Wood is used for so many different products we need to be conversant with a range of properties that can vary. However once a manager establishes his market niches, a much smaller array of properties may be important to control. Here, we will survey the full array of variable wood properties over which one might want to have some influence.
During millions of years of evolution, trees have adapted to environmental change by adjusting physiological and biomechanical responses and these are reflected in the structure of the xylem, or wood (Baas et al., 2004; Niklas, 1992; Carlquist, 1975). In parts of the wet tropics where growing conditions are similar year round, distinct annual growth rings may be absent (Record and Hess, 1943). We will consider these after first examining the cross-sectional structure of trees with defined growth rings – an exercise that provides revealing insights in relation to growth rate management.
Coniferous trees (generally called softwoods) produce a single cell type, the tracheid, that carries out the functions of both support and water conduction (Alden, 1997; Panshin and deZeeuw, 1980; Jane et al., 1970). The simplest version of this is found in conifers characterized by a relatively uniform cross-sectional structure composed of tracheids with similar diameters and thickness of walls, but usually with some gradual increase in wall thickening near the end of the growth ring (latewood). This kind of conifer wood is termed gradual transition (Figure 1). More specialized conifers develop a structure with large diameter, thin-walled tracheids in the earlywood and decidedly thicker-walled, smaller-diameter tracheids in the latewood (Wardrop and Preston, 1950). These conifers that partially separate the roles of conduction and support are called abrupt transition (Figure 2 ) softwoods. Sometimes it is difficult to easily place a conifer in one of these two categories because the dense latewood is a significant portion of the ring, but the transition is gradual (Figure 3 ). In these cases, we usually group the wood with abrupt transition species since, in terms of growth rate and mechanical properties, it behaves more like those woods (Anonymous, 2002).
Dicotyledonous woods, or hardwoods, have evolved to produce completely different cells for conduction and support. Water transport is carried out by relatively large diameter vessels (referred to as pores when viewed in cross-section), while mechanical support is achieved with thick walled fibers. Because of this division of labour, the size and distribution of vessels can be adjusted to meet a wide range of environmental habitats, and as a consequence hardwoods have become the dominant tree species throughout much of the world (Carlquist, 1988).
In regions of the world where climate is distinctly seasonal, with long cold periods that halt tree growth, a type of vessel arrangement called ring-porous often develops (Carlquist, 1988). A few ring-porous hardwoods are also found in the tropics, especially in response to monsoonal conditions that provide a relatively short growing season (Chudnoff, 1984). The characteristic features of ring-porous hardwoods are clustered large pores (vessels) at the beginning of the growing season followed by more scattered, smaller diameter pores embedded in fibers in the latewood (Figure 4). Some temperate region trees and most tropical species that are classified as ring porous may have only slightly larger and few larger pores at the beginning of the growth ring, or have a more gradual change in pore size transitioning into the latewood (Figure 5). These will be referred to as weakly ring porous.
Diffuse-porous hardwoods have similar size vessels arranged in various patterns throughout the growth ring, but typically in a relatively uniform distribution of pores (Figure 6). Diffuse-porous hardwoods dominate in the tropics, but are also common in many temperate zone habitats. They are readily distinguished from ring-porous hardwoods by lacking the distinct concentration of larger pores at the beginning of the growth ring. Tropical hardwoods that lack distinct annual growth rings are classified as diffuse-porous.
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Abrupt Transition Softwoods
When growing in natural forests, these species generally have basic specific gravity (SG) values (green wood volume, oven-dry weight) that exceed 0.40, producing lumber that is strong and generally favoured for construction purposes (Anonymous, 2002; Summit and Sliker, 1980). When grown in short-rotation plantations these trees may have lower SGs and different wood properties, discussed subsequently. Examples of species in this grouping are (SG in parentheses):
• Warm Temperate - Pinus caribea (0.68), Pinus oocarpa (0.55), Pinus patula (0.45), Pinus palustris (0.55), Pinus elliottii (0.54), Pinus taeda (0.47), Pinus echinata (0.47), Pinus radiata (0.42);
• Cool Temperate - Pseudotsuga menziesii (0.46), Pinus sylvestris (0.42), Pinus resinosa (0.41), Pinus banksiana (0.40), Tsuga heterophylla (0.42), Larix species and hybrids (0.49+ or -).
