全文获取类型
收费全文 | 136篇 |
免费 | 3篇 |
专业分类
林业 | 99篇 |
农学 | 2篇 |
28篇 | |
综合类 | 8篇 |
畜牧兽医 | 1篇 |
植物保护 | 1篇 |
出版年
2023年 | 1篇 |
2020年 | 3篇 |
2019年 | 3篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 3篇 |
2014年 | 1篇 |
2013年 | 6篇 |
2012年 | 11篇 |
2011年 | 10篇 |
2010年 | 10篇 |
2009年 | 17篇 |
2008年 | 9篇 |
2007年 | 8篇 |
2006年 | 5篇 |
2005年 | 6篇 |
2004年 | 4篇 |
2003年 | 4篇 |
2002年 | 1篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有139条查询结果,搜索用时 15 毫秒
81.
82.
Torsten W. Berger Erich Inselsbacher Franz Mutsch Michael Pfeffer 《Forest Ecology and Management》2009,258(11):2578-2592
Studies on the combined effects of beech–spruce mixtures are very rare. Hence, forest nutrition (soil, foliage) and nutrient fluxes via throughfall and soil solution were measured in adjacent stands of pure spruce, mixed spruce–beech and pure beech on three nutrient rich sites (Flysch) and three nutrient poor sites (Molasse) over a 2-year period. At low deposition rates (highest throughfall fluxes: 17 kg N ha−1 year−1 and 5 kg S ha−1 year−1) there was hardly any linkage between nutrient inputs and outputs. Element outputs were rather driven by internal N (mineralization, nitrification) and S (net mineralization of organic S compounds, desorption of historically deposited S) sources. Nitrate and sulfate seepage losses of spruce–beech mixtures were higher than expected from the corresponding single-species stands due to an unfavorable combination of spruce-similar soil solution concentrations coupled with beech-similar water fluxes on Flysch, while most processes on Molasse showed linear responses. Our data show that nutrient leaching through the soil is not simply a “wash through” but is mediated by a complex set of reactions within the plant–soil system. 相似文献
83.
Karl-Heinz Häberle Angela J. Nunn Ilja M. Reiter Herbert Werner Werner Heller Günther Bahnweg Sebastian Gayler Cornelius Lütz Rainer Matyssek 《European Journal of Forest Research》2009,128(2):99-108
To prevent carbon (C) loss to consumers, trees need to defend their primary production. The tree-internal conflict in resource
allocation between growth and defence demands has been the subject of various hypotheses but still requires quantification.
A conceptual approach to approximating the C amount dispensable in favour of primary production at the expense of defence
is demonstrated which is based on nine defence-related metabolite groups. Quantification is exemplified at the level of sun
and shade leaves of adult Fagus sylvatica and Picea abies trees, two species contrasting in foliage type, under oxidative stress as induced by ozone exposure. The difference between
maximum and minimum metabolite levels sampled several times throughout four consecutive growing seasons were conceived as
dispensable between growth and defence-related metabolism and expressed in proportion of the mean annual gross primary production
(GPP) of the foliage. In both species, this proportion amounted to between 2 and 5% of GPP (on a molar C basis). Remaining
uncertainties are discussed as concerning functional overlap of substances between growth and defence-related metabolism,
estimated classification of metabolite turnover rates and “third-party” trade-offs across C demands. Given the complexity
of plant defence, simplification is needed for modelling allocation trade-offs in plants. The presented conceptual approach
meets this need in approximating C transfer capacities between competing physiological demands and in stimulating empirical
assessments towards mechanistic understanding.
This article belongs to the special issue: “Growth and defence of Norway spruce and European beech in pure and mixed stands”. 相似文献
84.
