Loess Region Research Articles

Spatial modeling of gully head erosion on the Loess Plateau using a certainty factor and random forest model

Gully head erosion significantly contributes to land degradation, and affects gully dynamics on the Loess Plateau of China. Modeling with a gully head erosion susceptibility map (GHEM) is an essential step toward appropriate mitigation measures. This study evaluates the effectiveness of two varieties of advanced data mining techniques—a bivariate statistical model (certainty factor (CF)) and a machine learning model (random forest (RF)) for the accurate mapping of gully head erosion susceptibility taking the Dongzhi Loess Tableland of China as an example at a regional scale. A database comprising 11 geographic and environmental parameters was extracted with 415 spatially distributed gully heads, of which 70% (291) were selected for model training and 30% (124) were used for validation. An accuracy evaluation using the area under the curve (AUC) value revealed that the CF model was 84.1% accurate, while the AUC value of the RF model map was 88.8% accurate. According to the RF method used to assess the relative significance of predictor variables, the most significant factors influencing the spatial distribution of the GHEM were the slope angle, slope aspect, topographic wetness index, and slope length. The GHEM can ultimately aid in decision making with respect to soil planning and management and sustainable development of the study area, which can be applied to other similar loess regions.

Effects of bacterial activity on the saturated hydraulic conductivity of remolded loess

Loess is widely used as a construction material in engineering projects on the Loess Plateau of China but is regarded as one of the most problematic soils due to water sensitivity. Hydraulic conductivity ( K ) is an essential parameter in assessment of water transfer in porous media. However, the influence of bacterial activity on the saturated permeability of remolded loess is still unclear and rarely being studied. We systematically studied the effects of Gram-negative bacterial activity (a common bacteria in the loess region) on the hydraulic properties of remolded loess, with the aim to better understand water flow through loess. A series of laboratory experiments were conducted to identify the variation in hydraulic conductivity under various bacterial inoculation and carbon constraint conditions. The results indicated that the K was relatively stable (0.14–0.16 m/d) at the initial phase and decreased by 81–93% reaching 0.01–0.05 m/d at the end of the experiments. The variation of hydraulic conductivity can be divided into the unaffected stage, linear reduction stage and stable stage. The growth of bacteria and the accumulation of extracellular polymeric substances occupied the pores between soil particles in the sample, which reduced the porosity and lead to the decrease of K . Higher available carbon contents and greater initial inoculum sizes speeded up the process of hydraulic conductivity reduction. The delayed effect was observed between rapid decline of hydraulic conductivity and exponential growth of bacteria. It should be highlighted that the reduction of hydraulic conductivity in the soil columns was mainly dependent on clogging in the inlet layer. Our findings provide a better insight into the change of hydraulic conductivity related to bacterial activities, and promote the reorganization of water cycle uncertainty and the prevention of engineering geological disasters in loess areas. • Saturated hydraulic conductivity decreased by 81–93% due to bacterial activities. • Delayed effect between linear reduction of hydraulic conductivity and exponential growth of bacteria. • Reduction of hydraulic conductivity is mainly due to bacterial growth in inlet layer.

Experimental Investigation into Compressive Behaviour and Preconsolidation Pressure of Structured Loess at Different Moisture Contents

This paper investigates the influence of the structured property of loess on its compressive behaviour and proposes a new method for determining the preconsolidation pressure of structured loess soil. A series of oedometer tests were carried out on undisturbed and remoulded loess samples prepared at various moisture contents. The effects of moisture content on the structured yield stress, the preconsolidation pressure, and the structural strength were also captured. It was found that the influence of the structured property of loess on the compression behaviour is divergent between undisturbed and remoulded loess samples. The discrepancy before and after structural yielding is more remarkable for the undisturbed soil. The Casagrande method realized through the MATLAB program can effectively eliminate human factors and accurately calculate the corresponding preconsolidation pressure for undisturbed soil. The effects of moisture content on the method for determining the preconsolidation pressure considering the structured property of loess were discussed. The determination method can accurately evaluate the loess consolidation state in loess regions. The influencing rules which the moisture content exerts on the structured yield stress, the preconsolidation pressure, and the structural strength all conform to exponential functions. The study is of great significance to correctly differentiating the foundation consolidation states and calculating the ground settlement in loess regions.