Gradual Transition Softwoods
The wood properties of these species are less affected by growth rate than the abrupt transition species, producing plantation grown wood that more closely resembles the properties of natural forest trees. Most of these species have SG values below 0.40. Many of these are also important construction timbers (Anonymous, 2002; Bootle, 1983: Mullins and McNight, 1981).
• Examples include: Pinus strobus (0.36), Pinus monticola (0.36), Pinus lambertiana (0.34), Pinus cembra (0.3-0.4), Abies species (0.33-0.34), Picea species (0.33-0.38), most members of the Cupressaceae, Araucariaceae, Podocarpaceae, Taxaceae.
Ring-Porous Hardwoods
Tree species that fall into this grouping respond to growth rate with distinct changes in strength properties. Those characterized as strongly ring-porous (Figure 4) are more affected by growth rate than ones categorized as weakly ring-porous (Figure 5). Listed here are tree genera that have at least some ring-porous members (Ilic, 1991; Martawijaya et al., 1986; Bootle, 1983; Keating and Bolza, 1982; Panshin and deZeeuw, 1980).
Key: S = strongly ring porous; W = weakly or semi-ring porous; N = some
members are not ring porous; Tr = tropical genera; Te = temperate genera
| Agonis (W, Tr) Acanthopanax (S, W, Te) Bombax (W, Tr) Carya (S, W, Te) Castanea (S, Te) Castanopsis (S, W, Te) Catalpa (S, W, Te) Cedrella (W, N, Tr) Celtis (S, W, N, Te) Diospyros (W, Tr, Te) Eucalyptus (W, N, Tr, Te) Fraxinus (S, Te) Gleditsia (S, W, Te) Guiacum (W, Tr) |
Juglans (W, N, Te) Maclura (W, Te) Melia (W, N, Tr) Morus (S, W, Te) Phellodendron (S, W, Te) Prunus (W, N, Te) Quercus (S, N, Te, Tr) Robinia (S, W, Te) Sapindus (W, N, Tr?) Sassafras (S, W, Te) Tectona (S, W, Tr) Toona (W, Tr) Ulmus (S, Te) Zelkova (S, Te) |
Diffuse Porous Hardwoods
Based on number of species, this is the largest of the four groups. Strength properties in these tree species are less influenced by growth rate than in ring-porous species (Lauridsen and Kjaer, 2002). However, development of internal stresses can be magnified by rapid growth, particularly in the denser species (Bootle, 2004; Cassens and Serrano, 2004; Archer, 1986). Most tropical hardwoods and several temperate genera fall into this category (Martawijaya et al., 1986; Summitt and Sliker, 1980; Record and Hess, 1943). A complete listing of the most commonly used tree species in planted forests (enumerated by country) can be found in James and Del Lungo (2005).
“Especially in the tropics, many forestry organizations are planting exotic species on a massive scale and are producing a large volume of “different” wood. Such wood is not desirable for some products and may be distinctly different from the wood that the same species produces in its indigenous environment. Huge amounts of it are now becoming available, requiring a reassessment of both manufacturing techniques and product type and quality” (Zobel, 1984).
In the 22 years since Bruce Zobel made the above pronouncement, many new planted forests have been established, often without recognizing the difficulties that could be faced at timber harvest time. This section examines how various properties may be different in wood grown in planted forests, especially if trees are growing more rapidly due to genetic manipulation, wide spacing, fertilization or irrigation, or are growing on different soils or in different climate zones (Mayhew and Newton, 1998; Zobel and Sprague, 1998; Bendtson, 1958). The emphasis is on describing wood property changes related to specific conditions and for different species groups. The issue of ‘wood quality’ is left to the discretion of the reader since, as Gartner (2005) has noted “wood quality is the weighted value that society gives to wood characteristics that affect properties” (see Appendix I for a cogent example).
Growth Rate, Density and Strength Properties
Strength is an important criterion for wood that will be used for structural purposes, whether for building construction, boats, furniture, tool handles, sporting equipment, etc. (Anonymous, 2001; Chudnoff, 1984; Record and Hess, 1943). A surrogate measure often used to assess strength is wood density. Except in the case of compression wood (see below), density is generally a reliable indicator of modulus of rupture in bending, but less so for modulus of elasticity (Jagels et al., 2003; Anonymous, 2001).
