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The Effects of Alnus subcordata Roots on Soil Detachment Rates in Forestland of Northern Iran

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23 May 2023

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24 May 2023

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Abstract
Plant roots have significant effects on the soil erosion rates, since they can strongly change the soil detachment capacity (Dc). This study quantifies Dc at different flow rates in soils with Alnus subcordata species, compared to three other species (Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species) in the Saravan forest park and develops regression equations for predicting Dc. Undisturbed samples collected from soils with the four tree species and subjected to five slopes (from 4.3 to 38.3%) and five water discharges (from 0.28 to 0.71 l m−1 s−1) using a hydraulic flume. The results showed that Dc was significantly lower in soils with Alnus subcordata species compared to Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species, as the consequence of the changes in the root characteristics, so that Dc was negatively correlated with root weight density, root length and root biomass. The unit stream power had high accuracy for predicting Dc for all of the studied species. The lowest value of rill erodibility (Kr) was obtained in soils with Alnus subcordata species using regression relationship between the Dc and the shear stress of the soil. This experiment helped to show the importance of plant roots in reducing the soil detachment rates and provided a contribution in understanding the choice of appropriate species for soil conservation.
Keywords: 
Subject: Environmental and Earth Sciences  -   Soil Science

1. Introduction

Soil erosion is one of the most important global problems that can influence forest ecosystems [1,2]. This influence is particularly felt in hillslopes with steep slopes, where soils are extremely vulnerable to detachment [3]. In this regard, soil detachment capacity (Dc) by overland flow can defined as the maximum value of the soil detachment rate in these slopes [4,5]. The expected increase in this important parameter will aggravate the forest soil degradation [6]. Therefore, the study of factors affecting soil detachment rates can be useful for controlling soil erosion in these sensitive ecosystems.
For various reasons, it is sometimes difficult to modify soil properties. Therefore, the use of vegetation is a more logical solution to prevent soil erosion, especially on steep slopes. The effects of plant and tree communities on soil erosion have been investigated in past researches [7,8,9,10]. In this case, the effect of plant roots on resistance of soils to detachment has been widely investigated [11,12,13,14]. The presence of root in soil has positive impacts on some soil properties, as it increases soil aggregate stability [15] and reduces soil detachment capacity [16], thereby decreasing soil erosion. For instance, Yao et al. [17] found that the roots could significantly reduce the soil detachment rates and rill erodibility (Kr) as an exponential function. These authors stated that due to the higher soil organic matter, roots played an important role in reducing the Dc. Herbrich et al. [18] observed that that plant roots improved the soil shear strength status and decreased soil erosion by reinforcing the soil mass. Among the root characteristics, root weight density, root length and root biomass are considered as the most important characteristics of root that affect the soil detachment rates [19,20]. Many investigations on the impacts of these three characteristics on soil erosion can be found in the literature. For example, Li et al. [21] reported a remarkable reduction in soil detachment rates with increasing root weight density. Also, it is reported that soil erosion decreases with the increase in root length. On this regard, as example, Burak et al. [22] showed that the impact of root hairs on soil erosion can be assessed from the differences in some important root characteristics such as root length. Moreover, the role of root biomass on soil stability is significant. Related investigations have shown that among root characteristics, root biomass effectively improves the stability of the soil and reduces the soil detachment rates on the steep slopes [23,24].
In addition to root characteristics, soil detachment capacity depends on the characteristics of the overland flow [25]. In this context, the hydraulic parameters of runoff including the shear stress, stream power and unit stream power, can be used to predict Dc [26]. Although the effects of hydraulic parameters on Dc have been discussed in previous studies [27,28,29], these relationships are complex and different in various ecosystems, since a specific land use influences soil hydrology by modifying physical and chemical properties of soil [30]. Therefore, it is required to pay exact attention to it in future studies.
Despite this large body of literature, due to the presence of diverse plant species, an understanding of the effects of these species on soil detachment rates from the perspective of root ecology and morphology is urgently needed. In other words, root effects on soil erosion processes are largely variable among various species with different size, distribution and density of the roots [13], and are difficult to explain without describing the root characteristics. Therefore, this study aims to quantify the soil detachment capacity in soils sampled under four species (Alnus subcordata, Brachythecium plumose, Gleditsia caspica and Sambucus ebulus) in Saravan Forest Park (Northern Iran), by an experimental flume.
We hypothesize that Alnus subcordata, with better root growth condition, is able to reduce the soil detachment capacity, compared to other species. Moreover, power regression equations for the studied species are suggested to simulate Dc from unit stream power under given experimental conditions. In more detail, we followed the results of the study of Parhizkar et al. [26], who reported that the above-mentioned hydraulic parameter is the best choice for predicting Dc in forest ecosystem. Following this approach, but at a smaller scale, our research question was: Are the models obtained by unit stream power credible for the studied species in the same study area?
The findings of this investigation provide insight into the Dc under different plant species and present a suitable species for soil erosion control.

