Acoustic emission related to strain energy during drying of Eucalyptus regnans boards

Summary During timber drying, surface differential shrinkage within a board causes a high surface tensile stress and associated instantaneous strain. Acoustic Emission (AE) is generated when localised irreversible stress release events act to reduce the strain energy imparted to the material. A nonlinear one-dimensional drying model was used to calculate through-thickness moisture, stress and strain profiles during drying. The surface stress and instantaneous strain were used to calculate the strain energy at the surface. AE was measured during drying and the relationship between strain energy and the cumulative count (total ringdown counts) was investigated. The cumulative count is related to the unrecoverable strain energy rather than the elastic or recoverable strain energy. The cumulative count is not a useful measure of the propensity for surface checking. However the peak AE rate values are closely related to the surface instantaneous strain.Symbols E Young's Modulus (GPa) - instantaneous strain - _m mechano-sorptive effect - _c creep strain - _u unconfined shrinkage - _n net shrinkage - _o set (residual) strain - _pl proportional limit strain - W_in strain energy (kJ/m³) - U_u unrecoverable strain energy (kJ/m³) - U_r recoverable strain energy (kJ/m³) - W _in ^* nondimensional strain energy - U _u ^* nondimensional unrecoverable strain energy - U _r ^* nondimensional recoverable strain energy - stress (MPa) The authors are pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, University of Tasmania, the Australian Furniture Research and Development Institute and the Tasmanian Timber Promotion Board.     

From clearfell coupe to old-growth forest: Succession of bird assemblages in Tasmanian lowland wet eucalypt forests

As forests undergo succession after major disturbance events their assemblages of birds also change. Thus the frequency and extent of wildfire or clearfelling in the landscape can potentially affect the species-richness and abundance of forest birds. We used a chronosequence approach to investigate succession of bird communities in Tasmanian lowland wet eucalypt forest, from shortly after disturbance through to old-growth forest aged approximately 200–250 years. The number of native bird species recorded per survey per site increased as a linear function of stand-age. However, succession did not involve a unidirectional transition in assemblage-composition because of differences in successional responses among individual species and also among guilds of birds that mostly inhabited different strata of the forest. This was exemplified by the crescent honeyeater, which was observed most frequently in the youngest (6–8 years) and oldest (200–250 years) forests that we surveyed, and by the superb lyrebird (introduced to Tasmania from mainland south-eastern Australia) which favoured mid-aged regrowth (42–43 years) after clearfelling. Forests aged 200–250 years had the greatest richness-per-survey of those native species that were observed mostly in the lower layer or mid-layer. However, the greatest richness-per-survey of canopy-dwelling species and the highest native species-richness across the survey period were found in forests aged around 150 years. Younger regrowth was generally less rich in birds, although regrowth in the first decade after clearfelling was the only seral stage inhabited by superb fairy-wrens. These results suggest that, in the Tasmanian lowland wet eucalypt forest landscape, species-richness of birds may be greatest when old-growth forest is interspersed with young regrowth forests. Hence for bird conservation, a challenge is to ensure that old-growth forest continues to prevail in the production forest landscape.     

Acoustic emission monitoring during partial compression to detect early stages of decay

Summary The acoustic emissions (AEs) under partially compression were monitored with two softwood and one hardwood specimens in very eary stages of decay by a brown- and a white-rot fungus. Even slightly decayed specimens emitted AEs immediately after beginning of loading, typically when partial compression stress was applied to brown-rotted wood with the flat-headed attachment. With sound specimens of three wood species, only a few AEs were generated until the load reached at the proportional limit. These results suggest that AE monitoring will be one of the feasible means to detect the incipient stages of decay in a field test.The authors thank the Weyerhaeuser Research Grants Program for its aid in this investigation     

Measurement of strain distribution in timber finger joints

Timber finger joints bonded with melamine urea formaldehyde and one-component polyurethane were strained in tension. The deformation of wood in the finger joint area during the mechanical experiment was observed by means of 2-D electronic speckle-pattern interferometry (ESPI). Owing to the application of a surface treatment procedure developed for ESPI experiments, high-quality maps of the distribution of axial, transverse, and shear strain in the finger jointed area were obtained.     

