Effect of controlling herbaceous and woody competing vegetation on wood quality of planted loblolly pine
Southern pine plantations are increasingly established using herbicides to control herbaceous and/or woody competing vegetation to enhance growth, but little is known about the effect on wood quality. A study was established at 13 southern locations in 1984 to examine the effects of complete control of woody, herbaceous, and woody plus herbaceous competition for the first 3 to 5 years on the growth and stand dynamics of loblolly pine (Pinus taeda L) plantations. After 15 years, herbaceous plus woody control increased pine merchantable volume per acre by an average of 23 to 121 percent compared to no competition control. Increment cores, 12 mm in diameter, were collected from 36 trees in each of the 4 treatments from each of the 13 locations. X-ray densitometry was used to determine annual growth, proportion of latewood, and specific gravity (SG) of earlywood, latewood, and annual rings. Woody plus herbaceous competition control significantly increased growth at all locations, did not significantly reduce ring SG of earlywood or latewood, and did not significantly affect proportion of latewood in the annual ring. Woody plus herbaceous competition control did significantly increase growth during juvenile wood formation in years 1 to 5 and thus increased the diameter of the juvenile wood core by an average of 19 percent. Cross-sectional weighted proportion of latewood decreased 10 percent and cross-sectional weighted SG decreased 3 percent as a result of increased growth during the juvenility period in trees receiving the woody plus herbaceous control treatment. However, growth gains substantially offset the slight reduction in percent latewood and SG.