Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO2 concentration
We examined effects of a first nitrogen (N) fertilizer application on upper-canopy needle morphology and gas exchange in ~20-m-tall loblolly pine (Pinus taeda L.) exposed to elevated carbon dioxide concentration ([CO2]) for 9 years. Duke Forest free-air CO2 enrichment (FACE) plots were split and half of each ring fertilized with 112 kg ha–1 elemental N applied in two applications in March and April 2005. Measurements of needle length (L), mass per unit area (LMA), N concentration (Nl) on a mass and an area basis, light-saturated net photosynthesis per unit leaf area (Aa) and per unit mass (A>sub>m), and leaf conductance (gL) began after the second fertilizer application in existing 1-year-old foliage (FO) and later in developing current-year first-flush (FC1) and current-year second- flush (FC2) foliage. Elevated [CO2] increased Aa by 43 and 52% in FO and FC1 foliage, respectively, but generally had no significant effect on any other parameter. Fertilization had little or no significant effect on L, LMA, A or gL in FO foliage; although Nl was significantly higher in fertilized trees by midsummer. In contrast, fertilization resulted in large increases in L, Nl, and A in FC1 and FC2 foliage, increasing a by about 20%. These results suggest that, although both needle age classes accumulate N following fertilization, they use it differently— current-year foliage incorporates N into photosynthetic machinery, whereas 1-year-old foliage serves as an N store. There were no significant interaction effects of elevated [CO2] and fertilization on A. Elevated [CO2] increased the intercept of the A:Nl relationship but did not significantly affect the slope of the relationship in either foliage age class.