Acoustic emission from softwoods in tension

Summary Acoustic emission (AE) monitoring is a non-destructive testing technique widely used to detect flaw development and crack propagation in metals, ceramics, polymers and composite materials.This paper relates the AE-strain characteristics from three softwoods tested in tension to mechanisms of deformation observed by scanning electron microscopy. All wood specimens are identical in size and radial-longitudinal in orientation, enabling the path of failure through planes of earlywood and latewood to be examined.It is found that the proportion of earlywood to latewood in each species has a marked effect on the shape of the AE-strain curves. Parana pine, containing very few latewood tracheids, exhibits a close to linear relationship between log cumulative emissions and strain until close to failure when the count rate increases rapidly. Douglas-fir, which has well-defined earlywood-latewood boundaries generates many AEs at low strain and there is greater variation in the shape of the AE characteristic between samples.Parana pine and Douglas-fir are tested at 20 °C (12.5 % EMC). Scots pine is also stressed at 20°C (12.5%EMC), 20°C (0.7%EMC) and 80°C (0.7%EMC), to assess the effect of moisture content on AE.Values of Young's modulus, stress at failure and work of fracture for the three softwoods are compared with the AE-strain data. Although the work of fracture is related to the total AEs to failure, no direct proportionality exists between the two parameters.Finally, the AE-strain data for plywood and glass-reinforced plastic (GRP), both man-made composite materials, are compared with those of wood, the natural composite material.S.R.C. support for this work under grant No. GR/A/13257 is gratefully acknowledged. The support of Professor Bryan Harris of the School of Materials Science, University of Bath and Dr. J. M. Dinwoodie of the Building Research Establishment, Princes Risborough is greatly appreciated. Mr. B. Dobraszczyk performed the impact tests.     

Succession of mosses,liverworts and ferns on coarse woody debris,in relation to forest age and log decay in Tasmanian wet eucalypt forest

In managed forest landscapes, understanding successional processes is critical to management for sustainable biodiversity. Coarse woody debris is a key substrate for forest biodiversity, particularly because it undergoes complex succession reflecting the effects of changes in both forest structure and substrate characteristics. The present study used a chronosequence approach to investigate succession of mosses, liverworts and ferns on coarse woody debris following clearfell, burn, sow native forest silviculture in wet eucalypt forest in Tasmania, focussing on discriminating between the effects of forest age and log decay. It also compared successional processes following wildfire with those following clearfell, burn, sow silviculture. Forest regenerating after the latter form of regeneration showed clear ecological succession up to 43 years (the limit of available sites), characterised by increasing diversity and cover, and clearly delineated specialisation among species with regard to successional stage. Analyses of subsets of the full data-set indicated that the effects of forest age dominated this succession, with minimal effects of substrate change independent of forest age. Analysis of within forest microenvironments were consistent with the inference that microenvironmental changes related to forest age drive major successional changes in these forests. Comparative analysis indicated similarity between successional states in post-wildfire and post- clearfell, burn, sow regeneration after 43 years for logs of the same decay stages, and continuing succession on post-wildfire sites to at least 110 years. Overall, these data suggest that management to sustain fern and bryophyte diversity should ensure that areas of forest beyond 110 years are represented in the landscape at appropriate spatial scales.     

Acoustic emission for tracing fracture intensity in lime wood due to climatic variations

Acoustic emission (AE) monitoring was used to trace directly the fracture intensity in cylinders of lime wood subjected to variations in temperature and relative humidity (RH) in their environment. High-frequency components produced by mechanical fracturing were extracted from the raw AE signals using the wavelet transforms. The accumulated energy of these components depended on the magnitude and rate of the RH variations. The AE activity correlated well with predictions of the numerical modeling carried out as the first part of the present investigations. In particular, the AE activity became negligible below the allowable magnitude for the rapid RH variation predicted by the simulation, or when the time interval allowed for the RH variation was long enough. Furthermore, AE proved capable of tracing the progressive evolution of damage at the microlevel, which preceded failure of wood discernible from the macroscopic perspective.     

