全文获取类型
收费全文 | 4986篇 |
免费 | 339篇 |
国内免费 | 1篇 |
专业分类
林业 | 398篇 |
农学 | 233篇 |
基础科学 | 44篇 |
1151篇 | |
综合类 | 167篇 |
农作物 | 229篇 |
水产渔业 | 617篇 |
畜牧兽医 | 1929篇 |
园艺 | 108篇 |
植物保护 | 450篇 |
出版年
2023年 | 49篇 |
2022年 | 87篇 |
2021年 | 160篇 |
2020年 | 151篇 |
2019年 | 168篇 |
2018年 | 283篇 |
2017年 | 225篇 |
2016年 | 223篇 |
2015年 | 172篇 |
2014年 | 220篇 |
2013年 | 315篇 |
2012年 | 334篇 |
2011年 | 350篇 |
2010年 | 205篇 |
2009年 | 223篇 |
2008年 | 288篇 |
2007年 | 262篇 |
2006年 | 227篇 |
2005年 | 211篇 |
2004年 | 192篇 |
2003年 | 152篇 |
2002年 | 154篇 |
2001年 | 99篇 |
2000年 | 93篇 |
1999年 | 69篇 |
1998年 | 33篇 |
1997年 | 21篇 |
1996年 | 18篇 |
1995年 | 27篇 |
1994年 | 9篇 |
1993年 | 13篇 |
1992年 | 22篇 |
1991年 | 21篇 |
1990年 | 25篇 |
1989年 | 20篇 |
1988年 | 10篇 |
1987年 | 17篇 |
1986年 | 11篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 14篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 14篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1973年 | 7篇 |
1969年 | 12篇 |
1967年 | 8篇 |
排序方式: 共有5326条查询结果,搜索用时 15 毫秒
171.
Matts Lindbladh Adam Felton Renats Trubins Ola Sallnäs 《European Journal of Forest Research》2011,130(5):861-869
As resources allocated specifically for conservation are limited, there is a need to ensure conservation policy initiatives
lead to effective conservation outcomes. In this study, we investigated the potential conservation benefits from alternative
spatial allocations of old deciduous stands to a landscape dominated by coniferous production forests owned primarily by non-industrial
private forest owners. As a target species, we used the long-tailed tit (Aegithalos caudatus), a species associated with deciduous forests and known to be sensitive to isolation. We used a previously published model
based on empirical data on the occurrence of this species, to assess the probability of occurrence of the bird in a 4,000 km2 area in southern Sweden for which we possess detailed spatial GIS data (kNN data) of tree species composition and age. We
assessed alternative scenarios where old deciduous forest was allocated with or without respect to distance from existing
old deciduous forests. Due to the long-tailed tit’s habitat requirement increasing the amount of old deciduous forests close
to existing habitats was the most effective strategy. However, the potential advantages of this strategy may in fact be overturned
in favor of the other scenarios if ownership structures and probable uptake rates of policy initiatives are also considered.
If a policy initiative is targeted toward owners with properties in close proximity to existing suitable habitat, when compared
to if all forest owners are targeted, a higher proportion of owners is needed to participate in order to achieve the same
degree of habitat creation for the species. Here, we discuss the potential benefits for effective conservation policy formulation
from integrating spatially explicit datasets and detailed ecological knowledge with land-ownership structures and policy uptake
scenarios. 相似文献
172.
Edival Ângelo Valverde Zauza Lúcio Mauro da Silva Guimarães Nilza de Lima Pereira Sales Samuel Alves dos Santos Rafael Ferreira Alfenas Acelino Couto Alfenas 《Forest Pathology》2023,53(2):e12806
An outbreak of a new and severe disease was observed in Eucalyptus plantations of Bahia state, Brazil. An Ascomycota fungus has been frequently associated with the main symptoms of the disease namely leaf spot, branch cankers, shoot blight, defoliation, and dieback. Based on morphological characteristics, phylogenetic analysis (ITS and TEF-1α genes), and pathogenicity test on Eucalyptus plants, Pseudoplagiostoma eucalypti was identified as the causal agent of the disease. Although P. eucalytpi has been known from in Brazil since 1998, this is the first report of it causing severe disease and die-back on Eucalyptus spp. and we also record new symptoms associated with the pathogen. 相似文献
173.
