Grade distribution and drying degrade of sweetgum and yellow-poplar structural lumber

The fact that the supply of southern pine timber is changing to include more lower quality plantation stock may provide incentive for utilizing lower density hardwoods for structural lumber. Yellow-poplar and sweetgum are potential substitutes for southern pine. A major problem in utilizing soft hardwoods for structural lumber is the difficulties associated with drying. A study of the grade distribution and drying degrade of yellow-poplar and sweetgum structural lumber was completed. The results indicate that grade distribution was much higher for yellowpoplar than for sweetgum, and grade loss after drying was lower for sweetgum. Proper drying is especially critical for reducing degrade of hardwoods. Dwindling reserves of high quality pine sawlogs may result in shortages of machine stress rated (MSR) lumber in the future. Yellow-poplar and sweetgum are potential substitutes for pine for use as structural lumber. Both species have “clear wood” strength properties comparable to southern pine(6). Currently, stand inventories are abundant and growth is exceeding removals(7). Distribution of yellow-poplar and sweetgum stands is sufficient to furnish current mills with an adequate supply of wood at least to the year 2000 (7). The use of yellow-poplar as dimension lumber has been investigated (1,4). Relatively little quantitative data are available on the use of sweetgum as structural lumber. The characteristics of interlocked grain, abundant knots, and high volumetric shrinkage would suggest that sweetgum may be difficult to utilize for structural lumber on a commercial basis. Sweetgum is listed as a moderately difficult species to dry (6). This study was part of a comprehensive study (2) to investigate the utilization of sweetgum as structural lumber. An important aspect of its utilization is to determine expected grade distribution and degrade in the drying process. Approximately 23 thousand board feet (MBF) of sweetgum and yellow-poplar structural lumber (2 by 4, 2 by 8,12 ft. long) was manufactured at a mill in the North Carolina piedmont region. Grade distribution based on defects (knots, splits, slope of grain etc.) and warp (bow and crook) was evaluated from this lumber. After drying, all lumber was regraded for warp and splits. Materials and procedures Timber was selected from stands of mixed hardwoods and stands containing both mixed hardwoods and softwoods. The intent was to select trees in a similar fashion to most commercial harvesting operations. Logs were bucked to nominal 12-foot lengths in the woods and transported to a modern hardwood mill for processing. Breakdown of the log is illustrated in Figure 1. A pithcentered cant (nominal 8 by 8 in.) was produced by the primary headrig (bandsaw). The cant was reduced by double arbor resaw into four 2 by 8’s. Two 2 by 4’s were produced by ripping a 2 by 8 on a linebar resaw. Sideboards were retained for higher grade furniture lumber and were not included in the tally. Again the intent was to process the logs in a manner similar to a pine dimension mill. The hardwood lumber was graded for defects and warp by a certified lumber grader. These grades were independent of one another. The defect grade accounted for knots, splits, and slope of grain, which affect strength properties. The warp grade accounted for bow, cupping, and crook, which affect its utilization for structural lumber. Lumber was graded according to pine grading rules (5) to determine their applicability to hardwood structural lumber and reference grade distribution to southern pine. The sweetgum and yellow-poplar lumber was dried in a steam-heated kiln according to a nominal 8/4 redgum The author is .4ssistant Professor, School of Forest Resources, Univ. of Georgia, Athens, GA 30602. Gratitude is expressed to the Georgia-Pacific Corp. for donation of lumber used in this study. This study was funded through a cooperative agreement between the USDA Forest Serv. and the Agri. Expt. Sta. at the Univ. of Georgia. This paper was received for publication in August 1989. © Forest Products Research Society 1990. Forest Prod. J. 40(5):18-20.