The Importance of Soil Characteristics in Determining Survival of First‐Year Cottonwood Seedlings in Altered Riparian Habitats

Most major rivers in the southwestern United States have been hydrologically altered to meet human needs. Altered hydrological regimes have been associated with declines in native riparian forests. Today, many riparian areas have little or no regeneration of native riparian species and are now dominated by exotic Saltcedar (Tamarix chinensis Lour.). Success of riparian restoration efforts at least partially depends on the number of seedlings surviving the first growing season. Seedling survival is influenced by many abiotic and biotic factors including competition from other plants and available soil moisture, which is partially dependent on soil texture. In this study, we evaluated the relative importance of four soil categories (sandy loam, loam, silt, and clay), rate of soil moisture decline, salinity, beginning‐ and end‐season Saltcedar density, initial Cottonwood (Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.) seedling density, percent vegetation cover by potential dominant competitors Pigweed (Amaranthus L.) and Barnyard grass (Echinochloa crusgalli L., Beauv.), and average total vegetation height to Cottonwood seedling survival. Factors influencing seedling survival differed among the four soil types. Rate of moisture decline was important in sandy soils, whereas vegetation height influenced seedling survival in loamy soils. Overall, models of seedling survival in all the four soil types indicated rate of moisture decline as the single most important variable influencing Cottonwood survival. High initial densities of Saltcedar were correlated to higher survival in Cottonwood seedlings. Therefore, it is important to identify soil texture and understand soil moisture decline rates when proposing riparian Cottonwood restoration.     

Do understorey or overstorey traits drive tree encroachment on a drained raised bog?

• One of the most important threats to peatland ecosystems is drainage, resulting in encroachment of woody species. Our main aim was to check which features – overstorey or understorey vegetation – are more important for shaping the seedling bank of pioneer trees colonising peatlands (Pinus sylvestris and Betula pubescens). We hypothesised that tree stand parameters will be more important predictors of natural regeneration density than understorey vegetation parameters, and the former will be negatively correlated with species diversity and richness and also with functional richness and functional dispersion, which indicate a high level of habitat filtering.
• The study was conducted in the ‘Zielone Bagna’ nature reserve (NW Poland). We assessed the structure of tree stands and natural regeneration (of B. pubescens and P. sylvestris) and vegetation species composition. Random forest and DCA were applied to assess relationships between variables studied.
• Understorey vegetation traits affected tree seedling density (up to 0.5‐m height) more than tree stand traits. Density of older seedlings depended more on tree stand traits. We did not find statistically significant relationships between natural regeneration densities and functional diversity components, except for functional richness, which was positively correlated with density of the youngest tree seedlings.
• Seedling densities were higher in plots with lower functional dispersion and functional divergence, which indicated that habitat filtering is more important than competition. Presence of an abundant seedling bank is crucial for the process of woody species encroachment on drained peatlands, thus its dynamics should be monitored in protected areas.

     

Role of the soil Seed Bank in the restoration of a native pinewood at Glen Garry,Inverness-shire

Summary

The potential recovery of ground vegetation in a pinewood at Glen Garry, from which introduced tree species had been removed, was studied by sampling the upper soil seed bank and comparing the species composition of germinating seedlings with extant vegetation. Mean numbers and species of seedlings emerging differed between a control site (not under-planted) and two formerly under-planted sites cleared at different times. Germinating seedling numbers also differed with depth (0–10 cm). Calluna vulgaris and Juncus spp. dominated fifteen species germinating from the seed bank, while some other pinewood species were not found. Although seedling pine occurred, their survival will be affected by competition from Betula spp. and Deschampsia flexuosa. Other elements of the vegetation will recover from the seed bank or by migration, their distribution being influenced by topographic variations and the nature of the developing tree stand. Timing of clearance of exotics in relation to pine seed production and soil scarification might accelerate recovery of the pinewood flora generally.     

