全文获取类型
收费全文 | 1002篇 |
免费 | 154篇 |
国内免费 | 475篇 |
专业分类
1631篇 |
出版年
2024年 | 12篇 |
2023年 | 39篇 |
2022年 | 35篇 |
2021年 | 81篇 |
2020年 | 72篇 |
2019年 | 93篇 |
2018年 | 74篇 |
2017年 | 76篇 |
2016年 | 69篇 |
2015年 | 68篇 |
2014年 | 68篇 |
2013年 | 88篇 |
2012年 | 68篇 |
2011年 | 60篇 |
2010年 | 52篇 |
2009年 | 83篇 |
2008年 | 78篇 |
2007年 | 86篇 |
2006年 | 61篇 |
2005年 | 50篇 |
2004年 | 57篇 |
2003年 | 39篇 |
2002年 | 25篇 |
2001年 | 24篇 |
2000年 | 21篇 |
1999年 | 12篇 |
1998年 | 23篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
排序方式: 共有1631条查询结果,搜索用时 15 毫秒
1.
Classification and ordination of plant communities along an altitudinal gradient on the Presidential Range,New Hampshire,USA 总被引:1,自引:0,他引:1
An analysis of vegetation along an altitudinal gradient on the Presidential Range, New Hampshire, USA, using the Braun–Blanquet approach followed by multivariate data analysis is presented. Twelve main plant communities have been distinguished. Floristic information is presented in twelve tables and one appendix. The relationships of the communities to complex environmental gradients are analyzed using Correspondence Analysis. Floristic composition and community structure are controlled primarily by the altitudinal gradient (temperature, precipitation), and by mesotopographic conditions (snow accumulation, exposure and cryoturbation, slope position, and soil moisture). 相似文献
2.
3.
Feces of wild chimpanzees in the Mahale Mountains, Tanzania, were inspected for intestinal parasites under a compound microscope.
Eggs or larvae ofOesophagostomum, Strongyloides, Trichuris, Prosthenorchis, andBertiella were found. Intestinal nematodes significantly increased in the mid-rainy season. This finding supports (or, at least, is
not in conflict with) the hypothesis thatAspilia leaves which are occasionally swallowed by chimpanzees may function as a vermicide, since ingestion of such leaves also increases
significantly in the mid-rainy season. 相似文献
4.
Introduction of T-dependent antigens into the subarachnoid space (isas) resulted in higher systemic antibody responses in mice than injections into the peritoneal cavity (ip) or other sites commonly used for immunization. Antibody production in isas immunized mice was not increased by treatment with cyclophosphamide (Cy) at doses known to abolish T-suppressor-cell activity, but such treatment increased antibody production in ip immunized mice toward the higher level which was observed in the isas immunized animals. Suppressor cell-dependent low zone tolerance (LZT) to TNP-BSA could not be induced by isas injections of deaggregated BSA (d-BSA). Conversely, mice which were unresponsive to ip injected d-BSA showed consistent systemic antibody responses when the antigen was injected isas. These observations indicate that immune responses initiated within the CNS are associated with relatively ineffective induction of systemic suppressor cell activity. 相似文献
5.
Vegetation change over a nine-year period was studied in Guadalupe Mountains, New Mexico. Permanent transects in desert shrub
vegetation were sampled in 1972 and 1980. Emphasis was given to shrubs because of their importance to big game diets. Univariate
paired t-tests and reciprocal averaging ordination were used to detect and display coordinated changes in species composition over
time. Despite apparently less browsing pressure in desert shrub vegetation in 1980 there were few significant changes in species
composition. In addition, preferred forage species showed reduced reproduction while species of intermediate and poor forage
value dis-played increased reproduction during this time. These data do not support traditional rangeland succession theory
which states that enhanced reproduction of preferred species should follow grazing or browsing pressure reduction. 相似文献
6.
Leaf tolerance to high temperatures, as determined by electrolyte leakage and chlorophyll a fluorescence, was compared for Artemisia tridentata (Asteraceae), a widespread shrub of the Great Basin, Colorado Plateau, and western slope of the Rocky Mountains, and Potentilla gracilis (Rosaceae), a herbaceous forb common to high-elevation meadows of the western United States. Species-specific and treatment-specific differences in leaf temperature, high-temperature tolerance and chlorophyll a fluorescence from photosystem II were compared, to test the hypothesis that plants at ecosystem borders will exhibit species-specific responses to climate change. Measurements were made for plants exposed to a climate change warming manipulation on a major ecosystem border at the Rocky Mountain Biological Laboratory, Colorado, United States, in July and August 1995. In July, daily maximal leaf temperatures were significantly higher for P. gracilis than for A. tridentata. Leaf temperatures were slightly lower in August than July for leaves of both species, on control and heated plots, despite the fact that daily maximum air temperatures were not significantly different for the two months. High-temperature tolerance was determined for leaves treated for 1 h at temperatures ranging from 15°C to 65°C. LT50 was approximately 46°C for both species on control plots, but was 43°C for leaves of both species from heated plots, contrary to the predictions of the hypothesis. No shift in LT50 (acclimation) was apparent between July and August. Changes in chlorophyll a fluorescence from photosystem II (F
V
/F
M
) were used to characterize the photosynthetic response to high temperatures. For both A. tridentata and P. gracilis in July, F
V
/F
M
was about 0.7, but decreased for temperatures above 40°C. The results suggest that plant responses to global warming at ecosystem borders may be influenced by factors other than leaf-level physiological tolerance to elevated temperatures. 相似文献
7.
