Fertilizer responses of longleaf pine trees within a loblolly pine plantation: separating direct effects from competition effects
Evidence is mixed on how well longleaf pine (Pinus palustris Mill.) responds to increased soil nitrogen via fertilization. We examined growth and physiological responses of volunteer longleaf pine trees within an intensive loblolly pine (Pinus taeda L.) fertilization experiment. Fertilizer was applied annually following thinning at age 8 years (late 1992) at rates tailored to maintain a high needle nitrogen concentration. We measured the annual growth of 20 volunteer longleaf pine trees per treatment. We took bimonthly gas exchange measures on 12 longleaf pine trees per treatment from June 1999 through June 2000, after which we estimated carbon isotope discrimination and foliar nitrogen concentration on foliage. The impact of fertilization in both growth and gas exchange was dependent on competition for light with neighboring loblolly pine trees. When fertilized longleaf pine trees were separated into categories with respect to being dominant or suppressed (relative to the loblolly pine trees), dominant-fertilized trees had the lowest carbon isotope discrimination increased photosynthesis, and decreased stomatal conductance, indicating greater water use efficiency in these trees. Compared to loblolly pine, longleaf pine growth is restricted less by poor soil nutrition. However, early rotation longleaf pine appears to have the potential to increase growth rate via fertilization to almost the same extent as loblolly pine.