Introduction
Development and construction projects often cause damage to native
vegetation. In abandoned quarries or surface mines, recolonization of plants
is very difficult (Ballesteros et al., 2012) because of the destruction of natural
soil structure and seed bank, as well as the limitation of nutrition and
water (Jim, 2001; Haritash et al., 2007). Even though technical restoration can
accelerate succession, it takes decades to achieve a complex
self-sustainable ecosystem (Zhang et al., 2013). During the succession, wind or
water erosion may occur when the vegetation coverage is still low, further
decreasing soil nutrient (Zuazo and Pleguezuelo, 2008) and thus hindering
the process of revegetation (Wang et al., 2005). Geologic hazards may also happen
if no protective measures are applied (Robbins et al., 2013). Heavy metals in
mineral waste may be transported by wind force and cause soil pollution
(Brotons et al., 2010). Besides, during construction of roads and buildings,
temporary dumps without covering may be eroded, resulting in soil loss.
Some chemical properties of the local soil used in the field
experiment.
Organic matter
Total N
Available N
Available K
Available P
(g kg-1)
(g kg-1)
(mg kg-1)
(mg kg-1)
(mg kg-1)
4.72
2.47
19.06
22.23
4.74
Soil covering is a useful measure to protect soil from wind and water
erosion (Mu, 2010), where vegetation plays an important role (Sterk, 2003).
The risk and intensity of wind and water erosion decrease with increased
vegetation cover (Cai, 2001; Maurer et al., 2009; Kefi et al., 2011; Houyou et al., 2014).
Plants increase soil surface roughness, decrease wind speed and as a
result the erosivity and erodibility decrease (Borrelli et al., 2014). Plants
increase concentration time during rainfall events and increase
infiltration, so less runoff is produced.
Characteristics of target species.
Species
Family
Life form
Average size
A. fruticosa
Leguminosae
deciduous shrub
1–4 m
F. arundinacea
Gramineae
cool-season perennial C3 species of bunchgrass
30–100 cm
O. violaceus
Cruciferae
annual or biennial herbs
15–60 cm
V. philippica.
Violaceae
perennial herbs
4–14 cm
Designs of seed mixtures.
No.
Species
Ratio by mass
T1
A. fruticosa : F. arundinacea
0 : 10, 1 : 9, 2 : 8, 3 : 7, 4 : 6, 5 : 5, 6 : 4, 7 : 3,
8 : 2, 9 : 1
T2
A. fruticosa : O. violaceus : V. philippica
The ratio of shrub to herbs was the same with
T1, and the masses of O. violaceus and
V. philippica were the same.
T3
A. fruticosa : O. violaceus
The design of T3 was the same with T1.
T4
A. fruticosa : V. philippica
The design of T4 was the same with T1.
T0
A. fruticosa
Note: the thousand-grain weight of A. fruticosa, F. arundinacea, O. violaceus and V. philippica are 6.163, 2.814, 2.175 and
0.981 g, respectively.
Each plot is denoted as Txty. The capital letter T indicates species, and
the following x ranges from 0 to 4, indicating different combinations of
species. The small letter t indicates the proportion of shrub seeds, and the
following y ranges from 0 to 9, indicating the percentage of A. fruticosa in the seed
mixtures by mass, which equals y/10.
Data of T2t0 are deleted because of deficient setting of the experimental
plot.
Monthly precipitation (bars) and average temperature (dots) during
the experimental period.
Different types of vegetation respond differently to wind and water erosion.
Trees with large canopy are more effective in reducing wind speed, whereas
shrubs are more effective in trapping transporting materials (Leenders et al., 2007).
Compared to herbaceous species, shrubs have more developed root systems to
improve soil structure and conserve water in deep layers, resulting in a
better effect on soil and water conservation (Huang et al., 2006; Wei et al., 2009), and
its effect is less affected by rain intensity compared to herbs (Zhang et al.,
2014). Trees develop slowly (Ji et al., 2011) and have limited effect on soil
protection during the early stage of development (Zhang and Shao, 2003),
while herbs germinate and grow fast, rapidly covering the ground to prevent
splash erosion and decrease runoff (Franklin et al., 2012).
Seed mixtures of shrubs and grasses take the advantage of both taxons, but
the competition for light, water and nutrition may affect vegetation cover
formation (Milton and Dean, 1995) and thus the effect of soil protection.
