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991.
992.
993.
Abstract. Many theories of forest succession imply that terrestrial plant community composition within a region tends to converge toward a climax community. That is, given similar climatic and edaphic conditions, succession at different sites within an area will lead to comparable species compositions, a pattern referred to as successional convergence. In this study, we examine changes in plant composition within forest canopy gaps over a 17-yr period to identify potential patterns of successional convergence and to ascertain the factors controlling the successional pathway. To do so, we: (1) sampled 36 forest canopy gaps in Hueston Woods Nature Preserve in 1977, 1981, 1985, 1989 and 1993, (2) evaluated changes in the similarity of gap composition over this period, and (3) examined gap composition in each year as a function of variables describing gap habitat, seed source proximity, and disturbance history. Results indicated an initial pattern of successional divergence, with gaps exhibiting increased dissimilarity over the first 10–12 years of succession. We attribute this initial period of divergence to the effects of differential seed inputs from edge individuals and heterogeneity of available light due to differences in gap size. Recent surveys, however, indicated that gap composition has become more similar as competition within gaps has become more intense. In these samples, gap composition is closely linked to site conditions, including slope, soil conditions, and site exposure. Finally, while these patterns may suggest equilibrium-oriented dynamics, non-equilibrium processes such as repeat disturbances are also evident at Hueston Woods and will likely play an important role in determining future successional pathways. 相似文献
994.
S. Yamamoto 《Plant Ecology》1996,127(2):203-213
Gap regeneration of major tree species was examined, based on the pattern of gap phase replacement, in primary old-growth stands of warm-temperate, cool-temperate and subalpine forests, Japan. Using principal component analysis, the gap-regeneration behavior of major tree species could be divided into three guilds and that of Fagus crenata (monodominant species of cool-temperate forests). The criteria used for this division were total abundance of canopy trees and regenerations and relative abundance of regenerations to canopy trees. The gap-regeneration behavior of species in the first guild was that canopy trees regenerate in gaps from seedlings or saplings recruited before gap formation; they had higher total abundance and more abundant regenerations relative to their canopy trees. The gap-regeneration behavior of F. crenata was same as species in the first guild, but F. crenata had less abundant regenerations relative to its canopy trees. Species in the second guild had lower total abundance and less abundant regenerations to their canopy trees. The guild contained species whose canopy trees regenerate in gaps from seedlings or saplings recruited after gap formation or regenerate following largescale disturbance. The third guild consisted of species with lower total aboundance and more abundant regenerations relative to their canopy trees. The gap-regeneration behavior of some species in this guild was that trees regenerate in gaps from seedlings or saplings recruited before gap formation, and grow, mature, and die without reaching the canopy layer, while the gap-regeneration behavior of other species was same as that of species in the first guild or F. crenata. Major tree species of subalpine forests were not present in the third guild. 相似文献
995.
996.
We report here the successful scale up of transient recombinant protein expression to litre scale using Semliki Forest Virus
System. The expression of bacterial β-galactosidase was initially compared in BHK and CHO cells and the conditions for optimal
infection of BHK cells were identified. 10% FCS in a medium at pH 6.9 and infection in small volumes were found to be optimal.
A high MOI results in an increased recombinant protein yield. Stirring does not affect the infection process. Finally we applied
these optimal conditions to the production of a microsomal enzyme, human cyclooxygenase-2 in suspension spinners. Five independant
productions at the 1 litre scale yielded reproducible substantial amounts of recombinant protein (16 mg microsomal protein
109 cells−1) with an average specific activity of 3942 ± 765 pg PGE2 μg−1 microsomal protein 5 min−1.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
997.
Vassili Davydchuk 《Ecological Engineering》1997,8(4):325-336
The ecosystems of Chernobyl nuclear accident zone, having been deeply transformed by normal long-term agricultural and forestry activity, have been damaged by irradiation after the nuclear accident of 1986 and then influenced by post-accident countermeasures. Ecosystem remediation is being undertaken in this area, including maintenance of the forests, artificial afforestation, and processes of spontaneous self-restoration of the natural coenoses. Both natural and artificial remediation of the landscapes and ecosystems at the abandoned territory improves their capacity to retain radionuclides and their ability to stabilise the processes of redistribution of radionuclides and their spread in the environment. The Chernobyl zone is a territory of great importance for studying ecosystem remediation processes and elaborating on ecological engineering technologies to correct serious environmental problems arising from radionuclides release. 相似文献
998.
999.
Leif Björkman 《Vegetation History and Archaeobotany》1997,6(2):79-90
A pollen record from a small alder carr located in the centre of aFagus stand near the hamlet of Flahult in southern Småland has shown thatFagus became established in a semi open cultural landscape about 900 B.P. Human disturbance seems to have controlled the local establishment ofFagus at this site through an expansion of pastoral farming. TheFagus dominance in the present stand seems to be of recent origin, asFagus pollen percentages and influx values have increased considerably only during the last 50 years. The modern composition and structure of theFagus stand are probably an effect of changes in land-use and decreased human activity at the end of the last century. Today, only occasionalPicea individuals occur in the studied stand, andPicea does not appear to have been more abundant in the recent past. The regional expansion ofPicea has probably occurred during this century and has been favoured by modern forestry during the last 50 years. 相似文献
1000.
Forest fire can affect arbuscular mycorrhizal (AM) fungi by changing the soil conditions and by directly altering AM proliferation.
We studied the effects of a severe forest fire at Margalla Hills near Islamabad on the number and viability of AM fungal propagules
in the burnt soil and their role in the re-establishment of post-fire infection in colonized plants. Compared with a nearby
control area, the burnt site had a similar number of total spores but a lower number of viable AM fungal propagules. The roots
of the two most frequent species at the burnt site, Dodonaea viscosa and Aristida adscensionis, showed a gradual increase in percentage root length colonized by AM fungi in general and hyphal infection in particular.
Our results indicate resumption of mycorrhizal activity following the fire, probably from AM hyphae in the roots of these
dominant shrubs.
Accepted: 18 July 1997 相似文献