2. Materials and Methods

2.1. Study area

The experiments were conducted in the Saravan Forest Park (37°08′10″N, 49°39′49″E). It is located in Guilan Province, Northern part of Iran and has a Mediterranean climate, with an average annual temperature and precipitation of 16.3 °C and 1360 mm, respectively [31]. Particle size analysis showed that soil texture in the study area was silty clay loam. More details about the geographical location of the study area can be found in previous work [6].
This area, which approximately 260 ha, is composed of various tree and shrub species including Carpinus betulus, Viburnum lantana, Artemisia annua, Fragaria vesca, Morus alba, Viola odorata, Zelkova carpinifolia, Danae racemosa, Hypericum perforatum, Parrotia persica and Danae racemosa [32]. In addition to the above species, Alnus subcordata, as a native, pioneer and fast growing species, is one of the abundant species in this area, especially on the slopes. However, despite its potential capabilities and rapid root growth, its effects on soil erosion processes have not been studied for soil protection purposes. For this study, four species, Alnus subcordata, Brachythecium plumose, Gleditsia caspica and Sambucus ebulus (hereinafter indicated by AS, BP, GC and SE, respectively) as prevalent species with the same slope gradient, slope aspect, elevation and soil type were selected. These conditions were considered the same, in order to minimize their impacts on experimental results. In the past years, deforestation has increased due to various reasons and has caused an increase in rill erosion in forestlands of Guilan province [6]. Therefore, proper management operations and conservation of plant species are essential actions in this area.

2.2. Soil Sampling and Root Characteristics Measurement

To begin measuring the soil detachment capacity (Dc), from January to February 2023, undisturbed soil samples were collected from the areas with the investigated species using a steel ring (0.1 m in diameter and 0.05 m in height) and then transported to the laboratory for hydraulic tests of flow by soil erosion flume. The complete specifications of this flume and the sampling procedure are reported in study of Ghasemzadeh et al. [33]. Briefly, five samples were collected around each species for each slope. Two quadrats (10 × 10 cm and 20 × 20 cm) were overlaid on the ground for each species. Soil samples were extracted from the vertices of the diagonals of the first and second quadrat.
In addition to the above samples, 100 soil samples (25 samples × 4 species) were randomly collected, in order to measure the root characteristics. Root weight density was determined by the washing method over a sieve with a 1-mm mesh. Then the roots of each soil sample were weighted after oven-drying at 65 °C for 1 day. Root length was measured by universal tape meter. Root biomass also was measured by oven drying method at temperature of 60◦C for 2 day [34].