Relationships of marsh seed banks to vegetation patterns along environmental gradients

• 1 The relationship of the seed bank to the vegetation of a freshwater marsh was studied along gradients of water depth and soil organic matter content. Characters examined included standing crop, seedling density, and species composition, distribution and richness.
• 2 The seed bank differed from the vegetation in that only nine of twenty-seven species were present in both, abundant seed-bank species were uncommon as adults, and adults showed different distributions along a gradient of soil organic matter content whereas their seeds were most abundant in soils with high organic matter.
• 3 The seed bank resembled the vegetation in that separate multivariate analyses of the communities revealed that variation in the species composition of each was significantly correlated with water depth and soil organic matter content. Further, species richness in both communities decreased with water depth and increased with soil organic matter content. Lastly, the standing crop of the vegetation and the number of seedlings both decreased with water depth and increased with soil organic matter.
• 4 Consideration of spatial patterns and environmental gradients revealed more similarities between vegetation and seed banks than were obtained by comparing species lists. The results suggest that artificial stimulation of seed bank germination for management purposes will not produce vegetation changes as large as those suggested by differences in species lists.

     

腾格里沙漠人工植被区掘穴蚁(Formica cunicularia)的生态功能

掘穴蚁（Formica cunieularia）营筑丘活动,并通过营巢活动影响土壤的物理化学特性,是中国干旱和半干旱地区营巢的优势蚁类。在沙坡头人工固沙植被区调查了掘穴蚁对不同固沙年代和不同地形人工固沙区土壤的影响;研究了掘穴蚁的营巢活动对土壤水分、理化性质和种子库的影响。 研究结果表明：随着固沙年代的延长,掘穴蚁的筑丘活动加强;掘穴蚁筑丘活动对地形的选择顺序依次分别为：丘间低地〉迎风坡〉丘顶〉背风坡;与邻近土壤相比,蚁丘的土壤含水量升高,土壤容重（0～10cm深）、有机质、电导率、全氮、全磷、全钾、可溶性氮、可溶性磷和可溶性钾在蚁丘富集;土壤容重（10～20cm深）和pH值下降;蚁丘对土壤种子库有明显影响（p〈0．01）,种子密度和多样性指数在不同取样点间的大小顺序为：苔藓结皮上的蚁丘〉苔藓结皮〉地衣结皮上的蚁丘〉地衣结皮。 在沙坡头人工固沙植被区,由于生态环境的改善,特别是土壤环境的提高,导致掘穴蚁大量定居,它的筑巢对土壤具有反馈作用,这种反馈作用导致土壤异质性增强,改善了土壤环境的状况,加速了土壤和植被的演变。     

Effects of Mulch on Seedlings and Soil on a Closed Landfill

Landfills are ideal locations for short‐rotation forestry, tree‐based bioremediation, and wildlife habitat reconstruction. However, tree survival is reduced by harsh growing conditions and poor soil quality. This study was designed to determine survival of tree seedlings and test effects of different mulch types and depths on edaphic properties and herb emergence on a closed Ohio landfill. In spring 2002, five species of hardwood tree seedlings (Fraxinus pennsylvanica, Robinia pseudoacacia, Prunus serotina, Quercus macrocarpa, and Populus sp.) were planted in a clustered fashion on 35 plots. Each plot received a mulch treatment consisting of one type of mulch (hardwood, leaf, or mixed) at a specific depth (5 or 15 cm) or was designated as a nonmulched control. The status of each seedling and edaphic qualities (soil moisture, temperature, and nutrients) were recorded for two growing seasons. Aboveground herbaceous biomass was sampled at the end of each growing season. We found that survival and growth were species dependent. At the conclusion of the experiment, Fraxinus had the highest survival rate (70%) and Prunus the lowest (7%). The use of leaf mulch (15 cm deep) increased organic matter and various edaphic properties (cation exchange capacity, P, Mg) after the second growing season. Mulch treatments did not significantly affect aboveground herbaceous biomass when compared to nonmulched controls. These results suggest Fraxinus as a good candidate for landfill plantings. Additionally, leaves and yard waste are plentiful and could quickly enrich poor soils on a landfill if left to decompose aboveground.     

