Inter-Rotational Effects of Fertilizer and Herbicide Treatments on the Understory Vegetation Community in Juvenile Loblolly Pine (Pinus taeda L.) Stands

Abstract

Two randomized, complete block design experiments were established in north Florida to examine the inter-rotational effects of fertilization and herbicide applications on understory community responses in 2-year-old Pinus taeda L. stands. One experiment was left untreated (carryover [C]; CC, CF, CW, and CFW) and the second received the same first-rotation treatments—control (C), fertilizer (F), herbicide (W), and fertilizer and weed control (FW)—in the second rotation. In both experiments, herbicide applications alone and when combined with fertilization suppressed woody species such as Ilex glabra (L.) A. Gray and Serenoa repens (W. Bartram) Small (CFW: 0.26; FW: 0.01 Mg ha−1 biomass) but favored graminoids such as Andropogon virginicus L. var. glaucus Hack. and Dichanthelium acuminatum (Sw.) Gould & C.A. Clark (CFW: 3.1; FW: 1.1 Mg ha−1 biomass). Although the CW (0.99) and W (0.99) treatments did not affect understory diversity (H′), the CFW (0.45) and FW (0.5) treatments exhibited reductions compared with the CF (1.24) and F (1.33) treatments, respectively. In both experiments, fertilizer alone did not affect understory composition and diversity compared with the control. These results suggest that intensive additions of either herbicide alone or in combination with fertilization affected understory composition and diversity in the subsequent rotation. Management and Policy Implications The maintenance of long-term site productivity and associated ecological functions are central tenets for developing sustainable forest management systems. In addition to the overstory, the composition and diversity of understory vegetation represents an important component of forest ecosystems. Understanding how forest practices affect the inter-rotational development of understory vegetation is critical for advancing future management systems used in southern pine plantation ecosystems because of possible impacts, for example, on species diversity and nutrient cycling processes. This study examined how fertilizer additions and herbicide treatments affected understory redevelopment in second-rotation juvenile loblolly pine stands growing in north Florida. One experiment remained untreated to examine carryover effects of previous treatments whereas the second was actively managed using the same treatments as the prior rotation. The results for both experiments showed that intensive weed control treatments applied alone or in combination with fertilizer additions suppressed woody understory species development and favored graminoid vegetation (grasslike morphology) early in the next rotation. However, fertilizer additions alone did not affect understory composition and diversity when compared with the untreated control. When making forest management decisions in loblolly pine plantations, land managers should recognize that historical and intensive weed control treatments can have inter-rotational effects on understory species richness, diversity, and composition.