2.3. Measurement of soil detachment capacity

Extracted soil samples were inserted in the lower part of flume. After adjusting the flow rate [Q, L m-1 s-1] and slope [S, %], flow velocity [V, m s-1] and depth [D, m] were measured using potassium permanganate dye technique and level probe, based on the methods used in the study of Zhang et al. [35]. Experiment stopped when the soured depth of the soil sample in the steel ring reached 0.015 m or after 5 min of duration. After finishing each experiment, wet soil was oven dried at 105 ◦C for 24 hr and then weighed.
Based on field measurements, for the studied species, five flow rates and five slope gradients (Table 1) were simulated. Considering 6 replications for each flow rate and slope and 4 species, overall 600 soil samples extracted and subjected to the experiments.
The shear stress [τ, Pa], unit stream power [f, m s−1] and Soil detachment capacity Dc [kg s−1 m−2] were obtained, using the following equations:
τ = ρ g R S
φ = S V
D c = Δ M A ⋅ Δ t
where r, g, R and S are the water density [kg m−3], acceleration of gravity [m s−2], hydraulic radius [m] and slope gradient [m m−1], respectively. Also ΔM, Dt and A are dry weight of detached soil [kg], test time [s] and the area of the soil sample [m2], respectively.
In addition to the calculation of Dc, rill erodibility [Kr, s m−1] was calculated by the linear regression relationship between Dc and τ [36]. Table 1 shows the mean values of hydraulic parameters for flow discharges in the slopes adjusted for measuring the soil detachment capacity under four studied species.

2.4. Data analysis

Mean comparison analysis was applied to Dc and root characteristics (root weight density, root length, and root biomass) to evaluate the statistical significance of the differences among the four tree species (at p < 0.01).
Then, Pearson’s correlation analysis was used to find correlations between Dc and the root characteristics. The principal component analysis (PCA) was also applied to the root characteristics to recognize the existence of significant variables and to cluster soil samples in different groups based on the studied species. Moreover, the relationships between the Dc and unit stream power for the studied species were developed by power regression method. The accuracy of these equations was evaluated by the coefficient of determination (R2) and Root mean square error (RMSE), as statistical measurements [37].
Statistical analyses were conducted by XLSTAT release 19.1 (Addinsoft, Paris, France) software.

3. Results

3.1. Variability of root characteristics among investigated species

According to mean comparison analysis, significant differences (p < 0. 01) were found for all of the root characteristics among investigated species (Table 2). In more detail, root weight density was higher (0.66 ± 0.12 kg m-3) in AS and lower (0.51 ± 0.13 kg m-3) in SE. The latter species showed the lowest root length (13.71 ± 3.11 cm), while the maximum value (27.04 ± 3.62 cm) was detected in AS (about 1.37-fold, 1.73-fold and 1.97-fold the values obtained in BP, GC and SE species, respectively). The differences also in root biomass value among the species were significant (p < 0.01). The AS species had the highest root biomass, while the minimum value (27.04 ± 3.62 cm) for this characteristic was recorded in SE. The differences in root biomass values were not statistically significant between BP and GC species (Table 2).
Significant differences in the values of soil detachment capacity among the four species were detected (p < 0.01) (Figure 1). The lowest mean value of Dc was found in the AS species (0.009 kg m−2 s-1); this value was significantly different compared to the values measured in other species (p < 0.01). Also, this soil property was the highest in GC species (0.027 kg m−2 s-1). The Dc of latter species was 1.2-fold, 1.6-fold and 2.8-fold greater than in SE, BP and AS species, respectively.
Pearson’s correlation analysis indicates that soil detachment capacity was negatively correlated with all of the root characteristics (p < 0.01, Table 3). These relationships were also confirmed by principal components analysis. PCA showed three principal components, which explained 78.21% of the total variance of the characteristics of the root for the four investigated species. The root length and root biomass had significant loadings on PC1 (over 0.7) and root weight density had significant loading on PC3. Moreover, Dc had a significant weight on PC2 (Table 4). In more detail, Dc was associated with low values of root weight density, root length and root biomass (Table 4 and Figure 2). Moreover, plotting the sample scores on the first three PCs, shows clear differences in root characteristics among the studied species. Two well-differentiated groups, one for AS species and another for BP, GC and SE species were evidenced (Figure 2).

