EFFECTS OF NITROGEN ON PINUS-PALUSTRIS FOLIAR RESPIRATORY RESPONSES TO ELEVATED ATMOSPHERIC CO2 CONCENTRATION

Indirect effects of atmospheric CO2 concentration [CO2], on longleaf pine (Pinus palustris Mill.) foliage respiration were studied by growing trees in a factorial arrangement of low and high [CO2] (369 and 729 mu mol CO2 mol(-1)) and low and high N (40 and 400 kg ha(-1) yr(-1)). Direct effects of [CO2] on leaf respiration were tested by measuring respiration rates of foliage from all treatments at two CO2 levels (360 and 720 mu mol CO2 mol(-1)) at the time of measurement. Elevated CO2 did not directly or indirectly affect leaf respiration when expressed on a leaf area or mass basis, but a significant increase in respiration per unit leaf N was observed in trees grown in elevated [CO2] (indirect response to elevated [CO2]). The lack of a [CO2] effect on respiration, when analysed on an area or mass basis, may have resulted from combined effects of [CO2] on factors that increase respiration (e.g. greater availability of non-structural carbohydrates stimulating growth and carbon export from leaves) and on factors that decrease respiration (e.g. lower N concentration leading to lower construction costs and maintenance requirements). Thus, [CO2] affected factors that influence respiration, but in opposing ways.