Physiology and Growth of a 6-year-old Loblolly Pine (Pinus taeda L.) Plantation in Response to Rain Exclusion and Fertilization Treatments
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The increase in atmospheric carbon dioxide (CO2) is expected to change climate in the southeastern United States. A hotter, drier climate is anticipated over the next century and could have detrimental effects on the productivity of southern forests. Loblolly pine (Pinus taeda L.) is the most widespread planted southern pine in the region and knowledge of how this species will respond to future climate is critical. To gain insight on how the growth of loblolly pine may be affected by changes in ambient precipitation, growth and physiological responses of a 6-year-old loblolly pine plantation to rainfall manipulation across a nutritional gradient were studied over a one year period. The experiment was a 2 x 2 factorial design with two levels of rainfall manipulation (ambient and 30% reduction) and two levels of fertilization (none and operational). Fertilization had the greatest influence on leaf area index (LAI) and intercepted photosynthetically active radiation (IPAR). During a drought in 2012, fertilization increased LAI by 17%, while the rain exclusion treatment decreased light saturated net photosynthesis (Pnet) and stomatal conductance (gs) by 12% and 21% respectively and increased stomatal limitations of Pnet. Average soil moisture and predawn water potential (ѰL) decreased in 2012 in the rain exclusion treatment. In 2013, a year with high ambient precipitation, fertilization increased LAI by 49%, while the rain exclusion treatment caused no reductions in leaf physiology. No interactions between rainfall manipulation and fertilization treatments were observed for LAI, IPAR, or leaf physiology. The primary effect of the fertilization treatment was on leaf area production and the primary effect of the rain exclusion treatment was on leaf-level physiology. Reductions in Pnet and gs in the rain exclusion treatment indicate that under future hotter, drier climates reduced carbon gain may occur. No interactive effects were observed between the rain exclusion and fertilization on LAI, IPAR and physiological processes, but the greatest gains in LAI in response to fertilization were observed in 2013 when drought was alleviated.