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Wood Quality from Plantation Forests

 Francides Gomes da Silva Jr., Esalq, Universidade do Saõ Paulo, Brasil

Luiz Ernesto George Barrichelo, IPEF, Brasil

Quality is a word or term that is used in many areas of the human knowledge and has many meanings. Focusing the term quality on the forest science it is possible to restrict its meaning but is also necessary to be more specific to define its real importance.
 
Wood quality is a technical term commonly used on the forest science and wood technology science but there is still a lack of a concrete definition of this expression. The concept of wood quality is changing in parallel with the evolution of the technical knowledge developed by the research in wood science and technology. Downes et al. (1997) defines wood quality as a term used to describe wood in regard to its influence on product performance. Hence, what is considered good quality in one product may not be in another one.  Quality is often poorly defined, but commonly is used loosely to refer to wood properties.
 
A modern concept of wood technology that is also valid for other forest non-wood products such as oils, seeds, resins among others is related to its suitability to the industrial transformation process and to the final product characteristics.  This means that a high quality wood shows all or almost all the requirements of a wood based mill and its costumer specifications in terms of final product performance in its final use. The levels of wood quality are directly related to the level of performance in the industrial process and customer use requirements.
 
Forest Plantations
   
For a long time, forest growth rates and total volume production rate were the only quality parameters considered in forest plantation activities.  Wood characteristics were of minor importance.  A common belief was that the wood-based industrial transformation process technology could optimize and absorb the effect of different kinds and quality of wood from different species and even different genus.  Based on this concept, most of the effort and attentions were focused on silvicultural techniques and practices that could lead to a higher volumetric yield.  Those practices included selection of superior trees for seed production to the silvicultural operations that seek to maximize the diameter and height of tress, linearity of the logs, form of the coppice, and others characteristics.
 
In any industry the quality of the raw material needs to be evaluated. The large inherent variability of the wood, within and between trees, can make quality evaluation costly and of limited usefulness.  A huge number of logs are delivered to a mill each day; they are hard to sample quickly; and their exact effect on product quality is uncertain, especially when mixed with other trees. The extent to which quality is evaluated depends on costs and benefits of that assessment. Usually a compromise between the cost and extent of the evaluation is reached.
 
Within the pulp and paper industry the aspects of interest are based on productivity and product quality. Downes et al. (1997) mention that the economics of the pulp and paper industry is driven by productivity. Whereas much is said about quality, little is known about what drives product quality and how wood quality affects this. This lack of information is largely because of the cost and difficulty of measuring wood properties.
 
Wood quality varies with the length of the timber rotation cycle and the application of the raw material.  Wood quality considerations are often most important in short rotation plantations, where the young juvenile wood may tend to be both weaker and less stable.  This creates some problems in pulp and paper making, and greater problems in sold wood manufacturing and products. In longer rotations and in natural stands, wood quality may be less important, because the greater amounts of mature wood provide a more uniform and stable material for products.  There still are large variations in quality and wood properties among species and genus, but the properties are more consistent and can be accommodated better in the generally solid wood products that large trees are used for. 
 
The concept of wood quality was until the 1970s was related mostly to taxonomic aspects.  In general terms, softwood species with long fibers and more strength were considered superior to hardwood species in terms of wood quality, for both pulp and paper and for solid wood products.  The evolution of this concept leads to the consideration of the concept of specie as a wood quality characteristics.  In the 1990s the use of wood characteristics parameters started to be used for wood quality evaluation.   Furthermore, more use of hardwoods for printing and writing, as well as high visual quality construction applications, increased.  And the development of uniform high quality plantation hardwood species such as Eucalyptus greatly increased its desirability.
 
Key Properties
 
One of the most important parameters in terms of wood quality is the basic density, which is the relation between oven-dry mass and green volume, i.e. the volume of the wood in the green state.  Wood density is easy measure and is related with the main use of the wood such as production of pulp and paper, charcoal, panels, or lumber.  Basic density is influenced by silvicultural practices such as soil amendments, fertilization, water availability, pest control, and others.
 
Basic density is important in planted forests because it has a high heritability index, i.e., once you select a specific genetic material for basic density, the next generation will have almost the same level of basic density as its predecessors.  Many other wood quality factors also have good correlations with many industrial parameters such as pulp yield, charcoal yield, panel density, wood specific consumption among many others.   Thus tree improvement programs can use wood properties for selection of preferred families, although measuring this still takes time and expense.
 
Besides the importance of wood basic density, the chemical characteristics of the wood are also of great importance, such as the contents of carbohydrates (cellulose and hemicelluloses), lignin, extractives and ashes. The evaluation of the wood chemical characteristics is time-consuming and as a consequence its costs are much more when compared to wood basic density. In general terms, for examples, the content of lignin is related to the pulp yield and chemical consumption on the pulping process, to the charcoal production yield and the charcoal heating value and others.
 
Fiber dimensions are another important group of wood quality parameters.  Fibers and also tracheids (softwoods) are the structural elements of the trees and the main wood based products such as pulp, paper, panels and lumber and its dimensions have a great influence on product performance.
 
Conclusion
 
In industrial terms, the knowledge about the wood characteristics that will be processed is quite important once the parameters of process control can be adjusted in order to obtain the best results in terms of process efficiency and product performance.
 
The wood quality evaluation is limited by the cost and time required for the assessment of the information related to wood characteristics. The cutting edge research that is being developed in terms of wood quality now focuses on the development of techniques and equipments that can help to minimize the cost and time necessary for wood characteristics evaluation.  This will enable better integration of plantation wood quality and desired product characteristics, delivered in a cost-efficient means.
 
Reference
 
Dowes, G.M., I.L. Hudson, C.A. Raymond, G.H. Daen, A.J. Michael, L.R. Schimleck, R Evans, and A. Muneri.  1997.   Sampling Plantation Eucalypts for Wood and Fibre Properties. Collingwood. 1997. 132p.
 
 
Posted 28 February 2008

 

 

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