全文获取类型
收费全文 | 5325篇 |
免费 | 560篇 |
国内免费 | 149篇 |
出版年
2023年 | 102篇 |
2022年 | 52篇 |
2021年 | 123篇 |
2020年 | 192篇 |
2019年 | 219篇 |
2018年 | 208篇 |
2017年 | 167篇 |
2016年 | 212篇 |
2015年 | 176篇 |
2014年 | 192篇 |
2013年 | 379篇 |
2012年 | 192篇 |
2011年 | 228篇 |
2010年 | 198篇 |
2009年 | 241篇 |
2008年 | 257篇 |
2007年 | 271篇 |
2006年 | 239篇 |
2005年 | 223篇 |
2004年 | 205篇 |
2003年 | 179篇 |
2002年 | 197篇 |
2001年 | 208篇 |
2000年 | 155篇 |
1999年 | 136篇 |
1998年 | 111篇 |
1997年 | 113篇 |
1996年 | 81篇 |
1995年 | 84篇 |
1994年 | 87篇 |
1993年 | 61篇 |
1992年 | 59篇 |
1991年 | 59篇 |
1990年 | 53篇 |
1989年 | 35篇 |
1988年 | 34篇 |
1987年 | 36篇 |
1986年 | 36篇 |
1985年 | 31篇 |
1984年 | 59篇 |
1983年 | 16篇 |
1982年 | 27篇 |
1981年 | 15篇 |
1980年 | 26篇 |
1979年 | 11篇 |
1978年 | 23篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1974年 | 2篇 |
1972年 | 3篇 |
排序方式: 共有6034条查询结果,搜索用时 190 毫秒
91.
热带林茎流收集及计算方法探索 总被引:5,自引:0,他引:5
热带林茎流收集及计算方法探索周光益,吴仲民,李意德,陈步峰(中国林科院热带林业研究所,广州510520)CollectionandCalculationMethodsforStemflowinTropicalForest.¥ZhouGuangyi;W... 相似文献
92.
Stomatal and environmental control of transpiration in a lowland tropical forest tree 总被引:11,自引:8,他引:3
F. C. MEINZER G. GOLDSTEIN N. M. HOLBROOK P. JACKSON J. CAVELIER 《Plant, cell & environment》1993,16(4):429-436
Stomatal control of crown transpiration was studied in Anacardium excelsum, a large-leaved, emergent canopy species common in the moist forests of Central and northern South America. A construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35-m-tall individual. Stomatal conductance at the single leaf scale, and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively. This permitted the sensitivity of transpiration to a marginal change in stomatal conductance to be evaluated using a dimensionless coupling coefficient (1-ω) ranging from zero to 1, with 1 representing maximal stomatal control of transpiration. Average stomatal conductance varied from 0.09 mol m?2 s?1 during the dry season to 0.3 mol m?2 s?1 during the wet season. Since boundary layer conductance was relatively low (0.4 mol m?2 s?1), 1-ω ranged from 0.46 during the dry season to only 0.25 during the wet season. A pronounced stomatal response to humidity was observed, which strongly limited transpiration as evaporative demand increased. The stomatal response to humidity was apparent only when the leaf surface was used as the reference point for measurement of external vapour pressure. Average transpiration was predicted to be nearly the same during the dry and wet seasons despite a 1 kPa difference in the prevailing leaf-to-air vapour pressure difference. The patterns of stomatal behaviour and transpiration observed were consistent with recent proposals that stomatal responses to humidity are based on sensing the transpiration rate itself. 相似文献
93.
Abstract. A model is proposed for the fitting of species-area curves to data from the Stellenbosch region, South Africa. Basic assumptions of the model are finiteness of the number of species in a finite area, and random distribution of plant species over the region. The model involves a distribution of densities of different species, and the parameters of this distribution are useful for describing and classifying communities. The data of the Stellenbosch region suggest that the assumptions of the model break down in areas greater than 500 m2. 相似文献
94.
R. J. FENSHAM 《Austral ecology》1994,19(2):189-196
Abstract Sucking insects constituted 79% of all phytophagous insects collected from woody sprouts in the ground layer of a tropical eucalypt forest. Mobile insect groups such as non-psyllid Hemiptera and Orthoptera were relatively frequent in this environment compared to temperate, Eucalyptus-dominated vegetation. The high fire frequency of the tropical eucalypt forest may favour mobile insect groups. The capture of sucking insects and caterpillars peaked in dry season samples. Other patterns of abundance of phytophagous insect groups showed little consistency in their seasonal trends between host species or between vegetation types within host species. Disparities between chewing insect abundance in daytime samples and the damage chewing insects cause, may result from disproportionate consumption by large, mainly nocturnal insects, such as members of the Orthoptera. In this study, 21% of insect species were specialists on single plant species. This study suggested that insect abundance reflected the growth patterns of woody sprouts after regular burning, rather than that plant growth and development were tuned to the pressures of insect herbivory. 相似文献
95.
