Plant Spatial Pattern Predicts Hillslope Runoff and Erosion in a Semiarid Mediterranean Landscape

Abstract The importance of the spatial pattern of vegetation for hydrological behavior in semiarid environments is widely acknowledged. However, there is little empirical work testing the hypothetical covariation between vegetation spatial structure and hillslope water and sediment fluxes. We evaluated the relationships between vegetation structural attributes (spatial pattern, functional diversity), soil surface properties (crust, stone, plant, and ground cover, and particle size distribution) and hillslope hydrologic functioning in a semiarid Mediterranean landscape; in particular, we tested whether decreasing patch density or coarsening plant spatial pattern would increase runoff and sediment yield at the hillslope scale. Runoff and sediment yield were measured over a 45-month period on nine 8 × 2-m plots that varied in vegetation type and spatial pattern. We grouped vegetation into functional types and derived plant spatial pattern attributes from field plot maps processed through a GIS system. We found that there was an inverse relationship between patch density and runoff, and that both runoff and sediment yields increased as the spatial pattern of vegetation coarsened. Vegetation pattern attributes and plant functional diversity were better related to runoff and sediment yield than soil surface properties. However, a significant relationship was found between physical crust cover and plant spatial pattern. Our results present empirical evidence for the direct relationship between the hydrologic functioning of semiarid lands and both the spatial pattern and the functional diversity of perennial vegetation, and suggest that plant spatial pattern, physical crust cover, and functional diversity may be linked through feedback mechanisms.     

Remote sensing data predict indicators of soil functioning in semi-arid steppes,central Spain

A substantial part of current research efforts on desertification are devoted to establish monitoring systems to evaluate the status of natural resources and the onset of desertification processes. Methodologies based on ground-collected soil and plant indicators are being increasingly used for this aim because they are affordable yet do not compromise accuracy. Despite their inherent value, these methods have limitations regarding the extent of the area that can be monitored using them. Such limitations can be overcome combining field-based approaches with remote sensing data, which allow the establishment of monitoring programs over large areas. In this article we tested the relationship between a field methodology based on indicators of ecosystem functioning, the landscape function analysis (LFA), and a vegetation index (NDVI) obtained from satellite images of the ASTER sensor using data gathered in Stipa tenacissima steppes from central Spain. LFA uses soil surface indicators to assess the condition of a given ecosystem by producing three numerical indices (stability, infiltration and nutrient cycling) reflecting the status of basic soil functions. We found a significant positive linear relationship between the NDVI, the three LFA indices and some key structural attributes of vegetation related to the cover of perennial plants. Our results indicate that NDVI can be used as a surrogate of ecosystem functioning in semi-arid Mediterranean steppes, and thus can be a helpful index to monitor the functional status of large areas in these ecosystems, and the possible onset of desertification processes.     

干旱半干旱区坡面覆被格局的水土流失效应研究进展

植被恢复与建设是控制水土流失的重要措施.在干旱半干旱地区,植被在空间上的分布呈现离散特征,在坡面上形成的裸地-植被镶嵌和植被条带分布等覆被格局对水土流失过程具有重要影响.覆被格局与水土流失关系研究是景观生态学格局与过程研究的重要内容.从植被斑块、坡面覆被格局对水土流失的影响与耦合覆被格局与水土流失的手段和方法三个方面对国内外相关研究进行总结分析.从中发现,植被类型、层次结构和形态特征是植被斑块尺度上影响径流泥沙的关键因素;不同覆被类型的产流产沙特征与覆被格局的准确描述是研究覆被格局水土流失效应的重要基础;坡面覆被格局对水土流失的影响关键在于其改变了径流泥沙运移和汇集的连续性,应重点关注径流泥沙源汇区的连通性和空间分布在水土流失中的作用;以坡面的精确覆被制图为基础,建立基于水土流失过程的覆被格局指数和耦合格局信息的径流与侵蚀模型是定量研究覆被格局影响的有效手段.今后应加强以下研究:以动态系统的观点研究覆被格局的变化对水土流失的影响,系统理解覆被格局与水土流失之间的相互联系与反馈机制,探讨两者间动态关系随尺度变化的规律性并发展尺度上推方法;构建过程意义明确且简单实用的格局指数,将覆被格局与水土流失过程有机联系起来;发展覆被格局的动态信息与产流产沙过程相结合的水文模型,加强对格局与径流泥沙反馈系统的耦合,建立真正意义上的覆被格局-水土流失过程耦合模型.     