Strength is generally reduced in abrupt-transition conifers that produce very wide rings in planted forests. The percentage of weaker earlywood increases and the width and density of latewood generally declines (McAlister et al., 2000; Haygreen and Bowyer, 1989). Pinus radiata is normally an abrupt transition species (Figure 2), but in plantation grown trees the wood may become gradual transition (Figure 1).The degree of strength loss is correlated with the loss in wood density. Abrupt-transition north temperate species such as Larix spp., Pinus sylvestris and Pseudotsuga menzessii will generally reveal some strength loss in plantations, but it will not be as severe as in species like the southern hard pines of the US that can adapt to plantations in tropical regions. Gradual transition conifers, with normally lower wood density will be affected to a lesser degree when grown under conditions favouring rapid growth, and since many of these species are north temperate trees, they do not adapt to tropical conditions.
The strength properties of ring-porous hardwoods (Figure 4)) are generally enhanced when grown to produce wider rings. This is because the weaker large-pore earlywood zone is fixed in width in these species, so all added growth is in stronger latewood (Panshin and deZeeuw, 1980; Haygreen and Bowyer, 1989). Figure 7 shows a wide ring in Carya, as well as two narrow rings occupying about the same area and consisting of mostly weak earlywood. Species that are classified as semi- or weakly ring porous (Figure 5) have the same relationship between ring width and strength, but the magnitude of the change is less. Baseball bat and tool handle manufacturers in the US often set upper limits on rings per inch that they will accept in ring porous woods like ash, and hickory destined for these high strength requiring applications.
Figure 7. Ring porous hardwood with one wide and two narrow rings (Carya ovata)
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Strength properties of diffuse porous hardwoods are often less prone to changes in density and strength related to growth rate, but those species that have large geographical ranges and rainfall and soil tolerances can display a considerable range in wood density and strength properties. Swietenia macrophylla, for example, has a natural range from the wet lowlands of Mexico and Guatamala through Panama, to the high and dryer altitudes of Bolivia. Within this natural geographic range basic specific gravity can range from 0.40 to 0.85. In planted forests where moisture and spacing encourage rapid growth, the density range is smaller and closer to the low end of the range (Mayhew and Newton, 1998).
A caveat needs to be noted here. Standard testing procedures for strength properties of wood are performed on small, clear specimens. Lumber grading rules downgrade this strength value as the number and size of knots increases. In the case of some planted forest trees, where no pruning was performed and trees are growing at wide spacing, the great number and size of knots may considerably reduce the use of the respective timber for construction purposes as is the case, for example, with Acacia mangium in South East Asia (Killmann, personal communication).
Growth Rate and Internal Stresses
In young saplings the weight of the crown, which is large in relation to stem diameter, induces compressive stress on the cambium in the tree trunk. As the tree increases in height and girth, axial compressive stress established near the centre of the tree is gradually replaced by axial tensile stress in the more recently formed outer wood. These opposing stresses may increase in magnitude with time or be partially relieved by elastic or inelastic strain (Bootle, 2004; Cassens and Serrano, 2004; Boyd, 1980; Boyd, 1972). Factors that can affect the magnitude of unrelieved opposing stresses are: (1) whether a tree is a conifer or hardwood, (2) whether the wood density is high or low, (3) if a hardwood, whether the wood is ring-porous or diffuse porous, and (4) how rapidly the tree grows.
Conifers, possibly due to the structure of tracheids or the different chemistry of the lignin, are less prone to develop large opposing internal stresses than are hardwoods, although star shake and ring shake can develop in some species like hemlock (Tsuga spp.). Hardwoods with low wood density generally relieve internal stresses in the living tree through inelastic strain (Boyd, 1980). Ring porous hardwoods, because of the very weak earlywood zone can also provide some stress relief in the living tree, although these species are not immune to stress-related defects (Cassens and Serrano, 2004). The species most prone to developing large unrelieved internal stresses are high density diffuse porous hardwoods – and this is greatly magnified when these trees grow rapidly, reducing the time during which some stress relief could occur.