Analysis of nitrogen saturation potential in Rocky Mountain tundra and forest: implications for aquatic systems 总被引:3,自引:1,他引:2
Jill S. Baron Dennis S. Ojima Elisabeth A. Holland William J. Parton 《Biogeochemistry》1994,27(1):61-82
We employed grass and forest versions of the CENTURY model under a range of N deposition values (0.02–1.60 g N m–2 y–1) to explore the possibility that high observed lake and stream N was due to terrestrial N saturation of alpine tundra and subalpine forest in Loch Vale Watershed, Rocky Mountain National Park, Colorado. Model results suggest that N is limiting to subalpine forest productivity, but that excess leachate from alpine tundra is sufficient to account for the current observed stream N. Tundra leachate, combined with N leached from exposed rock surfaces, produce high N loads in aquatic ecosystems above treeline in the Colorado Front Range. A combination of terrestrial leaching, large N inputs from snowmelt, high watershed gradients, rapid hydrologic flushing and lake turnover times, and possibly other nutrient limitations of aquatic organisms constrain high elevation lakes and streams from assimilating even small increases in atmospheric N. CENTURY model simulations further suggest that, while increased N deposition will worsen the situation, nitrogen saturation is an ongoing phenomenon. 相似文献
8.
R. T. Busing 《Plant Ecology》1994,115(1):19-27
Relationships between canopy cover and tree regeneration were determined for various species in cove forests of the Great Smoky Mountains. Old-growth stands were sampled with six plots covering a total area of 4.8 ha. Each plot was subdivided into contiguous 10×10 m quadrats. Canopy cover overlying each of the 480 quadrats was characterized with three different indices based on visual estimates of cover. Influences of: (1) overlying cover, (2) proximate openings, and (3) total area of proximate openings on quadrat regeneration densities were determined. Most species reproducing by seed and some species reproducing by vegetative means had higher densities in quadrats with openings, but only the intolerants were highly dependent on gaps. Tsuga canadensis, a very shade-tolerant species, was one of the few species with abundant regeneration beneath dense canopy cover. In general, understory areas near gaps had somewhat higher regeneration densities than other areas with overlying cover. Several shade-tolerant species showed a positive regeneration density response to canopy openings and an ability to regenerate in gaps 0.01–0.03 ha in area. These openings were too small for intolerant species. Many species exhibited a positive response to total size of the proximate opening(s). A sharp increase in regeneration density with area of the opening(s) was evident at approximately 0.04 ha for the shade-intolerant species. 相似文献
9.
Climate change has been unprecedented in the last half-century. Tree growth dynamics and responses to climate warming at different elevations vary by study area due to regional diversity in site-specific climatic conditions in the central Hengduan Mountains. A. georgei is the dominant species in high-elevation montane forests in the central Hengduan Mountains. To study the response of A. georgei radial growth to climate and identify tree growth trends at different elevations, tree-ring width chronologies at four elevations across the subalpine A. georgei forest belt were built and growth-climate relationships were analyzed. The primary findings of this study were as follows: (1) radial growth rates of A. georgei decreased with elevation; (2) warming alleviated the limitation of low temperatures and abundant precipitation on tree radial growth at the highest sampling site; and (3) unlike at other elevations, the trend of trees basal area increment (BAI) at the lowest sampling site showed a significant decline over the past 20 years. This suggests the presence of an elevational inflection point, likely between 3800 m and 4000 m, where tree growth trends diverge. These results confirmed that A. georgei at higher elevation in the central Hengduan Mountains currently benefits from higher temperatures. However, the effects of drought on A. georgei at lower elevations would cause radial growth to decrease with climate warming. Therefore, it is critical to establish effective management strategies based on how A. georgei responds to climate change at various elevations. 相似文献
10.
通过形态学及化学研究,结合nrDNA ITS序列的系统发育分析,该文报道了茶渍属(Lecanora Ach.)和石蕊属(Cladonia P. Browne)地衣4中国新记录种:莱氏茶渍(Lecanora layana Lendemer)、白头山茶渍(L. baekdudaeganensis B. G. Lee&Hur)、伪银茶渍(L. pseudargentata Lumbsch)和草皮石蕊[Cladonia caespiticia (Pers.) P. Gaertn., B. Mey&Scherb.]。该文对这些物种的分类学特征进行了详细描述,并与相似种进行了对比,同时提供了各种的地理分布图和形态图。这为中国茶渍属和石蕊属地衣物种多样性及资源利用提供了基础资料。 相似文献