The Influence of Typical Forest Types on Soil Erosion Resistance in the Water Source Areas of Central Yunnan

Yangyi ZHAO，Xu DUAN，Shumiao SHU

College of Environment Science and Engineering，Southwest Forestry University，Kunming 650224，China

1 Introduction

Soil erosion is amajor environmental hazard issue of common concern in theworld today［1－2］，and it can cause decline in land productivity.The eroded sedimentwould cause silting of downstream rivers，reservoirs and estuaries.Meanwhile，the sediment adsorbs the organic and inorganic pollutants，polluting the downstream water body［3－4］；there will bemore serious consequences if soil erosion occurs in thewater source areas.Yunnan Province is located in the southwestern mountainous areas，where farmers survive by deforestation and expansion of arable land.The sloping land is difficult to retain water，soil and fertilizer，resulting in low yields and economic poverty，thereby exacerbating thewater loss and soil erosion.According to studies［5－6］，more than 70%of sediment in the Songhuaba reservoir area in Kunming City is from the arable land reclaimed from the steep slope.Thus，the vegetation protection in water source areas is particularly important.Based on the special soil erosion in water source areas of Kunming，the vegetation construction has become the key to further improvingwater retention and soil conservation effect and thus indirectly promoting regional agricultural development，so the study of the influence of typical forest types on soil erosion resistance is of great significance to regional soil and water conservation.This paper studies the erosion resistance of soil under three typical forest typeswithin Yizhe small watershed in Kunming City，and explores the influence of different forest types on soil shear resistance，scourability resistance and erosion resistance，in order to provide a basis for the prevention and control of regionalwater loss and soil erosion.

2 Materials and methods

2.1 Overview of the study areaThe study area is in Yizhe smallwatershed in Central Yunnan（102°45′E，25°08′N），with elevation of 1985－2200m.It features a low-latitude plateau and mountainmonsoon climate，with the annual average temperature of 15℃.The annual precipitation is about 1031 mm，and the relative humidity is74%；the average annual sunshine is2200 h，and the annual frost-free period in recent years averages above 240 d.There aremany sunny days throughout the year，and the sunshine rate is 56%.The soil ismainly yellow loam soil.Themain forest types include three：evergreen broadleaf forest；mixed broadleafconifer forest；coniferous forest.The forest age is 15 years.The evergreen broadleaf forest is dominated by eucalyptus.The eucalyptus species is Eucalyptusmaideni，and the average tree height is 15.6－28.8m；the shrub is dominated by Myrsine Africana and Lysidice rhodostegia，and the average height is 0.3－1.5 m and 0.4－2.7 m，respectively；the herb is dominated by Eupatorium adenophorum.The canopy density is about0.55 to 0.75，and the cover degree is about 35%to 50%.Themixed broadleaf-conifer forest is mainly the mixed forest of eucalyptus and Pinus yunnanensis.The canopy density isabout0.65－0.80，and the cover degree is65%－75%.The shrub is dominated by Myrsine Africana and the herb is dominated by Eupatorium adenophorum.The coniferous forest is dominated by Pinus yunnanensis.The canopy density is about0.85－0.90，and the cover degree is about90%.

The shrub is dominated by Myrsine Africana，and the herb is dom-inated by Eupatorium adenophorum.

Table 1 Tree species composition and characteristics of p lot

2.2 Research methods

2.2.1Determination of soil physical properties.In July 2013，we set the standard plots（20m×20m）in the typical placeswith similar elevation and slope within the study area，and conducted stand survey.Wemeasured the density of trees，average height，average DBH and other stand characteristicswithin plots，and the basic information of plots is shown in Table1.Using5-pointmethod，we collect soil samples at 0－20cm and 20－40cm，and the test soil is yellow loam.The soil samples are packed and brought back to laboratory to be air-dried，ground and sieved for analysis.The cutting ringmethod is used for soil bulk density，porosity and other indicators，and the oven dryingmethod is used for soilmoisture content［7－8］.Soil shear strength ismeasured using the portable shear produced by EIJkelkamp Company，and three replications are set for each sample.

