Impact of Weed Control and Fertilization on Growth of Eastern White, Loblolly, Shortleaf, and Virginia Pine Plantations in the Virginia Piedmont
The relative growth response of planted loblolly (Pinus taeda L.), shortleaf (Pinus echinata Mill.), Virginia (Pinus virginiana Mill.), and Eastern white (Pinus strobus L.) pines to intensive silvicultural practices such as woody competition control and fertilization in the Piedmont Province of Virginia is unclear. To address this issue, during 1999, a mixed stand of Virginia pine and hardwoods was clearcut and site-prepared by herbicide application. Three replications containing strips of loblolly, shortleaf, Virginia, and white pines were planted at a 3 m x 1.5 m spacing during February-June, 2000. Four different sources of loblolly pine seedlings were used. The strips were subsequently split across to accommodate four different silvicultural treatments: (1) check (no treatment); (2) woody vegetation control; (3) fertilization; and (4) weed control plus fertilization. The weed control treatment used two directed spray herbicide applications in 2001 (triclopyr and glyphosate) and 2003 (glyphosate) and one mechanical cutting of the remaining hardwoods in 2004. Fertilizer containing N, P, K, and S was applied in 2001, only N in 2002, and N plus P in 2004. Crop tree survival was highest for loblolly pine, decreased in shortleaf and Virginia pines, and Eastern white pine had the poorest survival through age 5. Fertilization without controlling the competing hardwoods decreased survival in all planted pines due to the increased hardwood competition. Loblolly pine was tallest (4.7 m) through the five-year period, shortleaf and Virginia pines were shorter (2.95 m and 3.06 m, respectively), and white pine was shortest (1.7 m). When fertilized, hardwoods were taller than white pine seedlings, almost equal in height to shortleaf and Virginia pines, but shorter than loblolly pine seedlings. Silvicultural treatments had no significant impact on tree height. However, the weed control treatment increased fifth-year pine DBH and stem volume, while fertilization did not. When applied in combination with weed control, there was no additional increase in crop tree DBH and stem volume due to fertilization beyond that from weed control only. The increased availability of soil nutrients due to harvesting the previous stands and allocating them to the crop trees by controlling the competing woody vegetation was enough to meet the nutrient requirements of the young seedlings up to this age.