全文获取类型
| 收费全文 | 135篇 |
| 免费 | 12篇 |
| 国内免费 | 15篇 |
专业分类
| 测绘学 | 4篇 |
| 大气科学 | 1篇 |
| 地球物理 | 14篇 |
| 地质学 | 30篇 |
| 海洋学 | 39篇 |
| 综合类 | 10篇 |
| 自然地理 | 64篇 |
出版年
| 2022年 | 1篇 |
| 2021年 | 4篇 |
| 2020年 | 7篇 |
| 2019年 | 1篇 |
| 2018年 | 6篇 |
| 2017年 | 3篇 |
| 2016年 | 4篇 |
| 2015年 | 7篇 |
| 2014年 | 3篇 |
| 2013年 | 11篇 |
| 2012年 | 8篇 |
| 2011年 | 12篇 |
| 2010年 | 5篇 |
| 2009年 | 5篇 |
| 2008年 | 4篇 |
| 2007年 | 9篇 |
| 2006年 | 6篇 |
| 2005年 | 12篇 |
| 2004年 | 7篇 |
| 2003年 | 5篇 |
| 2002年 | 6篇 |
| 2001年 | 5篇 |
| 2000年 | 10篇 |
| 1999年 | 3篇 |
| 1998年 | 4篇 |
| 1997年 | 1篇 |
| 1996年 | 4篇 |
| 1994年 | 1篇 |
| 1993年 | 3篇 |
| 1992年 | 3篇 |
| 1991年 | 1篇 |
| 1984年 | 1篇 |
排序方式： 共有162条查询结果，搜索用时 795 毫秒
71.
72.
73.
74.
森林的不同更新方式对森林土壤理化性质有重要影响．本研究通过野外调查探讨中亚热带山区杉木人工林和米槠人促更新林2种不同更新方式下林地土壤理化性质的变化特征,并用土壤退化指数定量描述土壤质量变化．结果表明：杉木人工林土壤表层（0—10cm）容重显著高于米槠人促更新林（P〈0．05）;随土壤深度的增加,杉木人工林和米槠人促更新林土壤有机C、全N、全P、水解N和有效P均逐渐降低．米槠人促更新林表层（0—10cm）土壤有机c比杉木人工林高出15．6％,全N、全P以及水解N和速效P也均表现出人促米槠更新林高于杉木人工林的趋势;土壤退化指数的计算结果表明杉木林土壤质量显著低于米槠人促更新林． 相似文献
75.
The endangered river plain woodlands of semi-arid Central Asia provide numerous ecosystem services. Previous studies have focused mainly on changes in water supply and salinity as the underlying mechanisms of Populus euphratica woodland’s decline. We tested whether vegetative regeneration of P. euphratica serves as an alternative pathway to propagation from seeds for reproduction in a degraded tree steppe in northwestern China and whether this method also works on grazed sites. We measured the effects of different grazing pressures on tree growth, survival, and vegetative regeneration. When subjected to high or moderate grazing pressure, P. euphratica populations failed to regenerate vegetatively, indicating recruitment limitation. A 25% increase of grazing pressure decreased the ramet density to 50% and the ramet height to 25%, as well as reducing the average age of ramets to one year. As overgrazing seriously limits the potential of natural recovery of P. euphratica a balance between livestock grazing and the regeneration of the Tugai forests is needed to sustain positive effects of increasing water tables and floods on tree vitality and regeneration. To restore population structure, and to support early vegetative regeneration long-term livestock management interventions to exclude livestock from degraded and recovered land need to be developed. 相似文献
76.
In the present study, chemical oxygen demand (COD) removal by packed‐columns of activated carbon (AC) derived from two different materials (coal activated carbon, CAC and wood activated carbon, WAC) is reported as part of an on‐site wastewater treatment system for handling small volumes of wastewater generated at wood‐floor industries for which there are no proper on‐site treatment options available in the market. The performance of the sorbents, the effect of bed depth (0.19 and 0.57 m) and volumetric load (0.10 and 0.24 m h?1) on the breakthrough curve of sorption systems were studied. The results indicated the feasibility of using both ACs to treat these wastewaters. At the bed depth (0.57 m), volumetric load (0.24 m h?1), and 30% breakthrough, CAC and WAC showed treatment capacity of 40.5 L kg?1 in 250 h and 23.8 L kg?1 in 63 h, respectively. This indicated that CAC requires longer retention times to reach a performance similar to WAC. The experimental data was fit into the bed depth‐service time model showing that under the same conditions, CAC had higher maximum sorption capacity (N0) than WAC. Moreover, thermal regeneration at 500°C temperature could be a cost‐effective procedure since the reuse of spent AC through such regeneration process for further treatment could still achieve 90% of the initial sorption capacity, reducing then costs for the use of new sorbents and also the need for waste disposal. 相似文献
77.
