全文获取类型
收费全文 | 3621篇 |
免费 | 696篇 |
国内免费 | 1452篇 |
出版年
2024年 | 11篇 |
2023年 | 150篇 |
2022年 | 141篇 |
2021年 | 197篇 |
2020年 | 247篇 |
2019年 | 288篇 |
2018年 | 256篇 |
2017年 | 236篇 |
2016年 | 254篇 |
2015年 | 191篇 |
2014年 | 204篇 |
2013年 | 299篇 |
2012年 | 164篇 |
2011年 | 208篇 |
2010年 | 235篇 |
2009年 | 212篇 |
2008年 | 230篇 |
2007年 | 234篇 |
2006年 | 240篇 |
2005年 | 201篇 |
2004年 | 191篇 |
2003年 | 176篇 |
2002年 | 122篇 |
2001年 | 121篇 |
2000年 | 135篇 |
1999年 | 108篇 |
1998年 | 78篇 |
1997年 | 63篇 |
1996年 | 75篇 |
1995年 | 56篇 |
1994年 | 52篇 |
1993年 | 40篇 |
1992年 | 58篇 |
1991年 | 29篇 |
1990年 | 39篇 |
1989年 | 45篇 |
1988年 | 23篇 |
1987年 | 23篇 |
1986年 | 21篇 |
1985年 | 25篇 |
1984年 | 22篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1978年 | 13篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1958年 | 4篇 |
排序方式: 共有5769条查询结果,搜索用时 15 毫秒
111.
112.
Seasonal and spatial patterns of S,Ca, and N dynamics of a Northern Hardwood forest ecosystem 总被引:1,自引:1,他引:0
Seasonal dynamics of S, Ca and N were examined at the Huntington Forest, a northern hardwood ecosystem in the central Adirondacks
of New York for a period of 34 months (1985–1988). Solute concentrations and fluxes in bulk precipitation, throughfall (TF)
and leachates from the forest floor, E horizon and B horizon were quantified. Both above and below-ground elemental fluxes
mediated by vegetation (e.g. uptake, litter inputs, and fine roots production) were also determined. The roles of abiotic
and biotic processes were ascertained based on both changes in solute concentrations through the strata of the ecosystem as
well as differences between dormant and growing seasons. Concentrations of SO4
2−, NO3
−, NH4
+ and Ca2+ were greater in TF than precipitation. Forest floor leachates had greater concentrations of SO4
2−, NO3
− + NH4
+ and Ca2+ (9, 6 and 77 μeq L−1, respectively) than TF. There were differences in concentrations of ions in leachates from the forest floor between the dormant
and growing seasons presumably due to vegetation uptake and microbial immobilization. Concentrations and fluxes of NO3
− and NH; were greatest in early spring followed by a rapid decline which coincided with a demand for N by vegetation in late
spring. Vegetation uptake (44.7 kg N ha−1 yr−1 ) could account for the low leaching rates of N03
−. Within the mineral soil, changes with soil depth and the absence of seasonal patterns suggest that cation exchange (Ca+) or anion sorption (SO4
2−) are primarily responsible for regulating solute concentrations. The increase in SO4
2− concentration after leachates passed through the mineral soil may be attributed to desorption of sulfate that was adsorbed
during an earlier period when SO4
2− concentrations would have been greater due to elevated S inputs. 相似文献
113.
Abstract. Microscale variation of vegetation was studied in a Mediterranean oak savanna dominated by annual plants at Neve Ya'ar Experimental Station, in the lower Galilee of Israel. Species presence/absence data were recorded in 20 cm x 20 cm quadrats on a 500-quadrat transect. These data were ordinated with Detrended Correspondence Analysis. Soil nitrate, phosphorous, potassium, pH, soil moisture and salinity were determined at 2-m intervals on the 100-m transect, at monthly intervals from November 1981 to April 1982. Litter cover and canopy shade were surveyed in October 1981 and April 1982. A sharp rise in available nitrate occurred in November, following the first winter rains. Patterns in herbaceous vegetation, as expressed by sample ordination scores, correlated best with litter cover in October. This suggested that vegetation pattern in this community may be strongly influenced by environmental conditions at the time of seedling germination and establishment. 相似文献
114.
115.
116.
JOSEPH HELLER MICHAL VOLOKITA 《Biological journal of the Linnean Society. Linnean Society of London》1981,16(4):279-284
Along the coastal plain of Israel, shell darkness of the polymorphic land snail Xeropicta vestalis is positively related to the extent of perennial vegetation. It is not related to rain, temperature, geographic position or darkness of the ground.
White shells reflect more radiation than dark ones do, and therefore in Israel's coastal plain, where solar radiation is very strong, they are favoured. Perennial vegetation, where it occurs, shields the snails from the sun by absorbing radiation, so that snails amongst them can afford to be darker, and thus more cryptic. Hence, the more perennial vegetation in the habitat, the darker the shells.
Also during the Pleistocene, when temperatures were 5–10°C lower than today, the shells of the coastal plain were as pale as recent ones are. The distribution of a snail-predator, Gerbillus allenbyi , which is restricted to sandy biotopes where perennial bushes occur, and which was absent from Israel during the Pleistocene, could perhaps explain the distribution of X. vestalis morphs, both today and in the past. 相似文献
White shells reflect more radiation than dark ones do, and therefore in Israel's coastal plain, where solar radiation is very strong, they are favoured. Perennial vegetation, where it occurs, shields the snails from the sun by absorbing radiation, so that snails amongst them can afford to be darker, and thus more cryptic. Hence, the more perennial vegetation in the habitat, the darker the shells.
