共查询到20条相似文献，搜索用时 187 毫秒
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本文基于阔叶红松林30hm2(500m×600m)固定监测样地,分析了红松在各林层(主林层、次林层、林下层)的空间分布、各林层之间的关联性。结果显示:(1)红松在各林层都在小尺度上呈聚集性分布,大尺度上呈随机分布;且在低林层聚集性强度高于高林层。(2)红松各林层关联性表现为主林层与次林层在小尺度上呈正相关,大尺度上关联性不显著;主林层与林下层在小尺度上呈负相关,大尺度上关联性不显著;次林层与林下层在所有尺度上呈现显著正相关。说明红松种子扩散和红松个体间竞争对空间分布起着重要作用。 相似文献
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以浑善达克沙地固定沙丘与丘间低地2种生境下榆树种群为研究对象,采用点格局分析法对不同生长阶段榆树种群的空间分布格局及其空间关联性进行分析。结果表明:丘间低地种群整体在<10m的小尺度上呈现聚集分布,幼树和中龄树在大尺度上呈随机分布,而成熟树呈现聚集分布;各发育阶段间的空间关联主要表现在小尺度上,随着空间尺度的增加,空间关联性微弱;固定沙丘榆树种群均在小尺度上呈聚集分布,随尺度增大种群呈现随机分布趋势;中龄树和成熟树间在小尺度上呈正关联,各个发育阶段间在大尺度上空间关联性很小。 相似文献
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以25像元和150像元的目标分割值生成小尺度和大尺度的快鸟影像景观目标层,利用知识分类提取出绿地信息,计算分析城市绿地6种景观参数,并利用模糊数学方法构造景观综合指数,研究探讨多尺度城市绿地景观特征。结果表明:多分辨率分割方法能有效产生两种绿地景观尺度;小尺度景观指标大多大于大尺度景观指标,符合景观学规律。 相似文献
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文中从实木榫接合家具节点、构件和整体框架3个层面，总结、分析有限元法在实木榫接合家具结构设计中的应用现状，并对该领域存在的主要问题及未来研究趋势进行了阐释。目前主要存在以下问题:1）未形成系统性的有限元分析方法，不同研究者之间存在较大差异；2）分析准确度不高，大多研究仅为有限元软件的模拟分析，缺乏试验验证；3）未建立合理的榫接合节点有限元模型，通常将榫接合节点视为刚性节点处理。未来研究可围绕以下几方面开展:1）榫接合节点有限元模型的建立方法及简化方法；2）实木榫接合家具疲劳寿命的有限元分析方法；3）基于有限元法的实木榫接合家具结构优化设计方法；4）二次开发，在有限元分析软件的基础上，建立实木榫接合家具结构设计模块；5）实木榫接合家具智能设计与制造系统开发。将有限元法应用于实木榫接合家具的结构设计中，将有助于实木榫接合家具的结构优化设计、智能设计与制造。 相似文献
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家具造型特征定量模型的构建研究 总被引：1，自引：0，他引：1
针对家具造型中的复杂性和不确定性设计信息,以造型特征空间构建和造型信息层次分析为基础,提取了家具造型中的几何特征;并基于数量化理论Ⅰ的产品意象造型研究方法,分析家具造型语义特征,发掘感性意象与造型要素之间的映射关系,构建了基于造型特征、语义特征及其映射关系的家具造型特征定量化模型。 相似文献
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胶合竹家具,如落地衣架和落地灯,存在着主杆节点的强度不足和美观度不佳等问题。植筋连接技术被广泛认为可以显著提升竹木构件节点的抗拔性能,而且由于其内置的特性,不会影响外形美观。尽管胶合竹植筋连接技术在建筑领域的方形梁柱节点中得到了广泛应用,但本研究旨在将这一技术应用于产品领域的胶合竹家具圆杆节点。在改变胶合竹植筋节点的形状后,需要通过试验分析来确定各项合理化设计参数,以提升节点的抗拔性能。选取胶合竹植筋圆杆为研究对象,以植筋边距、胶层厚度、植筋长径比等因素为研究变量。采用拉-拉模式进行抗拔性能试验,记录试件破坏模式和节点极限荷载。在试验过程中,出现胶合竹劈裂破坏、植筋拔出破坏和植筋屈服破坏3种破坏模式。研究结果表明：为防止胶合竹劈裂破坏,胶合竹与植筋直径之比应不小于8;节点极限荷载受胶层厚度影响显著,但胶层厚度不宜过大,当植筋直径为4 mm时,胶层厚度可设置为1.0～1.5 mm;节点极限荷载随着植筋长径比增加而提高,对于直径为4和8 mm的植筋杆,屈服破坏的临界植筋长径比均可取为15。基于试验结果,本研究提出了胶合竹植筋节点极限荷载预测方程,计算值与试验值较为接近。 相似文献
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以现代实木家具中常见的45°斜角接合结构为研究对象,以实现斜角接合结构可拆装,提高其接合强度为目的,分析了现有斜角接合结构的特征与不足,提出一种通过五金连接件与分体榫相结合的45°斜角可拆装改良方案;采用有限元分析方法,从应力分布角度对现有节点和改良节点进行对比分析;通过力学试验方法测试了两种结构的抗弯力学性能。结果表明:改良方案的极限承载力平均值比现有方案高18.7%,通过有限元分析方法获得的节点应力云图与试验中试件的破坏形式具有较好的一致性。改良方案实现了斜角接合节点的可拆装,提高了实木家具斜角接合节点的强度,为后续进一步开发实木家具隐藏式五金连接件及榫卯外观下的节点可拆装设计提供了依据及实现方法。 相似文献
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《Scandinavian Journal of Forest Research》2012,27(5):427-435
In Scandinavia, moose (Alces alces L.) sometimes cause severe browsing damage to economically-important pine. Moose-vehicle accidents have spurred construction of fences along roads, and these may interfere with moose migration between summer and winter ranges, or the road alone may be a barrier. If this happens and moose build up along roads, landowners may suffer economically. Therefore, this study investigated whether roads, fences or other factors influence the use of young pine stands by moose. Eighty stands along roads in northern Sweden were evaluated in which individually-browsed branches were counted on 9972 pines. Moose browsing was not significantly related to birch (Betula pendula Roth, B. pubescens Ehrh.) density, nor did it differ between pines (Pinus contorta Douglas or P. sylvestris L.). However, increased pine density, site productivity and proximity to a highway were associated with increased browsing. Further large-scale studies are needed to understand moose habitat selection and the effects of roads. 相似文献
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《Scandinavian Journal of Forest Research》2012,27(3):206-217
