Leaf dark respiration as a function of canopy position in Nothofagus fuscatrees grown at ambient and elevatedCO(2) partial pressures for 5 years

1. Mass-based and area-based rates of respiration, leaf nitrogen content, l eaf total protein content, non-structural carbohydrates and leaf mass per u nit area (LMA) all decreased with depth in the canopy of Nothofagus fusca ( Hook. F.) Oerst. (Red beech) trees grown for 5 years at ambient (36 Pa) or elevated (66 Pa) CO2 partial pressures. 2. Elevated CO2 partial pressure had a strong effect on dark respiration, d ecreasing both mass-based and area-based rates at all canopy positions, but had little or no effect on leaf physical and biochemical properties. 3. Leaf sugars, starch, protein, N and LMA were all correlated with respira tion rate, and are therefore strong predictors of area-based dark respirati on rates. The y axis intercept of regressions of respiration rate on mean l eaf N, protein, starch and LMA was lower for plants grown at elevated compa red to ambient CO2 partial pressures because of the differential effect of growth at elevated CO2 partial pressure on leaf gas-exchange, chemical and physical characteristics. 4. The lower respiration rates for leaves from trees grown at elevated CO2 partial pressure resulted in a significant increase in the ratio of light-s aturated net photosynthesis to respiration, increasing the potential carbon -use efficiency of these leaves.