Effects of Intensive Silvicultural Treatments on Kraft Pulp Quality of Loblolly and Slash Pine

Intensive forest-management practices have been shown to greatly increase the growth rates of southern pines. A joint study was undertaken to evaluate the wood and pulp quality from fast-grown 14,year-old loblolly pine from the Piedmont and 17-year-old slash pine from the Coastal Plain. The properties were compared to 24-year-old plantation-grown controls. Three sets of chip samples, sawmill residual, top log, and whole stem, were prepared from each wood source. Two unbleached pulp grades, linerboard at kappa no. 90 and sack kraft at kappa no. 60, were prepared from each chip sample, and the pulp yields, fiber lengths, and handsheet properties were compared. A bleachable grade pulp, kappa no. 30, was also made and the pulp yields and fiber lengths measured. Within each chip type, there was not a significant adverse effect on pulp yield from the intensive management practices for the unbleached grades. Chips from the top log samples with a high proportion of juvenile wood had significantly lo-wer packed bulk density and lower pulp yields than pulps from the sawmill residual chips. For the unbleached grades, there was no overall signiftcant negative impact on handsheet strength from the intensive management practices. Pulps from the top wood chips had higher burst, tensile, and compression strength but lower tear. For the bleachable grade pulps, there was some loss in pulp yield for the for the accelerated-growth loblolly pine pulps while the accelerated-growth slash pine exhibited no loss in pulp yield compared to the control. INTRODUCTION The pulp and paper industry in the Southeastern U.S. is facing increased competition from imported products while paying higher stumpage fees for southern pine timber because of reduced acreage for timber production along with an increasing demand. Foresters are using more intensive management practices to increase the yield of wood fiber from a shrinking land base. These management practices include competition control, fertilization, and shorter rotations to accelerate growth. The typical loblolly pine plantation in the Southeastern U.S. produces 4.9 to 8.0 m3/hectare/year. Research has shown that with intensive management practices and genetically improved stock, growth rates can be increased 15 to 25 m3/hectare/year. These growth rates compare well with the fastest growing loblolly pine anywhere in the world.’ Accelerated growth and shorter rotations will produce a higher percentage of juvenile wood in the harvested timber which can have a major impact on pulping and papermaking operations. Juvenile wood of southern pines has a lower specific gravity, higher moisture content, less cellulose, and more hemicellulose and lignin2 This can result in lower packing densities in digesters and reduced pulping yield.3Y4 Fibers from juvenile wood are shorter and have thinner cell walls.’ These juvenile fibers have some benefits with increased sheet bonding and higher tensile and burst properties but lower tear strength! A joint study by the University of Georgia, the Institute of Paper Science and Technology, the USDA Forest Service, and the Rayonier Specialty Pulp Products Company along with collaboration from industry partners was undertaken to evaluate the wood and pulp quality from fast-grown loblolly and slash pine. The effects of planting density [988,1,483,1,977 TPH (trees/hectare)] and herbaceous competition control of 14-year-old loblolly pine in the Piedmont and herbaceous competition control and fertilization of 17-year-old slash pine in the Coastal Plain on wood properties, pulp yield, and paper properties were examined. The properties of the young trees were compared to that of a 24-year-old loblolly plantation in the Piedmont and a 24-year-old slash pine plantation in the Coastal Plain. WOOD GROWTH AND CHIP QUALITY The effects of the management practices on the growth and wood properties have been reported previously? Wood samples were quantified as to annual growth increments, percent latewood per growth ring, and the proportion of juvenile wood. A summary of the wood growth is shown in Table I. Controlling competing vegetation dramatically accelerated the growth of both the 14-year-old loblolly pine and 17-year-old slash pine. The mean annual growth increment (MN) of the 14-year-old loblolly pine was 61 to 71% higher than that of a 24-year-old control. For the 17-year-old slash pine, the MAI increased 106-125% compared to a 24-year-old control. Table I. Wood Growth by Treatment for Loblolly and Slash Pine Plantations Loblolly 1,483 TPH, age 14 1 1,322 1 20.1 1 280 I 19.2 1 Loblolly 1,977 TPH, age 14 1 1,730 1 18.3 [ 298 1 20.2 1 ~ Loblolly control, age 24 I 509 1 2812 ] 303 I 11.8 1 Slash herbicide, age 17 1 1.139 1 17.8 1 198 I 10.1 I Slash herbicide & fertilizer, age 17 1 1,189 1 18:3 T215 I 11.0 1 Slash control, age 24 1 927 I 16.8 I 137 I 4.9 1 The proportion of juvenile wood in the saw log stem volume is shown in Figure 1. The loblolly and slash pine controls have 10 to 25% less juvenile wood than the accelerated-growth trees. The Coastal Plain slash pine had significantly less juvenile wood than the Piedmont loblolly pine but this is due more to differences in summer rainfall and length of growing season than a species difference.8 Loblolly Loblolly, 988 Loblolly, Loblolly, Control 1,483-rPl-l 1,977lPl-l Slash Control SIash , Herb. Slash, Herb/FM. Figure 1. Proportion of Saw Log Stem Wood Classified as Juvenile Wood Three types of chips were prepared from selected trees from the growth plots. These were sawmill residue chips (28 cm to 15 cm top) , top log chips (15 cm to 5 cm top), and whole stem chips. The grade, yield, and strength of the lumber produced from the study trees was also determined and will be reported separately.’ The packed bulk density of the chips and the wood disk specific gravity are shown in Table II. The top log chips are 12.20% lower in packed bulk density, and the whole stem chips are 7-15% lower than the sawrnill chips. Within each chip type, however, samples from the intensively managed stands had either higher or comparable bulk densities. For the loblolly samples, there was little difference in the specific gravity among the treatments within each chip type and between the sawrnill and whole stem samples. The specific gravity of the top wood samples was 9-12% less than the sawmill residual samples. The specific gravity of the slash pine samples from the managed stands was 5-8% lower than the control, while the top wood had a specific gravity 12-15% lower than the sawmill samples. Table II. Packed Bulk Density (kg/m3) and Specific Gravity of Chip Samples Loblolli 1,483 TPH LoblolIy 1,977 TPH Loblolly control Slash herbicide Slash herbicide & fertilizer Slash control 162.1 0.43 128.6 0.39 141.0 0.42 155.9 0.42 137.4 0.38 151.8 0.43 153.9 0.44 120.8 0.39 131.4 0.41 193.6 0.53 160.2 0.46 177.6 0.52 189.3 0.53 156.6 0.45 175.3 0.52 197.2 0.56 156.2 0.49 179.2 0.55 KRAFF PULPING UNBLEACHED GRADES Kraft pulping of the chip samples for the unbleached grades was done at the IPST laboratory. Two grades were targeted, a sack kraft grade at kappa no. 60 and a linerboard grade at kappa no. 90. Actual batch pulping experiments resulted in 150 pulps covering a range of kappa nos. from 37 to 124. Fewer cooks were completed for the whole stem chips due to project timing. Pulping Yield The total yield and kappa no. results from all the cooks were subjected to multiple regression analysis. The predicted total yields at each of the two target kappa nos. of 60 and 90 are shown in Table III. The young loblolly samples had the same or higher yields for the sawmill and top log chips compared to the control. The loblolly 988 and 1,977 TPH whole stem chips appeared to result in lower yields, but this is based on only a few cooks that were completed for these samples. The slash pine herbicide sample had a somewhat lower yield than the slash control. The top log samples resulted in lower yields overall because of the higher proportion of juvenile wood. In general, there does not appear to be a significant yield penalty for the accelerated-growth younger trees compared to the controls within each chip type. Table III. Predicted Total Yields for Unbleached Grades LoblolIy 988 TPH Loblolly 1,483 TPH 49.8% 53.4% 47.8% 51.4% 47.8% 51.5% 49.8% 53.4% 47.8% 51.4% 49.8% 53.4% LoblolIy 1,977 TPH 49.8% 53.4% 48.8% 52.5% 46.7% 50.3% Loblolly control 49.8% 53.4% 47.8% 51.4% 49.8% 53.4% Slash herbicide 49.8% 53.4% 47.8% 51.4% 49.8% 53.4% Slash herbicide & fertilizer 51.2% 54.9% 47.8% 51.4% 51.6% 55.2% Slash control 50.8% 54.9% 48.8% 52.9% 50.8% 54.9% The whole stem pulps at the higher kappa nos. had considerably more rejects after refining and screening, typically 8-15% of the dry chip charge. The chip size of the whole stem samples was more variable than the other chip samples with a number of larger chips. The sawmill and top log chips had screen rejects of l-5% at the higher kappa nos. Unbleached Kraft Pulp Quality Pulp samples at the target kappa nos. of 60*3 and 9Ok4 were analyzed for fiber length distribution and handsheet strength properties. Fiber Length The length-weighted average fiber lengths are shown in Figure 2 for the loblolly pine samples and in Figure 3 for the slash pine samples. The fiber length is somewhat lower for the kappa 60 pulps than the kappa 90 pulps due to the greater degree of delignification. For the loblolly samples, the 988 TPH sample has the shortest fiber length while the 1,977 TPH sample is nearly equal to or greater than the control. There is not a significant difference among treatments for the slash pine samples within a given chip type. For both the loblolly and slash pine, the top log samples have average fiber lengths that are 12-20% shorter than the sawmill samples while the whole tree samples that were completed are similar to or somewhat longer than the top log samples. cj sacit kraft 60 ..-~--.--.-.-..-------~ ---.--. .Kqw m---w; H Linerboard Kappa 90 .-------~------. --.” . . .-.--i Control, 988 1,483 1,977 Control, 988 1,483 1,977 Control, 988 1 ,gn sawnill TF+& lHA, FH, m= w w lPH, whole TPH, whole saw mill saw till saw till ws ws ws WhOk Figure 2. Length-Weighted Average Fiber Lengths for Loblolly Pine Unbleached Grades