Soil Loss and Runoff in Semiarid Ecosystems: A Complex Interaction Between Biological Soil Crusts, Micro-topography, and Hydrological Drivers

Biological soil crusts (BSCs) cover non-vegetated areas in most arid and semiarid ecosystems. BSCs play a crucial role in the redistribution of water and sediments and, ultimately, in the maintenance of ecosystem function. The effects of BSCs on water infiltration are complex. BSCs increase porosity and micro-topography, thus enhancing infiltration, but, at the same time, they can increase runoff by the secretion of hydrophobic compounds and clogging of soil pores upon wetting. BSCs confer stability on soil surfaces, reducing soil detachment locally; however, they can also increase runoff, which may increase sediment yield. Although the key role of BSCs in controlling infiltration–runoff and erosion is commonly accepted, conflicting evidence has been reported concerning the influence of BSCs on runoff generation. Very little is known about the relative importance of different BSC features such as cover, composition, roughness, or water repellency, and the interactions of these attributes in runoff and erosion. Because BSC characteristics can affect water flows and erosion both directly and indirectly, we examined the direct and indirect effects of different BSC features on runoff and erosion in a semiarid ecosystem under conditions of natural rainfall. We built structural equation models to determine the relative importance of BSC cover and type and their derived surface attributes controlling runoff and soil erosion. Our results show that the hydrological response of BSCs varies depending on rainfall properties, which, in turn, determine the process governing overland flow generation. During intense rainfalls, runoff is controlled not only by rainfall intensity but also by BSC cover, which exerts a strong direct and indirect influence on infiltration and surface hydrophobicity. Surface hydrophobicity was especially high for lichen BSCs, thus masking the positive effect of lichen crust on infiltration, and explaining the lower infiltration rates recorded on lichen than on cyanobacterial BSCs. Under low intensity, rainfall volume exerts a stronger effect than rainfall intensity, and BSC features play a secondary role in runoff generation, reducing runoff through their effect on surface micro-topography. Under these conditions, lichen BSCs presented higher infiltration rates than cyanobacterial BSCs. Our results highlight the significant protective effect against erosion exerted by BSCs at the plot scale, enhancing surface stability and reducing sediment yield in both high- and low-magnitude rainfall events.     

植被与水土流失关系研究进展

水土流失是世界性的环境问题之一,对人类社会可持续发展构成威胁,控制水土流失成为迫切需要,有许多水土流失控制措施,而生物措施尤其植被一直是人们研究的焦点.根据前人的研究,从斑块、坡面和流域/区域尺度总结了植被与水土流失的关系.斑块尺度植被对降雨和径流侵蚀能量具有很大的减弱或消除作用,可以改变植株底部的土壤性质,改善其结构,进而降低土壤可蚀性,增加入渗能力,从而减轻土壤侵蚀程度.不同植被类型、植被的不同层次结构,不同植被的形态结构具有不同的土壤侵蚀控制作用.坡面尺度主要从坡位、坡度、坡向对植被生长和分布格局的影响、对水土流失过程和格局的影响以及裸地-植被镶嵌格局、植被的条带格局对水土流失的影响和反映水土流失过程的景观格局指数的构建等方面进行了研究.更多是从植被恢复及其水土流失效应方面进行了探讨,为退化生态系统恢复和格局设计提供了极其有用的信息.流域/区域尺度植被与水土流失的关系更大程度上受到气候、地貌特征的影响,因此研究多从一定气候条件控制的土地覆盖（植被覆盖）及其格局的水土流失效应方面进行的.由于大尺度监测非常困难,研究多从遥感监测、GIS集成和模型模拟方面开展,是流域、区域等大尺度生态安全格局设计的有力支持.前人的研究为生态环境建设和保护提供了大量参考信息,但仍有一些问题需要进一步探讨,对此进行了初步的概括和归纳,希望能够对植被和水土流失关系的研究起到一定促进作用.     