As shown by some research, the competition from grasses might cause severe
growth decline of woody plants, especially during their early stage of
development (Gordon et al., 1989; Denslow et al., 2006). Because the interaction between
woody species and herbaceous species is complicated, it was proposed that
traits such as niche breadth and competitiveness for different resources of
different species should be thoroughly studied, and the selection of species
should be based on environmental condition including soil, water and light
(Heneghan et al., 2008; Abe et al., 2015; Oliveira et al., 2014). By means of species selection
and controlling seeding density, a positive effect can be attained for shrub
establishment (Franklin et al., 2012).
In this research, a measure of fast revegetation by means of sowing seed
mixtures of shrub and herbaceous species was tested using field experiment.
We focused on (1) which seed mixtures of shrub and grass (which species and
what proportion) could provide a fair or good coverage for a long period;
(2) how different proportions of species affect the speed of cover formation
and the stability of coverage (specifically, we tested the effect of the
ratio of shrub to herbaceous seeds, Rs/h); and (3) how different herbaceous
species affect the growth of shrub. Based on our research, advice will be
proposed on species selection and determination of their proportion in seed
mixtures during the practice of revegetation in (1) plains where wind
erosion occurs, (2) gentle slopes where water erosion occurs and plant
growth is not significantly affected by the slope and (3) seriously degraded
sites such as abandoned mines where measures such as topsoil covering have
been applied to improve soil quality.
Materials and methods
Study area
The research was conducted in the Ecological Restoration Research Base of
Environmental Protection Research Institute of Light Industry (EPRILI),
located in Changping County, Beijing (40∘9′56.73′′ N,
116∘9′1.04′′ E, 57 m a.s.l.). Beijing has a continental monsoon
climate with a rainy season from June to September. The mean annual
precipitation is 620 mm (historical data). Monthly precipitation and average
temperature during the experimental period were measured using Davis Vantage
Pro2 Weather Station, and the data are shown in Fig. 1.
The study area was a flatland. The local soil used for the experiment was
sandy loam. The pH value was 7.44. The chemical properties of the soil are
shown in Table 1 (Liang, 2013).
Experimental design
Four native species were studied, including a shrub species, Amorpha fruticosa L., and three
herbaceous species, Festuca arundinacea Schreb., Viola philippica Car. and Orychophragmus violaceus O. E. Schulz. These species are
commonly seen in the North China Plain, and former research has shown their
tolerance against water or nutrient deficiency. The characters of target
species are shown in Table 2 and the designs of seed mixtures are shown in
Table 3.
Every design of the seed mixtures was tested in a 4.5 m long, 1.3 m wide plot, so
there were altogether 41 plots. Seed mixtures with a seeding density of 15 g m-2 were manually sowed without fertilizer in May 2013.
Non-woven fabrics (a planar, permeable, polymeric textile material) were used
as soil cover to protect the seeds from erosion and enhance humidity.
Irrigation was applied until mid-June, after which precipitation became the
only water source for plants.
Data collection and analysis
From July to October 2013, three 1 × 1 m2 sample plots were
randomly taken in each plot three times a month to measure the coverage of A. fruticosa and the total
coverage of all species. Invaded native species were
recorded. The same measurement was also made from April to August 2014. In
this study, a coverage of 60 % was assumed to be fair, and a coverage of
80 % was assumed to be good, because erosion risk was low in slopes under
30∘ with a fractional vegetation cover of 60–80 %
based on an erosion model (Vrieling et al., 2006). The duration of total coverage
higher than 60 and 80 % were calculated by the following
equation.
Durationoffairorgoodcoverage=the number of days when total coverage ≥ 60 or 80% the number of days for coverage measurement×120days
Coefficient of variation (CV) of total coverage during the experimental
period was calculated to describe the stability of total vegetation
coverage. Each CV value of different Rs/h was taken as a sample, and the Friedman
test for non-parametric paired samples was used to test the significance of
variation between the CV values of different combinations of shrub and
herbaceous species.
At the end of October 2013, 15 individuals of A. fruticosa in each plot were randomly
taken to measure height and ground diameter. ANOVA was used to test the
effect of herbaceous species and Rs/h on the growth of A. fruticosa where T0 was
used as control. Normality of samples was tested before the significance test,
and when the effect was significant (P < 0.05) Least Significant Difference Test (LSD) was used to test
comparisons among different seed mixture designs. Statistic analysis was
performed using SPSS program.
