer made of disintegrated black marl fragment on
the slopes and black marl sediment mainly on gully beds. The top layers are
made of coarse marl fragments within a fine silty matrix and present low
carbonate content, from 20 to 35 %, with pH varying from 7.8 to 8.1
(Wijdenes and Ergenzinger, 1998). The slope gradient is relatively steep,
reaching 75 % in most cases on the steepest parts of the gullies.
Spontaneous vegetation is present in a dispersed manner, mainly on the lower
and higher parts of gullies and rarely on the slopes. The dominant tree species
is Pinus nigra, and the shrub layer is mainly composed of Juniperus communis, Hippophae rhamnoïdes and Buxus sempervirens. Even more dispersed
are Ononis fruticosa, Lavandula officinalis and Robinia pseudo acacia.
Plant and seed materials
We conducted two experiments to study two modes of revegetation over three
growing seasons: (1) planting seedlings of 10 plant species, hereafter
designated as “plant experiment',' and (2) seeding of nine plant species
(Table 1), hereafter designated as “seed experiment”. Six species were
used in both experiments – Acer campestre L., Alnus cordata (Loisel.) Duby, Buxus sempervirens L., Hippophae rhamnoïdes L., Juniperus communis L. and
Lavandula officinalis Chaix. Four species were tested only in the first experiment – Quercus pubescens Willd.,
Pinus nigra Arnold, Robinia pseudo acacia L. and Salix caprea L. – and three species only in the second
experiment –
Dorycnium pentaphyllum Scop., Anthyllis vulneraria L. and Ononis natrix L. This set consists of ligneous and semi-ligneous species,
mainly shrubs that commonly grow in eroded marly lands around the two
experimental sites (except Alnus cordata). The choice of the species and the method of
vegetation installation (young plants or seeds) depended on the plant
material availabilities in the local tree nurseries and accorded with
results of previous local studies (mentioned above, especially
Burylo et al., 2014).
List of species tested in the two experiments and seed density (for
species tested in the seed experiment).
Plant species Seed densityTested in one experiment(number/m2)In the plant experimentIn the seed experimentQuercus pubescens Willd. (Que)Dorycnium pentaphyllum Scop. (Dor)4000Pinus nigra Arnold (Pin)Anthyllis vulneraria L. (Ant)8000Robinia pseudo acacia L. (Rob)Ononis natrix L. (Ono)4000Salix caprea L. (Sal)Tested in both experimentsAcer campestre L. (Ace) 120Alnus cordata (Loisel.) Duby (Aln) 580Buxus sempervirens L. (Bux) 120Hippophae rhamnoïdes L. (Hip) 190Juniperus communis L. (Jun) 190Lavandula officinalis Chaix (Lav) 1600
In parentheses: species abbreviations.
Wood chips came from woody wastes of small branches after tree pruning in
local public parks. It was mainly composed of poplars, lindens and plane
trees. In the two cases, the wood chips were mixed and spread on the soil
surface after seedling or planting operations, and formed a homogeneous
5 cm thick layer. The total volume of wood chips used to cover the different
plots was 2 m3 (almost 1 m3 per experiment, corresponding to 500 m3 ha-1). The seed densities depended on the species tested and were
chosen according to the supplier's recommendations (Table 1). Because of the
soil hardness, plant establishment required boring a 5 cm diameter and
17 cm deep hole with a drilling machine. This size corresponded to the size
of the plant containers of all the species tested that were supplied by a
local tree nursery.
Experimental design
For the two experiments, half of the surface area was covered with wood
chips. The plant experiment consisted of five randomized completed blocks
composed of two plots (wood chips and control). Each plot was composed of
four replicates arranged in a row, and each row was composed of 10 single
trees of each species tested (Figs. 2 and 3). Each plot covered 4 m2. The seed experiment consisted of three randomized blocks
divided into nine plots corresponding to the plant species. Each plot was
divided in two 1 m2 half-plots corresponding to the soil
treatment (wood chips or control). Each half-plot was 1 m × 1 m and
divided into four 0.25 m2 sub-plots (Figs. 2 and 3). For
the two experiments the blocks were generally placed in separate gullies.