Acoustic emission analysis of industrial plywood materials exposed to destructive tensile load

Several plywood materials made from spruce wood and, for comparison, solid spruce wood were investigated focusing on the sub-macroscopic damage evolution during tensile loading of the specimens. The destructive tests were simultaneously monitored by the acoustic emission (AE) method and strain field deformation measurement using digital image correlation (DIC). The bilinear interpretation of exponential defect growth identified the start of significant nonlinear behavior at 70 % of ultimate strength for all plywood materials. However, already the preceding and more stable damage evolution at lower stress levels has indicated a variation in intensity of the source mechanisms evaluated by AE energy of the detected events. Additional information on the formation of strain field concentration, which correlates with discrete accumulation in AE events and increased spreading in the distribution of AE energy, reveals the complexity of pre-damage due to the variation in cracks’ magnitude and timescales involved. The correlation between ultimate tensile strength and damage accumulation below 70 % of ultimate strength is determined, as well as the influence of layered structures on damage size shown by the percentage distribution of AE energy.     

Non-lethal strategies to reduce browse damage in eucalypt plantations

Browsing by mammalian herbivores is a major problem in plantation forestry worldwide. This has traditionally been controlled through a reduction in herbivore numbers achieved by lethal means, namely shooting and poisoning. In recent years, however, there has been increasing social and political pressure to reduce reliance on such lethal controls, and consequently research into non-lethal controls is becoming increasingly important. In order for non-lethal methods to be efficiently incorporated into management, however, we must first identify those that are most effective under operational conditions. Different methods are rarely tested concurrently, and many have only been proven effective with captive animals or in small scale field trials. This study therefore sought to consolidate past research by combining non-lethal methods, which were known to significantly reduce browsing damage, into a single trial to enable their relative effectiveness to be compared under operational conditions.     

Numerical and experimental study of moisture-induced stress and strain field developments in timber logs

When solid wood dries from a green condition to a moisture content used for further processing, moisture-induced fracture and stresses can occur. The drying stresses arise because of internal deformation constraints that are strongly affected by the cross-sectional moisture gradient differential shrinkage and the inhomogeneity of the material. To obtain a better understanding of how stresses develop during climatic variations, the field histories of stresses (and strains) in cross sections in their entirety need to be studied. The present paper reports on experiments and numerical simulations concerned with analysing the development of strains and stresses during the drying of 15-mm-thick discs of Norway spruce timber log. The samples were dried at 23 °C and relative humidity of 64 % from a green condition to equilibrium moisture content. The moisture gradient in the longitudinal direction was minimised by use of thin discs simplifying the moisture history of the samples studied. The strain field history was measured throughout the drying process by use of a digital image correlation system. Numerical simulations of the samples agreed rather well with the experimental strain results obtained. The stress results also indicated where in the cross section and when fractures could be expected to occur during drying. More optimal drying schemes showed markedly reduced stress generation.     

Genotypic variation in response to low boron in eucalypt clones

Eucalypts are increasingly important in the tropics for meeting growing demand for timber, wood chips, paper pulp and biofuel. Many new plantations are planted on low boron (B) soils, with adverse effects on plant growth and productivity. Two experiments in sand culture with different levels of added B, from 0 to 10 μM B, examined the effect of B deficiency on growth, wood yield and morphology of fibres of three commercially available eucalypt clones: K7 (Eucalyptus camaldulensis × E. deglupta), K51 (E. brassiana × E. grandis) and K57 (E. camaldulensis). In plant height, dry weight and wood production, K7 was more tolerant of B deficiency, but K57 and K51 were more responsive to increasing B. At the level of B that depressed growth by up to 54% and wood yield by up to 65%, no significant effect of B deficiency was observed on fibre morphology. However, as the wood:shoot ratio in K51 and K57 increased with increasing B, there is a possibility that B has a direct effect on wood production in some genotypes, in addition to an indirect effect via better growth. These results indicate that attention to B nutrition in eucalypt plantations would be beneficial to plant growth and productivity before effects of B on individual wood fibres becomes detectable. Selection for B-efficient genotypes could be useful for plantations on low B soils, and the full potential of sites where B is not limiting could be better realised with B-responsive genotypes.     

Analysis of determination methods for characteristic timber properties as related to growth area and grade yield

The origin of the raw material is a key aspect for strength grading of timber. Large grading areas are favored by the sawmilling industry as they require less effort in handling and documentation during the production process. However, large growth areas can also cause problems, as too high mechanical properties can be declared or yields may become uneconomical. The presented study presents a method that should allow for timber from different countries to be combined into a single grading area. Additionally, the influence on the yield for guaranteeing timber properties for differently defined populations is analysed. In this process, a number of available calculation methods for characteristic values for modulus of rupture, modulus of elasticity, and density are considered as the determination method also influences the final yield. Non-destructive and destructive test data from 8487 spruce specimens from Europe tested in bending or tension are the basis for the presented study. Based on the grading results the presented method is able to simply identify countries that may be combined. The definition of pan-European grading areas seems problematic if characteristic timber properties need to be guaranteed separately for each individual country as it may result in a severe drop in yield. However, checking timber properties only for the European population is unsatisfying as calculated timber properties considerably vary depending on the origin. As for the calculation method, the preferred method itself seems to have less impact on bending class assignments then on tension class assignments.     