Evaluating the performance of multiple remote sensing indices to predict the spatial variability of ecosystem structure and functioning in Patagonian steppes

Assessing the spatial variability of ecosystem structure and functioning is an important step towards developing monitoring systems to detect changes in ecosystem attributes that could be linked to desertification processes in drylands. Methods based on ground-collected soil and plant indicators are being increasingly used for this aim, but they have limitations regarding the extent of the area that can be measured using them. Approaches based on remote sensing data can successfully assess large areas, but it is largely unknown how the different indices that can be derived from such data relate to ground-based indicators of ecosystem health. We tested whether we can predict ecosystem structure and functioning, as measured with a field methodology based on indicators of ecosystem functioning (the landscape function analysis, LFA), over a large area using spectral vegetation indices (VIs), and evaluated which VIs are the best predictors of these ecosystem attributes. For doing this, we assessed the relationship between vegetation attributes (cover and species richness), LFA indices (stability, infiltration and nutrient cycling) and nine VIs obtained from satellite images of the MODIS sensor in 194 sites located across the Patagonian steppe. We found that NDVI was the VI best predictor of ecosystem attributes. This VI showed a significant positive linear relationship with both vegetation basal cover (R² = 0.39) and plant species richness (R² = 0.31). NDVI was also significantly and linearly related to the infiltration and nutrient cycling indices (R² = 0.36 and 0.49, respectively), but the relationship with the stability index was weak (R² = 0.13). Our results indicate that VIs obtained from MODIS, and NDVI in particular, are a suitable tool for estimate the spatial variability of functional and structural ecosystem attributes in the Patagonian steppe at the regional scale.     

Indicators of landscape organization and functionality in semi-arid former agricultural lands under a passive restoration management over two periods of abandonment

Abandoned lands previously used for agricultural purposes may constitute an opportunity for understanding the variables involved in the restoration of native ecosystems over time. In this study, we assessed the functional status of an abandoned farmland currently used for conservation, using a methodology based on indicators of landscape organization and soil surface. We analyzed changes in plant cover, patch and interpatches structure and several soil surface properties during two periods of land abandonment (less and more than 40 years). Using this methodology, we characterize the potential of the ecosystem to capture and transfer resources, and the state of functional properties such as infiltration capacity, soil stability and nutrient cycling.We detected a significant development of the shrub layer and an increase of the number, type and area occupied by vegetated patches in old fields. The contribution of vegetated patches to the recovery of stability, infiltration and nutrient functions was significant at the local scale. Nonetheless, when analyzing the landscape as a whole only the nutrient cycling index was significantly higher in old fields. The lack of improvement of the stability and infiltration in old fields can lead to further degradation and indicates that even though vegetation cover may have improved over time in old fields, the current cover might not be enough to prevent further degradation by erosion. Our results reinforce the importance of using functionality indexes in future studies focused on adequate restoration measures to protect the function of desert ecosystems, especially when aimed for biodiversity conservation. Hence, the use of monitoring systems based on organization and function indicators is a useful tool to represent the current state and potential recovery of previously disturbed ecosystems, provided that the time scale is taken into account.     

The pervasive and multifaceted influence of biocrusts on water in the world's drylands

The capture and use of water are critically important in drylands, which collectively constitute Earth's largest biome. Drylands will likely experience lower and more unreliable rainfall as climatic conditions change over the next century. Dryland soils support a rich community of microphytic organisms (biocrusts), which are critically important because they regulate the delivery and retention of water. Yet despite their hydrological significance, a global synthesis of their effects on hydrology is lacking. We synthesized 2,997 observations from 109 publications to explore how biocrusts affected five hydrological processes (times to ponding and runoff, early [sorptivity] and final [infiltration] stages of water flow into soil, and the rate or volume of runoff) and two hydrological outcomes (moisture storage, sediment production). We found that increasing biocrust cover reduced the time for water to pond on the surface (?40%) and commence runoff (?33%), and reduced infiltration (?34%) and sediment production (?68%). Greater biocrust cover had no significant effect on sorptivity or runoff rate/amount, but increased moisture storage (+14%). Infiltration declined most (?56%) at fine scales, and moisture storage was greatest (+36%) at large scales. Effects of biocrust type (cyanobacteria, lichen, moss, mixed), soil texture (sand, loam, clay), and climatic zone (arid, semiarid, dry subhumid) were nuanced. Our synthesis provides novel insights into the magnitude, processes, and contexts of biocrust effects in drylands. This information is critical to improve our capacity to manage dwindling dryland water supplies as Earth becomes hotter and drier.     

