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171.
Climate change scenarios predict increases in temperature, changes in precipitation patterns, and longer drought periods in most semi-arid regions of the world. Ecosystems in these regions are prone to land degradation, which may be aggravated by climate change. Soil respiration is one of the main processes responsible for organic carbon losses from arid and semi-arid ecosystems. We measured soil respiration over one year in two steppe ecosystems having different degrees of land degradation under three ground-covers: with vegetation, bare soil, and an intermediate situation between plants and bare soil.The largest differences in soil respiration rates between the sites were observed in spring, coinciding with the highest level of plant activity. The degraded site had drier and hotter soils with less soil water availability and a longer drought period. As a result, vegetation on the degraded site did not respond to spring rainfall events. Soil respiration showed a strong seasonal variability, with average annual rates of 1.1 and 0.8 μmol CO2 m−2 s−1 in the natural and degraded sites, respectively. We did not observe significant differences in soil respiration rates associated with ground-cover i.e., the temporal variation was much larger than the spatial variation. At both sites, soil moisture was the controlling driver of soil respiration for most of the year, when temperatures were above 20 °C and constrained the response to temperature for the few months when the temperature was below 20 °C. An empirical model based on soil temperature and soil moisture explained 90% and 72% of the seasonal variability of soil respiration on the natural and degraded sites, respectively. For the first time, this study suggests that land degradation may alter the carbon balance of these ecosystems through changes in the temporal dynamics of soil respiration and plant productivity, which have important negative consequences for ecosystem functioning and sustainability. 相似文献
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173.
作物残茬覆盖对农田土壤风蚀的影响 总被引:3,自引:0,他引:3
为定量评价干旱半干旱地区农田土壤抗风蚀效果,采用移动式风洞及其配套测试系统对内蒙古武川县上秃亥乡农田地表进行了原位测试研究。结果表明,不同风速下土壤风蚀量随作物残茬盖度的增加呈指数规律减少;40%以上残茬盖度可明显提高土壤颗粒起动风速并减少风蚀量;当风速为14~18 m/s时,地表作物残茬盖度为60%~80%具有较好的抗风蚀效果。 相似文献
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The role of topographic factors in the invasion of hillslopes by Pteronia incana, an unpalatable shrub, was investigated. The study combined field observations with image analysis based on high-resolution infrared imagery. A Digital Elevation Model (DEM) of 20 m spatial resolution was used to derive terrain parameters. The Topographic Wetness Index (WI), a component of the TOPMODEL, was derived from the DEM and its relationship with the spatial distribution of P. incana was explored. The absence/presence of P. incana was noted to be strongly influenced by slope angle and aspect. The probability for P. incana occurrence increased with slope steepness and southerly slope orientation. Abandoned and grazing lands were identified as the main invasion hotspots on hillslopes. The combined influence of slope gradient and aspect, and land use was noted to have promoted the invasion. This is borne out by the concentration of the invasion on abandoned steep slopes with a southerly orientation. 相似文献
176.
为了探讨西北干旱绿洲区土地利用/覆盖类型景观格局特征,以及不同土地利用/覆盖类型植被覆盖度变化,本文以干旱典型绿洲区新疆生产建设兵团第八师为研究区,选取2000年、2005年的Landsat TM和2010年环境一号星CCD遥感影像数据,采用室内解译与野外核查相结合,运用RS和GIS技术分析了该区域的土地利用/覆盖和景观格局变化特征,并基于土地利用/覆盖类型选取3年的MODIS数据进行了植被覆盖度变化分析。结果表明:1)该研究区主要以耕地、草地和灌丛为主,占总面积的88.9%。在自然因素和人为因素的综合作用下,干旱绿洲区土地利用/覆盖类型发生明显变化,草地和灌丛呈减少趋势,面积分别由2000年的2 603.2 km2和1 166.2 km2减少至2010年的1 677.3 km2和933.8 km2;耕地和城镇逐步增加,面积分别由2000年的2 892.8 km2和209.2 km2增加至2010年的4 038.3 km2和259.1 km2;耕地主要由草地和灌丛转化而来,城镇主要由耕地转化而来,土地利用变化整体处于不平衡发展趋势。2)研究区景观特征趋于简单,土地利用变化改变了该区域景观格局,景观多样性、均匀度减少,优势度明显增大,破碎化程度降低,生物多样性也呈向单一方向转变的趋势。3)受气候等主要因素的影响,2000—2010年植被覆盖度呈先降低后逐步升高的趋势,不同土地利用/覆盖类型植被覆盖度表现为林地耕地草地灌丛荒漠,林地、灌丛、草地和耕地植被覆盖度分别增加6.7%、38.2%、15.6%和12.3%;较高和高等级植被覆盖面积增加,中、较低和低等级植被覆盖度面积减少。4)温度一定的条件下,降雨量是影响干旱绿洲区植被覆盖变化的关键因素。该研究结果可为新疆生产建设兵团生态环境保护管理以及合理规划生产模式提供科学的参考依据。 相似文献
177.