Many moderate to high density Eucalyptus species that normally grow slowly in moisture restricted native habitats in Australia, produce very large internal stresses when grown in wetter indigenous planted forests, or as exotics in Africa, Asia and Latin America (Bootle, 2004). Felling or milling of these trees is quite dangerous as internal stresses are relieved by sudden and forceful splitting. Even when trunks are banded during felling, and protective cages are built around milling saws, the wood yield is quite poor. A relevant example can be found in Eucalyptus camalduensis grown rapidly in Pakistan (Killmann, personal communication). By contrast, the low density Gmelina arborea when grown rapidly in plantations does not usually develop large internal stresses. Gmelina wood only has a density of about 410 kg/m3 (compared to 470 to 500 for Acacia mangium and hybrids; and over 600 for most Eucalypts). The low density of Gmelina is often a source of complaint among planted forest managers, and as a consequence researchers are attempting to breed higher density varieties or hybrids (Dvorak, 2004). However, this may run the risk of introducing unwanted internal stresses, greatly reducing yield.
Growth Rate and Production of Juvenile Wood
Wood production by the layer of cambial cells between the bark and xylem is controlled by a combination of biomechanical and physiological influences – most critically by the weight of the crown and the concentration of plant hormones (auxins) and sugars transported from the leaves in the crown to the cambial zone in the trunk. When a tree is young the weight of the crown relative to the diameter of the stem is large, and the transport distance for auxins and sugars is short. During this period the cambium produces a kind of wood that has been descriptively labelled “juvenile wood”, “crown wood” or “core wood” (Zobel and Sprague, 1998; Cave and Walker, 1994; McMillan, 1973; Dadswell, 1958). As trees age the relative weight of the crown lessens and the transport distance for auxins and sugars increases. The cambium near the base of the tree responds by beginning to produce mature wood, with properties that are more acceptable for most wood products (Bendtson, 1978). The differences between juvenile wood and mature wood are most pronounced in conifers and are of less importance in hardwoods (Zobel and Sprague, 1998; Bendtson, 1978).
The volume of juvenile wood produced in a stem is related to how rapidly it grows and whether the lower crown is pruned, dies due to shading, or remains alive (Zobel and Sprague, 1998; Larson, 1969). Trees that are planted with wide spacing to maximize growth and are not pruned of lower branches will produce the largest core of juvenile wood. In some species of abrupt transition hard pines planted in warmer climates, juvenile wood can occupy 80 to 100% of the stem volume if trees are widely spaced, un-pruned and harvested at less than 20 years of age. Some managers have suggested that it would be wise to consider these as different tree species due to pronounced differences in wood properties (Zobel, 1984).
Juvenile wood is significantly weaker, and unlike mature wood shrinks or swells in the axial direction, due to a high microfibril angle in the secondary wall of the tracheids (Meylan and Probine, 1969; Boyd, 1985). If both juvenile and mature wood are present in the same board, the differential transverse and longitudinal shrinkage of these two zones will lead to significant warping and twisting (Meylan, 1972). Juvenile wood processed for pulp will require more chemicals and produce paper that is not as strong - and the percentage of fines in the waste stream will increase substantially (Zobel and Sprague, 1998; Dinwoodie, 1965). Ideally, conifers should be established with moderately close and even spacing followed by a program of judicious pruning and thinning as the stand ages. This produces a greater quantity of knot free, higher value wood at harvest. Many managers have opted for wider spacing and no thinning or pruning in order to minimize costs per unit volume of wood produced. Unfortunately this precludes the possibility of marketing the logs for quality lumber or veneer, and the quality for pulpwood is significantly reduced. Often not calculated in this strategy are the further economic losses at the paper mill due to increased fines (due to shorter and weaker tracheids) that reduce the volume converted to paper and increase the cost of waste stream management (Groom et al., 2002; Anderson, 1951). One market for which juvenile conifer wood is acceptable is chips for board construction (with the possible exception of oriented strand board). However small diameter un-pruned trees can be difficult to debark and the high knot content may quickly dull knives and yield a low volume of acceptable chips.
