全文获取类型
收费全文 | 1481篇 |
免费 | 166篇 |
国内免费 | 8篇 |
出版年
2024年 | 1篇 |
2023年 | 78篇 |
2022年 | 94篇 |
2021年 | 82篇 |
2020年 | 65篇 |
2019年 | 56篇 |
2018年 | 97篇 |
2017年 | 149篇 |
2016年 | 115篇 |
2015年 | 110篇 |
2014年 | 117篇 |
2013年 | 106篇 |
2012年 | 75篇 |
2011年 | 58篇 |
2010年 | 97篇 |
2009年 | 80篇 |
2008年 | 40篇 |
2007年 | 35篇 |
2006年 | 40篇 |
2005年 | 28篇 |
2004年 | 17篇 |
2003年 | 12篇 |
2002年 | 29篇 |
2001年 | 11篇 |
2000年 | 17篇 |
1999年 | 12篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1988年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有1655条查询结果,搜索用时 31 毫秒
91.
Clanwilliam cedar (Widdringtonia cedarbergensis; WICE), a long-lived conifer with distinct tree rings in Cape Province, South Africa, has potential to provide a unique high-resolution climate proxy for southern Africa. However, the climate signal in WICE tree-ring width (TRW) is weak and the dendroclimatic potential of other WICE tree-ring parameters therefore needs to be explored. Here, we investigate the climatic signal in various tree-ring parameters, including TRW, Minimum Density (MND), Maximum Latewood Density (MXD), Maximum Latewood Blue Intensity (MXBI), and stable carbon and oxygen isotopes (δ18O and δ13C) measured in WICE samples collected in 1978. MND was negatively influenced by early spring (October-November) precipitation whereas TRW was positively influenced by spring November-December precipitation. MXD was negatively influenced by autumn (April-May) temperature whereas MXBI was not influenced by temperature. Both MXD and MXBI were negatively influenced by January-March and January-May precipitation respectively. We did not find a significant climate signal in either of the stable isotope time series, which were measured on a limited number of samples. WICE can live to be at least 356 years old and the current TRW chronology extends back to 1564 CE. The development of full-length chronologies of alternative tree-ring parameters, particularly MND, would allow for an annually resolved, multi-century spring precipitation reconstruction for this region in southern Africa, where vulnerability to future climate change is high. 相似文献
92.
93.
Spartina alterniflora, a species vegetating on inter-tidal flats that was introduced from the eastern coast of United States, has become a hot topic, focusing on its invasion within local species in the coastal zone of China. Impacts of S. alterniflora on the inter-tidal macrobenthos community in the Jiangsu coastland are addressed by comparing the macrobenthos characteristics in a mudflat and in a four-year-old Spartina salt marsh that had earlier been a mudflat. During the period October 2002–July 2003, we studied the distribution pattern and diversity of macrobenthos, and discussed their correlation with environmental factors caused by Spartina vegetation. The results showed that a total of 43 macrobenthos species were found, mainly consisting of Mollusca, Crustacea, and Annelida. Ten macrobenthos species were found in the Spartina salt marsh, and 36 species were found in the mudflat. Life forms and functional groups of macrobenthos in the Spartina salt marsh were obviously distrinct from that of the mudflat. The study showed that macrobenthos diversity in the Spartina salt marsh decreased, and the community structure altered obviously, whereas the biomass showed no differences in different seasons. Statistical analysis demonstrated that seasonal change of macrobenthos diversity in the Spartina salt marsh negatively related to content of sediment organic matter, total N, bulk density, height and biomass of Spartina vegetation, and positively related to the density of Spartina. All these differences suggested the obvious effects of the Spartina vegetation on the Jiangsu inter-tidal benthic macroinvertebrate ecology. Furthermore, the investigation also showed that the niche of the native macrobenthos living in the mudflat has been transferred down, seaward, due to the invasion of Spartina in our study site. 相似文献
94.
95.
The invertebrate fauna of the nests of three seabird species, black-legged kittiwakes (Rissa tridactyla), common eider (Somateria mollissima) and glaucous gull (Larus hyperboreus), were sampled in Kongsfjorden, Spitsbergen, Svalbard. The invertebrate community was species poor, consisting predominantly
of the flea, Mioctenopsylla arctica arctica (Insecta: Siphonaptera), but with six species of oribatid mite (Acari: Oribatida), Diapterobates notatus, Oribatula tibialis, Ameronothrus lineatus, Hermannia reticulata, Trichoribates trimaculatus and Ceratoppia bipilis, plus an occasional mesostigmatid mite. No Collembola or ticks (Acari: Ixodidae) were observed. With the exception of M. arctica arctica, the fauna of seabird nests consisted of opportunistic microarthropod species rather than specialised nest-dwelling or bird
parasitic species. Species diversity of soil oribatid mites was greater in nests of the common eider than compared to nests
of the black-legged kittiwake, which may be related to the ground nesting behaviour of the common eiders. No rare or unusual
microarthropod species for Svalbard were found in the seabird nests. The contentions that nests may facilitate microarthropod
colonisation of High Arctic regions via bird phoresy by providing a high-quality habitat at the point of arrival, or that
there might be a specialised microarthropod fauna exploiting this habitat, were not supported in this study. These are amongst
the first data on the microarthropod community of seabird nests in the High Arctic. 相似文献
96.