Continuous estimation of leaf area index and the woody-to-total area ratio of two deciduous shrub canopies using fisheye webcams in a semiarid loessial region of China

Automatic and accurate measurements of the leaf area index (LAI) can provide information about key parameters of vegetation structure in an ecosystem. Little attention, however, has been paid to LAI dynamics for shrub canopies in both arid and semiarid regions compared to forests and crops. One of the main sources of error when measuring shrub LAI from the indirect method is the woody part. The influence of woody parts on LAI estimations can be expressed as the woody-to-total area ratio, α; there are also relatively few studies on α value of shrubs, especially the dynamic changes of α. This study used a new cost-effective tool, the fisheye webcam, to derive daily LAI and α in the fields of Caragana korshinskii and Salix psammophila on the Loess Plateau in China. The LAI data were compared with measurements acquired by an LAI-2200 Plant Canopy Analyzer and direct methods. The LAI-2200 data were consistent with the effective plant area index (PAIeff) estimated using upward-pointing webcams (UPWs) (R2 = 0.89, root mean square error (RMSE) = 0.21) but not downward-pointing webcams (DPWs) (R2 = 0.52, RMSE = 0.68). Shrub PAIeff derived from the DPWs was much smaller than PAIeff derived from the UPWs over the season (y = 0.48x–0.03, R2 = 0.79, RMSE = 0.85). LAI derived from the direct methods was strongly linearly correlated with the PAI minus WAI obtained from upward digital hemispherical photography (R2 = 0.94, RMSE = 0.62 for C. korshinskii and R2 = 0.99, RMSE = 0.31 for S. psammophila). This result indicates that the upward digital hemispherical photography for deriving PAI and WAI performs good, which can be utilized to calculate LAI accurately. The woody-to-total area ratio (α) of deciduous shrub canopies was a dynamic parameter that varied throughout the season, decreasing as the canopy leaves developed and increasing when they fell. α = 1 for the leaf-off season and < 0.1 when the foliage peaked. α differed little between the two shrub canopies (R2 = 0.95, RMSE = 0.07), and it could be used to convert PAI to LAI for C. korshinskii and S. psammophila. The UPWs demonstrated excellent potential for continuous LAI measurement for the two shrub canopies, but the DPWs should be further tested before use. The fisheye-webcam is inexpensive and can easily monitor LAI at field scale and can be used to evaluate LAI products derived from remote sensing.

Numerical investigation of the tensile strength of loess using discrete element method

The tensile strength of loess is important for the tension-related crack and failure in the Loess regions. While the previous work on the tensile strength of loess mainly focuses on the test method and its variation with different internal and external factors, the micro-mechanical characteristics of the tensile experiments are seldom studied. Taking the unconfined penetrated test (UP test) as example, the tensile strengths and internal stress distributions of undisturbed loess samples under the loading plate-sample ratios (d/D) ranging from 0.05 to 0.95 are numerically investigated using discrete element method. The internal stress distribution, failure pattern of sample and variation of internal crack are clearly reflected by the discrete element modelling. The DEM simulation results also show that: 1) the d/D in the range of 0.15 to 0.35 is recommended for measuring the tensile strength of undisturbed loess; 2) the tensile strength of undisturbed loess increases with d/D recommended above ;3) the failure of sample is mainly controlled by the tensile stress when the d/D is equal or lower than a critical d/D 0.50, while that is mainly controlled by the compressive stress when the d/D is larger than a critical d/D 0.50. This study shows the capability of discrete element method to simulate the UP test, thereby providing us a relatively reliable way to investigate the tensile strength of loess numerically and to explore the internal stress and crack distribution of soil sample.

