首页 | 官方网站   微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   49篇
  免费   1篇
农业科学   50篇
  2024年   1篇
  2023年   2篇
  2022年   1篇
  2018年   1篇
  2015年   1篇
  2014年   2篇
  2013年   6篇
  2012年   4篇
  2011年   4篇
  2010年   4篇
  2009年   1篇
  2008年   2篇
  2007年   4篇
  2006年   4篇
  2005年   4篇
  2004年   1篇
  2003年   2篇
  2002年   1篇
  2001年   1篇
  1999年   1篇
  1998年   1篇
  1990年   1篇
  1989年   1篇
排序方式: 共有50条查询结果,搜索用时 15 毫秒
11.
This paper examines the historical phases of Finnish forest planning from the 1960s to the present day in support of the rational future development of forest planning for family-owned forest holdings. The ‘historical types’ of work—craft work, mass production, process enhancement, and mass customization have been recognized in the Finnish planning discourse to date. Indications of mass customization are typical in the present developmental discussion of forest planning. There are some signals in the current planning debate that are calling for the next activity principle, co-configuration. A forthcoming type of forestry extension activity, labelled adaptive planning, is described in this paper to explicate the demands imposed by a future planning system. The present analysis informs also other contexts outside of Finland, where public and private institutions that support family forest owners face post-industrial challenges.  相似文献   
12.
We studied methanogen activity—measured by in vitro methane production potential and by detection of the messenger RNA (mRNA) of a functional gene—in two boreal fens under high and deep water table (WT) level conditions resulting from a rainy growing season and a dry growing season. The depth of the highest CH4-producing layers differed between the years. In the wet year, the highest CH4 production rate was around 20 cm below the mean WT. In the dry year, the highest rates were measured close to the peat surface, well above the mean WT. The distribution of activity in the peat profiles of the two fens appeared to be site specific. Under deep-WT conditions, CH4 production potential was generally lower than that under high-WT conditions. Detection of the mRNA of the methanogen-specific mcrA gene indicated in situ methanogenesis in both water-saturated peat (below the WT) and unsaturated peat (above the WT). Analyses of DNA-derived and mRNA-derived methanogen community structures showed greater similarity between those two in water-saturated peat than in unsaturated peat. This suggested that favorable conditions promoted the activity of most members in methanogen communities, but unfavorable conditions showed differences between distinct community members in adaptation to adverse conditions.  相似文献   
13.
The effects of pollution on thecomposition of soil seed banks in four drained pinemires located along a pollution gradient in southernFinland were studied using the emergence method. Thespecies composition of the seed bank was compared withthe above ground vegetation. Dominant species in theseed banks were Calluna vulgaris, Betulaspp. and Ledum palustre. Seeds seemed to beviable despite a heavy pollution load. Especially inthe vicinity of the pollution source, seed bank andabove ground vegetation were weakly correlated.  相似文献   
14.
Understanding of how a large landscape or network of conservation areas and habitats of red-listed species change in time is an important topic when addressing the temporal interplay between protected areas and matrix. We developed models of habitat suitability indices (HSI) for saproxylic red-listed invertebrate and fungal species, accounting for roughly 70% of all red-listed boreal forest species of the study area in eastern Finland. By using a forestry planning program that incorporates various optimisation methods we analysed trade-offs between timber production and amount of habitats of saproxylic red-listed species within a 60-year period. We also produced production possibility frontiers that show how to increase quality of the matrix with least costs. Moreover, we analysed how habitat suitability criteria used in optimisations affect the area of different habitat quality classes.