Root vertical distributions of two Artemisia species and their relationships with soil resources in the Hunshandake desert,China

1. Plant root variations and their relations with soil moisture and nutrient supply have been well documented for many species, while effects of drought, combined with extreme poor soil nutrients, on plant roots remain unclear.
2. Herein, we addressed root vertical distributions of two typical xerophyte semishrub species, Artemisia sphaerocephala and A. intramongolica, and their relations with soil moisture, total soil nitrogen and carbon contents in arid Hunshandake desert, China. The two species experienced similar light regimes and precipitation, but differed in soil moisture and soil nutrients.
3. Root vertical distribution patterns (e.g., coarse root diameter, root depth and root biomass) differed considerable for the two species due to high heterogeneity of soil environments. Coarse and fine root biomasses for A. intramongolica, distributed in relatively moist fixed dunes, mainly focused on surface layers (94%); but those for A. sphaerocephala dropped gradually from the surface to 140 cm depth. Relations between root traits (e.g., diameter, root biomass) and soil moisture were positive for A. intramongolica, but those for A. sphaerocephala were negative.
4. In general, the root traits for both species positively correlated with total soil nitrogen and carbon contents. These findings suggest that both soil moisture and poor soil nutrients were the limiting resources for growth and settlement of these two species.

     

Initiating Autogenic Restoration on Shallow Semiarid Sites

Our objectives were to evaluate the use of microcatchments in the establishment of Leucaena retusa (little-leaf leadtree) and Atriplex canescens (four-wing saltbush) and their role in the initiation of autogenic landscape restoration processes on a shallow semiarid site. Three six-month-old seedlings of either Leucaena retusa or Atriplex canescens were planted in 1.5-m² microcatchments. An equal number of seedlings was planted in control plots (unmodified soil surface). The water collection effects on shrub survival, standing biomass, and the natural immigration of herbaceous vegetation were determined over 42 months. Planting in microcatchment basins doubled Leucaena seedling survival and resulted in a five-fold increase in standing biomass, compared to the control, during the first growing season. There was a significant increase in soil organic matter in the microcatchment basins within 32 months. At the same time, microcatchments planted with Atriplex canescens seedlings had a ten-fold increase in seedling standing biomass compared to the control. Forty-two months after transplanting, the herbaceous standing crop was significantly greater near Atriplex canescens or in microcatchment basins than in plots with unmodified surface soil. Basins containing Atriplex produced significantly more herbaceous vegetation than basins containing Leucaena, and empty basins produced the least herbaceous vegetation of three basin treatments. These data suggest that landscape-scale procedures that concentrate scarce resources (water, organic matter, nutrients, and propagules), establish keystone species, and ameliorate microenvironmental conditions can initiate autogenic restoration of degraded semiarid ecosystems.     

祁连山排露沟流域青海云杉天然更新与土壤水热的关系

青海云杉是祁连山区森林生态系统的建群种,其天然更新对维持祁连山森林生态系统功能具有重要意义。以祁连山排露沟流域青海云杉林天然更新为研究对象,于2021年6-9月测定海拔2700-3300m的0-40cm土层土壤温度和水分数据,并于7月中旬对样地天然更新进行每木检尺,将天然更新划分为幼苗(第一龄级)阶段和幼树(第二龄级)阶段,通过方差分析探究不同海拔更新苗胸径、株高的差异,再进一步建立回归模型拟合更新苗胸径、株高与各层土壤水热的关系,探讨更新苗胸径、株高对土壤水热的响应。结果表明:(1)更新苗胸径、株高随海拔升高而降低,海拔2700m幼苗胸径与3300m幼苗胸径存在显著性差异(P<0.05),海拔2700m、2800m幼苗、幼树株高与3300m幼苗、幼树株高存在显著性差异(P<0.05),幼树胸径在各海拔均不存在显著性差异,表明更新苗胸径对海拔的敏感性要低于更新苗株高。(2)更新苗胸径、株高与土壤温度呈正相关,与土壤水分呈负相关,幼苗胸径与20cm土壤温度和土壤水分的相关系数最大,幼树胸径与40cm土壤温度和土壤水分相关系数最大,幼树株高与40cm土壤温度及20cm土壤水分相关系数最大。(3)回归模型显示幼苗胸径和株高对土壤水热的利用模式相同,而幼树胸径和株高对土壤水热的利用模式存在差异。(4)青海云杉天然更新随着龄级的增加,影响生长的主要因素由土壤温度逐渐变为土壤水分,更新苗胸径对土壤水热的响应也随着龄级的增加在逐渐减弱。     