They were dispersed over an approximately 1 ha surface area, as far as
possible on similar ecological conditions (slope, soil, water erosion).
Experimental design of the two experiments (see species
abbreviations in Table 1).
Pictures of the seed experiment setup (left) and the plant
experiment setup (right).
Observations and measurements
The experiments were carried out over three growing seasons (2012–2014).
Measurements and observations were conducted three times a year in order to
divide the growing seasons: (i) at the end of May, (ii) at the end of July or
the beginning of August and (iii) at the end of September or the beginning of
October. This frequency allowed us to observe the rhythm of plant
development during the growing seasons and to evaluate the effect of certain
climatic parameters.
We quantified the seedling emergence of seeds by counting the number of
seedlings of plant species sown on each sub-plot. The possibility of
emergence of other plant species was also sought. For the plant experiment,
seedlings were considered alive if living tissues in leaves, buds or stems
were observed. We measured the plant height from the ground to the terminal
bud of the tallest stem. The latter parameters were measured three times a
year. The stem basal diameter was measured at the root–shoot junction only
once a year at the end of the growing season. Compared to growth in height,
we considered that growth in diameter was very low and did not need more
than one measurement per year. For each measurement, an empirical
observation of the state of the wood chip plots was made (export, covering,
degradation).
Statistical analysis
Most of the data were non-normal, following different distributions and
requiring nonparametric analysis. It was also necessary to involve random
effects. We fitted generalized linear mixed models (GLMMs; Bolker et al., 2009) that can be performed on both
normal and non-normal data and allowed us to analyze both fixed effects
(soil treatment, species in the two experiments) and random effects (plot).
Seedling emergence data were modeled using a GLMM with a Poisson
distribution. Data on young plant mortality were analyzed with a binomial
distribution. Young plant growth data (diameter, height) were analyzed using
the linear mixed model with a normal distribution. These analyses
allowed us to research the effect of soil treatment that is composed of only
two modalities (wood chips or control) with likelihood ratio tests. The
species effect was also indicated but the differences between species cannot
be known.
Seedling emergence data correspond to the maximum of seedlings out of the
initial number of seeds in each plot, during the first year, the second
year and all 3 years. Young plant survival data correspond to the
number of living trees after the first, second and third growing seasons.
The growth data were transformed to obtain relative data, with a difference
between final measurement and initial measurement divided by initial
measurement, in order to minimize the effect of initial plant size. All data
were analyzed with the R statistical packages lme4.
ResultsWood chip observations
Over the 3 years, we clearly noted that wood chips remained in place and
were hardly ever carried away by surface runoff. During the first few months
(first and second measurements), white-rot fungi presence was observed for
all the plots and seemed to increase the cohesion of wood material. For the
next 2 years, the white-rot fungi were not as visible. However,
the material was partly covered by sediment.
Plant experiment
Considering plant survival, the experiment showed a significant effect of
both species (p= 0.003) and soil amendment (p < 10-3, Table 2a).
The rate of survival quickly decreased with Alnus cordata, Lavandula officinalis and Salix caprea to almost 0 % from the
first growing season. They were removed for further analysis. The best
survival rates were observed with Acer campestre, Quercus pubescens, Pinus nigra, Robinia pseudo acacia and Buxus sempervirens, which exceeded 50 %. The two
other species, Hippophae rhamnoïdes and Juniperus communis, had intermediate results. A substantial positive
effect of wood chip amendment on the survival rate was clearly observed with
these two species: respectively, a 35 and 25 % higher survival rate with
wood chips. Most mortalities occurred during the summer period of the first
year (Figs. 4 and 5), which corresponded to the most severe drought in
comparison with the next 2 years.