3.2. Relationships between the soil detachment capacity and unit stream power

Figure 3 shows the power relationships between the soil detachment capacity and the unit stream power for the investigated species. Also, the values of the statistical indexes for the prediction capacity of proposed equations are shown in Table 5. The result showed that unit stream power (R2 from 0.77 to 0.90 and RMSE from 0.006 to 0.009), was good hydraulic parameter for predicting the Dc in species scale. Moreover, the regression relationships between the Dc and the shear stress for the various species are shown in Table 6. These equations showed satisfactory coefficients of determination (r2 > 0.50) for calculating rill erodibility. GC species showed the maximum Kr (0.0045 s m−1); so that this parameter was 1.15, 1.50 and 2.65 times greater than for SE, BP and AS soil, respectively. Thus, the Kr was the lowest for AS species (0.0017 s m−1).

4. Discussion

4.1. Impacts of plant species roots on soil detachment capacity

As a matter of fact plant roots influence the hydrological characteristics of a soil e.g., [38,39,40]. It is also known that soil detachment capacity, as an important hydrological characteristic, particularly in steep slopes, widely varies under different plant species [41,42,43].The investigated soils had the same texture (silty clay loam) and experimental conditions (water discharges and bed slopes) were same for the four species. Therefore, the effects of the root characteristics changes on the soil detachment capacity can be easily understood. In this study, Dc was significantly influenced by the four species. The lower Dc among the four species was detected in soils with AS species. This can be due to the highest root weight density, root length and root biomass. More specifically, when root weight density increases, Dc decreases from GC to AS species. It has been reported in many studies that this root characteristic had significant effects on Dc. For example, Yoshinori et al. [44] demonstrated that the soil detachment significantly decreased as root weight density increased. Zhang et al. [45] also found that among the characteristics of root, root weight density is considered as a main factor for reducing Dc. Moreover, based on the results obtained, when root length and root biomass increase, Dc decreases. This result agrees with the study by Burak et al. [22], who found that the Dc decreased with root length, and by De Baets et al. [46], who reported that Dc and rill erodibility reduced exponentially with increasing root biomass. Generally, more than 90% of the roots of Alnus subcordata are located at soil surface. Thus, there is the highest reinforcement for each soil layer and the aboveground biomass of this species can reduce the soil loss. Moreover, it is demonstrated that this species increases the Factor of safety in steep slopes and results in the stability of the study area [47].
In order to confirm the above relationships, the correlation analysis and principal component analysis (PCA) were applied between Dc and root characteristics. These two techniques showed that for the investigated species, the Dc is directly associated with the three studied root characteristics. In other words, when the root weight density, root length and root biomass (and thus some of important soil properties such as the aggregate stability and organic matter; Table S1) decrease, the Dc increases. Referring to study of Parhizkar et al. [4], all of the studied characteristics were selected as effective root parameters on Dc. Moreover, the clustering of soils in two groups (one for AS species and another for BP, GC and SE species) revealed a clear gradient according to the characteristics of the root. This leads to the conclusion that differences in Dc are clear not only among different land uses [48], but also among various tree and plant species, due to the significant variations in root characteristics.