半干旱黄土区坡面尺度柠条生长状况及影响要素分析

以半干旱黄土丘陵区典型小流域坡面大规模人工种植柠条林为例,基于坡面不同部位柠条生长状况和生境条件调查,定量分析了地形变化、土壤水分及灌木密度对柠条生长的直接、间接影响及其贡献率。结果表明:(1)东坡大株柠条生长明显好于南坡,下坡位柠条生长状况略好于中上坡位,其他各坡位之间柠条生长状况差异较小;(2)大株柠条生长与浅层土壤水分有正相关关系,而与灌木密度和深层土壤水分则呈负相关关系;大株柠条灌木高度、灌木纵截面积和冠幅体积对浅层土壤水分的响应敏感,冠幅长度对坡向和坡位的响应较为敏感,冠幅宽度对灌木密度的响应较为敏感;(3)地形和土壤水分变化解释了59.9%的大株柠条生长变异,其中坡向、坡位和浅层土壤水分是影响大株柠条生长的主导环境因子,它们分别解释了21.1%、16.0%和13.1%的柠条生长变化。研究认为半干旱黄土区人工植被恢复既要重视空间布局,也要在后期实施必要的管理措施以维持人工林地的稳定性。     

Soil seed bank of the waste landfills in South Korea

The restoration of urban landfill is a topic of growing interest in reclamation ecology as the acreage of abandoned sites near cities increases. The goals of this study were to assess the ecological status of waste landfills and to elucidate the role of seed banks in the establishment of vegetation at these sites. The study sites were located at five landfills around Seoul and Kyongki Province. On average, soils were sampled on 20 plots per landfill in 2001 to record species composition and to estimate the number of seeds in the soil. Soil seed bank vegetation and the individual number of seedlings that germinated were recorded using the seedling emergence method. Relative density per species was calculated from the number of individual seedlings. We conducted canonical correspondence analysis (CCA) using the program CANOCO to survey the relationships between 23 environmental variables and plant importance values. Environmental variables included categorical and numerical variables (landfill age, landfill size, distance from landfill edge, human disturbance level, slope, periodic management level) and soil physico-chemical variables (bulk density, soil moisture content, organic matter content, total N, available P, K, Na, Ca, Mg, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn). The mean seedling density per m ² differed significantly among sites (P < 0.05). As landfill age increased, the mean seedling density per m ² decreased. The mean seedling density of the Sangpaedong landfill, which was less than 1 year old, was higher than that found in 6- and 7-year-old landfills. The Sangpaedong landfill mainly contained seeds of Chenopodium albumL. and Digitaria ciliaris(L.) SCOP. With regard to early vegetative colonization in landfills, our results highlighted the importance of seed banks occurring in cover soils. Cover soils, derived from various sources, will determine landfill landscapes because of different seed banks present in them. The first axis of the CCA was correlated with landfill age, Na, and human disturbance level, while the second axis was correlated with landfill size, slope, periodic management level, Zn, total N, and organic matter content. Understanding seed banks in landfill cover soils is important, therefore, for proper landfill management and restoration.     