《自然地理学》2013,34(6):465-475
Saguaro cacti establish periodically in cohorts during years that are climatically favorable. Determining the establishment year is complicated by age-height relationships that vary considerably from site to site and require extensive data collection. Estimations of age-height relationships for only a few locales have thus limited the ability of researchers to estimate age in other populations, and consequently to relate periods of regeneration to climate. We collected data for 254 saguaros in the arid western portion of the species' range and employed a recently developed site-specific technique for estimating saguaro age to reconstruct the population structure at this locale. Second, we used discriminant analysis to determine which climate variables best predict regeneration years. We found that above-average temperatures in November coupled with concurrent high soil moisture levels, conditions typically associated with El Niño years, are associated with years characterized by high regeneration. Accordingly, these results suggest that El Niño events may be related to years of favorable regeneration for saguaros. 相似文献
78.
《自然地理学》2013,34(6):505-518
Casual observations suggest that saguaro populations are densest in southeastern Arizona, although data have not been collected and no study has been done to address this topic. In addition, the topic of reproductive density has similarly never been addressed. Saguaro reproductive output is directly related to the number of adult individuals and the number of branches in the area. Thirty saguaro populations over their U.S. range were sampled to consider two variables: population density and reproductive stem density. Stepwise regression using climate and vegetation (e.g., availability of nurse plants) to predict density yielded tree + Ambrosia cover and maximum July precipitation. Nurse cover, however, is also influenced by summer rain. The partial correlation results suggest that high saguaro densities are linked with high quality nurse cover (i.e., not Larrea tridentata) in addition to summer rainfall. Total cover and mean annual precipitation are the best predictors of reproductive stem density. Mean annual precipitation may be a good predictor of reproductive stem density, because population density is linked with summer rain while branching is linked with winter rain. The plots were also divided into climatic regions. One-way ANOVA shows that the northeast (high winter precipitation) and west (dry) have lower saguaro densities than the southeast (high summer precipitation), while the northeast and southeast both have very high reproductive stem densities relative to the west. The warmer west is less susceptible to periodic freezing mortality, while previous work has shown that the southeast generally regenerates more successfully. Thus in the colder northeast, which is also outside of the primary summer rain and best nurse plant belt, low density populations seem to be maintained only with high reproductive density. 相似文献
79.
In 1987,a catastrophic fire burned over 1330000 ha in the densely forested area of the Da Hinggan Mountains in the northeastern China.After the fire,intensive management including burned trunk harvesting and coniferous tree planting had been conducted to accelerate forest restoration.To study the long term effect of these activities on forest recovery,we used a simulation modeling approach to study long-term(300 years) forest dynamics under current planting and natural regeneration scenarios.Results indicat... 相似文献
80.
Wildfires in the sub‐alpine belt of the Austrian Limestone Alps sometimes cause severe vegetation and soil destruction with increased danger of secondary natural hazards such as avalanches and debris flows. Some of the affected areas remain degraded to rocky slopes even decades after the fire, raising the question as to whether the ecosystems will ever be able to recover. The mean fire interval, the duration of recovery and the role of geomorphic processes for vegetation regeneration are so far unknown. These questions were tackled in a broad research approach including investigation of historical archives to determine the frequency of historical wildfires, mapping vegetation regeneration on 20 slopes of different post‐fire ages, and soil erosion measurements on two slopes. To date, > 450 historical wildfires have been located in the study area. The mean fire interval per square kilometre is c. 750 years, but can be as low as 200–500 years on south‐facing slopes. Vegetation regeneration takes an extremely long time under unfavourable conditions; the typical window of disturbance is between 50 and 500 years, which is far longer than in any other wildfire study known to us. Soil erosion constantly increases in the years after the fires and the elevated intensity can be maintained for decades. A two‐part vegetation regeneration model is proposed depending upon the degree of soil loss. In the case of moderate soil erosion, spreading grassland communities can slow down shrub re‐colonization. In contrast, after severe soil destruction the slopes may remain degraded for a century or longer, before rather rapid regeneration occurs. The reasons are not fully understood but are probably governed by geomorphic process intensity. The interdependence of vegetation regeneration and geomorphic processes is a paradigm of ecology–geomorphology interaction, and is a unique example of a very long‐lasting disturbance response caused by wildfire in a non‐resilient ecosystem. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献