Earthen mounds in the Głubczyce Forest (SW Poland) – are they prehistoric long-barrows? Geoarchaeology of the Silesian soil record and human-environment interplay in the Holocene

Abstract Two earthen mounds, trapezoid in shape, oriented along the W-E/NW-SE axis and located in prominent landscape positions, were recently discovered in the Głubczyce Forest in the loess area of the Głubczyce Plateau (SW Poland). Their resemblance to long-barrows of the Funnel Beaker culture, as yet unknown in this part of Silesia prompted low-invasive research, involving ALS data analysis, magnetic prospection and a study of soil properties. The objective was to determine if these are indeed anthropogenic structures and if so, how and when were they built. The results indicate: 1) a transformation from chernozemic (Phaeozem) to clay-illuvial soil (Luvisol/Retisol) in the Głubczyce Forest area. Similar processes were identified in neighbouring Central European loess regions and linked with prehistoric climate/vegetation changes (the spread of dense, beech-dominant forests). Human management of the landscape (involving sustained deforestation), enabled the patchy preservation of chernozemic soils until the present-day, 2) both mounds are anthropogenic features, built on a Phaeozem using chernozemic soil. Their construction occurred before the soil transformation, i.e. most likely in prehistory. The development of the Głubczyce Forest may have taken place during the Migration period – a time of settlement decline in Silesia, and 3) the Głubczyce Forest bears further traces of anthropogenic activity: ancient agriculture (field systems), funerary practices, forest management and WWII combat.

Soil faunal community composition alters nitrogen distribution in different land use types in the Loess Plateau, China

Environmental factors and human disturbance can affect the richness and diversity of soil fauna and nutrients. However, few studies have systematically evaluated the soil faunal community composition and N distribution and the relationships between them in different land use types in the Loess region of China. In this study, we assessed the correlation between these factors and their variation among three land use types: woodland, grassland, and abandoned land. The results revealed Formicidae (34.9%), Isopoda (26.1%), and Lumbricidae (11.2%) to be the dominant groups of soil fauna in the Loess area. The soil faunal community composition was related to both the soil and land use types (p < 0.05). The effects of different land use types on soil physicochemical properties change the community distribution of soil fauna, thus, affecting the role of soil faunae in the N cycle. The differences in soil organic matter, soil moisture content, and bulk density affect the composition of the soil faunal community. The diversity index of soil faunae in the three land use types was the highest in woodland (0.59–2.31) and the lowest in abandoned land (0.41–1.92). The contents of total N, nitrate N, ammonium N, and microbial biomass N were the highest in woodland followed by grassland and abandoned land. Redundancy analysis indicated significant positive correlations between individuals of Lumbricidae and NH4+-N, individuals of Stylommatophora and NO3−-N, individuals of Acariformes and total N, and individuals of Isoptera and microbial biomass N. Different land use types, therefore, alter the diversity and richness of the soil faunae in the Loess region, and the soil faunal community composition plays a significant role in promoting the content of different N forms in the soil environment.

In situ field measurements of air permeability in eroded loess formations

Loess is an unsaturated soil and is widely distributed in arid and semi-arid areas in northwest China. Air permeability is a key parameter for determining the permeation performance of unsaturated soil. At present, the air permeability of loess is mostly determined by laboratory measurements with few studies using in situ measurements. In this paper, an improved loess air permeameter was used to measure the air permeability of in situ loess and undisturbed loess samples in three typical loess regions. The in situ air permeability measurement results obtained with the improved air permeameter were compared with laboratory (in-lab) permeameter measurements of undisturbed loess samples since the in-lab-based method is relatively mature. Experiments were conducted to investigate the influence of the void ratio, the water content and other parameters of the eroded loess formation on the air permeability of the in situ loess samples. The results showed that the in situ air permeability measurements obtained from the improved air permeameter are highly reliable. The air permeability of loess in different regions was highly correlated with the particle size distribution. The higher the clay content, the lower the number of effective permeation pores and the lower the air permeability of the loess. Increasing depth is positively associated with the water content and the dry density, but negatively related to the void ratio and air permeability. The air permeability was the highest near sinkholes, fissures, and trenches; it initially decreased with increasing distance from these features and stabilized eventually. The results of this study successfully provide technical information and support for engineering applications in loess areas.