Total coverage of different combinations of shrub and herbaceous
seeds (±SE) from July to October 2013; see Table 3 caption for code
identification.
Results
The effect of species on total coverage
As shown in Fig. 2, from July to October 2013, T4 had the highest total
coverage, regardless of the ratio of shrub to herbaceous seeds. The performance of other seed mixtures differed with time. In
July, when Rs/hs were 1 : 9, 3 : 7, 4 : 6 and 5 : 5, T2 had the second highest
total coverage, and when Rs/hs were 2 : 8, 6 : 4, 8 : 2 and 9 : 1, T3 had the
second highest total coverage. T1 had the lowest total coverage in July.
From August to October, when Rs/hs were 1 : 9–3 : 7, T2 had
a higher total coverage than T1, and when Rs/hs were
6 : 4–9 : 1, T1 had a higher total coverage value than T2. T3 had
a relatively low total coverage from August to October, which was also shown
in T2t8 and T2t9.
The effect of Rs/h on total coverage
As shown in Fig. 3, Rs/h had different effects on the dynamics of
total coverage in different species combination.
T0: A. fruticosa took a longer time to form a fair coverage and maintained a fair or good
coverage for a much shorter period compared to herbaceous species. Total
coverage of T0 was lower than 60 % in July and October but higher than
80 % in August and September.
T1: in July, 6 out of 10 plots including t2 and t5–t9 had a total
coverage higher than 60 %, among which t7 had a total coverage higher than
80 %. In August, except for t0 and t1, all plots had a total coverage
higher than 80 %. From September onward, all plots had a total coverage higher
than 80 %.
T2: in July, 7 out of 9 plots including t1 and t3–t8 had a total
coverage higher than 60 %, among which t3 and t5 had a total coverage
higher than 80 %. In August, except for t9, all plots had a total coverage
higher than 80 %. From September onward, t1–t7 had a total
coverage higher than 80 %. The total coverage of t8 and t9 was good in
September, but both fell to 77 % in October.
T3: in July, 9 out of 10 plots including t0 and t2–t9 had a total
coverage higher than 60 %, among which t6 had a total coverage higher than
80 %. In August, all plots had a total coverage higher than 60 %, among
which t0 and t6–t9 had a total coverage higher than 80 %. The
total coverage of most plots was maintained until October except for t2,
which enhanced total coverage from September, t1 and t3–t5,
which enhanced total coverage in October, and t9, which decreased total
coverage to a value lower than 60 % in October.
T4: since July, all plots achieved a total coverage higher than
80 %.
Dynamics of total coverage (±SE) from July to October 2013;
see Table 3 caption for code identification.
Duration and stability of total coverage
From July to October (counted as 120 days), duration of fair coverage was 76,
107, 112, 112 and 120 days (mean values of different Rs/hs, the same
below) from T0 to T4, respectively. Duration of good coverage was 65, 84,
95, 82 and 109 days from T0 to T4, respectively. In this respect, T4 had the
best performance, followed by T2, T3, T1 and T0. T1 and T3 had relatively
poor performance compared to T4 and T2, but T1t5, T1t7, T3t7 and T3t8
maintained a good coverage of more than 100 days. Even though T2 had the second
best performance in general, T2t2 and T2t9 maintained a shorter period of
fair or good coverage compared to T1 or T3 of the same Rs/h.
Remarkably, when Rs/h was 6 : 4 and 7 : 3, all combinations of shrub and
herbaceous seeds maintained a fair coverage for 120 days, i.e., the whole
experimental period. As a result, this ratio of shrub to herbaceous seeds is
proposed for seed mixtures applied in rapid revegetation.
Coverage of A. fruticosa sowed with different herbaceous seeds. Note: each spot with an error bar is the mean value of coverage of A. fruticosa during the
experimental period. The dotted line indicates a predicted coverage of A. fruticosa
under different seeding densities based on the assumption that the coverage
is proportional to the amount of seeds sowed.