基于SIMWE模型的典型水土保持措施侵蚀阻控路径分析——以通双小流域为例

针对东北黑土区长缓坡地形条件下坡面产汇流集中易加剧土壤侵蚀的问题,本研究基于GIS和SIMWE(SIMulated Water Erosion)模型,引入连通性指数和水深空间分布作为水文连通性与径流路径的衡量指标.通过量化不同典型水土保持措施对土壤入渗速率和地表曼宁糙率的影响,构建梯田数字高程模型(DEM)模拟表征地表...     

Ecohydrological Source‐Sink Interrelationships between Vegetation Patches and Soil Hydrological Properties along a Disturbance Gradient Reveal a Restoration Threshold

Vegetation, soil, and hydrology in drylands often collectively exhibit strong ecohydrological interrelationships in which vegetation both influences and is influenced by runoff, particularly on sites with more gradual slopes. These two‐way relationships have important implications for ecological restoration of disturbed sites, such as those being reclaimed following mining, yet studies from both ecological and hydrological perspectives specifically evaluating how the strength of ecohydrological interrelationships varies for a range of natural and restored conditions are still missing. We assessed two‐way relationships between vegetation and soil hydrological properties by evaluating patterns of both plant community structure and soil hydrological characteristics related to runoff for natural sites and restored sites following mining. At the plot scale, we identified eight ecohydrological units based on interrelationships between vegetation communities and hydrological properties associated with runoff along a progression from source to sink patch types. Similarly, at the hillslope scale, which included patches of different types, we found a correspondence between the proportions of source and sink patches and both vegetation community and hydrological properties. The relative strength of ecohydrological interrelationships in hillslope mosaics decreased with decreasing disturbance except for rilled hillslopes, likely because parts of the hillslope become isolated from the others. Our results highlight, in general, how ecohydrological interrelationships are related with degree of disturbance, and in particular, how rilling alters ecohydrological interrelationships, thereby precluding effective restoration.     

黄土高原景观格局与水土流失关系研究

采用DCCA排序法对黄土高原腹地泾河流域12个子流域的景观格局与流域水土流失关系进行了定量分析.结果表明,DCCA排序的前4轴分别与农业用地比率、景观多样性指数、森林比率显著相关.各子流域的水土流失特征具有明显的梯度变异.在森林比率占65%的三水河子流域,景观相对简单、多样性低,流域年径流量大、输沙小、含沙量低,径流相对稳定;随着森林比率减小,农业用地比率增大,景观多样性升高,产流系数增高,径流深度、输沙量和含沙量增大;在森林比率很低、农业用地53.41%的洪河子流域,景观格局复杂、多样性较高,河流含沙量高、输沙率大,月输沙和径流变异极大;在农业用地比率减小,其他景观类型比率增大,景观相对简单的环江上、下游子流域,输沙量和含沙量减小,但输沙和径流的年际变化极大.排序分析结果较清晰地解释了黄土高原典型地区水土流失特征沿景观梯度的变化规律.     

Patch Configuration Non-linearly Affects Sediment Loss across Scales in a Grazed Catchment in North-east Australia