Spacing and Reaction Wood
Trees exposed to windy conditions or when planted on steep slopes will produce an abnormal wood in an attempt to maintain vertical stem axes. In hardwoods this is called tension wood, and forms on the upper side of leaning stems. In conifers reaction wood forms on the underside of leaning stems, and is called compression wood (Panshin and deZeeuw, 1980; Jane et al., 1970). As spacing between trees is widened the wind loading on saplings increases and the proportion of reaction wood increases. Also, uneven spacing can lead to unbalanced crowns and result in leaning stems and greater proportions of reaction wood (Bootle, 2004). Tension wood can affect machining properties, producing a wholly surface, and in some species, notably Eucalypts, can result in non-recoverable collapse during drying (Bootle, 2004). Compression wood greatly reduces many mechanical properties, and increases lignin content and density (Panshin and deZeeuw, 1980; Wardrop and Dadswell, 1950). Since it forms mostly in the first several years of growth, when trees are most susceptible to bending, it further degrades the juvenile wood portion of the tree. The higher lignin content makes the conifer wood more difficult and costly to pulp for paper production.
Drying Defects in Timber
Several drying defects can be enhanced in rapidly grown plantation wood. As noted in the discussion of juvenile wood, warping, cupping, and twisting during drying will be greater in boards that contain both juvenile and mature wood. As an example, much rapid growth southern pine plantation wood in the US readily accepts pressure-treatment with water-borne chemicals for use in exterior decay or termite exposure situations. Often the wood is immediately bundled and wrapped with steel straps after treatment and shipped to retailers. Partial drying occurs during shipment and when the steel straps are cut much of the surface lumber is released, like a wound spring, as warped and twisted waste. In order to limit their losses, retailers generally store pressure treated wood outside exposed to rain and cooler temperatures, so that further drying is limited. The consumer is then burdened with the warping and twisting that occurs as the wood continues to dry after purchase. Because the juvenile wood of Southern pine is so easily pressure treated with biocides, many marine structures have been installed with pilings of this wood throughout the world. A number of these have failed, not because of decay or marine borer attack, but because the weak juvenile wood cannot withstand the normal impact stresses imposed by docking ships. These problems have led to finding alternatives in either non-wood pilings or stronger, naturally durable wood from natural forests, such as Borneo ironwood (Eusideroxylon zwagerii), or, when available, Demerara greenheart (Ocotea rodiaei).
The presence of juvenile wood on only one surface ply of plywood can also lead to future warping, especially if during manufacture the plywood was produced to a moisture content standard that was higher than the equilibrium moisture content of the end user. The author has observed this problem with floor underlayment buckling and warping after installation during dry winter conditions in heated buildings in cold climates, particularly if the plywood had been manufactured in a warmer, more humid tropical country.
Dense hardwoods, especially those with low permeabilities, are often difficult to dry even when harvested from closed canopy natural forests (Siau, 1995; Henderson, 1951) but if this wood comes from rapid growth plantations, where internal stress differentials are enhanced, drying defects such as collapse (enhanced by the presence of tension wood), splitting and honeycombing are likely to be much greater (Bariska, 1992; Kauman, 1958).
Growth Rate, Soils and Durability
The natural resistance of processed wood to biodeterioration from insects, bacteria, fungi or marine borers depends primarily on chemicals produced during sapwood to heartwood conversion in certain tree species. Sapwood of any species is generally prone to attack by biodeteriorating agents. A few examples of species for which the heartwood has gained a high market valuation for resistance to biodeteriorating agents are: mahogany (Swietenia spp., Khaya spp.), teak (Tectona grandis), kauri (Agathis australis), redwood (Sequoia sempervirens) and cedars (Cedrus spp., Thuja spp., Chamaecyparis spp., Juniperus spp., etc) for decay resistance; longleaf pine (Pinus palustris) and bald cypress (Taxodium distichum) for termite resistance; and turpentine (Syncarpia glomulifera), greenheart (Ocotea rodiaei), and Borneo ironwood (Eusideroxylon zwagerii) for resistance against marine borers (Teredo navalis).