K. A. McGUINNESS 《Austral ecology》1994,19(3):283-289
Abstract Many molluscs in tidal wetlands climb trees as the tide rises, a behaviour usually assumed to be a means of avoiding subtidal predators. Some species are more active during neap tides, when the access of subtidal predators to the forest is limited, but rest on trees during spring tides. Cerithidea anticipata, which inhabits the mangrove forests around Darwin Harbour (Northern Territory, Australia), displayed almost exactly the opposite pattern. This species climbed higher, and was less active, during neap tides that did not flood the forest than during spring tides. In experiments with tethered snails, individuals prevented from climbing died during neap tides, apparently from physiological stress. Further, individuals resting on trees around clearings, usually selected shaded sites. These results suggest that the major reason C. anticipata climbed was to avoid physiological stress during neap tides, not subtidal predators during spring tides. There was some evidence of predation under the canopy, but the rate was relatively low and the species responsible appeared to be resident in the forest. 相似文献
96.
Population studies on a South African carrageenophyte: Iridaea capensis (Gigartinaceae,Rhodophyta) 总被引:1,自引:1,他引:0
An intertidal population of Iridaea capensis in the south-western Cape Province of South Africa has been studied in detail for a year. The species is generally restricted to sand-inundated rocky shores, which appears not to be the case for Iridaea species in the Pacific. In early winter (June) biomass, plant density and plant size were at a minimum. A small increase in these parameters occurred in August, but there was a massive increase in plant numbers and biomass in October. The largest plants were recorded in summer (February). The population was over 70% sporophytes in April, but switched to almost 80% gametophytes following heavy recruitment between August and October, and remained significantly gametophyte-dominated through April of the following year. Carrageenan levels were high (ca 42% of dry weight) in late winter-spring, and lowest (ca 30–35%) in summer-autumn. Strong offshore wind conditions in early summer caused a reduction in biomass, and it is hypothesized that the associated upwelling of nutrient rich water may be implicated in the lower carrageenan levels at this time. 相似文献
97.
Artificial destratification of a small tropical reservoir: effects upon the phytoplankton 总被引:3,自引:3,他引:0
Seasonal changes in the phytoplankton community of a small tropical reservoir were monitored over a four year period comprising
of an initial two seasonal cycles during which the water column stratified strongly for extended periods each year, and two
further seasonal cycles after installation of a mechanical aeration system to induce artificial destratification.
In the unmanaged reservoir, the concentration of chlorophyll a at 0.5 m reached maximum values (on one occasion > 90 mg m−3) when the water column was stratified and the epilimnion was very shallow (ca 2 m depth). The hypolimnion at this time was anoxic (less than 2% oxygen saturation) and had a high concentration of bacteriochlorophyll
(100–200 mg m−3).
The phytoplankton community of the unmanaged reservoir was generally dominated by cyanobacteria (Cylindrospermopsis raciborskii, Anabaena tenericaulis) during the warmer months of the year (November–March) (but replaced by chlorophyta, dinophyceae and euglenophyceae after
periods of intense rain) and by bacillariophyceae (Synedra ulna var. chaseana, S. tenera) during the cooler, dry months.
In the artificially destratified reservoir (8 h aeration day−1), the phytoplankton community was largely dominated by diatoms except after depletion of the silica content of the water
column which caused diatoms to be replaced by cyanobacteria (dominated by A. tenericaulis) and a range of chlorophytes.
The changing pattern of stratification and circulation of the water column in the unmanaged reservoir caused repeated disruption
of the established phytoplankton assemblage with peaks of high biomass associated with transient cyanobacterial blooms. Continuous
aeration and the consequent increase in the ratio mixed: euphotic depth provided conditions suitable for dominance of the
phytoplankton by diatoms, as long as silica was available, and resulted in average chlorophyll levels higher than in the unmanaged
reservoir (120 ± 10 v. 64 ± 9 mg m−2).
Hierarchical fusion analysis based on the biomass of species differentiated the phytoplankton samples into cluster groups
that could be related primarily to stratification or mixing of the water column. 相似文献
98.
SIGRID LIEDE F.L.S. ULRICH MEVE 《Botanical journal of the Linnean Society. Linnean Society of London》1993,112(1):1-15
LIEDE, S. & MEVE, U., 1993. Towards an understanding of the Sarcostemma viminale (Asclepiadaceae) complex. All names in Sarcostemma sensu Bullock (Asclepiadaceae, Asclepiadeae, Cynanchinae) are listed and their current status evaluated. Chromosome numbers are given for most taxa. The typification of S. viminale is clarified. A new subspecies of S. viminale is described from the Orange Free State, South Africa, and relationships of the Namaqualand/Namibia populations are discussed. 相似文献
99.
Hydrobiologia - In 1988–89 a 200 m core was drilled down to granite bedrock in the Pretoria Saltpan as part of a project to ascertain the origin of this unusual feature. The top 90 m of the... 相似文献
100.
Fernando Luís do Rêgo Monteiro Starling 《Hydrobiologia》1993,257(3):143-152
A mesocosm experiment was conducted to assess the impact of moderate silver carp (Hypophthalmichthys molitrix) biomass (41 g m–3 or 850 kg ha–1) on the plankton community and water quality of eutrophic Paranoá Reservoir (Brasília, Brazil). Microzooplankton (copepod nauplii and rotifers <200 m), netphytoplankton (> 20 m), total phytoplankton biomass (expressed as chlorophyll-a) and net primary productivity were significantly reduced by silver carp. Apart from increased nitrogen in the sediment, nutrients and chemical properties of the water were not affected by fish presence. The observed improvements in water quality suggest that stocking silver carp in Paranoá Reservoir to control blue-green algae is a promising biomanipulation practice. 相似文献