Response of mid-rotation loblolly pine (Pinus taeda L.) to fertilization and reduced water availability in southeastern Oklahoma

Abstract

Loblolly pine (Pinus taeda L.) is the most commercially important tree species in the southeastern United States and driver of timber productivity for the region. The positive of loblolly pine to forest management applications such as fertilization and competition control have led to such increases in productivity. However, a better understanding of impacts from a more variable climate on loblolly pine growth and productivity is needed. Across the southeastern United States, a shift towards a warmer and drier climate is predicted to occur. Increasing temperatures and more variable precipitation is expected to impact southern pine plantation growth and productivity. This impact is projected to stem from larger precipitation events with longer dry down periods which will ultimately lead to an increased intensity and duration of drought. Therefore, the interactive effects of fertilization and reduced water availability, as well as long-term moderate drought on a mid-rotation loblolly pine plantation in southeastern Oklahoma were studied. The first study examined the effects of nutrient availability and decreased soil water availability on leaf gas exchange, LAI, and stand growth. Results showed that fertilization increased productivity of throughfall reduction stands such that it was similar to ambient throughfall stands not receiving fertilization. Fertilization caused a reduction in stomatal conductance while net photosynthesis rates maintained, indicating increased water use efficiency. The second study examined the effects of nutrient availability and decreased soil water availability on stand-level water use efficiency of stem volume production by 18 %. The final study examined the effects of long-term moderate drought on leaf gas exchange, whole-tree water use, and individual tree growth. Results showed that throughfall exclusion did not significantly reduce leaf gas exchange, water use, or tree growth. However, throughfall exclusion significantly reduced leaf biomass. Results from all three studies indicate that fertilization can be beneficial in loblolly pine plantations experiencing reduced water availability on the western limits of its commercial range by increasing water use efficiency and that long-term moderate drought may cause reduced leaf area to conserve water rather than reducing leaf-level water use.