Growth Response From Herbicide, Prescribed Fire, and Fertilizer Treatments in Midrotational Loblolly Pine: Fire-Year Response

This study was initiated to determine growth response resulting from the application of prescribed fire and herbicide, with and without fertilizatio.n. In. southeast Texas, herbicide, prescribed fire and fertilizer treatments were applied in m1d:rotat1onal loblolly pine plantations 1.5 years after thinning. Five replications were established at. each of two study sites located on similar soils, aspects and slopes. Half of each replication. was randomly selected and fertilized. Eight treatment plots were established 1n e~ch replication with one of each of the four treatments of control , herbicide, fire, and herb1c1de/flre randomly applied to fertilized plots and one of each of the four treatments randomly applied to non-fertilized plots. Pre-treatment measurements were taken in a 0.04 ha measurement plot nested within each treatment plot. A late season herbicide treatment of. lmazapyr and Arsenal was applied in October 1999. Burning was conducted in early spring of 2000 followed by fertilizer applications of diammonium phosphate and urea. ~osHreatment measurements were taken in December 2000. Growth response and s1gnif1cant treatment differences are presented in this paper. INTRODUCTION Loblolly pine (Pinus taeda) plantations often receive little or no treatment between the time of stand establishment and harvest (Nyland 1996). However, studies have shown the benefit of mid-rotation manipulation in terms of increased pine growth rate, improved species composition, and wood quality (Zutter and Miller 1998, Haywood and others 1998, Borders and Bailey 1997, Cain and Yaussy 1984). Intermediate treatments include release cuttings to improve species composition, the application of prescribed fire to remove competition and reduce crown fire hazard (Nyland 1996), the application of herbicides to re~ov~ competition (Haywood and others 1997), and fert1hzat1on to improve growth (Young and Giese 1992). Because loblolly pine is naturally found on low and moist sites, it has evolved with no special adaptation to fire in its early years (Wright and Bailey 1982). Therefore, the use of fire in loblolly pine stands is often limited to site preparation or competition control and fire hazard reduction at midrotation. Although loblolly pine is less fire resistant when young, as trees age, bark thickens (Villarrubia and Chambers 1978, Cooper and Altobellis 1969) resulting in a higher tolerance to moderate fires. In addition, sunlight deprived lower limbs will fall, causing the tree crown to be less accessible to damaging flames. Both of these factors increase the tolerance of loblolly pine to moderate fire (Wade and Lunsford 1988). Herbicides may be used as an intermediate treatment to remove competing woody vegetation, herbaceous vegetation, or both woody and herbaceous vegetation (Borders and Bailey 1997). Mid-rotational loblolly pine benefits from the removal of woody competition that severely limits its diameter growth and its ability to completely occupy a site (Hodges 1990). However, growth response may vary due to site quality, season of treatment, and type and density of competing vegetation (Lauer and Glover 1990, Hodges 1990). Herbicide and prescribed fire are often applied together as a mid-rotational treatment in loblolly pine stands (Borders and Bailey 1997). Fertilizer may be used to improve pine tree growth in midrotational loblolly pine plantations. Studies over the past 20 years have shown increases in tree growth due to the use of fertilization at mid-rotation (Allen and others 1983, Gent and others 1986). Fertilization may also be best used at mid-rotation when the stand has filled most of the growing space and more nutrients are becoming tied up in living and dead plant material (Smith 1986). Fertilization alone may result in a shift toward competing vegetation, causing increases in pine mortality (Borders and Bailey 1997). It is possible that the addition of fertilizer may result in further reductions in the thickness of loblolly's already moderately protective bark (Tiarks and Haywood 1993). Growth response to fertilization may vary from site to site depending on pre-treatment soil conditions such as nutrients, soil type, and water availability (Borders and Bailey 1997). Chemical herbicide control of competing vegetation may be 'Graduate Student, Arthur Temple College of Forestry, Stephen F. Austin State University, Nacogdoches, TX; Associate Professors, Arthur Temple College of Forestry, respectively. Citation for proceedings: Outcalt, Kenneth W. , ed . 2002. Proceedings of the eleventh biennial southern silvicultural research conference. Gen. Tech. Rep. SRS-48. Asheville , NC: U.S. Department of Agriculture, Forest Service , Southern Research Station. 622 p.