木材顺纹压缩率与PDR的影响机制研究

木材顺纹压缩弹塑性和PDR(永久变形率)变化规律及机理研究是攻克木材顺纹压缩与多维弯曲技术的核心.从木材的树种、密度、软化处理条件、化学组分和相对结晶度以及压缩速度几个方面,分析了木材顺纹压缩过程中这些因素对木材顺纹压缩率和PDR的影响机制.     

加压热处理对表层压缩杨木变形回弹的影响

以水热控制法对人工林毛白杨木材进行间歇式压缩,获得表层密度1.0 g/cm~3、芯部密度基本不变的表层压缩木;再在常压和0.3 MPa压力下,分别对表层压缩木材进行热处理,检测热处理后木材的变形回弹率。结果表明,随着热处理温度升高,表层压缩木材变形回弹率呈降低趋势;加压热处理比常压热处理,对表层压缩木材的变形固定效果更优。200℃、0.3 MPa加压热处理,能够使杨木压缩材的厚度变化率降低67.38%。     

木材湿热软化压缩技术及其机制研究进展

木材压缩是提高软质木材密度、强度和硬度,改善木材物理力学性能,扩大木材应用范围的有效方法。本文针对湿热软化下的木材压缩问题,从木材软化机制、软化特性、软化点的确定、热板加热下的传热传质特性、层状压缩的形成和压缩变形固定等方面分析木材压缩技术的研究现状、进展以及存在的问题。木材细胞壁的成分和组织构造是影响木材软化和压缩变形的主要内在因素,而湿和热则是影响木材压缩变形的外在因素。木材是一种具有弹塑性的天然高聚物。干燥木材缺乏塑性,水分和热量都能对木材组分起到增塑作用,特别是在湿热共同作用下增塑作用更加显著。木材细胞壁主要成分纤维素、半纤维素和木质素的特性及所占比例直接影响木材的可塑性,其中木质素的含量和软化特性是木材软化的主要影响因素。玻璃化转变温度和应力屈服点是表征木材软化最常用的参数。在木材弹塑性分析中,应力屈服点控制了木材在塑性区域的应力-应变关系,同时也决定了塑性变形潜能,但由于木材成分和结构非常复杂,应力-应变关系的拐点并不明显,因此应力屈服点和屈服应力的确定是木材塑性变形表征的关键点,也是一个难点。木材的组织构造主要影响木材的传热传质过程。利用木材3个断面渗透性的显著差异,通过干燥、浸水、放置、热板加热等处理,可使木材内部各个层面上形成差异显著的含水率梯度分布和屈服应力差,压缩后形成层状压缩木材。层状压缩木材压缩层的密度可达0.8 g·cm-3以上,未压缩层仍然保持原有的密度,而且压缩层的形成部位是可控的。层状压缩技术可以解决整体压缩木材损失大的问题,但目前木材压缩变形机制的研究都是围绕木材整体压缩开展的,缺乏木材软化点和屈服应力随含水率变化规律以及热板加热下木材内部屈服应力差变化规律的基础研究。要实现层状压缩的可控性,还需要在热板加热下的传热传质规律及木材湿热梯度分布的形成与调控等方面开展深入研究。     

木材湿热软化径向压缩变形特征研究进展

压缩变形特征是压缩木研究的重要内容。文中针对木材湿热软化径向压缩变形,从压缩变形的影响因素、压缩变形位置对其力学性能的影响和压缩变形固定3个方面综述国内外研究进展,包括树种、生长轮构造和压缩工艺对木材压缩变形的影响,压缩变形位置对力学性能的影响,以及热处理对压缩变形的影响,指出木材湿热软化径向压缩研究中关于压缩变形需要继续解决的主要问题以及发展方向,以期为湿热软化径向压缩木材的进一步研究和应用提供参考。     

水热控制下杨木的表层密实化及固定技术

通过改变木材的表层水分含量,并利用热压机压板的热量对木材进行水热控制,开展毛白杨木材的表层密实化研究,分析毛白杨木材表层压缩密实化后的密度、硬度和恢复率等能指标。结果表明:表层密实化后的毛白杨木材的表层密度达到0.92g/cm3,为对照材的1.8～2.1倍;硬度达到22.3N/mm2,为对照材的2.4倍;压缩木经180℃、4h的热处理后,变形恢复率可降低30%。     

