Two-year throughfall and fertilization effects on leaf physiology and growth of loblolly pine in the Georgia Piedmont

Loblolly pine is the most extensively planted and productive commercial pine species in the southeastern United States, but future increases in drought frequency and intensity may affect plantation productivity. To better understand the potential impacts of reduced precipitation on loblolly pine productivity, the interactive effects of throughfall (ambient versus an approximate 30% reduction) and fertilization (no fertilization versus one time fertilization with 224kgNha^-1, 28kgPha^-1, 56kg Kha^-1, and micronutrients) on growth, leaf area index (LAI), intercepted photosynthetically radiation (IPAR) and leaf-level physiology in a 7-year-old loblolly pine plantation were studied over two years. During a year with below average annual precipitation (831mm), the throughfall reduction treatment decreased predawn leaf water potential, leaf gas exchange rates, needle elongation, basal area increment and stemwood production, and effects were independent of fertilization treatment. Leaf light-saturated net photosynthesis and stemwood production were on average 12% and 13% lower, respectively, in response to reduced throughfall treatment. During the following year with higher precipitation (1413mm), reduced throughfall treatment decreased only leaf water potential. One-time fertilization had the largest impact on growth through increases in LAI, IPAR and total absorbed photosynthetically active radiation rather than changes in growth efficiency. Enhancement of growth by fertilization was independent of the reduced throughfall treatment in both years.