全文获取类型
收费全文 | 29151篇 |
免费 | 1068篇 |
国内免费 | 4篇 |
专业分类
林业 | 4754篇 |
农学 | 1730篇 |
基础科学 | 212篇 |
5816篇 | |
综合类 | 1501篇 |
农作物 | 2650篇 |
水产渔业 | 2787篇 |
畜牧兽医 | 6384篇 |
园艺 | 1344篇 |
植物保护 | 3045篇 |
出版年
2023年 | 152篇 |
2022年 | 176篇 |
2021年 | 354篇 |
2020年 | 357篇 |
2019年 | 265篇 |
2018年 | 3153篇 |
2017年 | 3106篇 |
2016年 | 1711篇 |
2015年 | 547篇 |
2014年 | 584篇 |
2013年 | 863篇 |
2012年 | 1836篇 |
2011年 | 2987篇 |
2010年 | 2518篇 |
2009年 | 1602篇 |
2008年 | 2104篇 |
2007年 | 2287篇 |
2006年 | 685篇 |
2005年 | 757篇 |
2004年 | 702篇 |
2003年 | 713篇 |
2002年 | 509篇 |
2001年 | 315篇 |
2000年 | 304篇 |
1999年 | 231篇 |
1998年 | 74篇 |
1997年 | 66篇 |
1996年 | 54篇 |
1995年 | 68篇 |
1994年 | 42篇 |
1993年 | 49篇 |
1992年 | 92篇 |
1991年 | 71篇 |
1990年 | 66篇 |
1989年 | 69篇 |
1988年 | 67篇 |
1987年 | 49篇 |
1986年 | 48篇 |
1985年 | 49篇 |
1984年 | 43篇 |
1983年 | 36篇 |
1979年 | 36篇 |
1974年 | 29篇 |
1973年 | 22篇 |
1972年 | 24篇 |
1971年 | 23篇 |
1970年 | 27篇 |
1969年 | 26篇 |
1968年 | 29篇 |
1967年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Kenji Ono Motohiro Hasegawa Makoto Araki Masahiro Amari Masakazu Hiraide 《Journal of Forest Research》2007,12(4):255-261
We examined the relationships between the absorptional characteristics in the near infrared region and the chemical changes
of decomposing beech (Fagus crenata) and pine (Pinus densiflora) litters. Spectra as well as the concentrations of chemical substances approached each other and converged with decomposition,
although both initial characteristics differed markedly between beech and pine. This indicated that the fundamental chemical
structures were almost the same, although their organochemical composition differed. Specific absorption bands for lignin,
polysaccharide, and protein were identified at 2,140 and 1,670 nm, 2,270, 1,720, 1,590, and 1,216 nm, and 2,350 nm, respectively.
Absorbance at 1,670 nm, peculiar band of aromatics, showed a positive correlation with lignin concentration, which suggested
the relative increment of aromatics due to condensed lignin in decomposing litters. Absorbance at 2,140 nm, characterized
as the C–H bond in HRC = CHR, showed a negative correlation with lignin concentration, which suggested the decrements of some
structures such as side-chains in lignin polymers unrelated to aromatics. Absorbance at 2,270, 1,720, and 1,216 nm, specified
to O–H/C–O/C–H bonds in saccharide, might reflect the change of polysaccharide during decomposition because they showed a
positive correlation to polysaccharide concentration. In the same way, absorbance at 2,350 nm, identified to the C–H/CH2 bonds in protein, showed a negative correlation to nitrogen concentration in decomposing litters, which might indicate that
the C–H/CH2 bonds in protein decreased with decomposition due to microbial consumption of carbon in protein. Our findings suggested the
possibility that the spectral changes indicate the litter digestibility during decomposition and that also explain the compositional
change in decomposing litters. 相似文献
993.
This study examined how boiling and drying treatments influenced various physical properties of the tension wood with gelatinous
fibers (G-fibers) of a 29-yearold Zelkova branch. By boiling treatment, tension wood with numerous G-fibers contracted considerably in the longitudinal direction and
the longitudinal Young’s modulus decreased in spite of the water-saturated condition. The drying treatment caused green tension
wood and boiled tension wood with numerous G-fibers to shrink longitudinally and increased their longitudinal Young’s moduli.
These specific behaviors in tension wood were highly correlated with the proportion of G-fibers in a specimen and were probably
caused by the microscopic behavior of cellulose microfibril (CMF) in the gelatinous layers (G-layers). The longitudinal shrinkage
of tension wood due to drying suggests the existence of a hygro-sensible, noncrystalline region in the CMF, which is abundant
in the G-layer. Furthermore, the noncrystalline region in the CMF softens during boiling treatment, resulting in the reduction
of the longitudinal Young’s modulus in tension wood. The longitudinal contraction of tension wood with G-fibers by boiling
might be caused by the tensile growth stress remaining in green G-layers. However, no changes were detected in the 004 d-spacing of cellulose crystal in tension wood from the boiling and drying treatments, regardless of the proportion of G-fibers. 相似文献
994.