Intrinsic biotic factors and microsite conditions drive seedling survival in a species with masting reproduction

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Intercanopy community structure across a heterogeneous landscape in a western juniper‐encroached ecosystem

Questions: How does landscape position influence biotic and abiotic attributes of western juniper (Juniperus occidentalis) encroached ecosystems? How does intercanopy plant community structure respond to changes in soil moisture and temperature based on juniper cover and topographic position? Location: Steens Mountain, southeast Oregon, USA. Objectives: Competition with western juniper modifies plant community composition, alters soil hydrology, and reduces plant productivity. Research is needed to understand these influences across heterogeneous landscapes. This study characterizes the relationship between juniper encroachment and soil water, soil temperature, topographic position, and intercanopy plant community structure. Methods: Using a completely randomized block design, plant density and cover, percent bare ground, percent soil moisture, soil temperature, heat accumulation, and elevation were sampled in 10 m² plots representing low (<1%), moderate (～14%), and high (～27%) juniper cover at four aspects. The relationship and difference between vegetation patterns and environmental variables were analyzed using AOV, NMS, and MRPP (α=0.1). Indicator species analysis tested for shifts in dominant species along ecological gradients. Results: Soil moisture remained higher in low juniper cover sites than moderate and high juniper cover sites. North‐facing sites had highest soil moisture at 5 cm depth with low and moderate juniper cover levels. With increasing soil temperature from May to June, soil moisture declined by 19.7% at 5 cm depth. Achnatherum lemmonii and Pseudoreogneria spicata occurred in closed juniper stands while Achnatherum occidentale and Leymus cinereus were common when encroachment was limited. Application: This approach can be used to predict ecosystem response to western juniper encroachment across heterogeneous landscapes.     

山西太岳山小流域土壤水分空间异质性及其影响因子

以山西太岳山华北落叶松林地为主的小流域作为研究对象,采用地统计学方法结合地理信息系统(GIS)技术手段,研究了接石沟小流域土壤水分(0—60cm)的空间变异特征,以及植被分布和地形因子对其影响规律。结果表明:在时间稳定性的前提下,土壤水分含量和变异系数随土层加深逐渐降低。三层土壤水分半方差函数的最优拟合模型为球状模型,变程范围在1.1—1.4 km,均具有强烈的空间自相关性,其中0—20 cm和20—40 cm层土壤水分的空间异质性程度高于40—60 cm土层,以中间层的结构因素占总变异比例最大。自然结构因素(地形、母质、植被和土壤等)对不同土层土壤水分的总空间变异性起主导作用(81.4%—91.3%),而随机因素(取样误差、人为干扰等)的影响相对较小(8.7%—18.6%)。沿着集水线由西-东方向,从边缘的土壤水分高值斑块区逐渐过渡到明显的低值斑块区,梯度变化明显。研究发现,在植被覆盖异质性小的山地,土壤水分的空间异质性主要由地形因素引起,具体表现为其与坡向指数(TRASP)、坡度、海拔和土壤有机碳、全氮呈极显著相关关系(P0.01),而与植被指数(NDVI)呈弱的负相关关系。叠加分析显示,在阴坡、坡度较缓(15°)及高海拔叠合的区域土壤水分含量较高。研究结果可为山地人工林构建和植被恢复中土壤水资源的利用以及水分管理策略的制定提供理论依据。     

秦王川湿地柽柳分布与环境因子的关系

环境因子与植被分布的关系是生态学研究的重要内容,对揭示植物种群的形成机制和生态适应策略、预测植被的空间分布具有重要意义。为探究小尺度下干旱区湿地生境中环境因子对柽柳(Tamarix gansuensis H.Z.Zhang)空间分布的影响,在秦王川国家湿地保育区,基于遥感影像和数字高程模型,运用Maxent模型分析了海拔、坡度、坡向、坡位、离河流距离、土壤含水量、土壤含盐量等7个环境变量对柽柳空间分布的影响。结果表明:柽柳在秦王川国家湿地保育区内呈聚集分布,多分布在土壤含水量15%—30%、土壤导电率≤5 ms/cm、离河流18—70 m、海拔1890—1913 m、沟间地或沟坡地上;其空间分布主要受土壤含水量、含盐量和离河流距离3个环境变量的影响;海拔、坡度和离河流距离3个环境变量与土壤含水量、含盐量负相关(P0.05),坡位变量则与之相反。土壤水、盐含量等土壤因子是影响湿地植被分布格局形成的主要环境因子,水文因子和地形因子通过控制湿地土壤水、盐含量,改变局部环境条件,间接影响湿地植被的分布。     