T0 not only had the shortest duration of fair or good coverage but also had
the highest coefficient of variation (46 %), indicating that it had the least
stability among all plots. The coefficient of variation from T1 to T4 were
19, 15, 19 and 9 %, respectively. The coefficient of variation of T4 was
significantly lower than those of T1, T2 and T3 (P < 0.05). In T1,
plots with a Rs/h of 5 : 5–9 : 1 had a relatively low
coefficient of variation, ranging from 10 to 16 %. In T2, plots with a
Rs/h of 1 : 9 and 3 : 7–8 : 2 had a relatively low coefficient
of variation, ranging from 10 to 15 %. In T3, plots with a Rs/h of
2 : 8, 3 : 7, 6 : 4–8 : 2 had a relatively low coefficient of
variation, ranging from 11 to 15 %.
Average height and ground diameter of A. fruticosa.
Height (cm)
Ground diameter (cm)
T1
T2
T3
T4
T1
T2
T3
T4
t1
34∗
30∗
35∗
–
0.423∗
0.431∗
0.668
–
t2
31A∗
44B∗
45AB∗
–
0.328A∗
0.672B∗
0.679B∗
–
t3
52A∗
33B∗
39B∗
49A∗
0.639AB
0.563A∗
0.594AB∗
0.781B
t4
65A
35B∗
43B∗
50AB∗
0.807
0.594∗
0.626∗
0.737∗
t5
73A
36C∗
49BC∗
61AB∗
0.795A
0.515B∗
0.705AB∗
0.834A
t6
63AB∗
50A∗
55A∗
82B
0.679A
0.652A∗
0.756AB∗
1.063B
t7
53A∗
56A∗
66AB∗
73B∗
0.593A∗
0.669A∗
0.853B
0.889B
t8
55A∗
63AB∗
70B∗
89C
0.715
0.687∗
0.837
0.899
t9
79AC
54B∗
73A∗
93C
0.849A
0.571B∗
0.761A∗
0.858A
T0
92
0.926
Note: the superscript ∗ indicates a significant difference compared to T0
(P < 0.05). The subscript of the same letter or the absence of
subscript indicates that the mean values of height or ground diameter in the
same row were not significantly different.
No A. fruticosa survived in T4t1 and T4t2, and only five and two individuals of A. fruticosa survived in
T2t1 and T3t1, respectively.
The effect of herbaceous species on the coverage of A. fruticosa
The average coverage of T0 during the experiment period was 74.6 %.
Assuming the coverage was proportional to the amount of seeds we sowed, some
seed mixtures had a positive effect on the coverage of A. fruticosa, including
T1t1–T1t7, T3t8–T3t9 and T4t7–T4t9, while other seed mixtures had a negative effect on the coverage of A. fruticosa,
as shown in Fig. 4.
When A. fruticosa was sowed alone, fair coverage was achieved on 30 July. When
herbaceous species were sowed with A. fruticosa with a Rs/h range of 1 : 9 to 3 : 7,
the coverage of A. fruticosa was lower than 60 % during the whole experimental period
in any combination of seed mixtures. When the Rs/h ranged from 4 : 6 to
7 : 3, the coverage of A. fruticosa in T1 reached 60 % first, on 30 August,
20 July, 30 July and 10 August. When the
Rs/h ranged from 8 : 2 to 9 : 1, the coverage of A. fruticosa in T4 reached 60 %
first, on 20 July. In plots of T1t9, T2t7–T2t9,
T3t6–T3t9 and T4t6–T4t7, A. fruticosa also achieved a
coverage of 60 % but at a later period of the rainy season.
The effect of Rs/h and herbaceous species on the growth of A. fruticosa
There was a negative effect of herbaceous species on the growth of A. fruticosa, as
shown in Table 4. In T1, height growth of A. fruticosa was significantly lowered when
Rs/hs were 1 : 9–3 : 7 and 6 : 4–8 : 2, while
ground diameter was significantly lowered when Rs/hs were 1 : 9, 2 : 8 and
7 : 3 compared to T0. In T2, height and diameter growth of A. fruticosa were
significantly decreased in all Rs/hs compared to T0. In T3, height
growth of A. fruticosa was significantly lower than T0 in all Rs/hs, while ground
diameter was significantly lower than T0 when Rs/hs were
2 : 8–6 : 4 and 9 : 1. In T4, height growth of A. fruticosa was significantly
lowered when Rs/hs were 3 : 7–5 : 5 and 7 : 3, while ground
diameter was significantly lowered when Rs/h was 4 : 6, compared to T0.