Collapse Potential of Loess under Unloading Effect

Collapsible loess is generally characterized by a sudden and substantial decrease in volume that occurs when is applied under constant stress. To evaluate the loess collapse potential, the self-weight collapse and collapse coefficients have been defined by the code for building construction in collapsible loess regions. However, the method in the code does not account for the vertical stress variation. The loess collapse process commonly occurs with stress variation in practice. This paper documents a low-cost, quantitative evaluation scheme using regression analysis to evaluate the loess collapse potential by varying the unloading levels. The results show that the factors that prominently account for loess collapse deformation are the initial pressure, unloading ratio, and collapse completed-ratio. At a constant collapse-completed ratio, the remnant collapse coefficient significantly decreases with the decreasing unloading ratio; at a constant unloading ratio, the remnant collapse coefficient increases with a decreasing collapse-completed ratio. Decreasing unloading and collapse-completed ratios decreased the loess collapse potential with an initial pressure that exceeds the threshold value. Finally, an unloading collapse deformation calculation of loess was prepared to analyze practical project problems of loess based on the unloading collapse test.

Experimental Study on Landslides in Terraced Fields in the Chinese Loessial Region under Extreme Rainfall

Due to the development of the scale of tractor-ploughed terraces, terraces have been increasing in number, while global climate change is causing frequent extreme rainfall events in the Loess Plateau, resulting in many terrace landslides. To study the mechanism and process of shallow landslides and deep slip surface of terraces induced by extreme rainfall in loess hill and gully area, we conducted a laboratory model test of a terrace under artificial rainfall and used the Swedish arc strip method. The research results are as follows. The mechanism of shallow landslides in terraces is rill erosion accelerating rainfall infiltration, suspending the slope, and increasing its bulk density. The destruction process of shallow landslides can be roughly divided into six processes, and the earth volume of the landslide is 0.24 m3. The mechanism of the deep sliding surface in terraces occurs under the combined action of water erosion and gravity erosion. The soil moisture content increases, which decreases the anti-sliding moment and increases the sliding moment, and the safety factor becomes less than the allowable limit for terraces. The deep sliding deformation area of the terrace was 0~1.0 m below the slope surface, slip surface radius was 1.43 m, the slip surface angle was 92°, and the deep sliding surface began to form earlier than terraced shallow landslides. The displacement of the characteristic points increased from the slope top, to the slope center, and to the slope foot, with maximum displacements of 40.3, 15.5, and 6.0 mm, respectively.

Multifractal parameters of soil particle size as key indicators of the soil moisture distribution

Determining the soil moisture content (SMC) distribution is indispensable for field management, especially in arid and semiarid regions. Effective parameters can contribute to the optimization of SMC models for accurate SMC prediction. Therefore, in this study, the relationships between the SMC and multifractal parameters (D2, Dv, Ds, Av, and Fv denote the correlation dimension, property of small probability and large probability, spectrum width, and symmetry of spectrum shape, respectively) of the soil particle size distribution (PSD) were explored in the hilly loess region of China. A grid method was adopted (20 m × 20 m, total = 384 points) to sample the SMC in the 0–40 cm and 40–80 cm soil layers on the second, eighth, and twelfth days after the first rainfall, which were defined as the early sampling (ES), medium sampling (MS), and late sampling (LS), respectively. We found that the variation in the SMC explained by the multifractal parameters increased as SMC decreased, where they accounted for 40.07% and 75.75% of the SMC in the 0–40 cm and 40–80 cm soil layers in LS, respectively. The variation in the SMC in the 0–40 cm explained by the multifractal parameters was lower than that in the 40–80 cm soil layer in all sampling stages. The equations fitted for the whole soil layer were significant in all sampling stages (P < 0.01). Av (31.95%) and Dv (13.59%) had negative correlations with SMC, and their relative importance values with respect to SMC were high in all sampling stages (P < 0.05). SMC at higher values had a positive correlation with Fv, whereas SMC at lower values had a significant positive correlation with D2 and negative correlation with Ds , and these two parameters in the MS and LS stages explained more than 75% of the variation in the SMC in the 40–80 cm soil layer. These results suggest that the characteristics of the soil PSD can be described in detail by the multifractal parameters, which can directly reflect the SMC. Thus, we conclude that SMC is closely related to the multifractal parameters of the PSD (Av, Dv, Ds, D2, and Fv) and it can be applied to optimize SMC models.

