Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term
exposure to elevated CO2 concentration
Chris A. Maier (1, 2), Sari Palmroth (3) and Eric Ward (3)
1. Southern Research Station, USDA Forest Service, 3041 Cornwallis Road, Research Triangle Park, NC 27709, USA / 2. Corresponding author () / 3. Nicholas School of the Environment and Earth Sciences, Box 90328, Duke University, Durham, NC 27708, USA / Received May 11, 2007; accepted October 9, 2007; published online February 1, 2008
Summary
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 (Am), 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 Aa 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.
