The Effects of Fertilization and Four Years of Throughfall Reduction on Leaf Physiology of Loblolly Pine (Pinus taeda L.)

Abstract

Loblolly pine (Pinus taeda L.) is the most widely planted pine species in the southeastern United States. Increased frequency of drought associated with projected climate change may negatively impact loblolly pine productivity. The objective of this study was to evaluate the main and interactive effects of reduced water availability, achieved by throughfall reduction, and one-time fertilization on factors that affect loblolly pine productivity. Leaf-level physiology, and growth were monitored over the third and fourth year of treatment. After four years of throughfall reduction treatment, no interactive treatment effects on light-saturated net photosynthesis (Pnet), stomatal conductance (gs), or growth were observed. Neither fertilization nor throughfall reduction treatment effected growth efficiency, midday leaf water potential (ΨL), ΔΨL, or leaf-specific hydraulic conductance. Fertilization, regardless of throughfall reduction treatment, increased all growth variables except for height, increased radiation use efficiency of stemwood production (εstem), increased intercepted photosynthetically active radiation, and increased leaf area index. However, fertilization decreased Pnet and gs likely due to increased leaf evaporative demand associated with higher LAI. Throughfall reduction decreased LAI and, in 2014, throughfall reduction reduced the leaf area to sapwood area ratio (AL:AS) thereby increasing water supply per unit foliage resulting in no effect of throughfall reduction on leaf-level gas exchange. Conversely, both treatments decreased Δ13C discrimination indicating higher water use efficiency (WUE) and reduced gs in response to throughfall reduction treatment not detected by instantaneous leaf-level gas exchange measurements. The results of this study indicate that while fertilization can increase growth regardless of throughfall reduction, higher LAI with fertilization may increase water use and increase short-term leaf-level drought susceptibility. In a more drought prone future climate, management strategies that utilize fertilization to increase growth may result in diminished fertilization responses due to the impact of fertilization on leaf-level drought susceptibility. In order to maintain future productivity, management strategies such as density management may be required to reduce fertilization’s impact on water use.