Abstract A principle of the cross-scale interaction (CSI) framework is that disturbance-induced landscape changes resulting in coarser-grained spatial structure may non-linearly amplify transfer processes across scales. We studied suspended sediment losses at two spatial scales (0.24 m² plots and ca. 0.25 ha hillslopes of about 140 m in length) in a semiarid savanna landscape to determine whether the spatial structure of grassy and bare soil areas introduced a non-linear amplification of sediment loss. Sediment loss rates from 0.24 m² bare plots averaged 1.527 t ha⁻¹ y⁻¹, which was 23 times the loss rate from nearby grassy plots (0.066 t ha⁻¹ y⁻¹). These rates were then extrapolated linearly to two hillslopes separated by only 200 m and having similar total grass cover, slope and soil type but differing in the spatial structure of bare soil patches. The coarse-grained hillslope had a large bare patch on its lower slope, whereas the fine-grained hillslope had no bare soil patches when quantified at a 4 m grid-cell resolution. Measured sediment loss from the fine-grained hillslope averaged 0.050 t ha⁻¹ y⁻¹, whereas the average sediment loss from the coarse-grained hillslope was 2.133 t ha⁻¹ y⁻¹. By linearly extrapolating from the plot scale, the expected sediment loss for the fine-grained hillslope was 0.066 t ha⁻¹ y⁻¹, which is similar to that observed. The expected sediment loss for the coarse-grained hillslope was 0.855 t ha⁻¹ y⁻¹, where linear extrapolation assumed a 46:54 ratio of bare to grassy plots and that the spatial arrangement of plots does not affect sediment loss processes. For the coarse-grained hillslope observed sediment loss is 2.5 times greater than that expected by linear extrapolation from the plot scale. This result indicates a cross-scale interaction related to spatial configuration of patches. We suggest that there were non-linearities in hillslope ecohydrological transfer processes (runoff, erosion) across scales due to a specific patch configuration that greatly amplified sediment loss because the pattern failed to slow runoff and retain sediment before it entered a creek. This example supports the CSI framework and indicates the importance of considering the effect of spatial structure when predicting system dynamics at different scales.     

自然降雨下黄土丘陵区草灌植物垂直覆盖结构的减流减沙效应

黄土高原是我国水土流失和生态环境问题最为严重的地区之一,植被恢复是防治水土流失的重要措施。植物垂直覆盖结构包括地上冠层、地表枯落物和地下根系,各组分具有不同的水土保持作用,是研究植被与水土流失关系的基本单元。目前,关于植物垂直覆盖结构不同组分对土壤侵蚀影响的研究主要是基于人工模拟降雨,缺少自然降雨条件下不同植物的垂直覆盖结构对产流、产沙和入渗等多过程影响的系统研究。本研究以黄土丘陵区典型的草本(须芒草)、半灌木(铁杆蒿)和灌木(绣线菊)为研究对象,每种植物进行三种处理(自然状态、去除枯落物和仅留根系)以及裸地对照,观测2015—2016年降雨事件的产流产沙量和入渗量,分析植物不同垂直覆盖结构的减流减沙效益及其相对贡献。结果表明:三种植物均具有较好的减流(45.9%—73.2%)、减沙效益(87.5%—94.6%)和增加入渗作用(4.7%—10.8%),灌木的减流效果(73.2%)显著高于草本(45.9%)和半灌木(63.5%),但三种植物间的减沙效益没有显著性差异。冠层的减流作用最大,贡献率接近一半(48%—50%),草本枯落物的减流贡献率与根系基本一致,而半灌木和灌木枯落物的减流贡献...     

生物结皮分布格局对坡面流水动力特征的影响

生物结皮的分布格局是影响坡面产流产沙的因素之一,但其格局对侵蚀动力的影响机理尚不明确,妨碍了进一步解析生物结皮影响坡面产流产沙的机制。本研究通过室内模拟降雨试验,采用景观生态学方法,对带状、棋盘和随机3种分布格局下生物结皮坡面景观指数和水力学参数的变化规律与裸土坡面进行对比研究。结果表明: 生物结皮显著影响坡面侵蚀动力,其分布格局是影响坡面侵蚀动力的因素之一。与裸土相比,生物结皮显著降低了坡面流速(54.6%)和弗劳德数(67.0%),显著增加了径流深(86.2%)和阻力系数(10.68倍),但对雷诺数和径流功率的影响较小;除径流深外,3种格局的生物结皮坡面的各水力学参数均存在显著差异,其中,随机格局对坡面侵蚀动力的影响最大。基于因子分析和聚类分析,筛选出斑块占景观面积比例、斑块密度、景观形状指数、斑块连结度和分离度5个指标作为描述生物结皮斑块分布特征的指标。其中,生物结皮斑块间的斑块连结度和分离度是影响坡面侵蚀动力的主要格局因子。随着生物结皮斑块间斑块连结度减小、分离度增大,坡面流速增加,阻力减小,坡面侵蚀加剧。     

Crust Composition and Disturbance Drive Infiltration Through Biological Soil Crusts in Semiarid Ecosystems