半干旱黄土区苜蓿退化对坡面草本植物分布及多样性的影响

半干旱黄土丘陵区土壤水分亏缺引起人工苜蓿草地退化会显著影响其他草本植物的分布及多样性,然而地形驱动下的苜蓿草地退化及植被群落多样性响应还尚不清楚。以典型半干旱黄土丘陵区龙滩小流域为研究区,对不同地形条件下退化苜蓿草地地上生物量、草本多样性及生长季内0—200 cm土壤水分进行了定位监测,利用方差分析、相关分析和典范对应分析(CCA)明确坡面地形、苜蓿生长状况和土壤水分与其他草本植物分布及多样性之间的关系。结果表明:(1)地形显著影响植被群落特征,西坡、东坡和北坡样带苜蓿地上生物量明显不同,西坡和东坡样带中、下坡位苜蓿地上生物量明显高于上坡位,而其他草本的生物量、物种丰富度和多样性指数的变化趋势则与苜蓿相反;(2)苜蓿地上生物量与80—200 cm土壤水分显著正相关,而与0—20 cm和20—80 cm土壤水分的相关性较小;(3)地形特征、不同深度土壤水分和苜蓿地上生物量解释了退化苜蓿草地其他草本群落变异的87.8%,其中坡向、苜蓿地上生物量、0—20 cm和20—80 cm土壤水分4个因子解释了79.3%的群落变异。研究认为,半干旱黄土丘陵区不同地形条件引起坡面土壤水分变化,进而影响退化苜蓿草地地上生物量,使得苜蓿退化程度不同,而苜蓿退化程度和0—80 cm土壤水分决定了不同部位草本分布及多样性。     

Survival,growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient

In herbaceous dominated patches and ecosystems, tree establishment is influenced partly by the ability of woody seedlings to survive and grow in direct competition with herbaceous vegetation. We studied the importance of season long wet and dry spells on the competitive interactions between herbaceous vegetation and oak seedlings along a light and nitrogen gradient in an infertile secondary successional grassland in central North America. We conducted a field experiment in which seedlings of bur oak (Quercus macrocarpa) and northern pin oak (Q. ellipsoidalis) were exposed to two levels of light (full sun and 80% shade), three levels of nitrogen input (0, 5, 15 g m^–1 yr^–1), and three levels of water input (low, medium and high). In addition, seedlings were grown with and without the presence of surrounding herbaceous vegetation under both light and all three water levels. Seedling survival, growth, and rate of photosynthesis were significantly affected by competition with herbaceous vegetation and these effects varied along the multiple resource gradient. Overall, seedling survival of both species was significantly greater in wetter and shaded plots and when surrounding herbaceous vegetation was removed and was lower in nitrogen enriched plots. We found that soil water was significantly affected by varying inputs of water, light, and the presence or absence of herbaceous vegetation, and that seedling survival and rate of photosynthesis were highly correlated with available soil water. Our findings show that the impact of season long wet and dry spells on tree seedling success in grasslands can be affected by light and soil nitrogen availability.     

Impact of early root competition on fitness components of four semiarid species

Summary Plant demographic and root exclusion approaches were used to examine the influence of roots of adult Artemisia tridentata, Agropyron desertorum, and Agropyron spicatum individuals on seedling survival of four C₃ semiarid species, three perennials, Ar. tridentata, Ag. desertorum, Ag. spicatum, and an annual, Bromus tectorum. Furthermore, height of Ar. tridentata seedlings and seed production of B. tectorum were assessed. The probability of a seedling being alive significantly depended on the seedling species, the neighboring adult species, and on the depth to which root competition was excluded. As seedlings, survival of Agropyron species did not differ, whereas survival of Ar. tridentata seedlings was higher than Ag. desertorum and was similar to Ag. spicatum. Bromus tectorum maintained significantly higher survival rates than perennial seedlings. Established individuals of Ar. tridentata reduced seedling survival more than established individuals of either Agropyron species. Seedling survival significantly increased with greater depth of root exclusion for the perennials but did not significantly affect seedling survival of B. tectorum. Height of Ar. tridentata seedlings and seed production of B. tectorum significantly increased with depth of root exclusion. Seed production of B. tectorum was highest when competing with Ag. desertorum and was lowest with Ar. tridentata. Root competition decreased the seed population of B. tectorum in the next generation even though it had no impact on survival. Competition in the upper soil horizon occurs between seedlings and established adults early in the growing season and potentially restricts root growth of seedlings. In arid and semiarid ecosystems, soil moisture is depleted from the upper horizons first, resulting in the death of seedlings that do not have access to moisture.     