2.2.2Determination of soil anti-scouribility.Using the undisturbed soil scouring flume and experimentalmethod designed by Jiang Dingsheng［9－10］，we take 15×10×10 cm undisturbed soil samples from the soil surface layer by self-made soil sampler and soak the undisturbed soil samples prior to testing for24 h.The soil samples are kept still for 1 min and weighed after removing the gravitationalwater.They are placed in 50×10×10 cm scouring flume，and the slope is set at 20°for purposes of comparison.Scouring time is fixed at10 min.The soilwashed away is filtered and weighed after scouring，to calculate the impact factor.Antiscourability of soil（C）is used to evaluate the soil erosion resistance，defined as the product of the amount of water（Q／L）needed towash away 1 g of soil and time（t／min）.

where C is anti-scourability of soil and w is the weight of soil washed away（g）.

2.2.3Determination of soil erosion resistance index.We use a sieve with aperture diameter of5mm to select75 5－7mm soil aggregates and put them on the sievewith aperture diameter of5mm to be immersed in water，25 each time.The number of soil particles collapsed is recorded every 1min，and it is recorded continuously for10min.The process is repeated three times，and it isaveraged to calculate erosion resistance index.The specific formula［11］is as follows：

Erosion resistance index＝（total number of soil particlesnumber of soil particles disintegrated）／total number of soil particles.

3 Results and analysis

3.1 Soil physical properties of different types of forest land

We perform the analysis of soil physical properties under three typical forest types（Table 2）.For the same layer of soil in three plots，in the 0－20cm soil，Pinus yunnanensis forest land has the highest soil bulk density（1.57 g／cm3）；mixed broadleaf-conifer forest land has the lowest soil bulk density（1.30 g／cm3）；mixed broadleaf-conifer forest land has the highest total soil porosity，capillary porosity and non-capillary porosity（51.07%，36.89%and 14.18%，respectively）.In the20－40 cm soil，mixed broadleaf-conifer forest land has the highest soil bulk density（1.65 g／cm3）；eucalyptus forest land has the highest total soil porosity（44.65%）；Pinus yunnanensis forest land has the highest soil capillary porosity（42.21%）；eucalyptus forest land has the highest non-capillary porosity（12.14%）.For different layers of soil in the same plot，the soil bulk density ofmixed broadleaf-conifer forest land increaseswith increasing soil depth while the soil bulk density of eucalyptus forest land and Pinusyunnanensis forest land will decrease with increasing soil depth，mainly because of frequent human disturbance；the soil capillary porosity of mixed broadleaf-conifer forest land and Pinus yunnanensis forest land decreaseswith increasing soil depth while it increases for eucalyptus forest land.It can be found that themixed broadleaf-conifer forest land plays amore significant role in improving soil structure，protecting soil texture，increasing infiltration，reducing runoff and loweringwater loss and soil erosion，followed by eucalyptus forest land and Pinus yunnanensis forest land；mixed forest land plays a better role in improving soil structure than pure forest land.

3.2 Analysis of soil shear strength of different forest land typesTable 3 shows that there are significant differences in the shear strength values of soil in the same layer for eucalyptus forest，mixed broadleaf-conifer forest and Pinus yunnanensis forest land，and it shows an increasing trend.The shear strength values of soil in 0－20 cm layer is smaller than in 20－40 cm layer，possibly because the forest land subsurface soil ismore stable than the topsoil under root retaining effect.The soil shear strength of three typical plots is in the order of Pinus yunnanensis forest land＞mixed broadleaf-conifer forest land＞eucalyptus forest land，indicating that Pinus yunnanensis forest land has better soil shear strength than the other two types of forest land.