As a measure of estimating humidity control capacity of materials in an airtight room under sinusoidal temperature variation,
we used the Cb value, which is the ratio of the range of variation in relative humidity in a steel box lined with the material of interest
to the range in an empty steel box. In order to clarify the factors that affect the Cb value, we focused on both the temperature variation rate and the area that is lined in the box. It was found that changing
the rate of temperature variation over one period strongly affected the Cb value, although the amplitude of temperature variation did not. We further noticed that the difference between the time when
peak temperature was reached and the time when peak absolute humidity was reached (peak time difference), was found to be
useful for estimating the humidity control capacity of materials. Knowing the difference between the phase when peak temperature
was reached and the phase when peak absolute humidity was reached (phase angle difference) was also useful. Because the Cb value was affected by both the variation period and the lined area in the box, we can draw a contour diagram of the period
and the lined area for materials to give an overview of the humidity control capacity of a material. The materials of primary
interest in this study were Japanese cedar and porous ceramics. 相似文献
995.
To study the bark beetle attacks on Turkish fir stands and to determine the principle site and stand factors influencing beetle
attacks, trap log method was used in the Western Black Sea Region of Turkey. The study was carried out in ten different locations
prior to flying time in February 2003. On each experimental plot, three healthy fir trees with 30–50 cm diameter and 200 m
apart were cut down with a chainsaw. From each sampling tree, needles were sub-sampled for determination of specific leaf-area
and macro-nutrient concentrations. In addition, on each experimental plot, crown closure, basal-area, and stand density were
measured. To determine the bulk density and nutrient concentrations of the soil, samples were taken around trees on each plot.
The relationship between measured stand variables, the number of beetle species and beetle populations were evaluated using
correlation and stepwise multiple regression procedure. Eight different species belonging to four different families from
Coleoptera were identified in examining the trap logs. Six of these eight species (Pissodes piceae Illig., Pissodes notatus Fabr., Cryphalus piceae Ratz., Pityophthorus pityographus Ratz., Xyloterus lineatus Oliv., Pityokteines curvidens Germ.) were identified as harmful beetles (HB). The other two (Thanasimus formicarius L. and Rhizophagus dispar Payk.) were identified as predator species. The analysis showed that the total number of HB increased as the site’s slope
and needle specific leaf area increased. However, as needle N concentrations increased, HB population decreased. Increased
Ca concentration in fir needle reduced total insect attack. 相似文献
996.
Carly Green Brian Tobin Michael O’Shea Edward P. Farrell Kenneth A. Byrne 《European Journal of Forest Research》2007,126(2):179-188
Reporting carbon (C) stocks in tree biomass (above- and belowground) to the United Nations Framework Convention on Climate
Change (UNFCCC) should be transparent and verifiable. The development of nationally specific data is considered ‘good practice’
to assist in meeting these reporting requirements. From this study, biomass functions were developed for estimating above-
and belowground C stock in a 19-year-old stand of Sitka spruce (Picea sitchensis (Bong) Carr.). Our estimates were then tested against current default values used for reporting in Ireland and literature
equations. Ten trees were destructively sampled to develop aboveground and tree component biomass equations. The roots were
excavated and a root:shoot (R) ratio developed to estimate belowground biomass. Application of the total aboveground biomass function yielded a C stock
estimate for the stand of 74 tonnes C ha−1, with an uncertainty of 7%. The R ratio was determined to be 0.23, with an uncertainty of 10%. The C stock estimate of the belowground biomass component was
then calculated to be 17 tonnes C ha−1, with an uncertainty of 12%. The equivalent C stock estimate from the biomass expansion factor (BEF) method, applying Ireland’s
currently reported default values for BEF (inclusive of belowground biomass), wood density and C concentration and methods
for estimating volume, was found to be 60 tonnes C ha−1, with an uncertainty of 26%. We found that volume tables, currently used for determining merchantable timber volume in Irish
forestry conditions, underestimated volume since they did not extend to the yield of the forest under investigation. Mean
stock values for belowground biomass compared well with that generated using published models. 相似文献
997.
Due to a processing error, first author's name is missing in the HTML version of this article abstract page. The correct authors
are given below:
Jun Mu, Tohru Uehara, Takeshi Furuno
The online version of the original article can be found at 相似文献
998.
Radial variation in sap flow in five laurel forest tree species in Tenerife,Canary Islands 总被引:1,自引:0,他引:1
Variations in radial patterns of xylem water content and sap flow rate were measured in five laurel forest tree species (Laurus azorica (Seub.) Franco, Persea indica (L.) Spreng., Myrica faya Ait., Erica arborea L. and Ilex perado Ait. ssp. platyphylla (Webb & Berth.) Tutin) growing in an experimental plot at Agua García, Tenerife, Canary Islands. Measurements were performed around midday during warm and sunny days by the heat field deformation method. In all species, water content was almost constant (around 35% by volume) over the whole xylem cross-sectional area. There were no differences in wood color over the whole cross-sectional area of the stem in most species with the exception of E. arborea, whose wood became darker in the inner layers. Radial patterns of sap flow were highly variable and did not show clear relationships with tree diameter or species. Sap flow occurred over the whole xylem cross-sectional area in some species, whereas it was limited to the outer xylem layers in others. Sap flow rate was either similar along the xylem radius or exhibited a peak in the outer part of the xylem area. Low sap flow rates with little variation in radial pattern were typical for shaded suppressed trees, whereas dominant trees exhibited high sap flow rates with a peak in the radial pattern. Stem damage resulted in a significant decrease in sap flow rate in the outer xylem layers. The outer xylem is more important for whole tree water supply than the inner xylem because of its larger size. We conclude that measurement of radial flow pattern provides a reliable method of integrating sap flow from individual measuring points to the whole tree. 相似文献
999.
1000.