关于学分制几个问题的探讨     

范会敏  刘景会 《河北林果研究》1995,(2)

为了促进学分制的全面推行，本文对学分制的本质与宗旨、学分制产生和发展的社会和历史条件、学分制的优越性和弊端以及中国特色的学分制等问题进行了新的探讨。  相似文献   

Spatial pattern of soil carbon and nutrient storage at the alpine tundra ecosystem of Changbai Mountain,China   总被引：2，自引：2，他引：2  

魏晶  吴钢  邓红兵  赵景柱 《林业研究》2004,15(4):249-254

气候变化对土壤碳和养分的影响越来越受到生态学家的关注,尤其是对高山冻原生态系统的影响。本研究目的是长白山高山冻原系统土壤碳和养分的空间异质性。结果表明:不同土层深度的土壤碳在Meadow alpine tundra vegetation (MA)中显著地高于其他植被类型(p<0.05);在每一植被类型中,表层(010 cm)碳含量显著地高于1020 cm的土层;土壤氮的分布格局是氮素在表层和1020 cm土层变化规律相似;不同土层深度的土壤磷在Lithic alpine tundra vegetation (LA) 中显著地低于其他植被类型(p<0.05);土壤钾浓度在Felsenmeer alpine tundra vegetation (FA) 和LA中显著地高于其他植被类型,但在每一植被类型中土壤钾浓度随土壤深度没有明显的变化;不同深度土壤硫在MA中显著地高度其他植被类型;对每一植被类型而言,C: N, C: P, C: K 和C: S随土壤深度增加而降低。除了在SA的表层外,表层的土壤C: N高于1020 cm的土层。在长白山高山冻原系统中,随植被类型的变化,土壤碳和养分储量有着显著的空间异质性。参29表3图1。  相似文献   

Assessing raindrop impact energy at the forest floor in a mature Japanese cypress plantation using continuous raindrop-sizing instruments     

Kazuki?NankoEmail author  Norifumi?Hotta  Masakazu?Suzuki 《Journal of Forest Research》2004,9(2):157-164

The raindrop size distribution of throughfall and open rainfall was monitored continuously during a rainfall event using laser raindrop-sizing instruments (LD gauges), in order to calculate the raindrop impact energy in a plantation of mature Japanese cypress (Chamaecyparis obtusa), where surface erosion at the forest floor had been a problem. Data from two rainfall events were analyzed. The LD gauges recorded qualitative raindrop size distribution, and the capture rate during each rainfall event was used to manipulate raindrop data quantitatively. Throughfall and open rainfall comparisons revealed several important differences. First, throughfall raindrops were fewer in number and larger in size than open rainfall drops. In one rainfall event, for example, throughfall raindrops were less than one-fifth as frequent as open rainfall raindrops; in addition, the maximum throughfall raindrop diameter was 6.35mm compared to 3.31mm for open rainfall raindrops. Second, throughfall raindrops that were larger than the largest open rainfall raindrops comprised 63.8% of the throughfall precipitation by volume. Third, total raindrop impact energy from throughfall was over twice that of open rainfall. Moreover, comparison of throughfall events implied that throughfall raindrops did not always have a uniform distribution between different events or among different periods of time in one rainfall event, in contrast to findings in previous studies which showed that throughfall raindrops had a uniform size distribution independent of rainfall intensity. It is possible that an abrupt transition of throughfall intensity from low to high changes the distribution of throughfall raindrops.  相似文献   

Characterization of the correlation structure of lumber strength properties     

William?M.?BulleitEmail author  Richard?A.?Chapman 《Wood Science and Technology》2004,38(4):285-296

Spatially averaged data from the literature for along-the-length correlation of lumber flatwise bending modulus of elasticity and tensile strength was analyzed to determine the correlation of the base process for each property. The cross correlation between the two properties was also examined. The exponential correlation function, equivalent to a first-order auto-regressive process, was the best model for both correlation and cross correlation, based on the sum of squared errors between the correlations from the test data and the fitted model. The fitted functions can be used to generate correlation and cross correlation information for spatially averaged processes averaged over lengths different from the lengths used to obtain the test data.  相似文献   

