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151.
Potential impacts of global change on dryland ecosystems are reviewed from four perspectives. First, results are reviewed from recent research on vegetation change, land degradation and desertification. The role of disturbances and low recurrence events in triggering non-linear changes by driving ecosystems beyond their resilience thresholds is outlined. Particular attention is paid to the development of spatial structures as feedbacks that tend to buffer degradation. Secondly, the synergetic operation of climatic and anthropogenic forcing factors of dryland degradation is discussed in the light of some case histories. Changes in one group of those factors often render the ecosystems particularly sensitive to changes of those in the second group. Thirdly, off-site effects of dryland degradation are summarized, including feedbacks to the atmosphere, changes in biological diversity and downstream impacts in river catchments. Finally, the implications of global change for land degradation control policies are outlined. The importance of prevention is emphasized, as well as the need to carefully consider where to apply rehabilitation and restoration. Prevention includes soft and cheap measures based on management practices, while restoration calls for massive and expensive interventions on soil or vegetation. © 1998 John Wiley & Sons, Ltd.  相似文献   
152.
Shrub species are used in restoration projects on dryland for their facilitation effects, which include environmental improvements and protection from herbivore feeding. Facilitation effects on forage grasses are potentially important in improving grazing capacity on rangelands. However, the morphology-dependent performance of benefactor plants in facilitating forage species growth and supplementation under moderate grazing intensity remains unclear. Here, our main purpose was to measure facilitation performance in terms of the survival of a native forage grass, Agropyron cristatum (L.) Gaertn. (Gramineae)., in accordance with the growth conditions of a sand-fixing benefactor shrub, Caragana microphylla Lam., in the Hulun Buir Grassland, northern China. Six study sites with patches of A. cristatum and C. microphylla were established at the foot of fixed sand dunes. At each site, five quadrats were set in places where C. microphylla coverage was 100% and A. cristatum grew among the shrubs (shrub quadrats), and another five were set where A. cristatum grew alone without C. microphylla (grass quadrats). We measured the morphological traits of C. microphylla and A. cristatum in all 60 quadrats, along with the soil water content and soil temperature. The data were compared between the shrub and grass quadrats by generalized linear mixed-effect models to assess the shrub's facilitation effects. We also used such models to elucidate the relationship between the average height of C. microphylla and the morphological traits of A. cristatum in the shrub quadrats. The maximum height, average grazed height, and the number of seed heads of A. cristatum were greater in the shrub quadrats than in the grass quadrats. The soil surface temperature was lower in the shrub quadrats. The maximum height and seed head number of A. cristatum were positively associated with the average height of C. microphylla. These results suggest that the grazing impact and heat stress were smaller in shrub quadrats than in grass quadrats, and that the degree of this protective effect depended on the shrub height. The shrub canopy seemed to reduce the increase in soil temperature and keep the grass vigorous. Livestock likely avoided grazing grasses in the C. microphylla patches because of the shrub's spiny leaves; only the upper parts of the grass stems (including the seed heads) protruding from the shrub canopy were grazed. The sand-fixing shrub thus moderates the grazing impact and soil temperature, and contributes to vegetation restoration and grazing system sustainability.  相似文献   
153.
以中甘21号甘蓝为试验材料,研究了覆膜时间及覆膜方式对山旱地春甘蓝产量和水分利用效率的影响.结果表明:秋季全膜双垄沟显著提高山旱地春甘蓝定植时0~100 cm土层内和结球期0~60 cm土层内的土壤含水量,促进了甘蓝的生长;秋季全膜双垄沟的产量和水分利用效率为41 000kg·hm-2和210.61 kg·mm-1·hm-2,较秋季全膜平铺、春季全膜双垄沟、春季全膜平铺、秋季半膜平铺、春季半膜平铺(CK)分别提高14.42%~89.23%和20.67%~88.58%,差异均达极显著水平;秋季全膜双垄沟显著提高了山旱地春甘蓝的产量和水分利用效率,可作为天水市山旱地春甘蓝的主要栽培方式.  相似文献   
154.
The harvester ant Messor barbarus can be responsible for high weed seed losses in dry land cereals in Spain. Because weeds occur in patches, harvester ants have to be able to find and exploit patches. However, seed patches can differ in size and may, therefore, differ in the probability of being discovered and exploited. Here, 90 patches varying in size from 0.25 to 9 m2 were created in three 50 × 50 m subareas in a cereal field. Oat seeds were sown as weed seed surrogates in the patches at 2000 seeds m?2. After 24 h, those remaining were collected and the exploitation rate (the percentage of seeds removed per patch discovered by ants) was estimated. Harvester ant nests and the location of the seed patches were georeferenced and used to estimate distances between them. The patch encounter rate (the proportion of patches discovered by the ants) decreased slightly, but significantly, with decreasing patch size, though not the exploitation rate, which was lowest in the smallest patches (78–94%) and highest in the largest (99–100%). Seed patches that were not found or partially exploited were mostly located in subareas with a lower ant nest density or a longer distance away from the nearest nest than seed patches that were fully exploited. The results of this study indicate that the interaction between the spatial distribution of ant nests and the patchy distribution of seeds can create opportunities for seeds to be subjected to lower levels of predation.  相似文献   
155.
旱稻抗旱性鉴定方法与指标研究--Ⅰ鉴定方法与评价指标   总被引:14,自引:2,他引:14  
概述了旱稻的抗旱性概念及抗旱类型,分析比较了田间鉴定法、人工模拟鉴定法和实验室鉴定法等不同方法的优缺点,并对抗旱性鉴定指标(形态指标、生长发育指标)和抗旱性评价指标(抗旱系数、抗旱指数)进行了分析讨论,认为抗旱指数可作为旱稻抗旱性鉴定的首选指标。  相似文献   
156.