Table 2 Physical properties of soil under different forest types

Table 3 Soil shear strength values of different forest land types

3.3 Soil anti-scourability and erosion resistance of different forest land types

3.3.1The changes in soil anti-scouribility with slope.The antiscourability to a certain extent reflects the ability of soil to resist runoff erosion.As can be seen from Table 4，the average antiscourability of soil for various forest types is in the order of eucalyptus×Pinus yunnanensis mixed forests＞Pinus yunnanensis pure forest.Except Pinus yunnanensis forest land，the anti-scouribility coefficient in the 0－20 cm soil is greater than in the 20－40 cm soil；with the increase of slope，the anti-scourability shows a decreasing trend.The significance analysis shows that the antiscouribility coefficient of soil under different vegetation types at the same slope is significantly different，and with the increase of slope，the significant difference is also increasing.In the 0－20 cm soil layer（Table 4），the anti-scourability of soil undermixed broadleaf-conifer forest at slope of10°is1.08 and 1.25 times as high as that of soil under eucalyptus forest and Pinus yunnanensis f orest，respectively；when the slope becomes 25°，it is 1.09 and 1.26 timesashigh as thatof soil under eucalyptus forestand Pinus yunnanensis forest，respectively；when the slope becomes30°，the anti-scouribility coefficient of soil under mixed broadleaf-conifer forest is1.70 and 1.49 timesashigh as thatof soilunder theother forest types.The variation in the 20－40 cm soil layer is consistentwith that in the topsoil under three degrees of slope.From the overall slope change in the 0－40 cm soil，when the slope is10°，the anti-scourability of soil under mixed forest is 12.52%and 18.43%higher than under eucalyptus forestand Pinusyunnanensis forest，respectively；when the slope is30°，it is64.24%and 28.25%higher than under eucalyptus forestand Pinusyunnanensis forest，respectively.The above conclusion suggests that with increase of slope，mixed forest plays a better role than pure forest in improving the anti-scouribility of soil.

Table 4 Soil anti-scourability of typical forest land

3.3.2Analysis of soil erosion resistance under different forest types.Table 5 shows thatmixed broadleaf-conifer forest has the highest soil erosion resistance index（39.0%），followed by eucalyptus forest（37.0%）and Pinus yunnanensis forest（24.0%）；there is a small difference in the soil erosion resistance index between eucalyptus forest andmixed broadleaf-conifer forest（2%），while there is a large difference in the soil erosion resistance index between Pinus yunnanensis forest and eucalyptus forest，between Pinus yunnanensis forest and mixed broadleaf-conifer forest（13%and 15%，respectively）.There are significant differences in the soil erosion resistance index between different soil layers in the same plot，and the soil erosion resistance index decreaseswith increasing soil depth；there are no significant differences in soil erosion resistance index in the 0－20cm layer in different plots be-tween eucalyptus forest and mixed broadleaf-conifer forest，while there are highly significantdifferences in soilerosion resistance index between Pinus yunnanensis forest and the other two forest types；in the 20－40cm layer，there are significant differences in erosion resistance index among three forest types，indicating that the mixed eucalyptus and Pinus yunnanensis forest plays a better role than pure forest in improving soil erosion resistance.

Table 5 The soil erosion resistance index for different forest types

Fig.1 shows the changes in soil erosion resistance index in 0－20 cm and 20－40 cm soil layers for different forest land types within tenminutes.It can be found thatwithin ten minutes，the soil particles of two layers in three plots disintegrate completely，and the soil erosion resistance index is zero at the tenth minute.The soil particlesof Pinus yunnanensis forest land disintegrate rapidly，and the soil erosion resistance index is relatively small and turns to0 within five to sevenminutes for two soil layers.The soil particles ofmixed broadleaf-conifer forest land and eucalyptus forest land disintegrate slowly，and the soil erosion resistance index gradually decreases to 0 after eight to nineminutes.The soil antidisintegration property of Pinus yunnanensis forest land is poor，while the soil anti-disintegration property ofmixed broadleaf-conifer forest land and eucalyptus forest land is a little better，indicating that during the actual rainfall，the soil disintegration and loss are not serious in the early rainfall for eucalyptus forestand Pinus yunnanensis×eucalyptusmixed forest land，butwith the increase of rainfall，the probability ofwater loss and soil erosion is gradually increased for eucalyptus forest，and Pinus yunnanensis and eucalyptusmixed forest can significantly improve the erosion resistance of soil underneath.

3.4 Comprehensive evaluation of erosion resistance of soil under different forests

3.4.1Selection of soil erosion indicators.This paper selects seven indicators commonly used for the study of soil erosion resistance at home and abroad，namely soil bulk density（X1），total porosity（X2），capillary porosity（X3），non-capillary porosity（X4），anti-scourability of soil（X5），soil erosion resistance index（X6）and soil shear strength（X7）.The above indicator system is comprehensive，but it is burdensome and complex and the information of some indicators overlapswith each other.Therefore，we can use fewer new indicators to replace the original indicators and save the information of the original indicators asmuch as possible.Here we use principal component analysis to determine the soil erosion resistance of three forest land types，and grasp the influence of three forest types on soil erosion resistance.