木材压缩变形固定处理技术研究进展

压缩密实化是木材提质增优的处理方法之一。木材作为一种具有形状记忆效应的天然有机高分子材料，当被压缩或弯曲后，在温度和湿度等环境因子的影响下容易发生蠕变回复。木材压缩变形的回复对木材制品的尺寸稳定性和力学性能均有重要的影响，研究木材压缩变形固定处理技术对木材加工改性和高效利用具有重要指导意义。概述了木材压缩变形固定处理技术在国内外的研究进展和现状，重点梳理归纳了水热（蒸汽）预处理、压缩后热处理以及化学处理对木材压缩变形固定的研究进展。在此基础上，分析和提出了木材压缩变形固定处理技术的思路和未来发展方向，以期为压缩材的高效加工利用提供科学指导。     

软质木材的表面密实化

探讨软质木材表面密实化的理想状态和途径,研究用酚醛树脂浸渍、再进行不同程度压缩木材的表面密实化效果.试验结果表明,浸渍压缩后木材密度从表层到内层的分布呈现一定的梯度,表面密度均高于内层密度和平均密度;试件的表面硬度、耐磨性和尺寸稳定性均有不同程度的提高.     

木材薄板横纹压缩强化的微观结构变化

以锯切大青杨、旋切白桦、刨切紫椴3种木材薄板为研究对象,用常温水浸泡至饱水状态后不经热软化预处理进行横纹压缩强化,通过扫描电镜观察分析横纹压缩强化前后3种木材的未压缩材和压缩材横、纵切面上的微观结构变化,从微观层面探讨此方法强化3种木材薄板的可行性和有待改进、完善之处.结果表明:3种木材的压缩材横切面上整体变形较均匀,横、纵切面上均未发现压缩裂纹或破坏,纵切面上导管壁均有因压缩产生顺纤维方向的横向挤压折皱条纹;横切面上,大青杨、紫椴压缩材管孔沿压缩方向变形最大,白桦压缩材管孔沿其长轴方向变形最大,并使两侧沿压缩方向、径向排列的木射线细胞和木纤维细胞发生侧向扭曲变形.     

压缩木制造技术的研究概述

目前 ,我国已进入大量应用人工林木材的阶段 ,为了实现人工林木材和间伐材的高效利用 ,首先应从软质人工林的材性 ,如强度、耐久性等方面进行改良 ,以满足使用功能的要求。压缩木、人工林软质木材的表面密实化加工、压缩整形木、木材横纹压缩变形的固定等一系列课题在压缩技术领域进行了广泛深入的研究 ,并取得了一定的进展。压缩木是通过对实体木材进行压缩 (热压 )处理而获得的一种材料 ,这种材料的特点是密度大、质地坚硬、强度高、耐磨性好 ,同时其尺寸稳定性也得到提高。本文主要介绍了压缩木制造工艺中几项关键技术的研究成果 ,希望对…     

压缩矩形材的动态热力学分析

水蒸气处理对压缩木材变形固定和提高木材尺寸稳定性均有很好效果,水蒸气处理使压缩木内部应力释放及结晶结构增加,压缩变形得以固定(井上雅文等,1994);使压缩木内部形成凝聚结构是变形固定的主要原因(東原貴志等,2000).     