Lennart Noordermeer Terje Gobakken Erik Næsset Ole Martin Bollandsås 《Scandinavian Journal of Forest Research》2021,36(1):55-67
ABSTRACT Forest productivity is a crucial variable in forest planning, usually expressed as site index (SI). In Nordic commercial forest inventories, SI is commonly estimated by a combination of aerial image interpretation, field assessment and information obtained from previous inventories. Airborne laser scanning (ALS) and digital aerial photogrammetry (DAP) data can alternatively be used for SI estimation, however the economic utilities of the inventory methods have not been compared. We compared seven methods of SI estimation in a cost-plus-loss analysis, by which we added the expected economic losses due to sub-optimal treatment decisions to the inventory costs. The methods comprised direct and indirect estimation from combinations of ALS, DAP and stand register data, and manual interpretation from aerial imagery supported by field assessment and information from previous inventories (conventional practices). The choice of method had great impact on both the accuracy and the economic value of the produced estimates. Direct methods using bitemporal ALS and DAP data gave the best accuracy and the smallest total cost. DAP was a suitable and low-cost data source for SI estimation. Estimation from single-date ALS and DAP data and age obtained from the stand register provided practical alternatives when applied to even-aged stands. 相似文献
174.
This study investigates the optimal conditions for the vegetative propagation of the multipurpose leguminous shrub Colutea istria from leafy stem cuttings. The conditions examined included 1) the origin of the cutting (apical, medial or basal), 2) indole-3-butyric
acid (IBA) treatment (0, 50, 200 or 400 mg/l), 3) the relative humidity of the propagation environment (high or low), 4) wounding
base of cutting, and 5) the season in which the cutting was taken. The rooting ability of the cuttings was significantly influenced
by all these factors. Winter cuttings obtained from the middle and base of the branch showed a greater rooting capacity than
those harvested from the apical area, while autumn cuttings harvested from the middle and apical areas of the branch showed
a greater rooting capacity than those harvested from the base. Winter cuttings showed the best rooting capacity than autumn
cuttings. IBA treatment (200 mg/l), high humidity (98-100%), and wounding increased the rooting capacity of the cuttings. The sprout
of new leaves was higher in basal cuttings, while IBA and wounding also significantly enhanced the root number of rooted cuttings.
A method for vegetatively propagating C. istria plants is given. 相似文献
175.
176.
Eugenio Díaz-Pinés Andreas Schindlbacher Michael Pfeffer Robert Jandl Sophie Zechmeister-Boltenstern Agustín Rubio 《European Journal of Forest Research》2010,129(1):101-109
We conducted a trenching experiment in a mountain forest in order to assess the contribution of the autotrophic respiration
to total soil respiration and evaluate trenching as a technique to achieve it. We hypothesised that the trenching experiment
would alter both microbial biomass and microbial community structure and that fine roots (less than 2 mm diameter) would be
decomposed within one growing season. Soil CO2 efflux was measured roughly biweekly over two growing seasons. Root presence and morphology parameters, as well as the soil
microbial community were measured prior to trenching, 5 and 15 months after trenching. The trenched plots emitted about 20
and 30% less CO2 than the control plots in the first and second growing season, respectively. Roots died in trenched plots, but root decay
was slow. After 5 and 15 months, fine root biomass was decreased by 9% (not statistically different) and 30%, (statistically
different) respectively. When we corrected for the additional trenched-plot CO2 efflux due to fine root decomposition, the autotrophic soil respiration rose to ~26% of the total soil respiration for the
first growing season, and to ~44% for the second growing season. Soil microbial biomass and community structure was not altered
by the end of the second growing season. We conclude that trenching can give accurate estimates of the autotrophic and heterotrophic
components of soil respiration, if methodological side effects are accounted for, only. 相似文献
177.
