Effects of nitrogen on Pinus palustris foliar respiratory responses to elevated atmospheric CO2 concentration

Indirect effects of atmospheric CO₂ concentration CO₂], onlongleaf pine (Pinus palustris Mill.) foliage respiration werestudied by growing trees in a factorial arrangement of low andhigh CO₂] (369 and 729µmol CO₂ mol^–1) and low andhigh N (40 and 400 kg ha^–1 yr^–1). Direct effectsof CO₂] on leaf respiration were tested by measuring respirationrates of foliage from all treatments at two CO₂ levels (360and 720µmol CO₂mol^–1) at the time of measurement.Elevated CO₂ did not directly or indirectly affect leaf respirationwhen expressed on a leaf area or mass basis, but a significantincrease in respiration per unit leaf N was observed in treesgrown in elevated CO₂] (indirect response to elevated CO₂]).The lack of a CO₂] effect on respiration, when analysed onan area or mass basis, may have resulted from combined effectsof CO₂] on factors that increase respiration (e.g. greateravailability of non-structural carbohydrates stimulating growthand carbon export from leaves) and on factors that decreaserespiration (e.g. lower N concentration leading to lower constructioncosts and maintenance requirements). Thus, CO₂] affected factorsthat influence respiration, but in opposing ways. Key words: Pinus palustris, elevated CO₂, nitrogen, foliar, respiration