Following the measurements of gas exchange, the shoots were carefully removed from the cuvette, harvested, placed in plastic bags and refrigerated (−20 °C). Projected needle area was measured using WinSeedle (Version 2007 Pro, Regent Instruments, Québec, Canada). Samples were then oven-dried for 72 h at 56 °C and their dry mass was determined. Specific leaf area (SLA) was calculated as the ratio of projected needle area (cm2) to needle dry mass (g). Dried needles were ground to a fine powder in a ball mill. Needle nitrogen concentration (Nmass, mg g−1) was determined using a LECO elemental analyser (LECO Corporation, St Joseph, MI, USA). Nitrogen on a projected area basis (Narea) was calculated as Nmass divided by SLA.