全文获取类型
收费全文 | 1950篇 |
免费 | 107篇 |
国内免费 | 140篇 |
专业分类
林业 | 770篇 |
农学 | 75篇 |
基础科学 | 166篇 |
222篇 | |
综合类 | 654篇 |
农作物 | 42篇 |
水产渔业 | 56篇 |
畜牧兽医 | 126篇 |
园艺 | 36篇 |
植物保护 | 50篇 |
出版年
2024年 | 5篇 |
2023年 | 18篇 |
2022年 | 45篇 |
2021年 | 63篇 |
2020年 | 53篇 |
2019年 | 79篇 |
2018年 | 45篇 |
2017年 | 75篇 |
2016年 | 88篇 |
2015年 | 108篇 |
2014年 | 100篇 |
2013年 | 120篇 |
2012年 | 161篇 |
2011年 | 160篇 |
2010年 | 115篇 |
2009年 | 125篇 |
2008年 | 121篇 |
2007年 | 80篇 |
2006年 | 99篇 |
2005年 | 56篇 |
2004年 | 52篇 |
2003年 | 47篇 |
2002年 | 23篇 |
2001年 | 41篇 |
2000年 | 37篇 |
1999年 | 38篇 |
1998年 | 29篇 |
1997年 | 29篇 |
1996年 | 30篇 |
1995年 | 26篇 |
1994年 | 27篇 |
1993年 | 28篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 14篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1976年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有2197条查询结果,搜索用时 15 毫秒
151.
152.
It is well established that increasing soil bulk density (SBD) above some threshold value reduces plant root growth and thus may reduce water and nutrient acquisition. However, formation and elongation of maize seminal roots and first order lateral (FOL) roots in various soil layers under the influence of SBD has not been documented. Two studies were conducted on a loamy sand soil at SBD ranging from 1.25 g cm–3 to 1.66 g cm–3. Rhizotrons with a soil layer 7 mm thick were used and pre‐germinated plants were grown for 15 days. Over the range of SBD tested, the shoot growth was not influenced whereas total root length was reduced by 30 % with increasing SBD. Absolute growth rate of seminal roots was highest in the top soil layer and decreased with increasing distance from the surface. Increasing SBD amplified this effect by 20 % and 50 % for the top soil layer and lower soil layers, respectively. At the end of the experiment, total seminal roots attributed to approximately 15 % of the total plant root length. Increasing SBD reduced seminal root growth in the lowest soil layer only, whereas FOL root length decreased with SBD in all but the uppermost soil layer. For FOL, there was a positive interaction of SBD with distance from the soil surface. Both, increasing SBD and soil depth reduced root length by a reduction of number of FOL roots formed while the length of individual FOL roots was not influenced. Hence, increasing SBD may reduce spatial access to nutrients and water by (i) reducing seminal root development in deeper soil layers, aggravated by (ii) the reduction of the number of FOL roots that originate from these seminal roots. 相似文献
153.
154.
Guillermo Trincado Curtis L. VanderSchaaf Harold E. Burkhart 《European Journal of Forest Research》2007,126(2):253-262
A height–diameter mixed-effects model was developed for loblolly pine (Pinus taeda L.) plantations in the southeastern US. Data were obtained from a region-wide thinning study established by the Loblolly
Pine Growth and Yield Research Cooperative at Virginia Tech. The height–diameter model was based on an allometric function,
which was linearized to include both fixed- and random-effects parameters. A test of regional-specific fixed-effects parameters
indicated that separate equations were needed to estimate total tree heights in the Piedmont and Coastal Plain physiographic
regions. The effect of sample size on the ability to estimate random-effects parameters in a new plot was analyzed. For both
regions, an increase in the number of sample trees decreased the bias when the equation was applied to independent data. This
investigation showed that the use of a calibrated response using one sample tree per plot makes the inclusion of additional
predictor variables (e.g., stand density) unnecessary. A numerical example demonstrates the methodology used to predict random
effects parameters, and thus, to estimate plot specific height–diameter relationships. 相似文献
155.
156.
Wenwen Wang Yanfeng Bai Chunqian Jiang Haijun Yang 《Journal of Sustainable Forestry》2020,39(5):526-541
ABSTRACT An individual-tree basal area increment model was developed for masson pine based on 26276 observations of 13,138 trees in 987 sample plots from the 7th (2004), 8th (2009), and 9th (2014) Chinese National Forest Inventory in Hunan Province, South-central China. The model was built using a linear mixed-effects approach with sample plots included as random effects since the data have a hierarchical stochastic structure and biased estimates of the standard error of parameter estimates could be a consequence of applying ordinary least square (OLS) for regression. In addition, within-plot heteroscedasticity and autocorrelation were also considered. The final mixed-effects model was determined according to the Akaike information criterion (AIC), Bayesian information criterion (BIC), log-likelihood (Loglik), and the likelihoodratio test (LRT). The results revealed that initial diameter (DBH), the sum of the basal area (m2/ha) in trees with DBHs larger than the DBH of the subject tree (BAL), number of trees per hectare (NT), and elevation (EL) had a significant impact on individual-tree basal area increment. The mixed-effects model performed much better than the basic model produced using OLS. Additionally, the variance structure of the model errors was successfully modeled using the power function. However, the autocorrelation structures were not defined because there was no autocorrelation amongst the data. It is believed that the final model will contribute to the scientific management of the masson pine. 相似文献
157.
158.
白桦胸径和材积生长率相关关系的研究 总被引:1,自引:0,他引:1
根据地位指数的大小将白桦林分划分为好、中、差3个类型,通过没类型样地上的解析木资料分析,证明白桦的胸径和材积生长率存在关良好的相关关系。经过优选,建立了不同类型白桦林分的胸径与材积生长率相关模型,并依此编制出白桦一元材积生长率表,为准确预测白桦积生长率提供了理论上的依据。 相似文献
159.
160.
Jochen Schngart 《Forest Ecology and Management》2008,256(1-2):46-58
Against a background of increasing human populations in developing countries, and global climate change, conservation of tropical forests remains one of the most important ecological challenges of our time. One of the biggest difficulties for ecologically sustainable management of tropical forests is obtaining reliable growth data for trees, which is a prerequisite for determining harvesting volumes and cutting cycles. GOL is the first concept for sustainable management of tropical timber resources in Amazonian floodplain forests (várzea) based on species-specific management criteria, such as minimum logging diameters (MLDs) and cutting cycles. From timber species with varying wood densities of different successional stages, volume stocks have been estimated in 1-ha plots and 12 growth models have been developed based on tree rings, which are annually formed as a consequence of the regular, long-term flooding. The MLDs of timber species vary between 47 and 70 cm and the estimated cutting cycles differ the 10-fold, from 3 to 32 years. These enormous differences in the growth rates between tropical timber species are not considered in current management practices, which apply only one diameter cutting limit and one cutting cycle to harvest many tree species. This practice risks the overexploitation of slow-growing timber species, while the fast-growing timber species with low wood densities cannot be efficiently used. Based on the timber stocks and lifetime growth rates, the GOL concept has been created as an aid to improve forest management in the Central Amazonian várzea. The model is unique for tropical silviculture. 相似文献