When different combinations of species with the same Rs/h were
compared (T1–T4), the values of height and ground diameter
were the highest in T3 when the Rs/hs were 1 : 9 and 2 : 8. When the
Rs/hs were 3 : 7–5 : 5, T1 had highest value of height and
generally the highest value of ground diameter. When the Rs/hs were
6 : 4–9 : 1, T4 had the highest values of height and ground
diameter.
Dynamics of the established plant communities in the subsequent
year
T1 was damaged in 2014 so the data are not reported. As to other plots, in
April 2014, T2t1–T2t5, T3t0–T3t8 and
T4t0–T4t5 had a total coverage higher than 80 %, and T3t9
had a total coverage higher than 60 %. In May, except for T2t6 and
T3t2–T3t5, all plots had a total coverage higher than 80 %.
In June, the total coverage of T2t1, T2t5 and T3t0–T3t5
decreased and was lower than 60 % because of the wilting of O. Violaceus since April.
Since May, T0 and all plots of T4 achieved a total coverage higher than
80 %. Since July, all plots of T2 achieved a total coverage higher than
80 %. In August, T3t0–T3t1 had a total coverage ranging
from 70 to 73 %, while the other plots of T3 achieved a total coverage
higher than 80 %.
Discussion
The effect of species selection and Rs/h on total coverage
Vegetation cover is one of the main factor controlling the effect of soil
protection from wind and water erosion (Ferreira and Panagopoulos, 2014).
An early recovery of vegetation cover can prevent water erosion during the
rainy season, while the stubble and litters can prevent wind erosion during
the following dry season. Two months after sowing, total coverage of T0,
T1t0, T1t1, T1t3, T1t4, T2t2, T2t9 and T3t1 were lower than 60 %, so they
are not proposed for rapid revegetation.
Based on the speed and the stability of coverage, sowing seed mixtures
performed better than sowing shrubs alone, which was consistent with
Gilardelli et al. (2013). Among the combinations of shrub and herbaceous species,
T4 showed its excellency in fast ground cover formation and high coverage
maintenance around the whole experimental period, most attributed to V. philippica.
According to our results, sowing V. philippica with a seeding rate of 1.5 g m-2 is efficient in rapid revegetation in northern China or regions
where the climate and soils are similar. A higher seeding rate may be a
waste of seeds and more seriously, the dense ground cover may hinder the
recolonization of other native species. In plots where O. violaceus instead of V. philippica was sowed
with A. fruticosa, a coexistence with local annual or perennial herbs such as Bidens pilosa L.,
Acalypha australis L., Amaranthus retroflexus L., Euphorbia humifusa Willd., Abutilon theophrasti Medic., Artemisia annua L., Convolvulus arvensis L. and Polygonum lapathifolium L. was observed
but not in T4.
Other than T4, T2 had a fast cover formation when Rs/h was low, and
T3 had a fast cover formation when Rs/h was high. Sowing F. arundinacea alone was
not appropriate for rapid revegetation because it covered the ground slowly.
But considering the whole experimental period, T1 had a relatively high
total coverage when Rs/h was high. As a result, F. arundinacea should be mixed with
other fast-growing species and a seeding rate of 1.5–6.0 g m-2 is proposed in order to achieve high value of total
coverage. T3, i.e., O. violaceus, covered soil rapidly but had the lowest total coverage
considering the whole experimental period. Because O. violaceus wilted when the
seeds were ripe, a significant decline of O. violaceus was observed, though total
coverage was hardly affected thanks to the development of A. fruticosa.
Before the experiment, we supposed that the stability of total coverage was
correlated with the tolerance to environmental stress. For example, because
of the stochastic nature of precipitation, wilting, defoliation or die off
during water deficiency may weaken the protective effect of vegetation when
a rain storm finally occurs (Zuazo and Pleguezuelo, 2008). Compared to herbs,
woody species were supposed to maintain a more stable coverage because they
could use the resource in the deep soil layers or at least they have longer
life (Wang et al., 2005). Contrary to expectation, results showed that T0 had the
highest coefficient of variation among all plots. If A. fruticosa could use the water
stored in the deep layers, its coverage would not fluctuate in spite of the
temporal water deficiency (the longest interval between rainfall events was
17 days during our experiment), and thus the coefficient of variation would
be small. The high value of coefficient of variation in T0 indicated that
the ability to conserve water and the resistance against environmental
stress was not fully developed in A. fruticosa.