Rill development and its change rate: a field experiment under constant rainfall intensity

Rill erosion is a widespread form of soil erosion on loess slope surfaces. The rill formation, evolutionary mechanism, and the quantitative expression of rill erosion dynamic changes in a micro-topographic level are currently lacking, primarily owing to lack of finer-scale measurements. This work presented the rill erosion evolutionary process and their dynamic changes at different slope gradients, (12°, 18°) in a millimeter-scale analysis. We achieved it by constructing an artificial excavation in the field and performed a soil pan with dimensions of 8 m in length and 2 m in width, which were subjected to a constant rainfall intensity of 100 mm h−1. The multiphase micro-geomorphologic changes during each rainfall phase were monitored using high-resolution terrestrial laser scanner (TLS). We noticed that the larger overland flow in the rill corresponded to more pronounced fluctuations in the cross-section lines of the rill bed. Also, with more the number of rills, the more easily the overland flow was dispersed, which required a larger slope gradient to resume the erosion and fill processes. Erosion rate and fill rate were highly correlated in general, and they increased abruptly at 70–90°. Due to the presence of swelling minerals, the loess surface expanded rapidly after encountering water in the early stage of rainfall. Therefore, the amount and rate of erosion on the slopes were underestimated. Understanding the detailed evolution process of rill erosion from a micro-perspective can provide a valuable reference for the development law of the initial stage of slope erosion in the loess region where erosion is extremely serious.

Ground Settlement analysis of Metro Interval using PBA method in the Loess region

In the construction of urban subway, the control of surface settlement has become the key and difficult point in the construction. As a common method in subway tunnel construction, PBA method has been widely used in many areas in China. This method performs well in settlement control. In addition, the supporting system of piles, beams, and arches formed during the construction process of this method effectively avoids the impact of the open-cut method on urban traffic. Therefore, this paper will combine the engineering conditions and the existing research results to simulate the various construction stages of the subway interval excavation in the loess area by establishing a Midas model. Combined with the results of numerical simulation, four increasing stages of surface settlement and corresponding settlement ratios are obtained, and the characteristics of surface settlement troughs are analyzed. This research is helpful to further understand the increasing law of ground settlement during the construction of metro interval in loess area.

A method for determining unsaturated strength parameters in stability analysis of loess slope

In recent years, with the rapid development of social economy and progress of human activities in loess area, the Loess Plateau become one of the areas with the most serious soil erosion and the most frequent occurrence of geological disasters in the world. Landslide, collapse, debris flow and ground subsidence are common geological disasters in the Loess Plateau, resulting in a more fragile ecological environment. Therefore, it is very important to accurately predict the stability of loess slope for engineering safety and ecological protection in loess region. But loess is a typical unsaturated soil. the formula of unsaturated strength is seldom used in practical applications. The reason is that the matric suction is difficult to measure. And it cannot be applied in engineering practice. In this paper, based on unsaturated soil shear strength formula of Fredlund, the direct shear test under different moisture content is conducted with the samples of Q1 loess. The effective cohesion and the effective internal friction angle are obtained. Through the matric suction test, the soil-water characteristic curve is plotted. Combined cohesion and matric suction, the strength parameters of unsaturated loess in formula of Fredlund can be informed.

Surface Settlement Analysis Induced by Shield Tunneling Construction in the Loess Region

The influence and prediction of shield tunneling construction on surface settlement (SS) and adjacent buildings is a hot topic in underground space engineering. In this work, several analytical methods are utilized to estimate the maximum surface settlement (MSS) and conduct a parametric sensitivity analysis based on Xi’an Metro line 2. The results show that there are mainly nine factors influencing the SS induced by shield tunneling construction in loess strata. The disturbance degree of the surrounding soil during the shield advancing stage has the largest influence on the SS, followed by the seepage of the shield lining segments or falling water levels, which lead to the overlying soil consolidation. After this is the grouting filling effect at the shield tail, followed by the reinforcement effect of the tunnel foundation and the track. The smallest influencing factors on the SS are the shield overexcavation and improper shield attitudes during the construction period. The sensitivity analysis results of the above influencing factors may offer a scientific guidance for the control of shield tunneling construction.