黄土丘陵沟壑区坡面尺度土壤水分空间变异及影响因子

土壤水分空间分布特征及其影响因子是土壤前期含水量模拟和小流域产流机制研究的重要内容,也是半干旱地区进行生态建设的重要参考。通过对黄土高原典型坡面雨季前后100 cm深度内土壤含水量进行观测,分析地形、植被和雨季对土壤水分空间分布的影响。基本统计分析显示,土壤水分的空间异质性在上层(<20 cm)较小,在下层(>40 cm)较大。坡面尺度上,土壤含水量的空间差异主要表现在不同植被类型之间,而不是坡位之间。各覆被类型的土壤含水量相对大小为荒草地>8年生刺槐林>20年生刺槐林>沙棘林。即使沙棘林和刺槐林位于更利于获取土壤水分的地形条件下,其土壤含水量仍然明显低于荒草地。地形对土壤水分的影响被植被类型的影响所掩盖。上述规律在雨季前后都有明显表现。因此,完全基于地形指数的土壤水分预测模型在黄土高原应该慎用,植被类型应该作为土壤水分空间预测的一个重要参数。雨季使土壤含水量整体提高,但是土壤水分空间分布格局并没有根本改变,高处仍高,低处仍低,各样点处的土壤含水量在雨季前后达到显著相关水平,说明土壤水分空间格局并不是瞬时状态,而具有明显的时间稳定性。     

Habitat degradation,topography and rainfall variability interact to determine seed distribution and recruitment in a sand dune grassland

Question: Understanding the mechanisms underlying how habitat degradation, topography and rainfall variability interactively affect seed distribution and seedling recruitment is crucial for explaining plant community patterns and dynamics. Interactions between these major factors were studied together in a semiarid sand dune grassland. Location: Eastern Inner Mongolia, China. Methods: The study system used four sites of fixed, semifixed, semishifting and shifting sand dune grasslands, representing a gradient of habitat degradation. We investigated the density of germinable seeds deposited in the top 5 cm of soil and in situ seedling emergence (number of seedlings emerging early in the growing season) and establishment (number of plants recruited at the end of the growing season) at three topographic positions (dune top, windward and leeward sides) within each site over 2 years that differed in rainfall. Habitat characteristics (i.e. vegetation cover, plant species composition and diversity, soil moisture and nutrient availability and soil erodibility) of the four sites were also measured. Results: Habitat degradation (i.e. decreased vegetation cover and enhanced wind erosion rate) significantly reduced the size of the germinable soil seed bank. On average, germinable seed number from the high‐vegetation cover fixed dune was 36‐fold larger than the low‐vegetation cover shifting dune, and eight‐ and two‐fold larger, respectively, than the semishifting and semifixed dunes with intermediate vegetation cover. We observed within‐habitat variability in seed distribution, but among‐topographic position variation differed among habitats. Seedling recruitment showed large between‐year, and among‐ and within‐habitat variability, but these variations varied significantly depending on the response variables evaluated (i.e. initial seedling density, final plant density, emergence rate and recruitment rate). Path analysis revealed complex density‐dependent positive and negative, direct and indirect effects of germinable seed density and initial seedling density on recruitment, but the relative importance of these density‐dependent effects varied depending on habitat type and rainfall availability. Conclusion: Our results suggest that habitat degradation, microtopography and rainfall availability interact in shaping sand dune seed bank and plant community recruitment patterns and dynamics. Their effects were mainly mediated through changes in both the biotic and abiotic environment during the process of habitat deterioration.