As Propriedades mecânicas de amostras individualizadas da madeira e do carvão

AS THE GROWING CONDITIONS OF TREES INFLUENCE THE WOOD PROPERTIES?

ABSTRACT

Aim of the study: Currently there is no information about how the growing conditions influence the spatial variation of the wood along the stem, since most studies have analyzed the radial or longitudinal variation in just one environment. Thus, it is necessary to generate a better understanding to what extent the planting density influences the spatial variation of wood properties in Eucalyptus plantations.

Main results: The characteristics of the wood may cause variations in their properties and performance in specific applications, and can be caused by both genetic and environmental factors. However, how genetic and environmental factor acts on wood variation along the trunk is still unclear. Another point is that even with new genetic breeding programs, the mechanical properties of wood have not been considered in those programs, since the selection of new material is always based on the growing rate, cellulose and lignin content and wood density.

Keywords: Wood quality; Spatial variation, NIR, Resonance

INTRODUCTION

Wood plays a very important role in the world because it can be an environmentally sustainable material and a renewable resource. The deforestation of natural forests is causing serious economic and environmental problems, not only locally, but also globally. The establishment of forest plantations for providing raw material with adequate quality for pulp and paper production, energy products, wood panels and sawnwood industry, is the key to reducing deforestation of natural forests. The forest sector has played a major role as a source of income for the national economy, generating these products, which are both for direct consumption and export.

The Eucalyptus has been the focus of several studies of genetic breeding due to its rapid growth and good adaptation to climatic and soil conditions in Brazil. However, for each intended use, whether pulp and paper, charcoal production, and others, the wood requires specific, sometimes contrasting features. The characteristics of the wood may cause variations in their properties and performance in specific applications, and can be caused by both genetic and environmental factors. However, how genetic and environmental factor acts on wood variation along the trunk is still unclear. Another point is that even with new genetic breeding programs, the mechanical properties of wood have not been considered in those programs, since the selection of new material is always based on the growing rate, cellulose and lignin content and wood density.

Currently there is no information about how the growing conditions influence the spatial variation of the wood along the stem, since most studies have analyzed the radial or longitudinal variation in just one environment. Thus, it is necessary to generate a better understanding to what extent the plant spacing influences the spatial variation of wood properties in Eucalyptus plantations, providing new accurate and reliable information on this issue, that remains still unclear.

INDUSTRIAL IMPORTANCE OF Eucalyptus WOOD

Considering the wide variety of species and clones of Eucalyptus cultivated in Brazil, it is necessary to search for new information on wood properties, so that the selection of genetically superior material may be successful (PEREIRA et al., 2012).

The term wood quality is widely used, but cannot be defined in one specific way. Wood quality is defined as the attributes that make logs and lumber valuable for a particular end use. In the production of pulp and paper, low wood density combined with long fibers results in collapsible, easy bonding fibers that exhibit low porosity and high strength. Conversely, structural lumber manufacturing requires wood with high density, small knots, and straight grain characteristics to ensure a high quality product (JOZA; MIDDLETON, 1994).

According to Downes et al. (1997) and Trugilho et al. (2007) the knowledge of the heterogeneity of the Eucalyptus woods is fundamental to indicate its correct use, predict their physical-mechanical behavior and establish an appropriate method of sampling.

Radial and longitudinal variation in wood properties observed along the trunk should be primarily due to differences in the proportion of juvenile and mature wood, the growth layer as well as by wood chemical change (MALAN; HOON, 2010).

According to Santos et al. (2011), high density, low levels of ash, and high lignin content are characteristics which may be considered as indices of wood quality for charcoal production.

In regard to pulp and paper production, Queiroz et al. (2004) emphasized the importance of wood basic density as the main characteristic influencing not only wood cost and industrial yield but, above all, final pulp and paper quality.

Considering the use of Eucalyptus as lumber, Zobel (1981) stated that one of the major problems is related to the existence of growing tensions present, and the highest prevalence occurs in juvenile wood. The growth stress forces are internally generated in the growing tree, it may cause wood defects such as cracks, warping and reduction in the strength of wood (CHAFE, 1979). Growth stress is a major cause of degradation and processing problems, especially in fast-growing Eucalyptus wood inducing warping and splitting of logs and boards (PANSHIN; DE ZEEUW, 1980).

Thinking on wood use for furniture, should be taken into account the density, volumetric shrinkage, bend strength, modulus of rupture, compressive strength, tensile strength, shear strength, stiffness, and other wood properties (SERPA et al., 2003).

EFFECT OF SILVICULTURE AND MANAGEMENT AT WOOD QUALITY

2. Tree growth

Many silvicultural operations can affect the tree growth, like plant spacing, fertilization, pruning, thinning and others (ZOBEL, 1992).

At closer spacings stand leaf areas and biomass develop more rapidly and the site is “fully occupied” sooner. Early stand growth rates will be faster at closer spacings and the peak current or mean annual increments can occur sooner (STAPE; BINKLEY, 2010; CHEN et al., 2011).

Trees with large crowns produce low quality wood because they produce more auxin (LARSON, 1962, SUNDBERG et al., 2000). Auxins produced in the stem apices, such as indolic-3-acetic acid (IAA), have been linked to the type (size and thickness) of wood cells formed (LITTLE; PHARIS, 1995), such that higher concentrations of IAA result in larger cell diameters with thinner cell walls, which is known as earlywood (LARSON, 1962; SUNDBERG et al., 2000).

The competition between trees begins when two or more trees are growing in close proximity and fighting for the same resources. Tree growth will be checked when any one of those resources becomes limiting, for example water or other vital nutrient elements, or light due to mutual shading by the tree crowns. The trees exist often in a state where several vital resources are at, or close to, critical limits which can change from time to time throughout the year depending on weather patterns (SHEPHERD, 1986).