Livestock‐exclusion duration required for restoring grassland in semiarid, loess region in China: Estimate based on species composition measured from small‐scale vegetation patterns

AbstractTo restore the degraded grassland in the Yunwushan Natural Reserve of Ningxia semiarid loess region, Ningxia Autonomous Region, China, managers have established grasslands with excluding livestock from one large area every 5 years since 1982. The annual precipitation at the site is around 400 mm, and heavy grazing by sheep/goats has been conducted in the past. In July 2009 in which vegetation surveys were conducted, seven grasslands with different exclusion durations such as 0, 2, 7, 12, 17, 22 or 27 years had already been established. The objectives of survey were to clarify the vegetation changes with exclusion duration, and to estimate the number of years required for recovering the vegetation. In each of these grasslands, 100 50 × 50 cm2 quadrats, each of which was further divided into four 25 × 25 cm2 sub‐quadrats, were set, and then each of the species occurring within each sub‐quadrat was recorded. Aboveground biomass and soil hardness within each quadrat were also measured as auxiliary variables. We used eco‐statistical survey and analysis methods of small‐scale spatial patterns of vegetation. The followings are results obtained: after 12–17 years of exclusion, (a) the soil had softened; (b) the aboveground biomass had increased to amounts &gt;200 dw g m–2; (c) the spatial uniformity in biomass increased to a stationary state; and (d) the dissimilarity of species composition between two successive exclusion durations decreased to a stationary state. We conclude from these observations that the degraded grassland required 12–17 years of exclusion to restore vegetation and productivity. We also proposed candidates of plant species to be seeded or planted for recovering degraded grassland in this area.

Relationship between soil organic carbon density and topography in three-dimensional space in a small watershed of loess hilly region in China

In order to display the three-dimensional spatial distribution of soil organic carbon density (SOCD) and analyze the relationship between SOCD and topography, this study took 290 soil samples at the depth of 0 to 100 cm in the Nangou watershed in the loess hilly region of China, simulated the spatial distribution of SOCD by the three-dimensional inverse distance weighting method, and compared the SOCD of several soil sections of different slope shapes and different slope aspects when the land use were all forests. The results showed that the SOCD tended to decrease with increasing soil depth; the mean varied from 0.52 kg m–2 (60 to 100 cm) to 1.03 kg m–2 (0 to 10 cm), and there was obvious stratification in the vertical direction. The variation coefficients of the SOCD were between 23.43% (60 to 100 cm) and 41.15% (10 to 20 cm), which were relatively close in the topsoil of the depth of 0 to 30 cm and gradually decreased with soil depth. The SOCD in the watershed was higher in the northeast, followed by the southwest and lower in the middle. The three-dimensional spatial distribution of the SOCD contained more information, and its prediction accuracy was higher than that of the corresponding two-dimensional method. In slopes where the land use were all forests, the SOCD was ranked as concave slope > straight slope > convex slope and shady slope > sunny slope, and its complexity was ranked as concave slope > convex slope > straight slope and shady slope > sunny slope. Our results highlighted the carbon (C) sequestration ability of concave slopes on the shady side and provided scientific reference for the development of slope C sequestration policies in the loess hilly region of China.