Growth of Phlebiopsis gigantea in wood of seven conifer species

Heterobasidion parviporum and Heterobasidion annosum are widely distributed root‐rot fungi that infect conifers throughout Europe. Infection of conifer stumps by spores of these pathogens can be controlled by treating fresh stumps with a competing non‐pathogenic fungus, Phlebiopsis gigantea. In this study, growth of three Latvian strains of P. gigantea and the biological control agent ‘Rotstop’ strain was evaluated in stem pieces of Norway spruce, Scots pine, lodgepole pine, Douglas‐fir, Weymouth pine, Siberian larch and Sitka spruce. The growth rates of one H. parviporum and one H. annosum isolate were also measured in the same stem pieces. The growth rate of P. gigantea varied greatly in wood of different conifer species. It was higher in the three pine species, lower in Norway spruce and lowest in Sitka spruce and Siberian larch, and in Douglas‐fir, this fungus did not grow. The largest area of wood occupied by P. gigantea was in lodgepole pine. Growth of Latvian isolates of P. gigantea in the wood of Pinus and Picea species was comparable to that of the Rotstop isolate. Consequently, stump treatment with local P. gigantea isolates should be recommended. However, our results suggest that Douglas‐fir stump treatment against Heterobasidion by P. gigantea may be ineffective and other stump treatment methods should be considered.     

Growth of Mixed Birch-Coniferous Stands in Relation to Pure Coniferous Stands at Similar Sites in South-eastern Norway

Mean age, mean and top heights and yield were studied in 20 mixed stands of birch ( Betula pubescens Ehrh. and B. pendula Roth) and Picea abies (L.) Karst. and nine mixed stands of birch and Pinus sylvestris L. in south-eastern Norway. Each mixed stand and the adjacent pure coniferous stand (control) were growing under the same site conditions and had not been commercially thinned. There were no significant differences in mean age at breast height or in top heights between birch and conifers in the mixed stands, while mean height was significantly higher for birch than for spruce. A growth index was calculated based on total volume and age at breast height. For the spruce sites the growth in young mixed stands (birch < 17 m) was superior to that of pure spruce, while the difference was insignificant in older stands. The growth index correlated positively with the ratio between generatively and vegetatively regenerated birch trees, and negatively with the age of the oldest species in the mixture and with site quality. For the pine sites there was no significant difference in the growth index between mixed birch-pine and pure pine stands. A mixture effect of birch on the volume yield of spruce or pine could not be demonstrated.     

Defoliation by the Needle-shortening Pine Gall Midge,Thecodiplosis brachyntera,on Pines in Central Sweden

An outbreak of the needle-shortening pine gall midge, Thecodiplosis brachyntera , occurred on Scots pine, Pinus sylvestris (L.) and lodgepole pine, P. contorta (Dougl.) in central Sweden during 1995-1998. The larval feeding of T. brachyntera kills current-year needles and heavy infestation causes severe defoliation. A pattern of fluctuating defoliation among years and sites was found on both pine species. Defoliation was evenly distributed in the crown of P. sylvestris during years of high as well as low infestation; apical whorls had as much defoliation as basal whorls. In P. contorta , however, there was a tendency for defoliation to be concentrated on branches in the most apical whorls. Growth ring increments on P. sylvestris with high levels of defoliation (71%) did not differ from trees with moderate levels (26%) of defoliation. Shoot lengths were significantly shorter, however, on heavily infested trees the year after defoliation.     

Decomposition and nutrient loss from prunings of different contour hedgerow species in tea plantations in the sloping highlands of Sri Lanka

Contour hedgerows of multipurpose tree species in the sloping tea lands of Sri Lanka are expected to reduce soil erosion and also add significant amounts of plant nutrients to the soil via periodic prunings. The objective of this experiment was to characterize the biomass decomposition pattern and quantify the amount of nutrients added through prunings of six tree species (Calliandra calothyrsus, Senna spectabilis, Euphatorium innulifolium, Flemingia congesta, Gliricidia sepium and Tithonia diversifolia) currently being used in hedgerows associated with tea. Withered leaf and stem prunings (50 g) were enclosed in 2-mm litter bags, placed at 5-cm depth and retrieved after one, three, six, nine and 12 weeks. Loss of initial dry weight, N, P and K was measured. Single exponential decay function adequately described both dry weight and nutrient loss. Tree species differed significantly in their rate of breakdown with decomposition constants (k) varying from 0.0299 to 0.2006 week⁻¹ for leaves and from 0.0225 to 0.0633 week⁻¹ for stems. Gliricidia showed the highest k for leaves with the rest in the following descending order: Senna > Tithonia ≥ Euphatorium > Calliandra > Flemingia. A similar pattern was observed for loss of all nutrients with Calliandra and Flemingia always having lower k values than the rest. Although N immobilization was not observed, immobilization of P and K was observed during the first week of incubation in some species, particularly in stem prunings. Annual biomass of prunings differed significantly between tree species in the following descending order: Calliandra > Senna > Flemingia > Tithonia > Gliricidia > Euphatorium. Calliandra added the greatest amount of nutrients annually to the soil with Euphatorium adding the least. Calliandra prunings provided the annual total K requirement and 49% of the N requirement of mature tea. However, none of the species provided more than 5% of the P requirement. It is concluded that among the tree species tested, Calliandra and Flemingia are the most suitable for contour hedgerows in tea plantations of this agroclimatic region because of their higher soil nutrient enrichment capacity and slower decomposition rates which would minimize leaching losses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of Seed Weight and Seed Type on Early Seedling Growth of Pinus sylvestris under Harsh and Optimal Conditions

