Sustained productivity of intensively managed loblolly pine plantations: Persistence of fertilization and weed control effects across rotations

Two randomized complete block design experiments were used to study the effects of fertilization and weed control treatments on the productivity of second rotation loblolly pine (Pinus taeda L.) plantations growing on poorly-drained Spodosols in north Florida. One experiment (actively managed retreated) received similar treatments as in the first rotation (Control, C; Fertilizer, F; Fertilizer + weed control, FW; Weed control, W), and the second was left untreated in the second rotation [untreated carryover (C-): C-C, C-F, C-FW, and C-W]. Comparisons of total height and current annual increment across rotations indicated that the second-rotation stands were more productive than the first-rotation. In the current rotation of the untreated carryover experiment, treatments that received fertilizer in the first rotation (C-F and C-FW) accumulated significantly more aboveground biomass compared to the C-C treatment [i.e. C-F (63 Mg ha(-1)) = C-FW (60 Mg ha(-1)) > C-C (40 Mg ha(-1))]. From the third to the fourth year, biomass accumulation in C-F exceeded C-FW, but by the fifth to the seventh years the two treatments were similar; a change that likely occurred because of root development into the lower solum for the C-FW or increased understory competition in the C-F treatment. In the actively managed retreated experiment, cumulative total aboveground biomass accumulation followed the trend: FW (90.6 Mg ha(-1)) > F (71.8 Mg ha(-1)) > W (55.1 Mg ha(-1)) > C (31.8 Mg ha(-1)). Comparison of upper quartile height gains due to fertilization between the first- and second-rotation experiments suggested that fertilizer added in the second rotation only provided growth gains after the fourth year. Our results suggest that management practices enhanced levels of productivity across treatments and rotations with some adjustment caused by carryover effects from past fertilization and weed control treatments.