4.2. Relationships between the soil detachment capacity and unit stream power

Unit stream power, as time rate of expenditure of stored energy in a particle size range per unit weight of overland flow [49], is an important hydraulic parameter affecting soil detachment capacity. Based on the results obtained from the power regression, relationships between unit stream power and Dc had high accuracy for all of the studied species. This result is consistent with several studies [50,51], showing that Dc had close relationship to flow characteristics, such as unit stream power. Also, comparisons between the measured results from the laboratory flume and the predicted results using unit stream power equations indicate that these equations are precise in predicting the Dc for the four species. This was likely due to the simultaneous existence of flow velocity and soil slope in unit stream power equation, which considerably affect the detachment rate of soil particles [26]. In this regard, some authors [52,53] showed that soil slope plays main role in driving the soil detachment rate. In other words, Dc is generally more sensitive to changes in slope gradient than in other hydraulic characteristics of water flow. It is worth noting that above power relationships can be developed for different species types and be used in combination with other process-based erosion models under various slope and soil conditions.
This study found that the Dc is linked to rill erodibility (Kr). This parameter, as a reflection of soil resistance to detachment, is an important input in some of soil erosion models [54]. In this case, it is necessary to compare the effects of the studied species on Kr in the study area. Therefore, in this investigation, Kr was obtained by the linear relationship between the Dc and flow shear stress for the four tree species. According to the link of Dc and Kr, as expected, the lower Dc detected in Alnus subcordata species, compared to the other species, can be due to some root characteristics and this also influences the values of Kr. This result is consistent with the conclusions of Zhang et al. [55], who reported that linear equation between Dc and τ can be used to predict Kr. The higher values of Kr in the Gleditsia caspica species, compared to the other species, show the less effects of GC roots on soil erosion.
As differences exist among the studied species and their root characteristics, the rill erodibility and the soil particles detachment rates differ for diverse vegetation [56]. Moreover, there are abundant and complex interactions among many significant soil and root parameters, thus the effect of one species on Kr is probably different from another. The great effectiveness of detachment reduction in soils under Alnus subcordata species was mainly caused by better development of root system in a long period of time. In other words, a well-structured soil could be formed in a long-term time scale and this leads to improvement of soil quality [57], increase soil resistance to erosion and reduction of Kr [58]. Considering the great variety of plant species and their different temporal and spatial scales for root development, more studies are needed to quantify the changes in rill erodibility for the same soil and even transfer the information obtained to other ecosystems.

5. Conclusions

Our investigation showed how the soil detachment capacity is affected by various tree species (Alnus subcordata, Brachythecium plumose, Gleditsia caspica and Sambucus ebulus). The root characteristics including root weight density, root length and root biomass were significantly different among the studied species. The results showed that the mean soil detachment capacity (Dc) in soils with Alnus subcordata species was lower than in soils with Brachythecium plumose, Gleditsia caspica and Sambucus ebulus species. This can be due to the more developed root structure of Alnus subcordata compared to the other species. In this study, Dc was negatively correlated with root weight density, root length and root biomass.
For all species a power equation with well accuracy was developed to predict the Dc from the unit stream power, as main hydraulic parameter for the computation of soil detachment rates. The rill erodibility, obtained by relationship between Dc and shear stress, was the lowest for Alnus subcordata species among the investigated species; this confirms higher resistance to detachment of soil of this species.
It was proposed that Alnus subcordata roots decreased the risk of soil erosion (mainly for soils with steep slopes), and protection of this species can be reasonable and suitable strategy to control soil erosion. Moreover, the findings of this study revealed that the models developed are particularly useful in forest ecosystems, when the best species for soil conservation purposes must be selected.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org, Table S1, Some properties (mean of four replicates ± standard deviation) of forest soil sampled under the studied species in the study area.

Author Contributions

conceptualization, methodology, and fieldwork, Z.G., M.P, M.Z., R.S., S.M. and M.S.; writing and review M.P.

Funding

Faculty of Agricultural Sciences, University of Guilan.