Andrés Dieste Andreas Krause Carsten Mai Holger Militz 《Wood Science and Technology》2010,44(4):597-606
Beech wood (Fagus sylvatica L.) was modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU). The equilibrium moisture content (EMC) of wood modified
with DMDHEU calculated on a dry modified basis is reduced. Previous results have shown that the modification with DMDHEU does
not alter the capillary condensation; therefore, the reduction in EMC seems exaggerated. The equilibrium constants of the
Hailwood–Horrobin model (K
d and K
h) and the molecular weight of a hypothetical polymer of modified wood capable of adsorbing one molecule of water (W
i) were calculated from the EMC on a dry modified wood basis (M) and on a dry wood basis (M
R). The hypothetical polymer was also calculated by stoichiometry (W
c) and compared to W
i to estimate the number of operative OH groups. The number of operative OH groups decreased when M was used, in contradiction with the previously obtained results of differential heat of adsorption (∆H
s). Therefore, the use of M
R is recommended for the analysis of moisture sorption in wood modified with DMDHEU. 相似文献
178.
María Angela Martín Juan Bautista Alvarez Luís Miguel Martín 《Annals of Forest Science》2010,67(6):603-603
– |
• The Spanish fir (Abies pinsapo Boiss.) is a conifer endemic to southern Spain and belongs to a Mediterranean basin group of firs considered to be relict.
Megagametophyte storage proteins are a useful tool for the evaluation of genetic variability due to the haploid nature of
megagametophytes in conifers. 相似文献
179.
Black locust (Robinia pseudoacacia L.) is one of the most important stand-forming tree species in Hungary and its importance is increasing in many countries.
Black locust plants are commonly produced by two methods, by seed and by root cuttings. Tissue culture propagation can be
considered as a relatively new method. Growing trees from seed is a relatively simple method for reliably producing seedlings
on a large scale under a variety of circumstances. Mechanization of the method is easy and the production cost is relatively
low. Propagation from root cuttings and tissue culture are valuable for reproduction of superior individuals or varieties.
By applying these methods, superior traits of the selected trees can be preserved in the clones. Recent experiments demonstrated
that micropropagated trees could be successfully transplanted into soil, hardened and grown in the field.
Biography: Karoly Redei (1952), male, doctor, deputy director general of Hungarian Forest Research Institute H-1023 Budapest, Frankel
Leó Str. 42 44, HUNGARY.
Responsible editor: Chal Ruihai 相似文献
180.
Whole-stand models normally require data on initial stand basal area and dominant height. Dominant height measurements are time-consuming and often imprecise, compromising subsequent predictions. Poplar plantations provide a special case where basal area correlates with site index; a whole-stand model could thus be based on stand basal area. We report a static model constructed by the generalized algebraic difference approach (GADA) for poplar plantations for three different hybrid poplars (Populus × euramericana (Dode) Guinier “I-214”, “MC”, and “Luisa Avanzo”) in northeast Spain. The transition function was based on current stand basal area and was fitted with data from 158 permanent plots ranging from 1- to 17-year-old plantations. Merchantable stand volume was estimated by a volume equation where height was predicted by a height–basal area relationship based on 458 temporary plots. The model differences between clones were compared using the nonlinear extra sum of squares method. Significant differences were detected, while Luisa Avanzo presented the highest merchantable volume at the end of the rotation. Errors in basal area predictions were below 20% within 6 years in the case of Luisa Avanzo and MC clones, and within 3 years in the case of I-214. Our research showed that satisfactory predictions can be obtained using GADA with a single transition function based on an easily measurable variable such as stand basal area. 相似文献
|