Some plots, including T2t2, T2t9 and T3t1, had a low total coverage, and no
pattern was observed between these plots and the adjacent plots. It was
supposed that random factors such as the variation of seeds and microsite
conditions accounted for the poor performance of these plots. However,
natural ecosystems are much more diverse than our study plots. Microsites
are spatially heterogeneous, weather events are stochastic by nature and
the inter- or intraspecific relationship may vary in different stages of
individual development and community succession (Zanini et al., 2006). To deal
with the spatial and temporal heterogeneity, more species should be used in
artificial revegetation because of their adaptation to different niches and
thus the reconstruction of the whole plant community is more likely to
succeed even if some species fail (Sheley and Half, 2006).
The effect of herbaceous species on the growth of shrub
The coverage of A. fruticosa in T1t5, T4t8 and T4t9 reached 60 % 10 days earlier than
T0, even though fewer seeds were sowed in these plots, indicating a positive
effect of herbaceous species on the coverage of A. fruticosa. Compared to T0, the
average coverage of A. fruticosa during the study were higher in T1t1–T1t7, T3t8–T3t9 and T4t7–T4t9, but average
height and ground diameter were lower in these plots, indicating that the
individuals were smaller, but the number of individuals was higher when
herbaceous species were sowed together. The result was consistent with the
research by Mason et al. (2013), which showed that ground cover was favorable for
shrub germination but disadvantageous to growth. Moreover, when a field
study was made in May 2014, it was observed that the stem number of each
individual of A. fruticosa ranged from three to five in T0, but more than six in T1t9 and T4t9,
which may partly account for the inconsistency between high coverage and low
growth in these plots.
Competition for resources, such as water, may explain the decline of growth
of A. fruticosa. Soil water content is determined by the input, such as precipitation and
irrigation, together with the output, such as infiltration and
evapotranspiration. Plants can increase infiltration rate (Ji et al., 2008) and
water holding capacity but also consume a large amount of water during
transpiration. As a result, soil water content may be increased or decreased
by coexisting species (Bréda et al., 1995, D'Odorico et al., 2007). Competition may
also exist for nutrition or light, but the relationship differs among
different species (Denslow et al., 2006; Mendoza-Hernández et al., 2014). Research
indicated a very comprehensive relationship between different coexisting
species; not only negative but also positive relationships were shown in
different studies (Harmer et al., 2011, Ballesteros et al., 2012; Zhang et al., 2013; Oliveira
et al., 2014).
Other than interspecific competition, intraspecific competition exists.
Competition for light between individuals of A. fruticosa was more intense in T0 than
other plots, especially when Rs/h was low. In T0, short and weak
individuals may be weeded out and only the tall and strong ones which have
access to light survive, leading to a higher mean value of growth. Compared
to height, ground diameter was less correlated to the competition for light,
so it was also less corrected to Rs/h. However, this hypothesis needs
to be tested.
Conclusions
Firstly, shrub cover was formed slower than ground cover and was maintained
for a shorter period at least in the early stage of development. When
herbaceous species were sowed with shrubs, total coverage increased and was
maintained for a longer period, but the growth of shrubs was hindered.
Secondly, in the practice of rapid revegetation in the North China Plain or
wherever the soil and climate are similar, the ratio of shrub to herbaceous
seeds is proposed to be 6 : 4–7 : 3 by mass. Thirdly, herbaceous
species have different traits. In our experiment, three different types of
herbs were found, i.e., slow-growing stable species (F. arundinacea), fast-growing unstable
species (O. violaceus) and fast-growing stable species (V. philippica). Slow-growing stable species
and fast-growing unstable species should not be used alone because they
cannot cover the ground fast or they cannot maintain a long period of
coverage. A small seeding rate of fast-growing stable species should be used
to ensure a fair coverage against erosion, and other species with different
traits should be added to enhance the stability of plant community.
Fourthly, in the practice of rapid revegetation in the North China Plain or
wherever the soil and climate are similar, seeding density of F. arundinacea is proposed to
be lower than 6 g m-2 and the seeding density of V. philippica is
proposed to be 1.5 g m-2.