The Impact of Agricultural Irrigation on Landslide Triggering: A Review from Chinese, English, and Spanish Literature

Landslides are considered a natural process, with hundreds of events occurring every year in many regions of the world. However, human activities can significantly affect how stable a slope or cliff is, increasing the chances of slope collapses. Moreover, agricultural irrigation has potential to saturate subsurface materials well below ground level and is known to be an important factor that can trigger landslides in many countries. A macroregional literature review of irrigation-induced landslides was developed in this investigation, considering what has been published in Chinese, English, and Spanish. A total of 115 peer-reviewed papers, books and book chapters, graduate and undergraduate theses, and government reports were found, including 82 case studies (23 in Chinese, 26 in English, and 33 in Spanish). Results from this analysis indicate that studies focusing on this important topic have increased exponentially since the 1960s, with most irrigation-induced landslides occurring in dry climates (precipitation less than 600 mm/year), with rainfall concentrated during summer months. The majority of studies have been done in the loess region of China (Asian region), followed by Peru (Latin American region), though cases were found from other macroregions (African, Indian, Russian, Angloamerican, and Indonesian). Based on this global review, new agricultural irrigation projects located in collapsible areas must include a landslide risk analysis. Cultivated areas can follow a series of measures to minimize the chances of triggering a landslide, which would put human lives, ecosystems, food production, and infrastructure at risk.

Developing Additive Systems of Biomass Equations for Robinia pseudoacacia L. in the Region of Loess Plateau of Western Shanxi Province, China

The accurate estimation of forest biomass is important to evaluate the structure and function of forest ecosystems, estimate carbon sinks in forests, and study matter cycle, energy flow, and the effects of climate change on forest ecosystems. Biomass additivity is a desirable characteristic to predict each component and the total biomass since it ensures consistency between the sum of the predicted values of components such as roots, stems, leaves, pods, and branches and the prediction for the total tree. In this study, 45 Robinia pseudoacacia L. trees were harvested to determine each component and the total biomass in the Loess Plateau of western Shanxi Province, China. Three additive systems of biomass equations of R. pseudoacacia L., based on the diameter at breast height (D) only and on the combination of D and tree height (H) with D2H and DbHc, were established. To ensure biomass model additivity, the additive system of biomass equations considers the correlation among different components using simultaneous equations and establishes constraints on the parameters of the equation. Seemingly uncorrelated regression (SUR) was used to estimate the parameters of the additive system of biomass equations, and the jackknifing technique was used to verify the accuracy of prediction of the additive system of biomass equations. The results showed that (1) the stem biomass contributed the most to the total biomass, comprising 51.82% of the total biomass, followed by the root biomass (24.63%) and by the pod and leaf biomass, which accounted for the smallest share, comprising 1.82% and 2.22%, respectively; (2) the three additive systems of biomass equations of R. pseudoacacia L. fit well with the models and were effective at making predictions, particularly for the root, stem, above-ground, and total biomass (R2adj &gt; 0.812; root mean square error (RMSE) &lt; 0.151). The mean absolute error (MAE) was less than 0.124, and the mean prediction error (MPE) was less than 0.037. (3) When the biomass model added the tree height predictor, the goodness of fit R2adj increased, RMSE decreased, and the accuracy of prediction was much improved. In particular, the additive system, which was developed based on DbHc combination prediction factors, was the most accurate. The additive system of biomass equations established in this study can provide a reliable and accurate estimation of the individual biomass of R. pseudoacacia L. in the Loess region of western Shanxi Province, China.

Effects of soil nutrients and stand structure on aboveground net primary productivity of oak secondary forests in the forest–steppe transition zone of Loess Plateau, China

Liaodong oak (Quercus liaotungensis Koidz.) dominated forests contribute to the few natural forest ecosystems in the semiarid Loess Plateau region in China. This study investigated the influence of soil nutrients and stand structure on aboveground net primary productivity (ANPP) in the oak communities. We aimed to understand the major factors and their possible pathways that determine the ecosystem productivity. The biomass and the ANPP were estimated using plot surveys and litterfall collection in fixed plots. Linear regressions and structural equation models were used to evaluate the relationships among soil nutrients, stand structure (including vertical and horizontal structure), and ANPP. The results showed that the effect sizes of soil nutrients on ANPP and leaf production were near 60% and 70%, respectively. More than 70% of the variation in woody production was explained by stand structure, while litterfall production was largely dependent on stand vertical structure with effect size of 64%. The effect of soil nutrients on stand structural indices was detected only in the case of the vertical structure. The results suggested that soil nutrients could affect forest productivity both directly and indirectly in coordination with stand vertical structure, and that the effect of stand structure was limited in these communities.