The ability of a tree to produce resins, terpenes, flavenoids and other chemicals that render heartwood toxic or unpalatable to biodeteriorating organisms depends on genetically controlled metabolic pathways and the availability of biosynthesis precursors. These precursor chemicals, as well as chemicals such as silica that are not further modified in the tree, are absorbed from the soil by tree roots. The chemistry of the soil is, therefore, critical for providing these chemicals. Soil pH and cation exchange capacity is also important because if the required chemicals are not in solution they cannot be absorbed by the roots. For example, soils under Pinus radiata trees in New Zealand often have lower pH, higher extractable aluminium and lower exchangeable calcium than the native soils prior to planting (Giddens et al., 1997).
Soils exposed to excessive rainfall, as is common in many parts of the wet tropics, may be rapidly leached of soluble chemicals, reducing the amount that can be absorbed by roots (Tiarks et al., 1998). Even under non-leaching conditions, but where moisture and tree spacing combine to accelerate wood production, the quantity of chemical precursors available at the sapwood/heartwood boundary may be reduced leading to the production of heartwood with reduced biodeterioration resistance. This ‘dilution affect’ can occur even with indigenous species planted on native soils where growth rate is greatly accelerated.
A further problem in intensively managed planted forests may be long-term depletion of soluble soil minerals after one or more harvests (Fox, 2000; Tiarks et al., 1998). Soil fertilization to ameliorate this problem and to maintain biomass production usually does not take into account loss of chemicals in the soil that are needed for producing optimum heartwood properties. Indicative of this problem is the greater variation seen in the heartwood of Swietenia macrophylla now that it is being harvested from a wider natural range as well as from planted forests all over the globe. Decay resistance of heartwood varies, with colour ranging from yellow to deep red, and black or grey spots, or chalky or black inclusions can also be found; and Fijian grown mahogany often has pocket rot or more serious heartrot (Mayhew and Newton, 1998; Lamb, 1966; Record and Hess, 1943).
Finally, it should be noted that trees have developed decay resistance in the heartwood not for our benefit as we use the wood for various products, but rather to fend off potential heart-rot pathogens. The greater propensity for certain species, such as some Eucalypts, Tectona grandis, Swietenia macrophylla, and, especially, Acacia mangium, to develop heart-rot in planted forest settings could be linked to the above cited factors that reduce heartwood extractives (Barber, 2004; Barry et al., 2004; Gales, 2002; Mayhew and Newton, 1998; See and Arentz, 1997).
Dimensional Stability and Chemical Resistance
For certain wood products a high level of dimensional stability is required. Examples include ship decking, musical instruments, scientific apparatus and tools (such as levels) for engineers, carpenters and masons. Teak and mahogany are woods with good dimensional stability. Resistance to caustic solution deterioration permits the use of some woods for vats or other chemical exposure situations. Redwood and some cedars are known for their chemical resistance.
The chemistry and quantity of extractives deposited in heartwood determine to a large degree the dimensional stability or chemical resistance of woods, and, as such, are subject to the same soil chemistry controls cited for bio-deterioration resistance. Growth rate can also be a factor as it contributes to the ‘dilution affect’ and, in denser woods, enhances internal stresses. In general, then, wood with wider rings may be less dimensionally stable, and may have lower chemical resistance.
The traditional uses of forest grown mahogany and teak in applications requiring good dimensional stability are being threatened by the recent substitution of rapidly grown plantation stock. Much teak planted outside its natural range is unsuitable for boat decking or garden furniture. Similarly, carpenters levels made from rapidly grown Swietenia macrophylla (or sometimes even Philippine ‘mahogany’) have led to warped or cracked tools. Plastics and metals are taking over much of this market, despite the fact that carpenters and masons prefer the better shock-absorbing properties of wood when a level is accidentally dropped.