Accurate determination of the limiting viscosity number of pulps     

G.?U?arEmail author  M.?Balaban 《Wood Science and Technology》2004,38(2):139-148

The viscosity of copper ethylenediamine (CED) solutions of pulps exhibits a decrease with time, while the viscosity loss is insignificant when the time between dissolution and measurement is just 15–20 min. Time of 1 h or longer would lead to noticeable differences from the initial viscosity. The viscosity of the solution at the beginning can be estimated from a series of measurements performed under strict conditions. In particular, tight control of the temperature of the solution and accurate measurement of the flow time are of importance. The plot of efflux times against elapsed times to the measurements results in a almost linear correlation, which enables to determine the initial viscosity.  相似文献   

绿秆黄槽白夹竹生物学特性的研究     

何俊  沈承权  樊季旻  傅必逊 《竹子研究汇刊》1995,(2)

绿秆黄槽白夹竹(Phyllostachys nidularia)是天然形成的园艺新品种,1989年以来,对其生长发育特性以及与环境条件的关系进行了研究,揭示了它的“地下茎—笋—幼竹—成竹”的生长规律以及出笋和退笋的特点,为制订科学的培育技术措施提供依据。  相似文献   

Erratum to: Tensile properties of bamboo in different sizes     

Hong?Chen  Haitao?Cheng  Ge?WangEmail author  Zixuan?Yu  Sheldon?Qiang?Shi 《Journal of Wood Science》2016,62(2):213-213

  相似文献   

Bird communities in tropical agroforestry ecosystems: an underappreciated conservation resource     

Skye?M.?GreenlerEmail author  James J.?Ebersole 《Agroforestry Systems》2015,89(4):691-704

Tropical conservation and research focus primarily on protected areas and often ignore conservation values of surrounding agricultural lands. Understanding how species utilize agricultural land will maximize conservation efforts. We compared bird community composition in four habitats in northeastern Costa Rica: shade-grown cacao, live fences, riparian forest buffers, and preserved late-successional rainforest. Point counts over 2 months found 167 species from 36 families. Rainforest contained the most species unique to a habitat although richness per point was lower than in agricultural habitats. Half, 31, of the rainforest species did not occur in other habitats, while 106 species, mostly those preferring open areas, occurred in agroforest habitats but not rainforest. While agricultural habitats had fairly similar species composition to each other as determined by distance in an ordination, each also contained significant numbers, 9–30, of unique species. While intact rainforest remains central to conservation of tropical birds, agricultural lands with substantial trees, e.g., live fences, riparian buffers, and plantations with shade trees, can support a high richness of birds. These avian communities are not simply subsets of the rainforest species but include substantial numbers of unique species. Conservation contributions of these lands to species richness and complexity should be considered in conservation, and trees in these habitats preserved.  相似文献   

Improvements in dimensional stability and lightfastness of wood by butyrylation using microwave heating   总被引：1，自引：0，他引：1  

Hui-Ting?Chang  Shang-Tzen?ChangEmail author 《Journal of Wood Science》2003,49(5):455-460

Microwave heating was used as the heat source for butyrylation of wood with the aim of reducing the reaction time. The photostability and dimensional stability of butyrylated wood were also investigated in this study. Chemical changes of wood were confirmed by cross polarization/magic angle spin ¹³C-nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) after butyrylation by microwave heating. Results from DRIFT with the Si-Carb sampling technique revealed that, using microwave heating, a higher degree of butyrylation of maple wood occurred in the middle of the specimen than on the outer surface. The increase in yellowness index of butyrylated wood treated with microwave heating was much less than that of untreated wood after the lightfastness test, indicating that photoyellowing of wood is effectively inhibited by butyrylation using microwave heating. The dimensional stability of wood was also improved after modification.  相似文献   

Plant antifreeze proteins and their expression regulatory mechanism     

LinYuan-zhen LinShan-zhi ZhangZhi-yi ZhangWei LiuWen-feng 《中国林学(英文版)》2005,7(1):46-52

Low temperature is one of the major limiting environmental factors which constitutes the growth, development,productivity and distribution of plants. Over the past several years, the proteins and genes associated with freezing resistance of plants have been widely studied. The recent progress of domestic and foreign research on plant antifreeze proteins and the identification and characterization of plant antifreeze protein genes, especially on expression regulatory mechanism of plant antifreeze proteins are reviewed in this paper. Finally, some unsolved problems and the trend of research in physiological functions and gene expression regulatory mechanism of plant antifreeze proteins are discussed.  相似文献