Concerning the growth of young plants, significant effects were observed
(i) for soil amendment with diameter and height measurements and (ii) for
species with diameter only (Table 2a). There was a marked difference in the
species ranking between diameter and height results (Fig. 6). For
instance, Pinus nigra showed higher relative growth in height than in diameter. On the
contrary, Quercus pubescens and Hippophae rhamnoïdes showed higher growth in diameter than in height. Differences
in plant architectural forms were obvious, with a shrubby form that showed
no apical dominance (especially Hippophae rhamnoides) and an arborescent form (especially Pinus nigra and
Robinia pseudo acacia). Moreover, some species (Acer campestre, Quercus pubescens) showed some dried apical stems, leading to
problems assessing the initial plant growth and the comparison between
species. Nevertheless, the results were very good for Robinia pseudo acacia and Pinus nigra, which doubled
in height (and doubled in diameter for Robinia pseudo acacia) before the end of the
third year, and for Buxus sempervirens to a lesser extent, which reached a 50 % relative growth
increase at that date. Conversely, we noted very low growth of Quercus pubescens, Juniperus communis and Acer campestre,
which did not show real growth after three growing seasons.
(a) Plant experiment: likelihood ratio tests for fixed effects (soil
treatment in species) based on (1) generalized mixed-effect models (GLMMs)
for plant survival and (2) linear mixed-effect model (LMM) for relative
growth during the first three growing seasons. (b) Seed experiment: likelihood ratio tests for fixed effects (soil
treatment and species) based on GLMMs for
maximum seedling emergence.
p values in bold indicate a significant effect
(< 0.05).
Mortality rate of seedlings (bottom: number of dead trees observed
from the next assessment) and climatic parameters during the three growing
seasons (ombrothermic diagram with monthly rainfall (mm) = 2 × monthly
temperature, ∘C). Black arrows indicate the observation dates.
Mortality rates were calculated from dead plants per stage referred to the
live plants at the previous stage.
Plant experiment: comparison of survival rates between “wood
chips” and “control” modalities, for three growing seasons after planting
(see species abbreviations in Table 1).
Relative increase in height (%, left) and relative increase in
diameter (%, right) after three growing seasons. Bars indicate SE (see
species abbreviations in Table 1).
Effect of wood chips and comparison between species on
seedling emergence for three growing seasons after seeding (see species
abbreviations in Table 1).
Seed experiment
In all sub-plots we did not identify other species than the species sown.
Analyses revealed that the effect of species and soil amendment were
significant in 2012 and 2013 (Table 2b). The date of emergence (Fig. 7)
separates two groups of species: (i) Alnus cordata, Hippophae rhamnoïdes, Dorycnium pentaphyllum, Anthyllis vulneraria and Ononis natrix, which sprouted mainly in the
first year, and (ii) Acer campestre, Buxus sempervirens, Juniperus communis, Lavandula officinalis, which sprouted mainly in the second year. In
both cases, no plant survived longer than one growing season. Three species
sprouted new leaves for only one season (Hippophae rhamnoïdes, Alnus cordata and Lavandula officinalis). The others sprouted shoots
over two seasons (Fig. 7). We observed an obvious positive effect of wood
chip amendment for most species during the first two growing seasons. Some
of them showed emergence capacity only with wood amendment: Alnus cordata, Lavandula officinalis, Buxus sempervirens, Acer campestre and Juniperus communis. The
significance was clear even if it almost disappeared and no plants survived
after the second growing season. Systematically the emergence rate decreased
over the growing season. Whatever the year and the species, a higher rate
always occurred in spring and it decreased during summer and autumn (Fig. 7).
Discussion
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.
Conclusions
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.
Vincent Breton, Freddy Rey and Yves Crosaz conceived the research and the experimental design (Yves Crosaz for the
seeding experiment, Vincent Breton and Freddy Rey for the planting experiment). Vincent Breton (assisted by
Yves Crosaz and Freddy Rey) took the measurements, made observations and analyzed the data.
The manuscript was written and edited by Vincent Breton and Freddy Rey.
Acknowledgements
Funding was received from the IngecoTech CNRS-Irstea program and from
Electricité de France (EDF), Agence de l'eau Rhône,
Méditerranée et Corse, Région Provence-Alpes-Côte-d'Azur and
the European Union (FEDER Programme “L'Europe s'engage en PACA avec le
Fonds Européen de Développement Régional”). We thank P. Bourduge (Zygène) and P. Boutteaud (Vilmorin) for the seed supply and
their technical advice. We also thank E. Bayle (ONF), N. Daumergue and P. Tardif for advice and assistance in setting up the
experiment. Irstea-Grenoble is part of Labex OSUG@2020 (ANR10 LABX56).
Edited by: A. Cerdà
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