3.4.2Principal component of soil erosion resistance and modeling.Table 6 is the resultof componentextraction after the principal component analysis.The characteristic root and contribution rate of component are the basis of selecting the common components，and the seven original variables of erosion resistance are transformed into seven components.It can be seen that the characteristic root of the first principal component is 5.35，indicating that the first principal component describes5.35 of total variance of the original variables，and the variance contribution rate is 76.42%，representing 76.42%of information of all components，so it is themost important component；the characteristic rootof the second component is 1.65，representing 23.575%of information of all components，second only to the first one；the contribution rates of other components descend one by one.The cumulative contribution rate of the first two components reaches 100%，indicating that the first two components have reflected all information of erosion resistance factors，so we can select the first two components as the comprehensive factors for erosion resistance evaluation.From Table 6，7，it can be found that for the first principal component，seven factors such as soil bulk density have great load；for the second principal component， except soil shear strength，all factors have great load.According to the load of factors in the two principal components，we can establish the princi-pal componentmodel of soil erosion resistance for different forest land as follows：

Table 6 All explanatory variables

Table 7 Principal component analysis of soil erosion resistance indicators for different forest land types

3.4.3Comprehensive evaluation of soil erosion resistance for different forest land types.The results of principal component analysis not only give the principal componentmodel of soil erosion resistance for different forest land types，but also derive the correlation coefficientsbetween variablesand factors.These correlation coefficients constitute the factor structure.Based on the weight of amount of information provided by the principal component，we calculate the composite scores of principal component［12－13］to evaluate the soil erosion resistance of different forest land types.The specific formula is as follows：

Using the score function of two principal components and composite score formula of principal components，we calculate the composite score of soil erosion resistance for three different forest land types.The comparison of soil erosion resistance is shown in Fig.2.The higher the comprehensive evaluation score，the stronger the soil erosion resistance.It can be found from Fig.2 that the soil erosion resistance of mixed broadleaf-conifer forest land is strongest，and the comprehensive evaluation score is0.150，while the soil erosion resistance of Pinus yunnanensis forest land isweakest，and the comprehensive evaluation score is negative（－0.0792）.The soil erosion resistance for three different forest land types is in the order ofmixed broadleaf-conifer forest land＞eucalyptus forest land＞Pinusyunnanensis forest land.The single forest type plays a limited role in improving physical and chemical properties of soil，soil structure，soil texture and soil erosion resistance，and its role in soil and water conservation is not significant.

4 Conclusions

In this paper，we study the influence of three types of typical forest on soil erosion resistance in water source area of Central Yunnan.The results show that there are significant differences in the shear strength values of soil in the same layer for eucalyptus forest，mixed broadleaf-conifer forest and Pinus yunnanensis forest land，and it shows an increasing trend.The shear strength values of soil in 0－20 cm layer is smaller than in 20－40 cm layer，possibly because the forest land subsurface soil ismore stable than the topsoil under root retaining effect.The soil shear strength of three typical plots is in the order of Pinus yunnanensis forest land＞mixed broadleaf-conifer forest land＞eucalyptus forest land，indicating that Pinus yunnanensis forest land has better soil shear strength than the other two types of forest land.There are no significant differences in the anti-scourability of soil for three different forest land types，and mixed broadleaf-conifer forest has the highest soil erosion resistance index（39.0%），followed by eucalyptus forest（37.0%）and Pinus yunnanensis forest（24.0%）.In the contextofheavy rain and long duration rainfall，the soilundermixed eucalyptus×Pinusyunnanensis forests playsamore significant role in resisting disintegration，that is，the erosion resistance is stronger.Using principal component analysis，we analyze the erosion resistance of soil under three types of forest land，and get principal component analysismodel of soil erosion resistance for three types of forest land：

Based on theweightofamountof information provided by the principal component，we calculate the composite scores of principal componentand get the comprehensive evaluation function：Y＝0.763Y1＋0.236Y2.It can be found that in terms of soil erosion resistance，the different types of forest land are sequenced in descending order ofmixed broadleaf-conifer forest land（0.150）＞eucalyptus forest land（0.127）＞Pinus yunnanensis forest land（－0.0790），which further indicates that themixed forests have betterwater loss and soil erosion control effect than pure forests.

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