The root systems of 2-yr-old Picea glauca, Picea mariana and Pinus banksiana seedlings were submitted to various frost temperatures during an artificial frost to induce different levels of root damage. Frost-damaged and control seedlings were placed in a greenhouse under high and low soil moisture regimes. Seedling growth and physiology were evaluated periodically. Seedling survival was reduced when root damage reached levels of 60-80%. Root systems of all three species showed partial to total recovery by the end of the experiment. In general, root freezing damage caused reductions in seedling growth, with these reductions becoming less significant over time. Root damage had little to no effect on black spruce and jack pine seedling physiology, while white spruce CO 2 uptake decreased with increasing root damage. Shoot nitrogen content of all three species decreased slightly with increasing root damage. 相似文献
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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. 相似文献
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《Scandinavian Journal of Forest Research》2012,27(2):118-130
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. 相似文献
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Steven Maranz Amadou Niang Antoine Kalinganire Djeneba Konaté Bocary Kaya 《Agroforestry Systems》2008,72(3):231-239
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. 相似文献
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《Wood material science & engineering》2013,8(2):76-84
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. 相似文献
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Astra Zaluma Lauma Bruna Darta Klavina Natalija Burnevica Kristine Kenigsvalde Andis Lazdins Talis Gaitnieks 《Forest Pathology》2019,49(6)
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. 相似文献
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《Scandinavian Journal of Forest Research》2012,27(1):53-61
This study investigated the stand structure in pine, spruce and deciduous forests in the border district of Finland and Russia. A total of 46 mature forest stands was selected as pairs, the members of each pair being as similar as possible with respect to their forest site type, age, moisture and topography. The stands were then compared between the two countries by means of basal areas and number of stems. The proportions of dominating tree species were 2-12% lower, and correspondingly the proportions of secondary tree species higher, in Russian forests. The density of the forest stock was also higher in each forest type in Russia. The forests in the two countries differed most radically in terms of the abundance of dead trees. The amount was two to four times higher in Russian deciduous and spruce forests, and in pine forests the difference was 10-fold. The stand structures indicated that Russian coniferous stands, in particular, were more heterogeneous than intensively managed pine and spruce stands in Finland. 相似文献
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《Scandinavian Journal of Forest Research》2012,27(1):15-27
The root collar diameter and the height:diameter ratio are of particular importance in container-grown seedlings where a high density in the containers may produce spindly seedlings. Temperature regimes and light quality are known to affect plant growth. The aim of this study was to identify responses in Picea abies (L.) Karst. seedlings grown with light providing different red:far-red ratios and under temperature regimes with alternating day (DT) and night temperature (NT) from negative (DT < NT) to positive (DT > NT) difference (DIF) between DT and NT. Experiments were conducted in controlled environment chambers and in a daylight phytotron. Only limited thermoperiodic responses appeared in P . abies seedlings with respect to seedling height and dry weight accumulation. The formation of terminal buds, however, was clearly delayed in seedlings grown at negative DIF. The results indicate a requirement for day extension light that is high in far-red, to prevent terminal bud formation under natural short-day conditions. An extended study should be conducted to clarify the minimum level of light intensity and the optimal light quality needed to prevent terminal bud formation under natural short-day conditions. 相似文献
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本文分析了CAD在设计中引起的正面和负面影响,并进行了系统的阐述,从而使设计者在应用中能够保持客观的态度。 相似文献