Finite element study of growth stress formation in wood and related distortion of sawn timber

Lack of straightness in timber is the most frequent complaint regarding solid (and laminated) timber products worldwide. Nowadays, customers demand higher quality in the shape stability of wood products than they did earlier. The final distortion of timber boards is mostly caused by moisture-related stresses in wood (drying distortions) and growth-related stresses (distortions appearing when logs are split up to timber boards by sawing). To get more knowledge on how these distortions can be reduced in wooden products, there is a need for improved understanding of this material behaviour through good numerical tools developed from empirical data. A three-dimensional finite element board distortion model developed by Ormarsson (Doctoral thesis, Publ. 99:7, 1999) has been extended to include the influence of growth stresses by incorporating a one-dimensional finite element growth stress model developed here. The growth stress model is formulated as an axisymmetric general plane strain model where material for all new annual rings is progressively added to the tree during the analysis. The simulation results presented include how stresses are progressively generated during the tree growth, distortions related to the redistribution of growth stresses during log sawing, and distortions and stresses in drying reflecting the effects of growth stresses. The results show that growth stresses clearly vary during tree growth and also form a large stress gradient from pith to bark. This in itself can result in significant bow and crook deformations when logs are sawn into timber boards. The distortion results from the simulations match well with the results observed in reality. The parametric study also showed that the radial growth stress distribution is highly influenced by parameters such as modulus of elasticity, micro fibril angle and maturation strain.     

Forest carbon sequestration changes in response to timber harvest

Forest succession contributes to the global terrestrial carbon (C) sink, but changes in C sequestration in response to varied harvest intensities have been debated. The forests of the Central Appalachian region have been aggrading over the past 100 years following widespread clear-cutting that occurred in the early 1900s and these forests are now valuable timberlands. This study compared the history of ecosystem C storage in four watersheds that have been harvested at different frequencies and intensities since 1958. We compared NPP, NEP, and component ecosystem C fluxes (g C m⁻² year⁻¹) in response to the four different harvest histories (no harvest, clear-cutting, single tree selection cutting, and 43 cm diameter-limit cutting). Clear-cutting had short-term negative effects on NEP but harvest did not significantly impact long-term average annual C sequestration rates. Average plant C (g C m⁻²) since 1950 was about 33% lower in response to a clear-cut event than plant C in an un-harvested forest, suggesting that the C sequestration associated with clear-cutting practices would decline over time and result in lower C storage than diameter-limit cut, selective cut, or un-harvested forests. Total C stored over a 55-year period was stimulated ∼37% with diameter-limit cutting and selective cutting relative to un-harvested forests.     

Damage to Brazil nut trees (Bertholletia excelsa) during selective timber harvesting in Northern Bolivia

The success of multiple forest management systems is contingent on a variety of social, economic, biophysical, and institutional factors, including the integration of timber and non-timber forest product (NTFP) extraction and management. Selective logging for timber is increasingly taking place in forests where the collection of Brazil nuts, a high-value Amazonian NTFP, also occurs. We report on logging damage to Brazil nut trees in three certified timber concessions in Northern Bolivia from which timber is harvested using reduced-impact logging (RIL) guidelines and nuts are gathered yearly from the ground by local people. Observed frequencies of logging damage to Brazil nut trees were low, likely mirroring the low intensity of timber harvesting (∼0.5 trees/ha and ∼5 m³/ha) being currently applied across the study area. Of the trees ≥10 cm in diameter at breast height about 0.1 Brazil nut trees and 0.4 timber species per hectare suffered some degree of logging damage. Crown loss was the predominant damage type for Brazil nut trees accounting for 50% of all damage. In spite of the observed low rates of tree damage, we further recommend that RIL guidelines be amended to include the pre-harvest marking of pre-reproductive Brazil nut trees along with the future crop trees of commercial timber species. Further refining directional felling to reduce crown damage to Brazil nut trees would also serve to help maintain nut yields in the long term.     

