Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO₂ concentration

Abstract

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 (N_l) on a mass and an area basis, light-saturated net photosynthesis per unit leaf area (A_a) and per unit mass (A>sub>m), and leaf conductance (g_L) began after the second fertilizer application in existing 1-year-old foliage (F_O) and later in developing current-year first-flush (F_C1) and current-year second- flush (F_C2) foliage. Elevated [CO₂] increased A_a by 43 and 52% in F_O and F_C1 foliage, respectively, but generally had no significant effect on any other parameter. Fertilization had little or no significant effect on L, LMA, A or g_L in F_O foliage; although N_l was significantly higher in fertilized trees by midsummer. In contrast, fertilization resulted in large increases in L, N_l, and A in F_C1 and F_C2 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:N_l relationship but did not significantly affect the slope of the relationship in either foliage age class.

Keywords

FACE, global change, Jmax, nitrogen availability, photosynthetic capacity, Pinus taeda, stomatal conductance, Vcmax

Citation

Maier, Chris A.; Palmroth, Sari; Ward, Eric. 2008. Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO₂ concentration. Tree Physiology, Vol. 28: 597-606