黄土高原是我国重要旱作农业区,冬小麦是该地区重要的粮食作物之一,如何进一步提高旱作小麦产量并降低由干旱缺水引起的产量波动是该区域未来小麦生产亟待解决的重要问题。由于黄土高原地区冬小麦生育期和降水季节不匹配,土壤水的利用对该区域小麦生产至关重要。基于黄土高原地区以往研究数据,分析表明当前冬小麦产量水平下土壤水利用并不充分;进一步分析表明,1~2 m深层土壤水残留量和产量高度相关,即在冬小麦收获期,当1~2 m深层土壤中有效水残留量为100~130 mm时,冬小麦产量为2 640~4 920 kg·hm-2,而当残留量减少至30~70 mm时,其产量可显著提高至5 250~6 576 kg·hm-2;此外,加强深层土壤水利用亦可显著提高高产概率。统计结果表明,在可用水量为666~766 mm(播前0~2 m土层储水量与生育期降水量之和)条件下,收获时如果1~2 m深层土壤水残留量从270~210 mm降低至150~90 mm时,小麦产量高于4 000 kg·hm-2和5 000 kg·hm-2的概率可分...  相似文献   
157.
旱作麦田秸秆覆盖的生态综合效应研究   总被引:59,自引:10,他引:59  
研究了秸秆覆盖春小麦田土壤温度、水分、养分和微生物数量的变化趋势,结果表明:秸秆覆盖对表层(0~10cm)土温变化有明显的调节作用,在低温时有增温作用,高温时有降温作用;覆盖有一定的抑蒸保墒作用,可提高作物水分利用效率9.61%~20.93%,其保水作用主要发生在上层土体0~40cm,并随覆盖量的增加而增强;覆盖后0~40cm土体中土壤全N、全P在一个生长季节后变化不明显,但土壤有机质、速效钾含量有所增加;覆盖处理后小麦生育时期土壤细菌、放线菌和真菌的数量均高于对照,形成了不同时期新的土壤微生物区系,改变了土壤生物学活性。适量秸秆覆盖有利于寻求适宜作物生长发育的水热组合,具有良好的农田综合效应,可提高产量12.47%~29.63%。  相似文献   
158.
施肥深度对旱地小麦花后根系干重及产量的影响   总被引:9,自引:1,他引:9  
用土柱栽培研究了施肥深度对旱地小麦花后根系干重及产量的影响,结果表明:较深层次(20cm ̄40cm)施肥,可增加中、下层根量及所占比例数,增加千粒重,稳定穗数和穗数,使籽粒产量中来自花后光合器官输送的比例增加,从而使产量提高,而对照(0cm ̄20cm)施肥则与此相反,由此可以认为旱地小麦早适施肥深度应在20cm ̄40cm,并可获得高产高效的结果,所以在旱地麦田应提倡分层深(20cm ̄40cm)施肥  相似文献   
159.
Streamflow forecasting in drylands is challenging.Data are scarce,catchments are highly humanmodified and streamflow exhibits strong nonlinear responses to rainfall.The goal of this study was to evaluate the monthly and seasonal streamflow forecasting in two large catchments in the Jaguaribe River Basin in the Brazilian semi-arid area.We adopted four different lead times:one month ahead for monthly scale and two,three and four months ahead for seasonal scale.The gaps of the historic streamflow series were filled up by using rainfall-runoff modelling.Then,time series model techniques were applied,i.e.,the locally constant,the locally averaged,the k-nearest-neighbours algorithm(k-NN)and the autoregressive(AR)model.The criterion of reliability of the validation results is that the forecast is more skillful than streamflow climatology.Our approach outperformed the streamflow climatology for all monthly streamflows.On average,the former was 25%better than the latter.The seasonal streamflow forecasting(SSF)was also reliable(on average,20%better than the climatology),failing slightly only for the high flow season of one catchment(6%worse than the climatology).Considering an uncertainty envelope(probabilistic forecasting),which was considerably narrower than the data standard deviation,the streamflow forecasting performance increased by about 50%at both scales.The forecast errors were mainly driven by the streamflow intra-seasonality at monthly scale,while they were by the forecast lead time at seasonal scale.The best-fit and worst-fit time series model were the k-NN approach and the AR model,respectively.The rainfall-runoff modelling outputs played an important role in improving streamflow forecasting for one streamgauge that showed 35%of data gaps.The developed data-driven approach is mathematical and computationally very simple,demands few resources to accomplish its operational implementation and is applicable to other dryland watersheds.Our findings may be part of drought forecasting systems and potentially help allocating water months in advance.Moreover,the developed strategy can serve as a baseline for more complex streamflow forecast systems.  相似文献   
160.
针对甘肃陇东旱作区年降水量少、季节分布不均,特别是玉米生产春旱严重等问题,以露地平播为对照(CK),研究了双垄面全膜覆盖沟播,双垄面半膜覆盖沟播,宽膜覆盖平播,窄膜覆盖平播,全地面覆盖平播,膜侧播种等6种覆膜种植方式对旱作玉米出苗率、干物质积累量、株高、叶面积指数、生育时期、产量和水分利用效率的影响。结果表明,全膜双垄沟提高了玉米出苗率和出苗速度,提早成熟,整个生育期玉米株高、叶面积指数和干物质积累量始终大于其它处理,显著提高了产量和水分利用效率,是旱作区进一步挖掘降水潜力和高产田创建的有效途径。  相似文献   
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