Soil crusts influence many soil parameters that affect how water moves into and through the soil, and therefore, critically influence water availability, erosion processes, nutrient fluxes, and vegetation distribution patterns in semiarid ecosystems. Soil crusts are quite sensitive to disturbance, and their alteration can lead to modification of the local hydrological regime, thus affecting general functioning of the ecosystem. The aim of this study was to analyze the influence of different types of soil crusts, physical, and biological in different developmental stages, as well as the impact of their disturbance, on infiltration. This was assessed by means of rainfall simulations conducted in two semiarid ecosystems in southeast Spain characterized by different lithologies, topographies, and soil crust distributions. Two consecutive rainfall simulation experiments (50 mm h⁻¹ rainfall intensity), the first on dry soil and the second on wet soil, were carried out in microplots (0.25 m²) containing the most representative soil crust types at each site, each crust type subjected to three disturbance treatments: (a) undisturbed, (b) trampling, and (c) removal. Infiltration in the crusts was higher on coarse- than on fine-textured soils and almost two times greater on dry than on wet soil. Biological soil crusts (BSC) showed higher infiltration rates than physical soil crusts (PSC). Within BSC, infiltration increased as cyanobacterial biomass increased and was the highest in moss crusts. However, late-successional crustose and squamulose lichen crusts showed very low infiltration rates. Trampling reduced infiltration rates, especially when soil was wet, whereas crust removal enhanced infiltration. But this increase in infiltration after removing the crust decreased over time as the soil sealed again due to raindrop impact, making runoff rates in the scraped microplots approach those registered in the respective undisturbed crust types. Our results demonstrate that water redistribution in semiarid ecosystems strongly depends on the type of crusts that occupy the interplant spaces and the characteristics of the soils which they overly, as well as the antecedent moisture conditions of the soil. Disturbance of these crust patches results in increased runoff and erosion, which has important consequences on general ecosystem functioning.     

壤中流和土壤解冻深度对黑土坡面融雪侵蚀的影响

融雪侵蚀是东北黑土区土壤流失的一种重要形式,而目前有关壤中流和土壤解冻深度对融雪径流侵蚀的影响研究较少。本研究采用室内模拟试验,设计两个融雪径流量(1和4 L·min^-1)和两个土壤解冻深度(5和10 cm),以及有、无壤中流处理,分析壤中流和土壤解冻深度对黑土区坡面融雪侵蚀的影响。结果表明: 1)壤中流处理下坡面融雪径流深度和侵蚀量分别是无壤中流处理的1.1～1.2倍和1.3～1.9倍。两个融雪径流量下,当土壤解冻深度由5 cm增加到10 cm时,无壤中流处理下坡面融雪径流深度和侵蚀量分别增加10.0%～13.5%和15.4%～37.1%;而有壤中流处理下坡面融雪径流深度增加6.5%～8.5%,融雪侵蚀量则无显著变化。2)坡面细沟发育受壤中流、土壤解冻深度和融雪径流量的综合影响,各处理下细沟侵蚀量占坡面融雪侵蚀量的72%以上。3)壤中流发生使坡面径流流速和径流剪切力分别增加20.3%～23.2%和37.0%～51.3%,Darcy-Weisbach阻力系数减少9.0%～21.4%,从而增加了坡面融雪侵蚀量;且壤中流发生促进了坡面细沟发育,其细沟侵蚀量较无壤中流处理增加43.6%～69.9%,也导致坡面融雪侵蚀量增加。无壤中流条件下,土壤解冻深度加剧坡面融雪侵蚀的主要原因是随着土壤解冻深度的增加,坡面径流侵蚀能力和可蚀性物质来源增加,导致融雪径流侵蚀量增加。此外,土壤解冻深度对壤中流条件下细沟形态发育也有明显的影响,土壤解冻深度为5 cm时,细沟横向加宽作用显著;而土壤解冻深度为10 cm时,细沟下切侵蚀作用更显著。本研究加深了对黑土区融雪侵蚀机理的认识,可为水蚀模型的研发提供理论指导。     