Philip Wipfler Thomas Seifert Peter Biber Hans Pretzsch 《European Journal of Forest Research》2009,128(2):135-144
Several findings indicate an impact of ozone on stem diameter growth leaving the question unanswered, if and how the intra-annual
growth pattern is changed. In this study the hypotheses are tested, that (1) ozone will alter the absolute growth and (2)
alter and shift the period of growth activity within a year. Our data originates from the free air ozone fumigation experiment
‘Kranzberger Forst’ in a mixed stand of Norway spruce and common beech near Freising/Germany. Annual and intra-annual growth
reactions of a sample of five adult beech and five spruce trees, exposed to double ambient ozone were examined and compared
to the same number of untreated reference trees. Diameter increments were measured with plastic diameter girth bands and high-resolution,
automatically logging micro-dendrometers, mounted at breast height (1.3 m). We used the increment data from the growth periods
2000 to 2005. The high-resolution micro-dendrometer data were examined by fitting a Weibull function to the standardized annual
growth profiles to obtain curve parameters for statistical tests. We estimated the parameters ‘T’ which represents the point of time, when 63% of the annual diameter increment is performed and the parameter ‘m’, the Weibull module, which was used as an indicator for the span of time needed to complete the annual growth. The statistical
significance of these curve parameters, together with the absolute diameter increment, was tested by use of mixed regression
models. The analysis of the growth curve parameters revealed a significantly altered intra-annual growth pattern of both species
induced by ozone. Spruce under ozone showed reduced absolute annual diameter increment and a preponed growth activity compared
to untreated trees. Beech’s absolute diameter increment was not affected under ozone, but its growth activity was delayed.
For both species, ozone fumigation did not alter the individual length of the annual growing season. These results are discussed
with respect to drought, tree ring anatomy and tree allometry. The study shows that ozone is able to change growth behaviour
of trees even if increment losses are not obvious.
This article belongs to the special issue "Growth and defence of Norway spruce and European beech in pure and mixed stands". 相似文献
85.
Roots can induce significant changes in the rhizosphere soil. The aim of the present study was to investigate the influence of beech (Fagus silvatica L.) roots on the chemistry of the rhizosphere soil solution. Special emphasis was given to the effect of the NH4+ supply since many forest soils presently receive high NH4+ inputs from atmospheric deposition. In a mature beech stand, a non‐mycorrhized long root was forced to grow into a rhizotrone filled with homogenized acidic forest soil from the Bw horizon of a Dystric Cambisol. Beside the control, a NH4+ enriched treatment was installed. Thirty micro suction cups of 1 mm diameter and 0.5 cm length were placed in a systematic grid of 5 × 10 mm in each rhizotrone to enable root growth through the grid. The water potential of the soil was kept constant by supplying a synthetic soil solution. Small amounts of soil solution were sampled periodically from May to October 1999 and analyzed by capillary electrophoresis for major cations and anions. Furthermore, pH and conductivity were measured by micro electrodes. In the laboratory experiments, beech seedlings were grown in rhizotrones in a control and in a NH4+ fertilized soil. The equipment for sampling soil solutions and the soil conditions in the laboratory was similar to the field experiment. In each rhizotrone a single long root grew through the lysimeter grid. The laboratory conditions induced higher rates of nitrification as compared to the field. Thus, the overall concentration range of the soil solution was not comparable between field and laboratory studies. In all treatments average soil solution concentrations of H+ and Al3+ were significantly higher in the rhizosphere than in the bulk soil. The NH4+ treatment resulted, in the field and laboratory, in a strong increase of the H+ and Al3+ concentrations in the rhizosphere, accompanied by an accumulation of Ca2+, Mg2+, and NO3—. The observed rhizosphere gradients in soil solution chemistry were highly dynamic in time. The results demonstrate that the activity of growing beech roots results in an acidification of the soil solution in the rhizosphere. The acidification was enhanced after the addition of NH4+. 相似文献
86.
87.
The light response curve and the intercellular CO2 concentration response curve of CO2 assimilation rate were investigated together with the light conditions at the four different heights within the beech crown
from 1995 to 1997 on Mt. Fuji in Japan. On the seasonal fluctuation, the CO2 assimilation rate at light saturated condition increased rapidly in May, and attained to the maximum between the end of June
and July, thereafter, slightly decreased until the middle of August and rapidly decreased in September and October. The daily
sum of photosynthetic photon flux density attenuated with deeping within the crown, and particularly, the relative value on
2nd position dropped to only 30%. TheA
max decreased from 10 to 5μmol m−2 s−1, approximately, with deeping within the crown. The light saturation point, quantum yield, light compensation point and dark
respiration rate also varied with deeping. These results suggest that the photosynthetic properties vary gradually from sun
to shade leaves along the light attenuation within a beech crown. At light saturated condition, the stomatal conductance and
mesophyll conductance were strongly correlated withA
max among the four different heights (r > 0.96, respectively). TheC
i/C
a ratio was around 0.8, and there were no remarkable differences among the four different heights. These results suggest that
the vertical gradient ofA
max depends on the variation of mesophyll conductance. The stomatal conductance may be also one of the major factor in the vertical
gradient ofA
max. However the intercellular CO2 concentration doesn’t influence the vertical gradient ofA
max within the crown.
