Early growth responses of slash and loblolly pine following fertilization and herbaceous weed control treatments at establishment

Data from 21 regional experiments, established using a common study design, were analyzed to examine 5 and 8 yr growth responses and interactions between fertilizer and herbaceous competition control treatments applied to newly established plantations of slash (Pinus elliottii Engelm. var. elliottii) and loblolly pine (P. taeda L.) in the southeastern lower Coastal Plain. Eleven experiments were with slash pine and the remaining 10 with loblolly pine. The study sites were predominantly found on CRIFF (Cooperative Research in Forest Fertilization, Univ. of Florida) A, B, C, D, and E group soils. The four treatments examined were (1) Control, no treatment; (2) Fertilizer (Fert) only, 250 lb/ac diammonium phosphate; (3) Herbaceous weed control (Herb) only, 1 quart/ac Velpar (hexazinone) + 4 oz/ac Oust (sulfometuron methyl); and (4) Fertilizer + Herbaceous weed control. Fifth-year survival rates for slash pine averaged 91% among all sites and were not significantly affected by silvicultural treatments or CRIFF soil groups. Loblolly pine survival rates were similarly unaffected by the Fert and Herb treatments; however, loblolly pine survival was lowest on CRIFF C group soils and most likely attributable to establishment practices rather than soil differences. The percentages of loblolly pine sites exhibiting significant volume responses at age 5 yr were 60% for Fert only, 40% for Herb only, and 100% for the Fert + Herb treatment; corresponding treatment responses for slash pine were 36%, 45% and 64%, respectively. In the majority of tests (76%, both species), the Fert and Herb treatments were additive in nature, suggesting that each treatment was independent relative to its effects on volume growth. After 8 yr, loblolly pine volume responses for the Fert + Herb treatment over the control averaged 100% on CRIFF A group soils (1430 ft(3)/ac vs. 713 ft(3)/ac) and 52% on CRIFF C and D group soils (688 ft(3)/ac vs. 452 ft(3)/ac); corresponding treatment responses for slash pine on similar soils averaged 29% (645 ft(3)/ac vs. 499 ft(3)/ac) and 10% (805 ft(3)/ac vs. 733 ft(3)/ac), respectively. The early growth responses due to treatment declined between ages 5 and 8 yr, especially for the Herb only treatment with loblolly pine. Results from these studies suggest that growth responses were generally commensurate with management intensity, except for slash pine on sites with good inherent growth potential. Additional silvicultural inputs, in the form of midrotation fertilizer applications, would be necessary to sustain the early growth benefits afforded by the time of planting fertilizer and herbaceous weed control treatments, especially with loblolly pine on comparable sites.