36 Drying of Wood

FIGURE 36.8 Tension wood in Pedunculate oak (Quercus rubra); note the existence of the G layer. (ESEM photograph:LERMAB–ENGREF.)wood formation lasts as long as living branches existat the corresponding height of the tree. The transitionfrom juvenile to mature wood takes place gradually.No clear demarcation exists between juvenile andadult wood.In juvenile wood the cells are smaller than those ofthe mature xylem. Particular differences exist in thelength of the cells as well as in the structure of thelayered cell wall. The microfibril angle in the S 2 layeris greater than in cells of the mature tissue. As forcompression wood, this causes a higher value of longitudinalshrinkage and a reduced tensile strength. Inaddition, the spiral grain (angle between the stem axisand the fiber orientation) is often large in the juvenilewood. Together with the high longitudinal shrinkage,this explains why the warping of timber containingjuvenile wood may be dramatic after drying. Juvenilewood is a major problem in processing wood fromplantations of fast-growing species (eucalyptus,radiata pine, etc.) that produce logs made up mostlyof juvenile wood.36.1.4 IMPLICATIONS FOR THE DRYING P ROCESSToinducetheascentofsapintree,themeniscipresentinthe leaf stomata pull up water (Zimmerman 1983).Because most trees are more than 10 m high, one candeduce that the absolute liquid pressure in the sapcolumn is negative. No gaseous phase can exist in suchconditions. The vascular system developed in trees hasmany other implications for the drying process:. Because the system is designed for longitudinalsap flow from the roots to the canopy, the woodmaterial is strongly anisotropic.. Because of negative pressure, the vascular systemmust be able to support a gas invasion dueto injury or cavitation. This is the role of borderedpits or vessel-to-vessel pits. These anatomicalfeatures may dramatically inhibit thefluid migration in the wood.. In heartwood, due to metabolite deposition, aspirationor closure of bordered pits, or tylosedevelopment, the permeability is often reducedby one or several orders of magnitude.. The wood is fully saturated in the sapwood partof logs (an air-free sap column is required toobtain negative pressures), whereas the heartwoodzone is generally only partly saturated.Table 36.3 indicates some orders of magnitudegenerally observed for the moisture content of greenwood.Indeed, because the sapwood part is fully saturated,the maximum moisture content in this zonecan be calculated by assuming that the entire porevolume is filled with water:X ¼ fr ‘(1 f)r swith f ¼ 1r 0r s(36:1)where f is the porosity, r 0 is the basic density (ovendrymass/green volume), r s is the density of the cellß 2006 by Taylor & Francis Group, LLC.