Acknowledgments

The authors thank to the Faculty of Agricultural Sciences, University of Guilan for their support and experimental assistance.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Soil detachment capacity (Dc) for the investigated species in the study area (lowercase letters show significant differences among the four species (p < 0.01).
Figure 1. Soil detachment capacity (Dc) for the investigated species in the study area (lowercase letters show significant differences among the four species (p < 0.01).
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Figure 2. Loadings of root characteristics and scores of soil samples on the first two principal components (PC) for four investigated species.
Figure 2. Loadings of root characteristics and scores of soil samples on the first two principal components (PC) for four investigated species.
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Figure 3. The relationship between the soil detachment capacity and unit stream power using power equations of Table 5 for the studied species.
Figure 3. The relationship between the soil detachment capacity and unit stream power using power equations of Table 5 for the studied species.
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Table 1. Mean of hydraulic parameters for flow discharges in the slopes adjusted for measuring the soil detachment capacity under four studied species.
Table 1. Mean of hydraulic parameters for flow discharges in the slopes adjusted for measuring the soil detachment capacity under four studied species.
Unit stream power Shear stress Flow velocity Flow discharge Slope percentage
0.07 1.29 0.31 0.28 4.2
0.09 2.44 0.41 0.37 9.8
0.11 4.09 0.53 0.48 19.9
0.14 6.28 0.65 0.59 29.1
0.16 8.79 0.75 0.71 38.3
Table 2. Root characteristics (mean ± standard deviation) of the investigated species in the study area.
Table 2. Root characteristics (mean ± standard deviation) of the investigated species in the study area.
characteristics The investigated species
AS BP GC SE
Root weight density (Kg m-3) 0.66 ± 0.12 a 0.56 ± 0.11 b 0.58 ± 0.11 b 0.51 ± 0.13 b
Root length (cm) 27.04 ± 3.62 a 19.78 ± 4.39 b 15.64 ± 3.05 c 13.71 ± 3.11 c
Root biomass (gr) 26.51 ± 1.42 a 16.88 ± 3.43 b 15.70 ± 1.76 b 13.62 ± 1.71 c
Notes: AS= Alnus subcordata; BP = Brachythecium plumose; GC = Gleditsia caspica; SE = Sambucus ebulus. Lowercase letters show significant differences among the four species (p < 0.01).
Table 3. Pearson’s correlation analysis between Dc and the root characteristics for the investigated species (AS, BP, GC and SE) in the study area.
Table 3. Pearson’s correlation analysis between Dc and the root characteristics for the investigated species (AS, BP, GC and SE) in the study area.
Root characteristics Root weight density Root length Root biomass
Dc -0.299** -0.278** -0.261**
Notes: Dc = Soil detachment capacity; **: significant level at 1%.
Table 4. Loadings of the root characteristics on the first two Principal Components for four investigated species (AS, BP, GC and SE).
Table 4. Loadings of the root characteristics on the first two Principal Components for four investigated species (AS, BP, GC and SE).
Root characteristics Principal Components
PC1 PC2 PC3
Dc 0, 289 0, 483 0, 228
Root weight density 0, 398 0, 160 0, 442
Root length 0, 764 0, 130 0, 017
Root biomass 0, 782 0, 123 0, 003
Note: Dc = rill detachment capacity; values in bold correspond for each variable to the factor for which the loading is the largest.
Table 5. The relationships between the soil detachment capacity and unit stream power and values of statistical measurements for the studied species.
Table 5. The relationships between the soil detachment capacity and unit stream power and values of statistical measurements for the studied species.
Hydraulic parameter Species Equation R2 RMSE
Unit stream power Alnus subcordata Dc = 0.072φ0.994 0.77 0.006
Brachythecium plumose Dc = 0.147φ1.075 0.87 0.007
Gleditsia caspica Dc = 0.275φ1.138 0.90 0.009
Sambucus ebulus Dc = 0.217φ1.125 0.87 0.009
Notes: φ = unit stream power (m s−1); Dc = soil detachment capacity; R2 = coefficient of determination; RMSE = Root Mean Square Error; the values of R2 were reported at p < 0.01.
Table 6. Linear regression between the soil detachment capacity and shear stress for the studied species.
Table 6. Linear regression between the soil detachment capacity and shear stress for the studied species.
Species Linear regression Kr [s m-1] R2
Alnus subcordata Dc = 0.0017τ - 0.0030 0.0017 0.57
Brachythecium plumose Dc = 0.0030τ - 0.0055 0.0030 0.64
Gleditsia caspica Dc = 0.0045τ + 0.0064 0.0045 0.66
Sambucus ebulus Dc = 0.0039τ - 0.0060 0.0039 0.62
Notes: Dc = soil detachment capacity; Kr = rill erodibility; R2= coefficient of determination; the values of R2 were reported at p < 0.01.
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