Also during the Pleistocene, when temperatures were 5–10°C lower than today, the shells of the coastal plain were as pale as recent ones are. The distribution of a snail-predator, Gerbillus allenbyi , which is restricted to sandy biotopes where perennial bushes occur, and which was absent from Israel during the Pleistocene, could perhaps explain the distribution of X. vestalis morphs, both today and in the past. 相似文献
117.
Measurements made in fresh water wetland have shown that temperature, pH, dissolved oxygen and bicarbonate alkalinity in the hyacinth infested area were lower, but dissolved free carbon dioxide concentration was excitingly greater than in the open water area. This extreme hypoxic and hypercarbic condition of hyacinth covered areas was caused by the thick coverage of Eichhornia crassipes which was crucial for the composition of the characteristic flora and fauna. 相似文献
118.
Shirley A. Crawford 《Hydrobiologia》1979,62(1):17-31
Four aged Madison County, New York farm ponds were selected to see if various treatments could be used to restore the water quality. One pond was untreated and used as a control; another pond was partially drained and exposed to the drying and oxidizing effects of the air over the fall and winter; the other two ponds were drained and the accumulated sediment removed by bulldozing. In these latter two ponds, Chara vulgaris vegetation was inoculated following the restoration process. C. vulgaris growth rapidly became the dominant producer where this inoculation was accomplished in the fall of 1976, and it is expected that the other pond will also become a C. vulgaris pond in 1978 — after its oogonia have undergone the requisite winter dormancy period.Early C. vulgaris growth was found to be associated with clear water conditions and lessened phytoplankton growth; short, bushy, light-inhibited growth by the algae stabilized the bottom against wind-caused turbidity because of its rhizoidal growth within the substrate. Pioneer C. vulgaris growth was also found to be highly productive, significantly lowering the pond's CO2 readings.Investigators of aquatic systems are cautioned to be cognizant of the effect of epiphytic growth on successional events in such environments. Such epiphytes are surely important, if not prime, causes of the demise of various aquatic macrophytes.The partial draining and exposing of a pond over the fall and winter did not yield significantly improved water conditions. 相似文献
119.
D. Sharp 《Plant Ecology》1976,31(2):103-119
Summary The vegetation of cultivated, fallow and ‘old’ fields of Scotland County, N.C. was both randomly and subjectively sampled
throughout the growing season of 1973. Relevés were placed in each of the above management units, and the data collected in
this relevé analysis were sorted into phytosociological charts, with the aid of the digital computer.
Weed associations delimited in the charts reflected the importance of edaphic factors, time (aspect), and man's agricultural
activities as causal factors in the determination of characteristic assemblages of weeds. Several character and differential
species were denoted, in a tentative classification of the weed associations. The cultivated and fallow stands in the spring
aspect were noted to be dominated by a group of spring annuals. Summer annuals were more prevalent in homologous situations
in the summer. It was determined that perennials, appearing mainly as seedlings in cultivated and fallow fields, attained
dominance in ‘old’ fields, one to five years in age. Within each of the major spheres of management, and in each season, associations
of weeds mainly reflected differences in edaphic factors. Many character species were noted to maintain affinities to an assortment
of soil types that ranged from moist loams to xeric, sterile sands.
A study of roadsides, hedgerows, field margins and other ecotonal areas inhabited by weeds was carried throughout the growing
season of 1973, also. Species presence was documented in each of these habitat types and reported in Sharp (1974).
The weed flora of Scotland County was found to be rich in species. A total of 506 species representing 248 genera and 80 families
were encountered throughout the study. Of the total number of species listed, approximately 25 percent were of foreign origin,
and 28 percent represented additions to the flora of Scotland County.
This paper is the short version of a masters thesis for the Department of Botany, University of North Carolina at Chapel Hill.
The author would like to extend his appreciation and gratitude to Professors H. Lieth, A. E. Radford and J. Hardin for their
generous advice and encouragement extended through the duration of the study. 相似文献
120.
Remote sensing can be the most effective means of scaling up grassland aboveground biomass (AGB) from the sample scale to the regional scale. Among the remote-sensing approaches, statistical models based on the vegetation index (VI) are frequently used to retrieve grassland AGB because of their simplicity and reliability. However, these types of models have never been comprehensively optimized to overcome VI insensitivity and soil effects. Because grassland AGB is related to grassland type, in our research the integrated orderly classification system for grassland (IOCSG) was used to differentiate grassland types. The study area, located in Inner Mongolia, China, included desert steppe, typical steppe and meadow steppe. A pure VI (PVI) was extracted from the normal VI using spectral mixture analysis (SMA). Using a proportional relationship, PVI models were then constructed based on grassland type. The results demonstrated that the PVI models can have clear advantages over the more commonly used VI models. They simplify the parameterization of VI models and thus enhance models constructed for different regions with different remote sensing data sources. Notably, detailed differentiation of grassland types can improve the accuracy of AGB estimates. The methodology proposed in this study is particularly beneficial for AGB estimates at a national scale, especially for countries such as China with many grassland types. 相似文献