The effects of seed weight and seed type on seedling growth of Pinus sylvestris (L.) were studied by seeding individually weighed orchard and stand seed in different mixtures under harsh (direct seeding in field) and optimal (seeding in nursery) conditions. In the nursery experiment an increase in the seed weight from 3 to 7 mg increased the seedling height by 10-27% and total weight by 27-113%, and decreased the height/diameter ratio by 5-6% after 2 yrs. With elimination of seed weight effects, orchard seedlings were 2% taller than stand seedlings in year 2. Without elimination of seed weight effects, orchard seedlings were 7-13% taller. In the field experiment an increase in the seed weight from 3 to 7 mg increased seedling height by 18-65%, stem volume by 81-274% and the number of top-buds by 23-34% in year 5. After elimination of seed weight effects, orchard seedlings were 7-13% taller than stand seedlings and without elimination of seed weight effects 20-21% taller after 5 yrs. Even after elimination of both seed weight and genetic effects orchard seedlings were 3-9% larger than stand seedlings in the field experiment. In conclusion, the influence of seed weight and seed type on growth traits and slenderness is highly significant and the influence seems to be greater in harsh conditions.     

Modelling of adequate pretwist for obtaining straight timber

Abstract

Wood in general and wooden studs in particular are often distorted owing to uneven shrinkage during the drying process in the sawmill. Twist is often the most detrimental of all types of distortion, and it is caused by spiral grain in combination with variations in moisture content. For sawmills, the objective is to produce dried, straight boards, and one method of dealing with boards with excessive spiral grain is to sort them out and then dry them in a pretwisted position to obtain straight boards after drying. A model using the finite element (FE) method for the simulation of drying twist distortions was first calibrated against laboratory experiments in which boards were dried with and without restraints and pretwists. After the calibration, the FE results were compared with industrial test results for boards that were dried without restraints or with restraints with zero pretwist, i.e. straight restraints. The FE model used an elastic–ideally plastic material model to obtain permanent deformations. The calibration was to set the yield stresses so that there was a good match between FE results and results from the laboratory experiments. The comparison between the industrial test results and the FE results showed that the FE model is capable of realistic simulations of drying boards with and without restraints and presumably also pretwists.     

Potential to harness superior nutritional qualities of exotic baobabs if local adaptation can be conferred through grafting

Baobab leaves form an important part of the local diet in Sahel countries and elsewhere in Africa. Existing leaf nutritional data and agroforestry performance information are based solely on Adansonia digitata L., the baobab of continental Africa. The introduction potential of Adansonia species from the center of diversity in Madagascar and from Australia remains untapped. To assess this potential, the mineral contents and B₁ and B₂ vitamin levels of dried baobab leaves were determined for five-year old trees of A. digitata, A. gibbosa (A. Cunn.) Guymer ex D. Baum, A. rubrostipa Jum. & H. Perrier (syn. A. fony Baill.), A. perrieri Capuron and A. za Baill. grown in an introduction trial in Mali. Nutritional data were evaluated against survival and vigor to identify promising germplasm. Leaf vitamin and crude protein contents were highest in the Madagascar species, especially A. rubrostipa (B₁ 88 mg 100 g⁻¹, B₂ 187 mg 100 g⁻¹, protein 20.7% dry weight). However, the local species far outperformed the introductions in survival, tree height, basal diameter and resistance to termites. We suggest grafting as a way of harnessing the vigor of well-adapted local baobab varieties to the superior nutritional profiles of A. rubrostipa and others. Cross-species grafting tests in Adansonia were successful, thus creating new agroforestry possibilities with different scion/rootstock combinations.     