Summary Tephra and underlying litter and soil were sampled in 1980, 1982, and 1987 beneath subalpine forests where 4.5 and 15 cm of tephra fell during the 1980 eruption of Mount St. Helens, Washington State, USA. Coarse pumice had a higher initial pH and less total N, less exchangeable K, Ca, and Mg, and less extractable B and S than finer textured layers. Tephra pH and concentrations of cations and S decreased rapidly with time, especially during the first 2 years in the finer layers. Total N, Bray-Kurtz P, and organic C concentrations in the tephra increased with time. Changes within a site in total N, pH, organic C, P, Ca, Mg, and S from 1980 to 1987 exceeded the differences among sites at any one time. By 1987 a forest floor covered much of the tephra surface, and differences in Ca associated with site vegetation and seepage had developed in tephra layers of a similar depth. In 15 cm deep tephra at one site in 1987, the tephra crust beneath the forest canopy was thicker and had higher concentrations of coarse particles, organic C, total N, and cations than beneath forest openings. In concave microsites the crust was thicker, with higher pH, organic C, and total N but lower S than in adjacent convex microsites. Spatial and temporal chemical differences are sufficient to affect patterns of vegetation recovery. 相似文献
178.
179.
In contrast to many other western European countries, the knowledge about trends in the Swedish butterfly fauna is poor. We studied the effects of habitat and species characteristics on species turnover in 13 grassland sites in southern Sweden by comparing species richness and compositions at two separate occasions with an interval of 21 years. The mean number of species per site decreased from 30 to 24, with a large variation between sites. The number of extinctions was highest in sites where the proportion of trees and shrubs had increased most, but there was no detectable effect of area or of the composition of landscapes surrounding the sites. Areas protected as nature reserves had lost as many species as unprotected areas had, indicating both the importance of proper management of nature reserves and that nature reserves alone may not be enough to inhibit regional extinction of butterfly species. Species dependent on nutrient-poor conditions tended to decrease while species dependent on nutrient-rich conditions tended to increase, indicating a negative effect of increased soil nitrogen levels resulting from active fertilizing of pastures and/or atmospheric nitrogen deposition. A regular monitoring program could show whether our results are representative for Sweden or Northern Europe. 相似文献
180.
J. Kros G. J. Reinds W. De Vries J. B. Latour M. Bollen 《Water, air, and soil pollution》1995,85(3):1101-1106
Changes in vegetation are often caused by changes in abiotic site factors, such as pH, nitrogen availability and soil moisture. It has been recognized that abiotic site factors are affected by atmospheric deposition and groundwater-table changes. In order to evaluate the effects of eutrophication, acidification and desiccation on site factors, the model SMART2 has been developed. For the Netherlands combinations of two acidification and two seepage scenarios (1990–2050) were evaluated with SMART2. The results are focused on pH, nitrogen availability and base saturation. Calculations were made for combinations of five vegetation structures on seven soil types and the five groundwater-table classes, using a 1 km2 grid. Results showed that deposition reductions lead to a relatively fast improvement of the site factors, increase in pH and base saturation and decrease in N availability. Whereas a reduction in groundwater abstractions of 25% has little or no effect on the pH and N availability. 相似文献