GIS在福建省用材林地利用规划中的应用

通过GIS在福建省用材林地利用规划中的应用,介绍了不同GIS软件平台数据的交换,GIS数据的更新,GIS的分析操作,报表的生成,以及专题图制作等方法。     

Damage to foliage and stems caused by fungal pathogens in young eucalypt plantations in Zambia

Plantations consisting of non-native tree species were established on the Copperbelt of Zambia to supplement timber supply from natural forests and to meet the increasing demand of round wood and fuel wood for the copper mines. In early 2004, a disease characterised by severe leaf defoliation and death of juvenile trees suddenly occurred in plantations of the Copperbelt Forestry Company. This study was undertaken to determine the nature of the disease and extent of damage caused with a view of providing a framework for sustainable management of the plantations. Disease incidence, severity and impact on plantations using the crown damage index were assessed for three consecutive years. Eucalyptus grandis was more affected than E. cloeziana and leaf spot incidence and severity were higher in Kalibu, Kitwe District, than in Kafubu, Kalulushi District. The incidence in Kalibu was 90.4%, causing tree crown damage of 18.6% at the end of the assessment in 2006 compared to 17.5% incidence and 1.02% crown damage recorded in Kafubu. Fungi collected from leaf tissues included species of Aulographina, Cryptosporiopsis, Cylindrocladium and fungi in the Teratosphaeriaceae. Stem pathogens were identified as fungi in the Botryosphaeriaceae and Kirramyces sp. Diseases induced by these pathogens are increasingly becoming major threats to young eucalypts, resulting in poor stem form. Factors favouring disease development are described and these need to be properly managed to minimise the impact of diseases in addition to the need to establish selection and breeding programmes to develop tolerant genotypes to ensure a sustainable future for the forestry industry in Zambia.     

Transpiration increases during the dry season: patterns of tree water use in eucalypt open-forests of northern Australia

Australian savannas exhibit marked seasonality in precipitation, with more than 90% of the annual total falling between October and May. The dry season is characterized by declining soil water availability and high vapor pressure deficits (up to 2.5 kPa). We used heat pulse technology to measure whole-tree transpiration rates on a daily and seasonal basis for the two dominant eucalypts at a site near Darwin, Australia. Contrary to expectations, transpiration rates were higher during the dry season than during the wet season, largely because of increased evaporative demand and the exploitation of groundwater reserves by the trees. Transpiration rates exhibited a marked hysteresis in relation to vapor pressure deficit, which was more marked in the dry season than in the wet season. This result may be attributable to low soil hydraulic conductivity, or the use of stored stem water, or both. Tree water use was strongly correlated with leaf area and diameter at breast height and there were no differences in transpiration between the species studied. These results are discussed in relation to scaling tree water use to stand water use.     

Ant community responses to experimental fire and logging in a eucalypt forest of south-eastern Australia

Sustainable forest management requires a sound understanding of the impacts of forestry management practices, especially prescribed fire and timber harvesting, on biodiversity. Many studies have examined the impacts of fire and logging separately, but few have considered them together. Here we describe the combined effects of selective logging and repeated prescribed fire on ants in eucalypt forest near Eden in south-eastern Australia. The study was conducted within 18 experimental coupes comprising three replicates of each of six treatments, representing combinations of two logging (selectively logged and unlogged) and three prescribed burning (unburnt, routine and high fire frequency over 20 years) practices. Ants were sampled using terrestrial and arboreal pitfall traps in two plots within each of the 18 coupes. A total of 92 species from 35 genera were recorded, with ant species richness increasing with both increasing fire frequency and selective harvesting. We also found significant differences in ant species composition between management treatments; however, these differences were rather modest, and apparent only between extremes of fire and harvesting combinations. Logging on its own was not a significant factor contributing to variation in ant species composition. However, the responses of two common species and a key functional group to fire depended on logging treatment; in each case their abundance decreased with increasing fire frequency at unlogged sites, but increased at logged sites. The general resilience of ant communities suggests that forests remain in good functional health even when subject to a combination of selective logging and frequent burning. However, our study under-sampled species with specialist microhabitat requirements, and these might have been more heavily impacted. We have also revealed some significant interactions between the effects of fire and logging, which highlights the importance of considering combined impacts of forest management practices.