Juvenileimature Wood Transition in Loblolly Pine as Defined by Annual Ring Specific Gravity, Proportion of Latewood, and Microfibril Angle

The length of juvenility or number of years a tree produces juvenile wood at a fixed height can be defined by the age of the wood at which properties change from juvenile to mature wood. This paper estimates the age of transition from juvenile to mature wood based on ring specific gravity (SG), proportion of annual ring in latewood, and ring average microfibril angle (MFA). The threshold method and the segmented modeling approach were used to estimate the age of transition. Twenty loblolly pine (Pirzus tnecln L.) plantations, 20-27 years old, were sampled across five physiographic regions in the southern United States. Increment cores were collected at 1.3 meters from 15 trees in each stand to determine ring specific gravity and proportion of latewood by X-ray densitometry and annual ring MFA by X-ray diffraction. Precisely determining the transition age between juvenile and mature wood was difficult because transition is gradual, not abrupt. The age of transition was found to differ by wood property because these properties mature at different rates due to genetic and environmental factors. Both the threshold and the segmented model approach showed that transition age varied among regions. Both approaches showed that length of juvenility based on ring SG was shorter in the South Atlantic and North Atlantic Coastal Plains (ranging from 5.5 to 7.9 years) compared to that in the Hilly Coastal Plain that ranged from 10.4 to 13.6 years. Using MFA to estimate the age of demarcation, both approaches showed the South Atlantic, Gulf Coastal. and Hilly Coastal Plains had shorter lengths of juvenility (ranging f r o ~ n 8.4 to 10.4 years) than thc Piedmont and North Atlantic Coastal Plain (ranging from 10.5 to over 20 years). Keyrijorzls: Juvenilelmature transition, P~IZLLS toe& L., specific gravity, percent latewood, MFA. 1 N T ~ o D U c T l o h : Greis 2002). In order to meet the growing demand for wood, it is estimated that by the year The South currently produces 60% of the 2040, 67% of the prodwtion in the wood harvested in the United States and 15% of south will come from intensively managed lot,the wood in the (Wealand lolly pine (Pitzus taeda L.) plantations. The high proportion of juvenile wood in intensively inani Member of SWST aged plantations makes plantation wood differClurk et ul.-JUVENILEIMATURE WOOD TRANSITION IN LOBLOLLY PINE 293 ent from that of pines harvested from older, slow-growing natural stands. Juvenile wood is produced by the young cambium in the live active crown and forms a cylinder of wood sur. rounding the pith that extends the length of the tree. The faster a tree grows during the first few years of a rotation, the larger the diameter of the juvenile core in the lower bole. Juvenile wood is low in stiffness and strength, and thus wood from fast-growing young trees is less desirable for solid wood products (Senft et al. 1985). It is important that the forest industry understand the impact of silvicultural practices on wood properties, and the volume of juvenile wood produced. However, to evaluate the impact of management practices on juvenile wood properties a thorough understanding of the age of transition between juvenile and mature wood is needed. Juvenile wood has a high proportion of earlywood-type tracheids and thus lower specific gravity (SG), thinner cell walls, wider microfibril angle (MFA), and less latewood than mature wood (Thomas 1984). Because of its tracheid characteristics, juvenile wood has significantly lower strength and stiffness, more longitudinal shrinkage, and less radial and tangential shrinkage than mature wood (Pearson and Gilmon 1971; Bendtsen 1978; Bendtsen and Senft 1986). It is important that the forest industry understand the impact of silvicultural practices on wood properties and the volume of juvenile wood produced. However, to evaluate the impact of management practices on juvenile wood properties, a thorough understanding of the age of transition between juvenile and mature wood is needed. To define the age of demarcation between juvenile and mature wood, researchers examine the rate of change of a given property in juvenile wood until the property reaches that of mature wood. Plots of annual ring SG at 1.3 m above ground over ring number from pith are significantly different in loblolly pine compared to Douglas-fir or western hemlock. In loblolly pine, ring SG is lowest for the first 2 -4 rings next to the pith in the crown-formed wood zone and then increases rapidly for the next 5 8 rings in the transition zone until it reaches the SG of mature wood (Clark and Saucier 1989). In Douglas-fir and western hemlock, ring SG is highest in the first 1 to 5 rings from the pith, then declines to a low point between rings 10 and 20, at which point it increases until leveling off in mature wood (Krahmer 1966; Megraw 1986; Jozsa 1998). In loblolly pine, MFA at 1.3 m is largest for 2 3 rings near the pith and then decreases in rings 412 and then levels off in mature wood. Researchers have found it difficult to define the age of demarcation between juvenile and mature wood because the change in wood properties is gradual and not abrupt, since wood properties do not mature at the same rate. The number of years the cambium of loblolly pine produces juvenile wood at a given height based on ring SG has been reported to range from 5 to 15 years (Zobel and McElwee 1958; Pearson and Gilmore 1980; Clark and Saucier 1989; Tasissa and Burkhart 1998; Mora et al. 2005). Researchers have reported that the length of juvenility in loblolly pine varies by geographic region (Clark and Saucier 1989; Tasissa and Burkhart 1998). Clark and Saucier (1 989) found the length of juvenility averages 5 to 8 years in the Atlantic Coastal Plain and 9 to 11 years in the Piedmont. Various methods have been used to determine the age of transition from juvenile to mature wood. The simplest method is the so-called threshold or graphic method where a value is selected from graphs to define when a property has reached that of mature wood (Clark and Saucier 1989; Jozsa 1998; Clark and Edwards 1999; Clark et a1 2005). Plots of a property over ring number from pith are visually evaluated to locate a ring number or age based on the property curve when the property reaches the threshold value for mature wood. A second method is to use linear segmented regression models (Loo et a1 1985; Szymanski and Tauer 1991 ; Sauter et al. 1999) or nonlinear regression models (Hodge and Purnell 1993; Tassisa and Burkhart 1998; Mora et al. 2005) to estimate the age of demarcation. An advantage of the threshold method is that mature wood can be defined as having properties of a minimum specified value that can 294 WOOD AND FIBER SCIENCE, APRIL 2006, V. 38(2) relate to the wood properties of a final product, whereas the segmented approach defines mature wood based on the rate of change of a property. This paper compares the threshold method and segmented modeling approach for estimating the transition age between juvenile and mature wood for loblolly pine based on annual ring SG, annual ring percent latewood, and annual ring MFA. MATERIAL AND METHODS Field and laboratory Twenty loblolly pine plantations, 20 27 years old, were sampled across five physiographic regions in the southern United States to determine annual ring SG and MFA (Fig. 1). The plantations sampled were planted at 1480 to 1980 trees per hectare (TPH) with nursery run seedlings. The stands sampled were thinned to 300 to 1200 TPH after age 15. No competition control or fertilization was applied at planting or during the rotation. Two increment cores, 12 mm in diameter, were collected at breast height (1.3 m) from 15 trees in each plantation. The sample trees were selected in proportion to the diameter distribution in each stand. One increment core from each tree was dried at 50°C for 24 h, glued to core holders, and sawn into 1.6mm-thick strips. Specific gravity of earlywood and latewood for each annual ring and radial growth of each ring were determined at 0.06mm intervals using an X-ray densitometer (Quintek Measurement systemsTM) with a resolution of 0.00001. A specific gravity value of 0.48 was used to distinguish earlywood from latewood. The densitometer was calibrated to express specific gravity on green volume and oven-dry weight basis. The second increment core was dried and shipped to silviscanTM in Australia for MFA determination. MFA was measured at 1-mm intervals from the pith to the