Our analysis shows that by adopting HSI models in long-term matrix management, it is possible to increase habitats for several red-listed species without substantial losses in timber production. The increase in habitat area is achieved mainly by decreasing the area that is thinned compared to intensive timber production plan. In the long term, this seems to be a novel cost-effective method to increase the quality of the matrix for red-listed saproxylic species. However, the selected optimisation method and the criteria or specification of the management objective for red-listed forest species have a strong effect on results when HSI models are used in conservation planning. Therefore any practical application must be performed with great care.  相似文献   

15.
For evaluating the applicability of the soil gradient method as a substitute for CO2‐, CH4‐, and N2O‐flux measurements in steppe, we carried out chamber measurements and determined soil gas concentration at an ungrazed (UG99) and a grazed (WG) site in Inner Mongolia, China. The agreement of the concentration‐based flux estimates with measured chamber‐based fluxes varied largely depending on the respective GHG in the sequence CO2 > CH4 >> N2O. A calibration of the gas‐transport parameter used to calculate fluxes based on soil gas concentrations improved the results considerably for CO2 and CH4. After calibration, the average deviation from the chamber‐based annual cumulative flux for both sites was 11.5%, 10.5%, and 59% for CO2, CH4, and N2O. The gradient method did not constitute an adequate stand‐alone substitute for greenhouse‐gas flux estimation since a calibration using chamber‐based measurements was necessary and vigorous production processes were confined to the uppermost, almost water‐saturated soil layer.  相似文献   
16.
With automatic plant identification methods, the amount of herbicides used in agriculture can be reduced when herbicides are sprayed only on weeds. In the present study, leaves of oat (Avena sativa) and dandelion (Taraxacum officinale, TAROF) were arranged so that there was overlap between the species, imaged with a pulse amplitude modulation fluorescence camera and photographed with a digital color camera. The fluorescence induction curves from each pixel were parameterized to obtain a set of features and from color photographs, texture features were calculated. A support vector algorithm that also performed feature selection was used for pattern recognition of both data sets. Fluorescence-based identification worked well with oat leaves, producing 92.2 % of correctly identified pixels, whereas the texture-based method often mis-identified the central vein of a TAROF leaf as oat, identifying correctly only 66.5 % of oat pixels. With TAROF that shows a clear dicot-type texture, the texture method was slightly better (96.4 % correctly identified pixels) than the fluorescence method (94.6 %). In fluorescence-based identification, the accuracy varied between entire TAROF leaves, probably reflecting the genetic variability of TAROF. The results suggest that the accuracy of identification could be improved by combining two identification methods.  相似文献   
17.
Greenhouse gas emissions from managed peatlands are annually reported to the UNFCCC. For the estimation of greenhouse gas (GHG) balances on a country-wide basis, it is necessary to know how soil–atmosphere fluxes are associated with variables that are available for spatial upscaling. We measured momentary soil–atmosphere CO2 (heterotrophic and total soil respiration), CH4 and N2O fluxes at 68 forestry-drained peatland sites in Finland over two growing seasons. We estimated annual CO2 effluxes for the sites using site-specific temperature regressions and simulations in half-hourly time steps. Annual CH4 and N2O fluxes were interpolated from the measurements. We then tested how well climate and site variables derived from forest inventory results and weather statistics could be used to explain between-site variation in the annual fluxes. The estimated annual CO2 effluxes ranged from 1165 to 4437 g m−2 year−1 (total soil respiration) and from 534 to 2455 g m−2 year−1 (heterotrophic soil respiration). Means of 95% confidence intervals were ±12% of total and ±22% of heterotrophic soil respiration. Estimated annual CO2 efflux was strongly correlated with soil respiration at the reference temperature (10 °C) and with summer mean air temperature. Temperature sensitivity had little effect on the estimated annual fluxes. Models with tree stand stem volume, site type and summer mean air temperature as independent variables explained 56% of total and 57% of heterotrophic annual CO2 effluxes. Adding summer mean water table depth to the models raised the explanatory power to 66% and 64% respectively. Most of the sites were small CH4 sinks and N2O sources. The interpolated annual CH4 flux (range: −0.97 to 12.50 g m−2 year−1) was best explained by summer mean water table depth (r2 = 64%) and rather weakly by tree stand stem volume (r2 = 22%) and mire vegetation cover (r2 = 15%). N2O flux (range: −0.03 to 0.92 g m−2 year−1) was best explained by peat CN ratio (r2 = 35%). Site type explained 13% of annual N2O flux. We suggest that water table depth should be measured in national land-use inventories for improving the estimation of country-level GHG fluxes for peatlands.  相似文献   
18.
The aim of the study was to compare model-based approaches inthe prediction of timber assortment recovery with bucking simulationbased on detailed stem data. A correction function for the totallength of saw log fragments and two optional saw log reductionmodels, that is, the MELA96 version and MELA05 version, wereapplied. In the bucking simulation, the volumes by timber assortmentswere calculated using a bucking-to-value simulator. The predictionof saw log recoveries varied between the bucking simulationand different versions of saw log reduction models. The levelof the reduction from the MELA96 version was at the same levelas from the bucking simulation where defects were taken intoaccount, but the saw log reduction had a very low variance dueto a small amount of independent variables. The saw log reductionof the MELA05 version included more variation although the levelof the reductions was higher and the variation did not meetwith the bucking simulation. As a conclusion, the model-basedapproaches seem applicable at least for the prediction of sawlog recovery in the large area forest inventories where thevariance of the standwise timber assortment recoveries neednot be predicted.  相似文献   
19.