Growth Rate and Sapwood/Heartwood Ratio
Sapwood in living trees provides the pathway for water transport from roots to photosynthesizing leaves. Many species, especially those that respond positively to increased moisture and wider spacing, may adjust the width of the sapwood to meet greater transpirational demand in an expanding crown (Waring et al., 1982). For some wood products wide sapwood is preferred; an example being the sapwood of ash for baseball bats. But if a species is being grown for the properties of its heartwood, such as teak (Tectona grandis), mahogany (Swietenia macrophylla) or kauri (Agathis australis), a wider sapwood will reduce heartwood volume at harvest (Steward and Kimberly, 2002). As previously noted, rapid growth that leads to wide sapwood may also induce the extractive ‘dilution affect’ in the heartwood. Teak (Tectona grandis) is an example of a species that responds positively to increased moisture by producing wider sapwood (Cordero & Kanninen, 2003). Similarly, sapwood of mahogany (Swietenia macrophylla) in rapid growth trees is wide (greater than 4-5 cm) compared to slower growing specimens (2.5 cm or less); and thinnings can have little value due to high sapwood proportion (Bulai, 1993; Streets, 1962). On the other hand, in black cherry (Prunus serotina), sapwood width is generally unresponsive to growth rate. In general, sapwood width in ring-porous hardwoods is somewhat more sensitive to growth rate (Panshin and deZeeuw, 1980).
Sapwood in a few species may have special attributes that enhance its product value. Mountain ash (Eucalyptus regnans) and several other Eucalypts growing in natural forests produce sapwood with very low starch content. Because of this the sapwood is highly resistant to Lyctus borer attack. Other Eucalypts that have higher sapwood starch contents are susceptible to Lyctid attack of wood products. Since this insect can attack soon after felling, it can prevent the export of logs or lumber to countries that have rigid sanitary import restrictions.
Climate Seasonality and Wood Properties
In the wet tropics, trees are often evergreen and photosynthesize year round producing wood on a continuous basis. In cooler, temperate climates or in monsoonal regions of the tropics, trees cease growth for a period each year, and this is often associated with the deciduous habit (periodic and simultaneous shedding of all leaves). Prior to the dormant period some tree species produce a different kind of wood – in conifers this is the denser latewood, triggered by shorter days and drier conditions (Hiller and Brown, 1967). Other trees produce a special wood when growth commences at the end of dormancy. Ring-porous hardwoods, such as oaks, produce large earlywood vessels at this time.
If seasonally adapted trees are planted in warmer regions where moisture and warm temperatures are available year round, they may lose some of the wood properties that are triggered by seasonality. Ring-porous woods, like oaks, that can adapt to tropical conditions can become diffuse-porous. For example oaks in Mexico and southern Japan are diffuse-porous. Monsoonal tropical species such as teak may also change when shifted to a non-seasonal environment. Conifers moved from seasonally warm temperate climates to the tropics (where daylength varies very little) may lose the capacity to produce a dense latewood. Pinus radiata exhibits this trait (Hiller and Brown, 1967). Each of these kinds of changes, as noted in earlier sections, can change wood properties.
Recent research suggests that, if seasonally adapted Acacia mangium are moved to conditions of continuous moisture, the incidence of heartrot increases (See and Arentz, 1997; Barry et al., 2004). This has become a major threat in a number of areas in Indonesia and Malaysia, where the majority of planted forests consist of this single species (Lee, 2002; Rimbwanto, 2002; Nair, 2000). This issue and its possible cause are discussed further under the insect and disease section below.
Genetics and Wood Quality
Just as different tree species growing in the same environment produce different kinds of wood, genetic strains or provenances of a single species can also respond differently to the same environment. This is the theoretical basis for provenance selection as a way of improving wood properties in trees. As a caveat it should be noted here that the majority of selection programs have focused on easily observable external tree features, notably height and volume growth, stem form and size and distribution of branches, with only small attention, if any, to wood quality issues (Evans, 2005).
Although some of the negative wood quality problems facing plantation species may be partially ameliorated by judicious provenance selection, this should not be viewed as a magic-wand panacea. Selection of different provenances for wood quality improvement may be coupled with a growth rate reduction, however, similar results may be more cheaply achieved through judicious silvicultural management.
In a study of five provenances of teak (Kjaer et al., 1999), the authors found considerable variation in silica content (0.27% to 66%) and heartwood percent, but concluded that they could not separate this variation into genetic or environmental effects. Like other studies that fail to use the ‘common garden’ approach, true genetic differences can be easily masked or falsely revealed, leading to unrealistic expectations for genetic tree improvement in planted forests. Model trial systems such as Danida (Denmark) and CAMCORE (USA) are necessary before investment in expensive provenances can be justified (Lauridsen, 2003; Dvorak, 2004).