Biogeochemical impacts of clearfelling and reforestation on blanket-peatland streams: II. major ions and dissolved organic carbon

Streams and drains in blanket-peatland forest in western Ireland were sampled weekly over 5 years, 1996–2000, using continuous, depth-proportional passive sampling. Analysis was for pH, alkalinity by Gran titration, anions by IC, metals by ICP, aluminium speciation by loaded-resin exchange, and dissolved organic carbon (DOC) by absorbance at 320 nm.

Effects of felling are identified graphically from two forest drains whose catchments (both about 1 ha) were clearfelled, and partial clearfelling of one larger catchment (somewhat over 1 km²), in summer 1999. Calcium concentrations and alkalinity were increased in the drain- and streamwater from the felled areas. The pH was increased in drains, while in the larger stream, pH range decreased while the mean increased. Phosphorus increased markedly with clearfell, in the absence of recent fertilising, and also increased with fertilising; these results are reported in an accompanying paper. Ammonium-nitrogen concentrations were increased in the two drains with felling, and temporarily decreased in the larger stream. Nitrate increased in some cases. Potassium and manganese concentrations also increased. Concentrations of DOC and organic monomeric aluminium increased gradually, subject to a continuing strong annual cycle. There were no clearfelling effects on concentrations of solphate, suspended solids or inorganic monomeric aluminium. In the two drains (fully clearfelled), concentrations of sodium, chloride and magnesium, and conductivity, were all reduced after felling. Fertilising effects other than for phosphorus were indistinct.

Effects of the combined clearfell–reforestation treatment were testable statistically, using randomised intervention analysis, between two larger streams (1 km²) as a treatment–control comparison. There were statistically significant positive responses in streamwater concentrations of nitrate, potassium, calcium, DOC and aluminium fractions. The implications for management practice depend on whether the loss of nutrients is sustainable for future crop productivity, and whether critical limits and loads of receiving ecosystems are exceeded.     

Effect of Drought on Growth and Mortality of Actively Growing Norway Spruce Container Seedlings Planted in Summer

Survival, root egress and height growth of 13-month-old actively growing Norway spruce ( Picea abies (L.) Karst.) container seedlings exposed to preplanting drying treatments (0, 4, and 8 days) and postplanting drought periods (1, 2, 3, and 4 weeks) were studied in the field between 5 July and 18 August 1999. The mortality of seedlings increased and the height growth and root egress decreased throughout the postplanting drought period. Postplanting performance was also affected by preplanting drying treatments. The results indicate that no risk of excessive mortality and growth restrictions occurs when actively growing seedlings are planted in summer, provided that seedlings are well watered before planting and the drought period does not exceed 3 weeks.     

Rehabilitation of diseased cacao fields in Peru through shade regulation and timing of biocontrol measures

The objective of this case study was to test the effectiveness of biocontrol for the rehabilitation of abandoned, diseased (moniliasis, witches' broom and black pod) cocoa and to develop testable hypotheses regarding the effect of shade and biocontrol timing. Three trial fields differed in their shade regimes. Biocontrol was administered starting either at flowering or pod filling. Disease incidence and yield were compared with cultural control alone. Biocontrol reduced moniliasis from 49% to 25%, independent of the shading regime and application time. Witches' broom was most severe without shade and unaffected by biocontrol. No significant effects on black pod were observed. Although biocontrol increased percentage healthy fruit independent of shading and timing, only the shaded plot with biocontrol from flowering onwards responded with increased yield. Our hypothesis is: 'In the rehabilitation of abandoned cacao, only fields under sustainable shade management and early onset of biocontrol can realise their production potential.' This revised version was published online in June 2006 with corrections to the Cover Date.