Background

Arthroscopy is widely used in various equine joints for diagnostic and surgical purposes. However, accuracy of defining the extent of cartilage lesions and reproducibility in grading of lesions are not optimal. Therefore, there is a need for new, more quantitative arthroscopic methods. Arthroscopic optical coherence tomography (OCT) imaging is a promising tool introduced for quantitative detection of cartilage degeneration and scoring of the severity of chondral lesions. The aim of this study was to evaluate the inter-investigator agreement and inter-method agreement in grading cartilage lesions by means of conventional arthroscopy and with OCT technique. For this aim, 41 cartilage lesions based on findings in conventional and OCT arthroscopy in 14 equine joints were imaged, blind coded and independently ICRS (International Cartilage Repair Society) scored by three surgeons and one PhD-student.

Results

The intra- and inter-investigator percentages of agreement by means of OCT (68.9% and 43.9%, respectively) were higher than those based on conventional arthroscopic imaging (56.7% and 31.7%, respectively). The intra-investigator Kappa coefficients were 0.709 and 0.565 for OCT and arthroscopy, respectively. Inter-investigator Kappa coefficients were 0.538 and 0.408 for OCT and arthroscopy, respectively.

Conclusions

OCT can enhance reproducibility of arthroscopic evaluation of equine joints.  相似文献   
20.
OBJECTIVE: To evaluate an arthroscopic indentation instrument (Artscan 200) for assessment of the health status of equine articular cartilage. STUDY DESIGN: In vitro experiment using equine isolated proximal phalanx (P1) specimens. SAMPLE POPULATION: P1 specimens from 39 horses (aged 1.5-22 years). METHODS: Reproducibility was tested by determination of the coefficient of variation (CV). Dynamic modulus and cartilage degeneration index (CDI) values were measured at 2 predefined sites (site 1, joint margin; site 2, joint center) to assess the accuracy and to evaluate the relation with surface integrity. RESULTS: CV was 9.0%. A significant decrease in indenter force was identified when dynamic modulus values decreased to <2.5 MPa (range of tested samples 0.9-8.1 MPa) and when CDI values at site 1 increased to >50% (range 5.4-72.8%). CONCLUSIONS: Technique reproducibility was adequate but accuracy was limited. The device identified degeneration-associated decreases in cartilage stiffness only when the mechanical properties of the cartilage were considerably changed. CLINICAL RELEVANCE: Usefulness of this indentation instrument during arthroscopic surgery would be limited in the initial phase of OA-like cartilage degeneration, but may yield important information in more advanced OA.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司    京ICP备09084417号-23

京公网安备 11010802026262号