Conservation of genetic resources should be another goal in planted forests. Past high-grading of natural forests through multiple harvesting rotations has led to the loss of some genetic potential, often leading to smaller, lower-quality trees. Salvaging what is left in seed banks or by other methods will help to stem further erosion of the gene pool and provide the greatest potential for developing valuable new provenances.
Insect and Disease Management
Planted forests, especially those that are monocultures, provide potential conditions for rapid population increases in insect pests or the spread of disease pathogens (Barber, 2004; Wingfield and Robison, 2004; Nair, 2000). Yet reduced growth or mortality attributable to biotic and abiotic causes can be less in properly managed planted forests than in natural forests (Gadgil and Bain, 1999). A well thought out plan of integrated pest management (IPM) is a necessary part of planted forest planning. Using a mixture of tree species, age classes or different genotypes may provide some control for insect and disease problems; and employment of targeted strategies such as overstory protection against apical shoot borers during early height growth can be effective (Barber, 2004; Tilakaratna, 2001; Mayhew and Newton, 1998). For more than a half century shootborers in the genus Hypsipyla have been major deterrents to managing trees in the Meliaceae family for wood production (FAO, 1958). Many of these Meliaceae, in the genera Swietenia, Khaya, Toona, Cedrela, Carapa, Entandrophragma and Lovoa, produce some of the most valuable wood in the world (Griffiths, 2001).
Although DDT was once the control measure of choice (FAO, 1958), biological control through canopy light management, often with different overstory species, is now considered to be a reasonably effective means of minimizing damage from Hypsipyla and has been effectively practiced for several decades in Sri Lanka (Mayhew and Newton, 1998). In the few places in the world where Hypsipyla borers are absent, such as the remote eastern Solomon Islands, Vanuatu, Fiji or Western Samoa, other insect and disease problems have arisen. In Fiji large numbers of trees develop pocket rot or severe heartrot, and pinholes caused by ambrosia beetles are very common (Mayhew and Newton, 1998). This problem has become so severe that market loss for Fijian mahogany has been predicted (Cown et al., 1989).
Pre-screening of soils for presence of pathogens or insects may avoid future problems, and this could be part of a general soil analysis. These and other plant protection strategies, including firebreaks or other control measures, and the expectation for the use of some chemical or biological controls should all be part of a comprehensive IPM program. Post-harvest measures may also be required, especially to protect recently felled logs from sap-staining fungi or insects, which can lower value or even lead to refusal by export markets (Kim et al., 2005).
The heartrot problem previously mentioned for Acacia mangium provides an interesting case study to examine in greater detail. This species is native to just three small islands in the Moluccas and parts of Irian Jaya in Eastern Indonesia (Pinyopusarerk et al., 1993), but has been planted widely in Indonesia (comprising more than 80% of planted forests) and Malaysia (Lee, 2002; Rimbwanto, 2002; Nair, 2000). Based on several studies, we know that the incidence of heartrot is greater in some areas than others. For example, it is greater in most of Malaysia than in Indonesia, but even within Indonesia the incidence can vary from 6.7% in East Kalimantan to 46.7% in West Java (Barry et al., 2004; Nair, 2000). In general the heartrot is greatest in areas that are continually moist with higher total rainfall than in areas that have less total precipitation and have a definite dry period (Barry et al., 2004; See and Arentz, 1997). See and Arentz (1997) have suggested that where rainfall is heavier and no seasonality exists, entry of decay fungi through dying branches might be enhanced. However, another explanation could be that heartwood extractives are reduced on the wetter sites due to greater soil leaching of precursors and the dilution effect, previously mentioned, for more rapidly growing trees. Evidence to support this theory can be found in the very recent research on heartwood extractives in affected and unaffected A. mangium, and comparisons with the more heartrot resistant Acacia auriculiformis (Barry et al., 2005; Mihara et al., 2005; Lange and Hashim, 2001). The take home lesson is to be very careful when establishing planted forests in areas that have different seasonality and precipitation regimes from the native habitat; the risks are often quite large.
2 The four basic wood types described here are illustrated in Figures 1 and 2, and can be seen directly on a smooth end surface of wood with or without slight magnification.
