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51.
早在有史以前,人们已经利用那些易于获得的树木提取物为人类提供了许多必需品,以后,不同国家的科学家们经过长期互相协作研究,分离出其有效组分,阐明了它们的结构,作到了化学合成和生物合成,还确定了控制它们在植物中生成的某些生物因素。今天在有机化学上的许多进展都是从175年以前的这些研究中发展而来的。近年技术的发展,质量标准的提高以及价格竞争都要求有特殊性质的原料以满足特殊的需要。当今技术的迅速发展使国际间的出口局限于那些价格低廉、质量较高的基本原料,或具有不能替代的特有成分的产品。在资源日益减少的今日世界必须更重视特定地区再生资源的保护和最大限度的发展,以及生产所需产品的最有效的技术。为此目的,对树木的生理和化学还需要更深刻的认识。 相似文献
52.
Ard G. Lengkeek Alice Muchugi Mwangi Caroline A. C. Agufa Joseph O. Ahenda Ian K. Dawson 《Agroforestry Systems》2006,67(3):293-300
It is possible that current tree domestication practices undertaken by farmers reduce the genetic base of tree resources on
farms, raising concerns regarding the productivity, sustainability and conservation value of agroforestry ecosystems. Here,
we assessed possible changes in genetic variation during domestication in the important and heavily utilised timber species,
Vitex fischeri Gürke (syn. Vitex keniensis), by comparing geographically proximate forest and farm material in central Kenya. Employing RAPD analysis, a total of 104
polymorphic markers revealed by five arbitrary primers were scored in a total of 65 individuals, 32 from forest and 33 from
farmland. Despite concerns of possible genetic erosion, forest and farm stands did not differ significantly in levels of genetic
variation, with H values of 0.278 and 0.269, respectively. However, Mantel tests did reveal greater geographically related associative genetic
structure among individuals in farm rather than forest material, with r
M values of 0.217 and 0.114, respectively. A more detailed analysis of structure suggested this could be due to local variation
in origin of some on-farm trees. Implications of data for the genetic management of V. fischeri stands during farmer-led tree domestication activities are discussed. At present, there appears little reason to reject on-farm
V. fischeri as a source of germplasm for future on-farm planting or for conservation purposes, although this situation may change and
will require monitoring. 相似文献
53.
In this research the relative importance of leaf area and microclimatic factors in determining water use of tree lines was
examined in sub-humid Western Kenya. Measurements of tree water-use by a heat-balance technique, leaf area, bulk air saturation
deficit, daily radiation, and soil water content were done in an experiment with tree lines within crop fields. The tree species
were Eucalyptus grandis W. Hill ex Maiden, Grevillea robusta A. Cunn. and Cedrella serrata Royle, grown to produce poles on a phosphorus-fixing Oxisol/Ferralsol with (+P) or without (−P) phosphorus application. Doubling
the leaf area of Cedrella and Grevillea doubled water use in a leaf area (LA) range of 1–11 m2 per tree. The response of Eucalyptus water use (W) to increases in leaf area was slightly less marked, with W = LAn, n<1. Transpiration rate per unit leaf area (Tr) was the other important determinant of water use, being affected by both tree
species and phosphorus fertilization. A doubling of the saturation deficit (SD) halved the water use of all trees except for
Cedrella +P, in which water use increased. A direct effect of soil water content on water use was only found in Grevillea -P,
with a small increase (60%) as available water increased from 1.4 to 8.9% above wilting point (32%). This low direct response
to soil water content is probably due to the extensive tree-root systems and the deep clayey soils supplying sufficient water
to meet the evaporative demand. Indirect responses to soil water content via decreases in leaf area occurred in the dry season.
The results showed that water use of tree lines was more determined by leaf area and transpiration rate per unit leaf area
than by micro meteorological factors. The linear response of tree water use to leaf area, over a wide range leaf areas, is
a specific characteristic of tree line configurations and distinguished them from forest stands. In tree lines light interception
and canopy conductance increase with leaf area much more than a similar leaf area increase would have caused in a closed forest
canopy. 相似文献
55.
56.
In the Philippines, smallholder farmers have become major timber producers. But the systems of timber production practiced
have several limitations. In intercropping systems, the practice of severe branch and/or root pruning reduces tree-crop competition
and increases annual crop yields, but is detrimental to tree growth and incompatible with commercial timber production. In
even-aged woodlots, lack of regular income and poor tree growth, resulting from farmers’ reluctance to thin their plantations,
are major constraints to adoption and profitable tree farming. In the municipality of Claveria, Misamis Oriental, the recent
practice of planting trees on widely spaced (6–8 m) contour grass strips established for soil conservation suggests ways to
improve the adoptability (i.e., profitability, feasibility and acceptability) of timber-based agroforestry systems. Assuming
that financial benefits are the main objective of timber tree farmers, we develop a simple linear programming (LP) model for
the optimal allocation of land to monocropping and tree intercropping that maximizes the net present value of an infinite
number of rotations and satisfies farmers’ resource constraints and regular income requirements. The application of the LP
model to an average farmer in Claveria showed that cumulative additions of widely spaced tree hedgerows provides higher returns
to land, and reduce the risk of agroforestry adoption by spreading over the years labour and capital investment costs and
the economic benefits accruing to farmers from trees. Therefore, incremental planting of widely spaced tree hedgerows can
make farm forestry more adoptable and thus benefit a larger number of resource-constrained farmers in their evolution towards
more diverse and productive agroforestry systems. 相似文献
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