Martyn Kelly Cathy Bennett Michel Coste Cristina Delgado François Delmas Luc Denys Luc Ector Claude Fauville Martial Ferréol Malgorzata Golub Amelie Jarlman Maria Kahlert John Lucey Bernadette Ní Chatháin Isabel Pardo Peter Pfister Joanna Picinska-Faltynowicz Juliette Rosebery Christine Schranz Jochen Schaumburg Herman van Dam Sirje Vilbaste 《Hydrobiologia》2009,621(1):169-182
The European Union (EU)’s Water Framework Directive (WFD) requires that all Member States participate in intercalibration
exercises in order to ensure that ecological status concepts and assessment levels are consistent across the EU. This paper
describes one such exercise, performed by the countries in the Central/Baltic Geographical Intercalibration Group stretching
from Ireland in the west to Estonia in the east and from the southern parts of Scandinavia to the northern regions of Spain
and Italy (but excluding alpine regions, which were intercalibrated separately). In this exercise, methods used to measure
ecological status of rivers using benthic diatoms were compared. Ecological status is estimated as the ratio between the observed
value of a biological element and the value expected in the absence of significant human impact. Approaches to defining the
‘reference sites’, from which these ‘expected’ values were derived, varied from country to country. Minimum criteria were
established as part of the exercise but there was still considerable variation between national reference values, reflecting
typological differences that could not be resolved during the exercise. A simple multimetric index was developed to compare
boundary values using two widely used diatom metrics. Boundary values for high/good status and good/moderate status set by
each participant were converted to their equivalent values of this intercalibration metric using linear regression. Variation
of ±0.05 EQR units around the median value was considered to be acceptable and the exercise provided a means for those Member
States who fell significantly above or below this line to review their approaches and, if necessary, adjust their boundaries.
Handling editor: J. Padisak 相似文献
97.
98.
Charudutt Mishra Herbert H. T. Prins Sipke E. Van Wieren 《Human ecology: an interdisciplinary journal》2003,31(4):595-609
We describe the diversity and dynamism of social, agricultural, and livestock husbandry practices in a traditional mountain production system in the Indian Trans-Himalaya. These are interpreted in the context of their role in mediating environmental risk. The production system is a little known Buddhist agropastoral system in the high altitude Spiti Valley (agriculture up to 4450 m, livestock grazing 4900 m, total area ca. 12,000 km2) in the Himalayan state of Himachal Pradesh. The local population (ca. 10,000) belongs to one of the three Buddhist sects Gelukpa, Shakyapa, or Ningmapa, is related by blood, and shares a common Tibetan dialect. Family is the basic unit of production, though families are highly dependent upon the community to meet production goals. A village council appointed on rotation and functioning democratically is responsible for village administration, and is the arbiter of all decision-making pertaining to collective work and settling disputes. The council ensures equal access of families to common resources, as well as equitable distribution of responsibilities among them. Systems of primogeniture, celibacy, and polygamy seem to have prevented the fragmentation of land holdings and limited population growth. The diversity of practices in the agropastoral system seems adapted to the risk-prone mountainous environment, the risks being climatic, geological, and those posed by wildlife. The system seems to aim at maximizing production while mediating environmental risk. The production system comes forth as highly dynamic, characterized by continuous innovation and experimentation. Recent changes in the production system are in response to both changes in local conditions as well as increasing integration of the local economy with regional markets, though many aspects of the traditional lifestyle continue to be maintained. 相似文献
99.
Current forest growth models and yield tables are almost exclusively based on data from mature trees, reducing their applicability to young and developing stands. To address this gap, young European beech, sessile oak, Scots pine and Norway spruce trees approximately 0–10 yr old were destructively sampled in a range of naturally regenerated forest stands in Central Europe. Diameter at base and height was first measured in situ for up to 175 individuals per species. Subsequently, the trees were excavated and dry biomass of foliage, branches, stems and roots was measured. Allometric relations were then used to calculate biomass allocation coefficients (BAC) and growth efficiency (GE) patterns in young trees. We found large differences in BAC and GE between broadleaves and conifers, and also between species within these categories. Both BAC and GE are strongly age-specific in young trees, their rapidly changing values reflecting different growth strategies in the earliest stages of growth. We show that linear relationships describing biomass allocation in older trees are not applicable in young trees. To accurately predict forest biomass and carbon stocks, forest growth models need to include species and age-specific parameters of biomass allocation patterns. 相似文献
100.