黄土丘陵沟壑区植被对不同空间尺度水沙关系的影响

根据晋西离石试验站的观测结果和相关文献的数据,研究了黄土丘陵沟壑区植被对坡面小区、全坡面、小流域及中大流域4个空间尺度水沙关系的影响。对梁峁坡坡面小区言,由于植被提高了土壤抗蚀性,因此植被不仅通过减水来减沙,也通过改变水沙关系来减沙。但在全坡面尺度,土壤侵蚀以沟蚀为主,由于植被措施难以改变沟道的各种水利参数,也难以有效控制切沟沟壁的重力侵蚀,导致水流进入沟道后仍然可以获取充足的泥沙,因此认为植被不会改变全坡面尺度的水沙关系。同样对于各级流域,由于植被措施难以改变沟道的输沙能力和黄土丘陵沟壑区流域泥沙来源充沛的特点,因此植被措施也不会改变其水沙关系,植被的减沙效应仅通过减水来实现。对离石试验站的一对水土保持对比沟的研究表明,即使在沟道已有茂密植被生长的情况下,高含沙水流的输沙能力也没有改变,这使得两者的水沙关系统计上可以认为完全一致。由于大流域水沙关系主要取决于沟道或河道的特性,而植被等坡面措施很难改变沟道或河道特性,因此认为流域尺度越大,植被越难以改变其水沙关系。     

Small-scale Environmental Heterogeneity and Spatiotemporal Dynamics of Seedling Establishment in a Semiarid Degraded Ecosystem

In semiarid environments, surface soil properties play a major role in ecosystem dynamics, through their influence on processes such as runoff, infiltration, seed germination, and seedling establishment. Surface soil properties usually show a high degree of spatial heterogeneity in semiarid areas, but direct tests to evaluate the consequences of this heterogeneity on seedling establishment are limited. Using a combination of spatial analysis by distance indices (SADIE) and principal components analysis (PCA) we quantified the spatiotemporal patterns of seedling survival of a Mediterranean native shrub (Pistacia lentiscus) during the first 3 years after planting on a semiarid degraded site in southeastern Spain. We used a variation partitioning method to identify environmental variables associated with seedling survival patterns. Three years after planting, only 36% of the seedlings survived. During the first summer, one-third of the seedlings died, with secondary major mortality in the 3rd summer after planting. The spatial pattern of survival became strongly clumped by the end of the first summer, with clearly defined patches (areas of high survival) and gaps (areas of low survival). The intensity of this pattern increased after subsequent high-mortality periods. Of the 14 variables, the ones most strongly coupled to seedling survival were bare soil cover, sand content, and soil compaction. These findings contribute to our understanding of the linkages between the spatial heterogeneity of abiotic factors and the response of plant populations in semiarid degraded ecosystems and can be used to optimize restoration practices in these areas.     

植被格局对土壤入渗和水沙过程影响的模拟试验研究

合理的植被格局能够提高土壤入渗性能和抗冲性,有效阻蚀减沙,平衡土壤保持与土壤水分保蓄二者关系,进而促进生态环境的改善。基于径流小区人工模拟降雨,研究了多种植被格局生物量密度（0、50%、100%）和分布方式（坡上分布、坡下分布、均匀分布）的土壤入渗、产流、产沙特征以及土壤储水量变化。结果表明：使用Philip模型、Kostiakov模型、Horton模型模拟坡面入渗过程,Horton模型的拟合结果最优。不同植被格局产流率的变化趋势基本一致,大体可分为两个阶段：初期阶段迅速增长,中后期阶段增长变缓并逐渐趋于稳定状态。产沙率的变化趋势随植被格局的不同而略有差异。相较于产流过程,产沙过程变化剧烈、规律性差。总体而言,降低生物量密度能够增加降雨期间的径流量（从19.21 mm到25.44-38.09 mm再到51.79 mm）和侵蚀量（从118.97 g/m²到237.57-597.90 g/m²再到1400.29 g/m²）,土壤水分得到更好的保蓄。从植被分布方式的角度来看,均匀分布的植被格局有利于更好地控制土壤侵蚀和径流,却促进了土壤水分的消耗。坡下分布的植被格局,例如植被过滤带的形式,能够最好地同时控制水土流失和土壤耗水。权衡考虑水土流失防治与土壤水分消耗,建议采用适宜密度（本研究为110 g/m²）且集中分布在出口附近的植被格局。