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. Land managers are using herbicide treatments to increase growth of southern pine plantations by controlling competing vegetation. Research has shown that herbicide applications during the first few years after establishment can reduce herbaceous and woody competition and thus significantly increase southern pine growth because of additional moisture and nutrients made available (Cain and Mann 1980, Nelson et al. 1981, Zutter et al. 1986, Glover et al. 1989). However, little information is available on the effects of competition control and rapid early growth on the wood properties. To determine the effect of near complete herbaceous, woody, and herbaceous plus woody competition control for the first 3 to 5 years on loblolly pine (Pinus taeda L.) growth and stand dynamics a south-wide study was established by a group of investigators with the forest industry, universities, and USDA Forest Service, Southern Research Station. This south-wide study was established in 1984 and called the Competition Omission Monitoring Project or COMP. This research project employed a unified protocol of four early competition control treatments over a wide range of physiography, topography, and commonly occurring soils (Miller et al. 1991, 2003a, 2003b; Zutter et al. 1995; Zutter and Miller 1998). COMP study results at age 15 (Miller et al. 2003a) showed that with near-complete herbaceous and woody control, pine merchantable stem volume reached 2,350 to 4,415 ft/ac by site compared to 1,132 to 2,965 ft/ac for the sites receiving no vegetation control. This represents a 23 to 121 percent increase in pine stem growth at age 15 and Miller et al. (2003a) reported that gains increased as hardwoods and shrubs increased on no control plots. Individual COMP pine tree data The authors are, respectively, Wood Scientist, USDA Forest Serv., Southern Research Sta., Athens, GA ([email protected]); Professor, Warnell School of Natural Resources, Univ. of Georgia, Athens, GA ([email protected]); and Research Ecologist, USDA Forest Serv., Southern Research Sta. ([email protected]. us).This paper was received for publication in July 2004. Article No. 9915. ✳Forest Products Society Member. ©Forest Products Society 2006. Forest Prod. J. 56(2):40-46.