This work is supported by the Sasagawa Scientific Research Grant from The Japan Science Society and Grant-in-Aid for Scientific
Research (C). 相似文献
88.
A. M. Kooijman J. M. van Mourik M. L. M. Schilder 《Biology and Fertility of Soils》2009,45(5):449-459
To test relationships between net N-mineralization, organic matter and soil organisms, we combined micromorphology with laboratory
incubation experiments over a soil gradient. Microbial biomass N generally increased with pH, and from sandy to loamy soil,
but net N-mineralization showed the opposite, and was highest in acid, sandy soil. Twenty-two micromorphological characteristics
were analyzed with principal component analysis. PC1 had high eigenvalue (0.70), and clearly separated fungi from earthworms,
microarthropods and bacteria. PC2 was less important (0.15). Organic layer and sand content clearly correlated with the fungi-end
of PC1, but pH and C-content of the Ah with the opposite. Microbial N also correlated with the earthworm–bacteria end, but
net N-mineralization did not. Efficiency of N-mineralization per unit microbe even correlated with the fungi end of PC1, in
both organic layer and mineral topsoil. The results support the hypothesis that high (or low) litter turnover and biological
activity can be counteracted by high (or low) microbial N-demand. 相似文献
89.
为阐明百山祖亮叶水青冈Faguslucida的种群结构与格局的形成原因.掌握种群数量动态及发展趋势,在百山祖北坡建立1个5hm2(250m×200m)固定样地,将样地划分成2000个5m×5rn的小样方进行调查,获得野外资料。对亮叶水青冈种群进行统计。绘制径级结构图.发现立木级1级和2级所占比例仅为4%。第4~9级处于峰值,表明种群内成熟个体数量所占比例最大(80%),种群处于成熟阶段.第10~14级则属于衰退级别,它们所占的比例并不大(13%);应用聚集强度指数(K)进行种群分布格局分析,亮叶水青冈除在小树期K=0.588较大外,其余K值在幼苗、中树,大树期分别为0.029,0.588,0.169,均较小。结果表明:亮叶水青冈种群结构呈纺锤型,幼苗严重不足,种群有衰退的趋势;幼苗、中树和大树阶段均为集群分布,聚集强度逐级降低,小树呈随机分布。分析认为,亮叶水青冈的种群结构和分布格局受到了多种因素的影响,如群落生境、亮叶水青冈生物学特性以及自然干扰等。图1表2参37 相似文献
90.
Mostafa Moradi Mohammad R. Marvie Mohadjer Kiomars Sefidi Mahmoud Zobiri Ali Omidi 《林业研究》2012,23(2):289-294
Oriental Beech is the most important commercial tree species in northern Iran. In recent years wood production companies interested in felling large beech trees for profit have challenged advocates of close-to-nature silviculture who favor conservation. Our study objective was to assess the economic value of over-mature beech trees by relating tree diameter (DBH) to amount of decay. Based on the location of onset of decay, we categorized three types of decay as stump, stem, and crown decay. Trees of greater diameter (age) typically showed greater decay in the stem. Percent of decayed volume, diameter of decayed tissue, and length of decay in tree stems varied between 0.5%?64.3%, 15 cm?75 cm, and 2.0?19.5 m, respectively. With increasing trunk diameter, the proportion of truck decay increased. Red heart and dark red heart constituted 25% and 14.3% of sampled trees, respectively. However, we found no correlation between intensity of stem decay and morphological characteristics of trees. Seedlings were not abundant around the bases of over-mature trees, suggesting that the trees did not contribute to regeneration of the stand. Beech trees of diameter >1 m do not provide valuable round wood for industries and cause to raise wood production costs. We recommend that these trees >1 m DBH should be retained in forest stands because of their low commercial value but high ecological and conservational values such as maintaining biodiversity in forest ecosys-tems. 相似文献